PATENT DOCUMENT

Abstract:
A switch-type power converter comprising an PET switch operating in a variable duty cycle mode under the control of a Unitrode 3846 integrated circuit controller. Indications of excess input voltage and reverse battery connections are provided by circuits including an element which permanently changes state. A cooling fan mounted on a finned heat sink is operated in a variable speed mode. A single thermistor sensor provides inputs to both the fan speed control and a thermal shutdown circuit connected to shut down the gate drives to the FET switch in the event of a high temperature condition. Another shutdown function is provided in response to an input overvoltage condition by way of an operational amplifier. The converter uses foldback for short circuit protection and is compatible with microprocessor units to selectively provide multiple output voltage levels.

Full Description:
RELATED APPLICATION 
     This application claims priority in part to provisional application Ser. No. 60/607,950 filed on Sep. 9, 2004. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to power converters, and more particularly to switch-type power converters designed for use in recreational vehicles as a regulated power supply for DC load devices and as a battery charger. 
     BACKGROUND OF THE INVENTION 
     As used herein, the terms “recreational vehicle” and “RV” should be construed to embrace motor homes, trailers, campers, van conversions, fifth wheels, boats, and similar products. The common characteristic of these recreational vehicles is an electrical system incorporating one or more batteries to provide power for DC load devices such as lights, refrigerators and motors. The more sophisticated recreational vehicles may also have alternating current systems and AC load devices such as stoves, televisions, microwaves and heating and ventilating systems. The AC load devices are typically powered from a 115 volt AC line voltage source brought to the recreational vehicle through a power cord and plug. Some recreational vehicles also carry generators powered by gas or diesel engines and capable of producing as much as 20 or more kilowatts of AC power. 
     It has become common to install power converters in recreational vehicles. A typical power converter converts 115 vac to 13.6 vdc and charges the RV battery or batteries as necessary. It has become more and more common to use “switch type” power converters rather than linear converters. There are numerous reasons for this including a substantial weight savings. Switch type power converters, often simply called “switchers” or “switching” power converters, typically use one or two power switching semi-conductor devices such as field effect transistors (“FET&#39;s”) and a controller such as the Unitrode UC 3846 for operating the semi-conductor devices in a variable duty cycle mode. Such devices further typically include a step-down transformer and a smoothing circuit between the transformer and the regulated output voltage terminal. 
     A designer of such converters faces numerous issues including heat dissipation, noise generation, tolerance to unstable or excessive supply voltages and protection of the expensive circuit components found therein. The manufacturer of such devices faces these and other issues including warranty claims based on alleged defects when, in fact, field failures are often caused by improper use such as (1) accidentally connecting the converter input to an excessive voltage source such as a 220 vac line or an improperly regulated or runaway generator; and (2) accidentally connecting the RV battery in reverse polarity 
     Power converters which deal with some of these issues are described in U.S. Pat. Nos. 5,600,550 and 5,687,066 issued to James Cook in February and November, respectively, of 1997 and assigned to Progressive Dynamics, Inc. of Marshall, Mich. The power converter described in the &#39;550 patent is of the switch type in which the switch includes two FET&#39;s operating in a push/pull fashion under the control of an integrated circuit controller such as the Unitrode UC 3846. The converter further comprises a fan powered by the converter output and a pair of thermistors mounted on a large heat sink along with the FET&#39;s. One of the thermistors is used in combination with a set-point device to turn the fan on and off and the other is used to shut the controller down in the event temperature reaches an extreme or intolerable level. 
     U.S. Pat. No. 5,687,066 describes a converter identical to that of the &#39;550 patent but adds overvoltage protection. This feature is provided by a Zener diode to sense an overvoltage condition in the dc output of a diode rectifier bridge used to convert an ac line voltage to dc. If the rectified supply voltage exceeds a predetermined limit, the Zener diode conducts and quickly sends a signal to a shut down pin of the Unitrode controller to prevent the controller from turning the FET&#39;s on. This protects the FET&#39;s from damage until the overvoltage condition subsides. 
     SUMMARY OF THE INVENTION 
     The subject invention has for its foundation a switch-type power converter/battery charger including a switch consisting of one or more FET&#39;s operating in a variable duty cycle mode. An integrated circuit controller such as the Unitrode UC 3846 is used with appropriate feedback and a rectifier and LC filter in the output stage to operate the switch to produce a regulated dc output. 
     The subject converter in a typical commercial embodiment includes a rectifier bridge so that the unit may be connected to a standard 60 cycle normal 115 volt ac line. This is typical of the line voltage made available by electric utility companies and/or commercial generators. The feedback system is used to cause the overall converter to operate in a current demand mode wherein the duty cycle of the switch is adjusted to maintain the desired output voltage. 
     In the preferred embodiment described herein, the converter further comprises a transformer for stepping voltages within the converter circuit down to a level suitable for use in connection with dc load devices and the charging of conventional storage batteries. Most of the reference voltages in the converter are taken from the primary side of the transformer. In addition, the fan supply and fan control are on the primary side of the transformer. By supplying the fan from the primary side, an undesirable drop in fan speed under heavy load conditions is avoided. 
     According to a first, more specific aspect of the present invention, a circuit is provided at or near the dc input of the converter; i.e., at or near the output of the ac-to-dc rectifier circuit, for providing a permanent indication of an abnormal over-voltage condition sufficient to cause circuit damage and likely to be the result of operator error. In general, the permanent over-voltage indicator comprises a circuit connected between the output of the ac-to-dc rectifier and ground and includes a device, such as a Zener diode, for establishing a very high breakdown voltage, and a device, such as a fuse, which permanently changes state in response to an over-current condition. The fuse and Zener diode are preferably chosen in the commercial embodiment to correspond to the conditions which might exist if the converter were accidentally connected to a 220 volt ac supply or to an unregulated or runaway generator. The permanent change of state in itself has no effect on converter operation, since it is not a shut down mechanism similar to that of the over-voltage protection feature. But it does provide the manufacturer or warrantor of the system with evidence that any damage occurring to the converter and/or its various circuit components was the result of an extreme over-voltage condition rather than system malfunction or component defects. 
     The permanent input over-voltage indicator is preferably used in combination with an over-voltage shutdown circuit also connected to the output of the ac-to-dc rectifier. The location and overall purpose of the over-voltage shutdown circuit is generally as described in the &#39;066 patent where it is referred to as an overvoltage “protection” circuit, but preferably uses an operational amplifier to establish the shutdown set point voltage in a way which is more precise than that available from the use of a Zener diode as described in the &#39;066 patent. The output of the over-voltage shutdown circuit is connected to a shut down pin in the variable duty cycle controller so as to prevent the switch transistors from turning on (and off again) while the over-voltage condition persists. This protects the expensive FET&#39;s and other components in the switch from damage. The set point of the over-voltage shutdown circuit in the illustrated embodiment is lower than that associated with the permanent over-voltage indicator device described above and the two circuits work in a cooperative fashion; i.e., the over-voltage shutdown circuit effects a shut down function at a first over voltage level whereas the permanent over-voltage indicator circuit changes state at a substantially higher over-voltage level likely resulting from, for example, owner/user error or generator runaway. However, the trip point of the overvoltage indicator could be set below or equal to the overvoltage shutdown circuit if the circuit designer wishes to do so. 
     Another aspect of the present invention in the foundation environment described above is a permanent reverse battery connection indicator circuit. This circuit detects a so-called “reverse” battery condition which results from the erroneous reverse polarity connection of the storage battery to the recreational vehicle electrical system after a period of disconnection for storage or service. Like the over-voltage indicator, the permanent reverse battery connection circuit includes a component which undergoes a permanent change of state when the battery is accidentally connected with the positive and negative terminals in reverse positions. Again, the permanent indicator does nothing to shut down or disable system operation, but simply provides an unequivocal indicator of owner/user error in the event a warranty claim is later made. 
     The converter of the present invention, like the converter described in the &#39;066 patent, uses a metal heat sink as part of the converter packaging structure and mounts certain components on or in contact with the heat sink. A thermistor sensor, preferably mounted on or in contact with the heat sink, is used to monitor converter temperature and provide an output signal which, also unlike the &#39;066 patent converter, is simultaneously supplied to two control circuits. The first control circuit operates the fan in a variable speed mode. These modes of operation are believed to not only extend fan life, but also reduce an annoying quality of fan noise. The thermistor sensor also furnishes a temperature-related signal to a second circuit including a comparator or “op-amp” to shut down the variable duty cycle controller in the event of a high temperature condition which may exceed the capacity of the fan. 
     Other aspects of the invention in the area of thermal control include a special mounting arrangement between the fan motor and the extrusion which provides the heat sink; i.e., a recess is machined into an end of the heat sink extrusion to provide an air gap between the extrusion and the fan motor so that the fan motor does not directly pick up heat from the extrusion. In addition, heavy wire leads are used in overlying relationship to the copper plating of a circuit board used to mount the elements of the circuit of  FIG. 2 . The wire leads are soldered to the board in high current connector areas. Numerous advantages flow from these packaging modifications as will be hereinafter explained in greater detail. 
     Still further aspects and advantages of the invention are described herein and will be best understood from a reading of the following specification which describes and illustrative embodiment in the form of an 80 amp power converter for use in recreational vehicles of the type using conventional storage batteries. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a switched power converter circuit according to the present invention; 
         FIG. 2  is a schematic circuit diagram of an illustrative switched power converter circuit embodying the inventive features described above; 
         FIG. 3  is a graph of temperature versus fan speed illustrating the operating curve of the fan according to the present invention; 
         FIG. 4  is a graph of fan voltage versus fan speed for a typical fan; 
         FIG. 5  shows various waveforms within the circuit of  FIG. 17 ; 
         FIG. 6  is a graph of output converter current versus temperature for a variety of fan speeds; 
         FIG. 7  is a partial schematic diagram illustrating a temperature-responsive input circuit according to the present invention; 
         FIG. 8  is a graph of V tempvar  of  FIG. 7  versus fan voltage showing the desired characteristic; 
         FIG. 9  shows partial schematics of a fan connected to an operational amplifier; 
         FIG. 10  shows a partial schematic of a fan connected to an open collector operational amplifier and a graph showing the resulting fan voltage curve with temperature changes; 
         FIG. 11  shows the partial schematic of  FIG. 10  with the addition of a gain amplifier and a graph showing the resulting fan voltage curve with temperature changes; 
         FIG. 12  shows the partial schematic of  FIG. 11  with the addition of circuit to shift the zero point of the fan voltage curve and a graph showing the resulting fan voltage curve with temperature changes; 
         FIG. 13  shows the equivalent circuit to the circuit to shift the zero point of  FIG. 12 ; 
         FIG. 14  shows the equivalent circuit to the open collector operational amplifier of  FIG. 12 ; 
         FIG. 15  is a schematic of a first embodiment of the control circuit according to the present invention; 
         FIG. 16  is a schematic of a second, alternative, embodiment of the control circuit according to the present invention; 
         FIG. 17  illustrates the two output current paths generated by the secondary-output side of transformer T 1 ; 
         FIG. 18  is a perspective view of a fully packaged power converter embodying the features described herein; 
         FIG. 19  is a cross-section of an illustrative heat sink showing a spring clip to hold a diode in the switch circuit against the heat sink; 
         FIG. 20  is an end elevational view of the power converter package of  FIG. 18 ; 
         FIG. 21  is an opposite end elevational view of the power converter package of  FIG. 18 ; 
         FIG. 22  is a top plan view of the switched power converter package of  FIG. 18 ; 
         FIG. 23  is a perspective view of a RV partially broken away to show the switched power converter according to the invention positioned therein; 
         FIG. 24  is a perspective view of the heat sink of  FIG. 19  showing a recess or relief in the fan mounting surface. 
         FIG. 25  is a photograph of one side of the circuit board used to support the components in the circuit of  FIG. 2  showing heavy wires connected from the center top of the transformer through the circuit board; and 
         FIG. 26  is a photograph of the reverse side of the circuit board, with the image reversed to coincide with the orientation of the  FIG. 25  photograph, showing the heavy wires from the transformer coming through the circuit board and soldered over the conductive traces leading to the negative output terminal. This photograph also shows additional heavy wires running from the fuses to the positive output terminal and also soldered to and in overlying relation to circuit board traces. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a block diagram of a circuit for a switched power converter embodying the features of the present invention. The block diagram includes an AC-to-DC rectifier circuit  10 , a switch circuit  12 , a transformer circuit  14 , a feedback circuit  16 , a controller  18 , an over-voltage shutdown circuit  20 , a permanent over-voltage indicator  22 , a permanent reverse battery indicator  24 , a thermistor circuit  26 , a variable speed fan  32 , fan control circuit  30 , an over-temperature shutdown circuit  28 , a current sensing feedback circuit  34 , a foldback circuit  42 , and an output rectifier and LC filter circuit  44  including the inductor L 2  referred to hereinafter. 
     The AC-to-DC rectifier circuit  10  converts a 115 v AC line voltage into an unregulated and time-varying dc signal with an average in the 170 volt range. It should be noted that the converter  46  can be plugged into a 170 vdc source, if available. In this case the rectifier  10  performs no rectification functions. The unregulated DC signal then enters the switching circuit  12  where the on/off states and duty cycle of the switching circuit  12  is determined by the controller  18  and feedback circuits  34  and  16 . The switching circuit  12  includes two field effect transistors (FET&#39;s). The output of the switching circuit  12  is a regulated waveform containing unidirectional pulses. 
     Current sensing feedback circuit  34  is connected to the output of the switching circuit  12  for the purpose of measuring the output current. The output of the current sensing circuit  34  is connected to controller  18 . Controller  18  adjusts the duty cycle of the FET&#39;s in the switching circuit  12  according to the current measured by the current sensing circuit  34  and the voltage measured by circuit  16 . Accordingly, duty cycle is controlled by two factors: voltage feedback via circuit  16  and current feedback via circuit  34 . 
     Over-voltage shut-down circuit  20  is connected between the output of the AC-to-DC rectifier circuit  10  and a shut-down pin of the controller  18  for the purpose of shutting off the switching circuit  12  in the event the rectified input voltage at 40 exceeds a pre-determined threshold voltage such as 195 vdc. The permanent over-voltage indicator  22  is connected to the output of the AC-to-DC rectifier circuit  10  for the purpose of triggering a permanent indicator in the event the voltage at 40 exceeds a second, higher threshold voltage, such as 220 vdc. As noted above, the second threshold voltage will typically be higher than the first, but could be lower or equal to the first threshold voltage. The over-voltage shut-down circuit  20  will protect the costly transistor components of the switching circuit  12  from being destroyed by the excessive input voltage conditions. The permanent over-voltage indicator  22  will provide evidence to the manufacturer that an undesirably high AC input voltage had been connected to the converter, e.g., a 220 VAC line voltage. The threshold voltage triggering the over-voltage shut-down circuit  20  is typically lower than the threshold voltage triggering the permanent over-voltage indicator  22 , but can be higher or equal to the overvoltage indicator circuit trigger voltage. 
     The regulated signal passes from the switching circuit  12  to the transformer circuit  14 . The transformer circuit  14  steps down the average of the unidirectional pulses to the level necessary for recreational vehicle use; e.g., ultimately to about 13.6 volts. The stepped down waveform is rectified and smoothed by circuit  44  before application to load devices. Feedback circuit  16  measures the voltage across the load. The output of the feedback circuit  16  is connected to controller  18 . Controller  18  then controls the on/off state and duty cycle of the switching circuit  12  based in part on the input received from the feedback circuit  16 . 
     A permanent reverse battery indicator  24  is also connected across the DC load for the purpose of providing a physical record that the operator connected a battery in reverse polarity. Such reverse battery connections may cause damage to the switched power converter, and the manufacturer may have an interest in knowing whether the damage was caused by the reverse connection of the RV battery as opposed to a manufacturing defect. 
     Thermistor circuit  26  senses the temperature of a heat sink  52  in the housing  70 , and provides a variable resistance based on temperature. Over-temperature shutdown circuit  28  receives a signal from the thermistor circuit  26  and, if a set-point is exceeded, sends a shutdown signal to the controller  18 . Controller  18  then terminates the operation of switching circuit  12 . The over-temperature shutdown circuit  28  will not permit the operation of the switching circuit  12  until the temperature sensed by the thermistor has fallen below the undesirable temperature limit. Hysteresis in the circuit makes the temperature at which operation is resumed lower than the shutdown temperature. 
     Fan control circuit  30  receives a signal from thermistor circuit  26 . The fan control circuit  30  produces a variable output based on the input from the thermistor  26 . A variable speed fan  32  is connected to the variable output signal of the fan control circuit  30 , such that the fan  32  will vary in speed based on the input signal. Accordingly, the speed of the fan  32  increases in response to increases in sensed temperatures. A low fan speed minimizes the annoying effects of fan noise at low to moderate power levels. The power supply for the fan  32  comes from the primary side of transformer circuit  14 . This feature eliminates the tendency of the fan supply voltage to droop, with a corresponding fan speed reduction, under heavy load conditions. 
     Having briefly described the overall block diagram of the switched power converter circuit, the schematic circuit of an illustrative, mechanical embodiment will be described in detail with reference to  FIGS. 2 ,  5  and  17 . The preferred values of all described electrical components are listed at the end of the detailed description. 
     Input Circuit 
     Input circuit  36  is connected to a conventional AC power supply through a cable having a conventional 3-prong connector. The 3-prong connector includes a ground conductor, a positive conductor, and a neutral conductor. The cable runs into the housing through an aperture  100 . The AC positive terminal is connected to AC positive input W 1 . The AC ground terminal is connected to AC ground W 2  identified by a “chassis ground” symbol. The AC neutral terminal is connected to AC neutral input W 3 . AC positive input W 1  is connected to thermistor RT 2 . Thermistor RT 2  is used as an inrush current protector for the purpose of protecting fully discharged capacitors from receiving a surge of current. Thermistor RT 2  initially (i.e. when cold) provides a high resistance but rapidly changes to a substantially lower resistance as the temperature increases, allowing an unrestricted AC signal to pass into the noise suppression circuit  38 . It should be noted that there is a primary ground, secondary ground and a “chassis” ground and that different symbols are used for these in  FIG. 2 . 
     Noise Suppression Circuit 
     The noise suppression circuit  38  includes capacitors C 15 , C 16 , C 26 , C 1 , C 2 , C 3 , C 30 , C 29 , and C 31 , inductor beads L 5 , L 6 , L 7 , and L 8 , jumpers J 6 , J 7 , J 8 , J 9 , J 10 , and J 11 , and common mode choke (CMC) transformers T 3 , and T 2 . These electrical components provide electromagnetic interference noise suppression, and filtering to prevent noise from within the converter from traveling back into the ac supply line. Noise transfer suppression is also provided by capacitors C 15 , C 16 , and C 26 . One plate of C 15  is connected to thermistor RT 2 , and one plate of C 16  is connected to AC neutral input line W 3 . The other plates of capacitor C 15 , and C 16  are connected to chassis ground, i.e., the ground of input W 2 . Capacitor C 26  is connected in parallel to C 15  and C 16 , where one plate of capacitor C 26  is connected to thermistor RT 2 , and the other plate of capacitor C 26  is connected to AC neutral input line W 3 . Both plates of capacitor C 26  are connected through CMC transformer T 3 . Winding  2 - 1  of CMC transformer T 3  is connected to thermistor RT 2 , and winding  3 - 4  of CMC transformer T 3  is connected to AC neutral input line W 3 . The output of winding  2 - 1  is connected to the input side of fuse F 1 . 
     Additional noise suppression is provided by capacitors C 1 , C 2 , and C 3 . One plate of capacitor C 1  is connected to the output side of fuse F 1 , and the remaining plate of capacitor C 1  is connected to winding  3 - 4  of CMC transformer T 3 . Capacitor C 1  is also connected to one plate of each of capacitors C 2  and C 3 . The remaining plates of capacitors C 2  and C 3  are connected to chassis ground. CMC transformer T 2  is connected in parallel to capacitors C 2  and C 3 . Winding  2 - 1  of CMC transformer T 2  is connected to the ungrounded plate of capacitor C 2 , and winding  3 - 4  of CMC transformer T 2  is connected to the ungrounded plate of capacitor C 3 . Jumpers J 6 , J 7 , J 8 , are connected in parallel to winding  2 - 1  of CMC transformer T 2 , and jumpers J 9 , J 10 , and J 11  are connected in parallel to winding  4 - 3  of CMC transformer T 2 . All jumpers provide the option of bypassing CMC transformer T 2 . 
     Additional noise suppression is provided by capacitors C 30 , C 29 , and C 31 . The windings of CMC transformer T 2  are connected in parallel to capacitor C 30 . Capacitor C 30  is also connected to one plate of each of capacitors C 29  and C 31 . The remaining plates of capacitors C 29  and C 31  are connected to chassis ground. High frequency noise suppression is provided by inductor beads L 5 , L 6 , L 7 , and L 8 . L 6  and L 8  are fitted onto the bridge connection wires by causing the wires to pass through the center opening of each inductor bead core, wrap around the bead core, and then pass again through the bead core. The wires passing through L 6  and L 8  are then connected between T 2  and a diode bridge DB 1  forming the AC-to-DC rectifier  10 . The inductor beads L 5  and L 7  are similarly mounted on the wires coming out of the diode bridge DB 1  (see  FIG. 2 ). 
     AC-to-DC Rectifier 
     The AC-to-DC rectifier  10  is composed of a diode bridge DB 1 , capacitors C 4   a , C 4   b , and C 4   c . The wires passing through L 6  and L 8  are connected to the input of diode bridge DB 1 . The wires passing through L 5  and L 7  are connected to the output of diode bridge DB 1 . Capacitors C 4   a , C 4   b , and C 4   c  are connected in parallel between wires passing through inductor beads L 5  and L 7 . The wire passing through L 5  is connected to the positive plate of each capacitor C 4   a , C 4   b , and C 4   c , and the wire passing through L 7  is connected to the negative plate of each capacitor C 4   a , C 4   b , and C 4   c . The negative plates of capacitors C 4   a , C 4   b , and C 4   c  are also connected to ground. Under optimal conditions capacitors C 4   a , C 4   b , and C 4   c  are charged by the output of diode bridge DB 1  to a desired voltage of 170 volts. Capacitors C 4   a , C 4   b , and C 4   c  provide an unregulated DC signal to unregulated DC terminal  40 . 
     Permanent Over-Voltage Indicator 
     The permanent over-voltage indicator  22  includes fuse Fx 1  and Zener diode D 23  connected in series between the output  40  of rectifier bridge DB 1  and ground. The permanent over-voltage indicator  22  receives the voltage developed across capacitors C 4   a , C 4   b , and C 4   c , and causes the fuse to change state if the voltage across the capacitors reaches an undesirably high level. The cathode of Zener diode D 23  is connected to the output of fuse Fx 1  and the anode of Zener diode D 23  is connected to ground. It will be noted in  FIG. 2  there are three different ground symbols. One is chassis ground, connected between C 2  and C 3  for instance. Another is primary circuit ground, connected to pin  12  of U 1  for instance. And lastly there is a secondary circuit ground, connected to the output P 1  terminal for instance. Each of these three ground symbols refer to separate voltage reference points and are isolated from each other. It will be further noted that the primary ground symbol is subdivided into an S, S 2  and P ground. The explanation is:
         S is the signal ground   S 2  is the current sensing circuit ground   P is the power ground
 
In practice, these grounds are separate except at one point in circuit board layout to avoid parasitic noise cross talk. Nevertheless, physically they are the same since they are connected by copper traces and wires. The same is the case for the S and P shown with secondary ground symbol.
       

     The purpose of the permanent over-voltage indicator  22  is to provide a permanent indication of receiving an undesirably high input voltage greater than that which triggers the over voltage shut down circuit  20 . If the permanent over-voltage indicator  22  changes state, it will be because the converter input receives an excessive voltage, caused, for example, by a 220 vac supply or runaway generator. If enough voltage is applied to Zener diode D 23  it will fail short creating a direct connection between fuse Fx 1  and ground. This short failure of D 23  causes Fx 1  to permanently change state, i.e., blow out to create an open circuit. The preferred voltage limit of the permanent over-voltage indicator  22  is normally 220 volts dc. It will be noted that because the indicator circuit  22  is a shunt, failing the diode and blowing the fuse Fx 1  does not disable the converter. The term “permanent” is used herein to mean device which does not reset by itself; i.e., it must be replaced to operate a second time. Since tripping the indicator does not shut down the converter, the owner has no reason to replace it and typically will not be aware of its presence. Therefore, it remains in the converter until the converter is returned for service or a warranty claim. For the majority of converters, this never happens. However, for the small percentage of converters returned for a warranty claim, the indicator helps the manufacturer evaluate the likelihood that circuit failures are the result of excessive input voltage other than manufacturing or material defect. If a converter is returned for service and the indicator fuse Fx 1  is failed, it will be replaced along with any other failed components and may, for example, signal the need to provide the owner with a cautionary message regarding the quality of the supply voltage source being used. 
     Over-Temperature Shut-Down Circuit 
     The over-temperature shutdown circuit  28  measures the heat sink temperature in the switched power converter and triggers a shutdown of the switching circuit  12  upon receiving an undesirably high temperature. The over-temperature shutdown circuit  28  includes Schottky diode D 3 , resistors R 8 , RN 1 B, R 7 , and RN 1 A, operational amplifier U 3 A, and thermistor RT 1 . Thermistor RT 1  changes in resistance based on sensed temperature. Preferably thermistor RT 1  is a negative-temperature-coefficient device and is mounted on or in contact with the converter heat sink  52  in the manner shown in  FIG. 19 ; i.e., a spring clip holds the sensor against a surface of the casting which makes up the sink  52 . Because the FET&#39;s in the switch  12  are also mounted in contact with the sink  52 , heavier load conditions cause the temperature of the sink  52  to rise. If turning the fan  32  on stabilizes the temperature, no further remedy is needed. It should be noted that the thermistor RT 1  does not have to be mounted on the heat sink, but can be mounted to measure, for example, air temperature or the temperature of some component such as the transformer  14  or the output inductor in circuit  44 . The illustrated arrangement is, however, preferred. 
     Operational amplifier U 3 A is used as a comparator for the purpose of triggering shutdown pin (pin  16 ) of controller  18  in the event that the internal temperature of the switched power converter exceeds a set-point temperature. Once shutdown pin (pin  16 ) of controller  18  is triggered the operation of switching circuit  12  is terminated. 
     Operational amplifier U 3 A includes the following connections: pin  1  is the output, pin  2  is the negative input, pin  3  is the positive input, pin  4  is connected to a 5 volt reference voltage  5 REF, and pin  11  is connected to ground. Pin  2  is connected to a temperature based variable voltage coming from a voltage divider circuit comprised of resistor RN 1 A and thermistor RT 1 . Pin  3  is connected to a reference voltage through a voltage divider circuit using resistors R 8 , R 7 , and RN 1 B. Pin  1  is connected to resistor R 8 , and Schottky diode D 3  leading to shutdown pin (pin  16 ) of controller  18 . 
     The output of operational amplifier U 3 A will remain at a low (ideally zero) voltage and will not trigger shutdown pin  16  of controller  18  as long as pin  3  input does not exceed the pin  2  input. When the internal temperature is sufficiently high, the voltage on pin  3  will exceed the voltage on pin  2  and the output of pin  1  will go high and trigger a shutdown. 
     The over-temperature shutdown circuit  28  will operate as follows under a cold temperature condition (i.e. a temperature condition where a thermal shutdown is not required). Resistor RN 1 A and thermistor RT 1  form a voltage divider circuit. Resistor RN 1 A is connected to a 5 volt reference  5 REF and thermistor RT 1  is connected to ground. Thus, pin  2  receives the voltage between resistor RN 1 A and thermistor RT 1 . Accordingly, the voltage applied to pin  2  will vary depending on the temperature of the heat sink  52 . 
     The value of resistor RN 1 A is 16.2K ohms, and the value of thermistor RT 1  is 100K ohms of 25° C. Thus, when the switched power converter is initially turned on and the temperature is cold the value of thermistor RT 1  will be about 100K ohms. At cold startup the voltage applied to the pin  2  of operational amplifier is roughly 4.3 volts. Further, at a cold (i.e. non thermal shutdown) temperature pin  1  will be near 0 volts because the voltage at pin  2  is higher than the voltage at pin  3 . When the voltage at pin  1  is near 0 volts, resistor R 8  is parallel with resistor R 7 . 
     In the illustrative embodiment, the values of resistors R 8 , R 7  and RN 1 B are 499K, 32.4K, and 47.5K ohms respectively. Because resistors R 8  and R 7  are in parallel, their equivalent resistance at 25° C. is 30.4K ohms. This resistance of 30.4K ohms will be called R coldtemp . Accordingly, the voltage at pin  3  will be the measured voltage between resistor RN 1 B and R coldtemp . Using a voltage divider, the voltage applied to pin  3  at a cold temperature is 1.925 volts. This voltage will be called V coldtemp . Accordingly, at a cold temperature the voltage at pin  3  will be V coldtemp  which is 1.925 volts. If the internal temperature significantly increases, the resistance of thermistor RT 1  will decrease and the voltage applied to pin  2  will fall below the voltage applied to pin  3 , the output of pin  1  will become positive, and the switched power converter will experience a thermal shutdown. 
     The over-temperature shutdown circuit  28  will operate as follows under a thermal shutdown condition (i.e. a temperature condition where a over-temperature shutdown is required). A shut down temperature is never reached if the load on the converter is within normal specifications because the fan  32  will provide sufficient cooling. If the load is very heavy and/or the operator has covered the converter  46  with blankets or the like, a shut down temperature may be reached. If this happens, the voltage applied to pin  1  will be approximately 5 volts. When pin  1  reaches 5 volts, resistors RN 1 B and R 8  will be in parallel (as opposed to resistor R 7  being in parallel with resistor R 8  at a cold temperature). The equivalent resistance of resistors RN 1 B and R 8  in parallel is 43.37K ohms. This resistance will be called R hottemp . Accordingly, the voltage at pin  3  will be the measured voltage between resistor R 7  and R hottemp . Using a voltage divider the voltage applied to pin  3  at a cold temperature is 2.138 volts. This voltage will be called V hottemp . Accordingly, in order for the switching circuit  12  to begin operation the voltage on pin  2  must rise above V hottemp  (rather than V coldtemp ). This hysteresis caused by resistor R 8  is important so that the switching circuit  12  will not be enabled until the internal temperature falls significantly below the temperature at which the thermal shutdown was triggered. 
     Over-Voltage Shutdown Circuit 
     The over-voltage shutdown circuit  20  measures the voltage of capacitors C 4   a , C 4   b , and C 4   c , and will shutdown the switching circuit  12  in the event of an over-voltage condition at point  40 . The over-voltage circuit  20  includes resistors R 38 , R 39 , R 40 , R 7 , and RN 1 B, operational amplifier U 3 B, and Schottky diode D 27 . The output of over-voltage shut down circuit  20  is connected to shutdown pin  16  of controller  18 , such that a high signal will terminate the operation of switch  12 . The over-voltage shut-down circuit  20  assures that transistors Q 2   a  and Q 2   b  are not damaged in the event of an undesirably high voltage at the output of the ac-to-dc converter; i.e., at point  40 . As discussed above, there are a number of factors which may cause high voltage conditions to exist. Lightning strikes or transients from other loads on the supply line, unregulated generators, runaway generators and the like may all cause over-voltage conditions. 
     Transistors Q 2   a  and Q 2   b  are rated at 500 volts. Because of the properties of transformer T 1 , transistor elements Q 2   a  and Q 2   b  will experience a voltage twice that imposed on capacitors C 4   a , C 4   b , and C 4   c . Accordingly, when capacitors C 4   a , C 4   b , and C 4   c  are at 250 volts, the transistor elements Q 2   a  and Q 2   b  will experience 500 volts. Accordingly, if the voltage of capacitors C 4   a , C 4   b , and C 4   c  exceeds 250 volts transistors Q 2   a  and Q 2   b  may be damaged. 
     Operational amplifier U 3 B includes the following connections: pin  7  is the output, pin  6  is the negative input, pin  5  is the positive input, pin  4  is connected to a 5 volt reference voltage  5 REF, and pin  11  is connected to ground. Pin  5  is connected to a voltage divider circuit comprised of resistors R 38 , R 39 , and R 40 . The voltage applied to pin  5  will vary depending on the line voltage of capacitors C 4   a , C 4   b , and C 4   c . Pin  6  is connected to a reference voltage through a voltage divider circuit comprised of resistors R 8 , R 7  and RN 1 B. Pin  7  is connected to resistor R 40 , and schottky diode D 27  leading to shutdown pin  16  of controller  18 . The output of operational amplifier U 3 B will remain as a low, ideally zero, voltage and will not trigger shutdown via pin  16  of controller  18  as long as pin  5  input does not exceed pin  6  input. When the line voltage of capacitors C 4   a , C 4   b , and C 4   c  is sufficiently high, the voltage on pin  5  will exceed the voltage on pin  6  and the output of pin  7  will trigger a shutdown. 
     The over-voltage shut-down circuit  20  will operate as follows under a normal voltage condition (i.e. a voltage condition that does not require an over-voltage shutdown). Resistors RN 1 B and R 7  form a voltage divider circuit, where resistor RN 1 B is connected to a 5 volt reference  5 REF and resistor R 7  is connected to ground. Accordingly, pin  6  receives the voltage between resistor RN 1 B and R 7 . Remember, that the voltage applied between resistors RN 1 B and R 7  will vary depending upon the operation of the over-temperature circuit  28  (i.e. when the temperature is cold resistor R 8  is in parallel with resistor R 7 , and when a thermal shutdown temperature is achieved resistor R 8  is in parallel with resistor RB 1 B). Accordingly, the voltage applied to pin  6  will vary depending on whether or not a thermal shutdown temperature is present. However, once a thermal shutdown has been triggered by over-temperature shut-down circuit  28  the operation of the over-voltage circuit  20  is irrelevant. Thus, for this explanation it will be assumed that the temperature is below shutdown level and resistor R 8  is in parallel with resistor R 7 . 
     In the illustrated embodiment, the values of resistors R 8 , R 7  and RN 1 B are 499K, 32.4K, and 47.5K ohms respectively. Remembering that at a low temperature resistors R 8  and R 7  are in parallel, their equivalent resistance is 30.4K ohms. This resistance of 30.4K ohms will be called R coldtemp . Accordingly, the voltage at pin  6  will be the measured voltage between resistor RN 1 B and R coldtemp . Using a voltage divider the voltage applied to pin  6  at a cold temperature is 1.925 volts. This voltage will be called V shutdownref . 
     In illustrative embodiment, the value of resistors R 38 , R 39 , and R 40  is 84.5K, 866, and 97.6K ohms, respectively. Prior to an over-voltage shutdown, pin  7  will remain at a low, ideally zero, voltage, causing resistor R 40  to be in parallel with resistor R 39 . The equivalent resistance of resistors R 39  and R 40  in parallel is 858.4 ohms. This resistance of 858.4 ohms will be called R normalvoltage . Pin  5  receives the voltage between the voltage divider circuit created by resistors R 38  and R normalvoltage . Accordingly, the unregulated DC terminal  40  voltage must exceed 195 volts for the voltage at pin  5  to exceed V shutdownref  (e.g. if unregulated DC terminal  40  carries a voltage of 195 volts, pin  5  will be at approximately 1.961 volts which sufficiently exceeds the 1.952 volts applied to pin  6 ). Thus, when the unregulated DC terminal  40  reaches a voltage of 195 volts the output of pin  7  will become positive causing controller  18  to shutdown the switching circuit  12 . Because capacitor voltage is approximately 1.4 times AC line voltage, the illustrative embodiment of the over-voltage shutdown circuit  20  will shut down the DC output if the AC input voltage exceeds 140 volts (i.e. the voltage of capacitors C 4   a , C 4   b , C 4   c  exceeds 195 volts). Remember that the preferred embodiment of the permanent over-voltage indicator  22  will be triggered at about 220 volts. Accordingly, the output of the switched power converter will be terminated by the over-voltage shut down circuit  20  at a lower over-voltage condition than that which changes the state of the fuse Fx 1  in the permanent over-voltage indicator  22 . 
     The over-voltage shutdown circuit  20  operates as follows under an over-voltage shutdown condition (i.e. the AC input voltage exceeds 140 volts). When a over-voltage shutdown condition is reached, the voltage applied to pin  7  is approximately 5 volts. When pin  7  reaches 5 volts, resistor R 40  is no longer in parallel with resistor R 39 , but will be used for a hysteresis effect. For example, when pin  7  is positive (i.e. over-voltage condition) resistor R 40  will provide feedback into pin  5 , which will in turn increase the voltage at pin  5 . Accordingly, once operational amplifier U 3 B triggers a shut down, the voltage at terminal  40  must be significantly lower than the 195 volts which triggered the initial shut down because resistor R 40  has temporarily increased the voltage measured by pin  5 . The purpose of the resistor R 40  hysteresis is to prevent the controller  18  from operating the switching circuit  12  until the voltage at terminal  40  has significantly fell below 195 volts. 
     Fan Control Circuit 
     The fan control circuit  30  includes resistors RN 1 A, R 4 , RN 1 C, R 2   a , R 1 , and R 20 , thermistor RT 1 , operational amplifier U 3 D, transistor Q 1 , capacitor C 5 , and Schottky Diode D 1   a .  FIGS. 2-16  are used to describe the operation of the fan control circuit  30  and the fan  32 . In this embodiment, the fan  32  is powered by a dc motor which varies in speed as a function of voltage amplitude, i.e., it is the control circuit which produces the variable speed characteristic. The fan control circuit  30  commands the fan  32  to come on at an initial (lowest) temperature. The speed of the fan  32  increases with temperature and will maximize at some point prior to the switched power converter being at full load. The fan control circuit is also described in the aforementioned provisional application, attorney docket no. PDY-106-A, the content of which is incorporated herein by reference. 
     An operating curve of the fan  32  using the fan control circuit  30  is shown in  FIG. 3 . (The slope is not necessarily linear as discussed in more detail herein.) The fan control circuit  30  will cause the fan  32  to come on at low speed when temperatures are over the set point by only a small amount. 
     The relationship between the voltage applied to the fan and the fan speed is shown in  FIG. 4 . Due to static friction the fan  32  does not start moving until a certain voltage is reached. Specifically, and as illustrated in  FIG. 4 , the fan blades will not move until the voltage at point  2  is reached. Compared to the thermal time constants, it more or less instantaneously starts moving, jumping to point  3  (initial turn on point). As the voltage increases, it moves to point  4 , where the fan  32  is operating at maximum speed. On the way down, the variable voltage controlled fan  32  follows from point  4  (maximum operation) to point  3  (initial turn on point) to point  1  (shut off). 
       FIG. 6  illustrates in principle how the fan control circuit  30  works. Temperatures T H  and T L  are the temperatures at which the fan  32  is ideally full on and full off, respectively. More accurately, T H  (line C) is the temperature at which full fan voltage is applied, and T L  (line D) is the temperature at which no voltage is applied to the fan  32 . Currents below point  14  have steady-state operating points on the “fan off” line (line B). currents above point  15  have steady-state operating points on the “fan full on” line (line A). Therefore, points  14  and  15  must be the beginning and end of the line of operating points when operating at currents where the variable voltage controlled fan  32  is in an intermediate state between full on and full off. Although a straight line (line E) is shown connecting these two points, the relationship is not necessarily a linear one. It is clearly, however, a strictly increasing (positive slope) function.  FIG. 6  illustrates the ideal case. 
     Assume the switched power converter starts cold at current operating point I OP2 , point  1 . The switched power converter will warm up and at point  2 , T L , the fan  32  will start to turn slowly. The heat sink  52  continues to warm up until it reaches its steady-state operating point, point  4 . Similarly, for current operating point I OP1 , the switched power converter will start at point  18 , the fan  32  will come on at point  6  and settle into a steady speed at point  7 . 
     Turning to  FIG. 4 , the operating characteristics of fan  32  are explained. Assume that T L1  corresponds to point  1  on  FIG. 4  and that T L2  corresponds to point  2  on  FIG. 4  (same as point  3 ). Thus, returning to  FIG. 6 , line G describes an actual fan  32 . Again, this relationship is not necessarily a linear one as shown, but it is a positive slope function. Starting cold with operating current I OP2 , the temperature increases. At point  2  (T L ), voltage starts being applied to the variable voltage controlled fan  32 , but it is not yet moving. At point  3  (T L2 ) the fan  32  begins to rotate. The switched power converter continues to heat up and eventually settles at point  5  (along line G). For I OP1 , the switched power converter would start cold at point  18  and heat up to point  6  (T L ). At point  6 , voltage begins to be applied to the fan  32 . The switched power converter will continue to heat up until point  9  (T L2 ), where the fan  32  begins moving. The fan  32  will now be moving faster than it needs to, the switched power converter will cool and eventually settle into a steady state speed at point  8  (along line G). 
     In both cases, the fan  32 , once started, continues to rotate. There is no discontinuance of operation. Notice further that variable voltage controlled fan  32  speeds are slower (and less noisy) for all current levels up to point  15  (T H ). Also notice the minimum current to turn the fan  32  on corresponds to point  17  (T L2 ), but if already on, it will stay on to a lower current, corresponding to point  16  (T L1 ). 
     A description of the fan control circuit  30  is illustrated in  FIGS. 2 , and  7 - 16 . The preferred embodiment of the fan control circuit includes resistors RN 1 A, R 4 , RN 1 C, R 1 , and R 2   a , thermistor RT 1 , transistor Q 1 , and operational amplifier U 3 D. Operational amplifier U 3 D includes the following connections: pin  14  is the output, pin  12  is the positive input, pin  13  is the negative input, pin  11  is connected to ground, and pin  4  is connected to a 5 volt reference voltage  5 REF. 
     As illustrated in  FIGS. 2 , and  7 , thermistor RT 1  is used as a temperature sensor for the fan control circuit  30  as well as the over-temperature shutdown circuit  28 . Thermistor RT 1  is connected to ground as well as resistor RN 1 A which also connected to a 5 volt reference  5 REF. Thermistor RT 1  and resistor RN 1 A are used to create a voltage divider circuit where V tempvar  is the output of the voltage divider circuit. V tempvar  is connected to pin  13  of operational amplifier U 3 D. Preferably RT 1  is a negative-temperature-coefficient thermistor. As the internal temperature increases, V tempvar  decreases. For the remainder of the fan control circuit  30 , a profile of a desirable fan voltage versus V tempvar  is shown in  FIG. 8 . 
     Because the components used in switched power converter (i.e. operational amplifier U 3 D) are powered by 5 volts, whereas the fan  32  requires a nominal 12 volts, a direct connection of an operational amplifier such as that shown in  FIG. 9  will not work. Simply stated an operational amplifier such as operational amplifier U 3 D cannot supply sufficient current or voltage to the fan  32 . Neither will transistor emitter follower-type circuits work because of voltage limitations. An open collector operational amplifier would work in a circuit such as that shown in  FIG. 10 , and a simple gain amplifier would almost provide the desired profile as shown in  FIG. 11 . Shifting the “zero” point will get the desired profile as shown in  FIG. 12 . Specifically, a Thevenin resistance and voltage coupled to the negative input of the operational amplifier would shift the zero point of the fan control. 
       FIG. 13  illustrates an equivalent of the Thevenin resistance and voltage, and the open collector operational amplifier is shown equivalently in  FIG. 14 . Using a conventional operational amplifier having an output connected to resistor R 1  and transistor Q 1  will result in a complete fan control circuit according to  FIG. 15 . In almost all cases, the fan  32  will be quiet, and only under extended high load or high ambient temperature condition will the switched power converter warm up enough to cause the fan  32  to be heard. 
     Because the circuit in  FIG. 15  has a linear range between full on and full off, significant power will be dissipated in transistor Q 1  at intermediate fan speeds. An alternative is to modify the linear circuit to act as a duty cycle control circuit as shown in  FIG. 16 . With duty cycle control, transistor Q 1  will be either full on or full off (zero voltage or zero current), but the duty cycle will vary to control the speed of the fan. 
     In  FIG. 16 , resistor R 3  adds hysteresis and causes operational amplifier U 3 D to behave as a comparator. As the switched power converter warms up, transistor Q 1  is off until it reaches a “low” temperature. The fan control circuit  30  then breaks into oscillation with low “on” duty cycle on transistor Q 1 . As the switched power converter continues to warm, the duty cycle gets larger. When an upper temperature is reached, the oscillation stops, and transistor Q 1  is always on and stays on as the temperature increases further. 
     The fan control circuit  32  as shown in  FIG. 2  includes resistors RN 1 C and R 4  acting as a voltage divider circuit connected to pin  12  of operational amplifier U 3 D. Resistor RN 1 C is connected to 5 volt reference  5 REF and resistor R 4  is connected to ground. The preferred value of resistor RN 1 C is 9.53K ohms, and the preferred value of resistor R 4  is 22.6K ohms. More exactly, the currents flowing through R 2   a  will also contribute to voltage at pin  12 . Analysis yields 
               VP   ⁢           ⁢   1   ⁢   N   ⁢           ⁢   12     =             +   5     ⁢           ⁢   VREF     RNIC     +       VQ   ⁢           ⁢   1   ⁢           ⁢   C       R   ⁢           ⁢   2   ⁢   a             1     R   ⁢           ⁢   2   ⁢   a       +     1   RNIC     +     1     R   4                 
where R 2   a  has the preferred value of 453K and VQ 1 C is the collector voltage of Q 1 .
 
     When Q 1  is off and no current flows through the fan, VQ 1 C can be as high as the voltage in C 5 , which can vary with line voltage. 
     Using a nominal value of 15 volts for the voltage on C 5  yields pin  12  voltages; 
     VPIN 12 =3.4657 for VQ 1   c =0 volts 
     VPIN 12 =3.6844 for VQ 1   c =15 volts 
     Thus VPIN 12  can more exactly have a range of voltages between 3.4657 and 3.6844 depending on the voltage at the collector of Q 1 . At pin  13  of operational amplifier U 3 D, resistor RN 1 A and thermistor RT 1  act as a voltage divider circuit. The preferred value of resistor RN 1 A is 16.2K ohms, and the preferred value of thermistor RT 1  is 100K ohms at a cold start up temperature (25° C.). Accordingly, the initial voltage applied to pin  13  at a cold temperature is approximately 4.3 volts, which will be called V tempvar . 
     At the initial startup of the switched power converter  46 , V tempvar  is greater than 3.68 v. Thus the output of operational amplifier U 3 D is near zero causing transistor Q 1  to be off and the fan  32  is not running. As the temperature increases, the resistance of thermistor RT 1  will decrease causing the value of V tempvar  to drop from the initial 4.3 volts. Eventually the temperature will increase such that the value of V tempvar  will fall slightly below 3.68 volts. When this occurs the circuit including operational amplifier U 3 D will enter the linear region. There will be a slight fan voltage but it will probably remain in the stalled condition. If the temperature continues to increase the value of V tempvar  will fall significantly below 3.68 v but above 3.46 v and operational amplifier U 3 D causes the fan to enter the mid speed range. As V tempvar  falls further, op-amp U 3 D turns transistor Q 1  full on and the fan  32  reaches full speed. 
     The fan control circuit  30  provides the variable voltage to control the speed of the fan  32 . The transformer circuit  14  provides steady power to the power input of the fan  32 . The power input for the fan  32  is connected to pin  3  of transformer T 1 , through Schottky diode D 1   a  and resistor R 20 . Pin  7  of transformer T 1  is connected to ground, completing the power input circuit for the fan  32 . Resistor R 20  is used for the purpose of preventing the voltage applied to the fan  32  from exceeding specifications. One plate of capacitor C 5  is connected to ground and the other plate is connected between resistor R 20  and Schottky diode D 1   a  for the purpose of providing a steady voltage to resistor R 20 . Capacitor C 5  is charged by transformer T 1  and carries enough voltage to power the fan  32 . Schottky diode D 1   a  prevents capacitor C 5  from discharging into pin  3  of transformer T 1 . 
     Because the power input to the fan  32  is connected to the primary side of the transformer circuit  14 , the fan control circuit  30  as well as the variable voltage controlled fan  32  will remain operational even when the output is heavily loaded or short circuited. Simply stated, this feature will permit the cooling system of the switched power converter to continue to operate in the event of an over-loaded output. Appropriately, the occurrence of this condition is when the operation of the fan  30  is most vital. 
     Transformer Circuit 
     Transformer circuit  14  is primarily inclusive of transformer T 1 . Pin  4  of transformer T 1  is the positive input line. Pin  4  is connected to unregulated DC terminal  40 , where the DC line voltage is approximately 170 volts. Pins  5  and  6  of transformer T 1  are connected to the switching circuit  12 . The switching circuit  12  provides a switching current to the primary-input side of transformer T 1 . For example, switching circuit  12 , which is controlled by controller  18 , allows current to flow between pins  4  and  5 , and between pins  4  and  6 . However, the current between pins  4  and  5 , and pins  4  and  6  will never flow simultaneously, but will alternate according to controller  18 . Operation is described below with reference to  FIG. 17 . 
     Switching Circuit 
     As illustrated by  FIGS. 2 and 17 , the preferred embodiment of the switching circuit  12  contains two transistors Q 2   a  and Q 2   b . When transistor Q 2   a  is turned on current I 1  will flow from pin  4  of transformer T 1  to pin  6 . Alternatively when transistor Q 2   b  is turned on current I 2  will flow from pin  4  of transformer T 1  to pin  5 . When transistor Q 2   a  is on transistor Q 2   b  will be off, and when transistor Q 2   b  is on, Q 2   a  will be turned off. The primary-input side of transformer T 1  is utilized in such a fashion so that the transistors within the switching circuit  12  may operate at up to a maximum 50% duty cycle, meaning that transistors Q 2   a  and Q 2   b  are never on more than 50% of the time. 
     As further illustrated in  FIGS. 2 and 17 , the secondary-output side of transformer T 1  includes pins  2 ,  8 , and  1 . When current I 2  flows between pins  4  and  5  of transformer T 1 , current I 4  will correspondingly flow between pins  8  and  2 . Alternatively, when current I 1  flows between pins  4  and  6  of the primary-input side of transformer T 1 , current I 3  will correspondingly flow between pins  8  and  1  of the secondary-output side. The switching circuit  12  further includes R 37 , R 23   a , R 23   b  and C 24  (shown only in  FIG. 2 ). 
     Transistors Q 2   a  and Q 2   b  provide two current loops. Transistor Q 2   a  is connected to pin  6  on the primary side of transformer T 1 , and transistor Q 2   b  is connected to pin  5  on the primary side of transformer T 1 . Controller  18  controls the on/off state of transistors Q 2   a  and Q 2   b . When transistor Q 2   a  is turned on, Q 2   b  is off. Current I 1  flows between pins  4  and  6  of transformer T 1 ; alternatively, when transistor Q 2   b  is turned on, Q 2   a  is off and current I 2  flows between pins  4  and  5  of transformer T 1 . The gate of transistor Q 2   a  is connected to resistor R 23   a  which is connected to AOUT (pin  11 ) on controller  18 . The gate of transistor Q 2   b  is connected to resistor R 23   b  which is connected to BOUT (pin  14 ) on controller  18 . When controller  18  applies a voltage to the gate of transistor Q 2   a , transistor Q 2   a  turns on and allow current I 1  to flow from pin  4  of transformer T 1 , through pin  6 , and then to ground through the drain and source of transistor Q 2   a . Alternatively, when controller  18  applies a voltage to the gate of transistor Q 2   b , transistor Q 2   b  will turn on and allow current I 2  to flow from pin  4  of transformer T 1 , through pin  5  and to ground through the drain and source of transistor Q 2   b.    
     Resistor R 37  and capacitor C 24  are connected in series between the drain of transistors Q 2   a  and Q 2   b  for the purpose of snubbing the transient drain voltage when transistors Q 2   a  and Q 2   b  are switching. 
     Controller 
     Controller  18  is used for controlling the output of the switching circuit  12  by controlling the duty cycles of switching transistors Q 2   a  and Q 2   b . Controller  18  receives input from the current sensing circuit  34 , over-voltage shut-down circuit  20 , over-temperature shut-down circuit  28 , feedback circuit  16 , and foldback circuit  42 . 
     As discussed previously, AOUT (pin  11 ) and BOUT (pin  14 ) are connected to transistors Q 2   a  and Q 2   b  respectively for the purpose of controlling the duty cycle and switching current of the switching circuit  12 . SHDN (pin  16 ) is connected to both the output of the over-voltage shutdown circuit  20  and the over-temperature shutdown circuit  28  for the purpose of terminating the operation of the switching circuit  12 . If SHDN (pin  16 ) receives a sufficient voltage AOUT (pin  11 ) and BOUT (pin  14 ) will turn off transistors Q 2   a  and Q 2   b , which will terminate the output across the DC load. 
     CS+ and CS− are connections to operational amplifier CS, which is internal to controller  18 . The output of operational amplifier CS corresponds to the instantaneous voltage output of current sensing circuit  34 . CS+ (pin  4 ) is connected to the output of the current sensing circuit  34 , which measures the current through transistors Q 2   a  and Q 2   b.    
     EA+, EA−, and COMP are connections on operational amplifier EA, which is internal to controller  18 . The output of operational amplifier EA is compared to the output of operational amplifier CS. EA+ (pin  5 ), is connected to the output of the feedback circuit  16 . EA− (pin  6 ) is connected to COMP (pin  7 ), acting as a voltage follower on operational amplifier EA. Accordingly, the output of operational amplifier EA will be the same as the voltage applied to EA+ (pin  5 ). 
     If the instantaneous output of operational amplifier CS exceeds the output of operational amplifier EA AOUT (pin  11 ) and BOUT (pin  14 ) transistors Q 2   a  and Q 2   b  are turned off. If the current generated by transistors Q 2   a  and Q 2   b  exceeds the limit set by feedback circuit  16 , controller  18  will temporarily terminate the gate drives to Q 2  and Q 2   b . This comparison/control function occurs on a cycle-by-cycle basis. 
     CLADJ (pin  1 ) is used to further limit the current output of the switching circuit  12 . The voltage applied to CLADJ (pin  1 ) limits the maximum current output of the switched power converter. As the voltage applied to CLADJ (pin  1 ) decreases so does the maximum current output of the switched power converter. CLADJ (pin  1 ) is connected to the output of the foldback circuit  42 , where the foldback circuit will cause the current limit to decrease (i.e. reduce the voltage applied to CLADJ) in a near short circuit situation. CLADJ (pin  1 ) is also connected between resistors R 14  and R 15  which act as a voltage divider circuit. Resistor R 14  is connected to 5 volt reference  5 REF and is in series with resistor R 15 . Resistor R 15  is also connected to ground. 
     VREF (pin  2 ) provides a 5.1 volt reference voltage which supplies power to various electrical components within the switched power converter. The output of VREF is identified as 5 volt reference  5 REF. VIN (pin  15 ) is connected to a power supply for the purpose of providing power to controller  18 . VIN (pin  15 ) is connected to Zener diode D 9  and capacitor C 10  which provide approximately 15 volts to controller  18 . Zener diode D 9  and capacitor C 10  receive voltage from unregulated DC terminal  40  through resistors R 24   a  and R 24   b.    
     VC (pin  13 ) is the power supply for the sales of transistors Q 2   a  and Q 2   b  through AOUT (pin  11 ) and BOUT (pin  14 ), respectively. VC (pin  13 ) is connected to VIN (pin  15 ) through resistor R 16 . Resistor R 16  is used to limit the current entering VC (pin  13 ). Schottky diodes D 16   a , D 15   a , D 15   b  and D 16   b  are used to prevent the voltage on AOUT (pin  11 ) and BOUT (pin  14 ) from exceeding VIN or from dropping below GND. 
     GND (pin  12 ) is connected to ground. Capacitor C 25  is connected to CT (pin  8 ) and resistor R 13  is connected to RT (pin  9 ) for setting the frequency and maximum duty cycle of controller  18 . Capacitor C 25  and resistor R 13  are also connected to ground. SYNC (pin  10 ) is not utilized. 
     Foldback Circuit 
     As briefly mentioned, foldback circuit  42  provides feedback to controller  18  for the purpose of reducing the duty cycle of transistors Q 2   a  and Q 2   b  under near short circuit conditions rather than allowing the output current across the DC load to increase out of control. Foldback circuit  42  includes, diode D 4 , resistors R 19   a , R 19   b , R 17 , and R 18 , capacitor C 8 , and operational amplifier U 3 C. Operational amplifier U 3 C has the following connections: pin  8  is the output, pin  9  is the negative input, pin  10  is the positive input, pin  4  is connected to 5 volt reference  5 REF, and pin  11  is connected to ground. 
     The foldback circuit  42  measures the duty cycle of transistors Q 2   a  and Q 2   b . Pin  10  is connected to AOUT (pin  11 ) and BOUT (pin  14 ) on controller  18  through resistors R 19   a  and R 19   b.    
     Capacitor C 8 , which is connected between pin  10  and ground, as well as in series with resistors R 19   a  and R 19   b  is used for the purpose of averaging the duty cycle controlled gate voltages of transistors Q 2   a  and Q 2   b . Resistor R 17  is connected between 5 volt reference  5 REF and pin  10 , and resistor R 18  is connected between pin  10  and ground for the purpose of creating a voltage divider circuit to reduce the voltage applied to pin  10 . Pin  9  is connected to pin  8  for the purpose of creating a voltage follower, such that the voltage at pin  8  will always equal the voltage applied to pin  10 . Pin  8  is also connected to the cathode of diode D 4 , and the anode of diode D 4  is connected to CLADJ (pin  1 ) of controller  18 . 
     As the duty cycle of AOUT (pin  11 ) and BOUT (pin  14 ) increases, the voltage of capacitor C 8  increases as well as the voltage on pin  10 . Accordingly, the voltage on pin  8  will be higher than the voltage between resistors R 15  and R 14 . When this occurs, diode D 4  will be reverse biased and the voltage at CLADJ (pin  1 ) of controller  18  will not be affected. In this situation the current limit of CLADJ will neither decrease nor increase because foldback circuit  42  is not pulling current from CLADJ (pin  1 ). 
     As the duty cycle of AOUT (pin  11 ) and BOUT (pin  14 ) decreases, the voltage of capacitor C 8  decreases as well as the voltage on pin  10 . Accordingly, the voltage on pin  8  will be lower than the voltage between resistors R 15  and R 14 . When this occurs, diode D 4  will be forward biased and the voltage at CLADJ (pin  1 ) of controller  18  will be pulled down. As the voltage applied to CLADJ (pin  1 ) decreases, the maximum current output of controller  18  will also decrease. Accordingly, in the event of a near short circuit at the DC load, the reduced current limitation of CLADJ will prohibit the current output from going unreasonably high and reduce the output current to less than its previous maximum rating. 
     Voltage Feedback Circuit 
     The feedback circuit  16  measures the voltage across the DC load and outputs a reference voltage to controller  18 . Controller  18  contains an internal voltage controller, for the purpose of providing a voltage controlled current source. Controller  18  will control the switching of transistors Q 2   a  and Q 2   b  accordingly. Feedback circuit  16  includes resistors R 28 , R 34 , R 32 , R 26 , R 25 , R 33  and R 30 , capacitors C 27 , C 22 , C 20 , and C 28 , and optical coupler U 2  which includes a LED, a photo-sensor and a 2.5 volt reference. 
     When the DC load is increased, there is an immediate drop in voltage across the DC output terminals of the power converter. This drop in voltage requires an increase of output current in the output circuit  44  in order to meet the new load demands. Alternatively, when the DC load is decreased, there is an immediate increase in voltage. This increase in voltage requires a decrease in the output current of the output circuit  44  in order to compensate for the load reduction. 
     For example, when the operator of the switched power converter brings an additional load on-line, the feedback circuit  16  first measures the voltage across the load and then scales the voltage down to a 2.5 volt range. Because a new load has been added the measured voltage will be below the 2.5 voltage range. Optical coupler U 2  will compare the measured voltage (scaled down) against a 2.5 volt reference. Because the measured voltage across the load will be below the 2.5 reference voltage, optical coupler U 2  will cause the LED to produce less light. When the LED produces less light the photo-sensor will cause the output of the feedback circuit to increase in voltage. The output of the photo-sensor is connected to EA+ (pin  5 ) on controller  18 . When the voltage input of EA+ (pin  5 ) increases, the voltage controller within controller  18  will temporarily increase the duty cycle of the switching circuit  12 . This in turn increases the load current to meet the new load demand (i.e. get the voltage across the DC load back up to 13.6 volts). 
     Alternatively, when the operator of the switched power converter removes a load, the feedback circuit  16  measures the voltage across the load and then scales the voltage down to a 2.5 volt range. Because a load has been removed the measured voltage will be above the 2.5 voltage range. Opto-coupler U 2  will compare the measured voltage (scaled down) against a 2.5 volt reference. Now, because the measured voltage across the load will be above the 2.5 reference voltage, opto-coupler U 2  will cause the LED to produce additional light. When the LED produces additional light the output of the feedback circuit will decrease in voltage. The output of the photo-sensor is connected to EA+ (pin  5 ) on controller  18 . When the voltage input of EA+ (pin  5 ) decreases, the voltage controller within controller  18  will temporarily decrease the duty cycle of the switching circuit  12 . This in turn, decreases the load current to meet the reduced load demand (i.e. get the voltage across the DC load back down to 13.6 volts). 
     Resistor R 25  limits current to opto-coupler U 2 . Resistor R 26  and R 28  are arranged as a voltage divider to provide a scaled output voltage in the vicinity of 2.5 volts. Capacitors C 20 , C 22 , C 27 , C 28 , R 34 , R 32  and R 33  are used for stability, do not affect the DC levels whatsoever as they carry no DC current. Resistor R 30  is used for providing an input voltage to EA+ (pin  5 ) of controller  18  based on the current output of opto-coupler U 2 . 
     Current Sensing Circuit 
     Current sensing circuit  34  is used to measure the current being drawn by transistors Q 2   a  and Q 2   b  and to send the measured current to CS+ (pin  4 ) of controller  18 . Controller  18  then compares this measured current to a reference level. The reference level is the output of feedback circuit  16 , which is connected to EA+ (pin  5 ) on controller  18 . Depending upon the measured current and the reference level, controller  18  will control the on/off state of transistors Q 2   a  and Q 2   b.    
     Current sensing circuit  34  includes transformer T 4 , diodes D 24   a , D 24   b , D 24   c , and D 24   d , resistors R 21 , R 21   a , R 21   b , R 21   c , R 21   d , and R 21   e , and capacitor C 9 . The drain of transistor Q 2   b  is connected to pin  4  of transformer T 4 , and the drain of transistor Q 2   a  is connected pin  6  of transformer T 4 . The output side of transformer T 4  (pins  1  and  2 ) is connected to a series of diodes and resistors and then to CS+ (pin  4 ) of controller  18 . 
     Diodes D 24   a , D 24   b , D 24   c , and D 24   d  make up a full wave rectifier bridge. Diodes D 24   c  and D 24   b  are connected in parallel to the output side of transformer T 4 , where the cathode of diode D 24   c  is connected to pin  1  of transformer T 4  and the cathode of diode D 24   b  is connected to pin  2  of transformer T 4 . The anodes of diodes D 24   b  and D 24   c  are both connected to ground. Diodes D 24   d  and D 24   a  are also connected in parallel to the output side of transformer T 4 , where the anode of diode D 24   d  is connected to pin  1  of transformer T 4  and the anode of diode D 24   a  is connected to pin  2  of transformer T 4 . The cathodes of diodes D 24   a  and D 24   d  are connected to CS+ (pin  4 ) of controller  18  as well as a series of resistors and a capacitor. 
     For example, when transistor Q 2   a  is turned on, the current from transformer T 4  will flow from pin  1  of the transformer, through diode D 24   d  and through resistors R 21 , returning through D 24   b  to pin  2 . A voltage representing the flow of this current through R 21  is connected to pin  4  of CS+ in controller  18 . When transistor Q 2   b  is turned on, the current from transformer T 4  will flow from pin  2  of the transformer through D 24   a  and through R 21  (to ground) and then returning through R 24   c  to ground to pin  1  of T 4 . Again, the voltage on R 21  resistors is fed to CS+, the op-amp in controller  18 . 
     Resistors R 21 , R 21   a , R 21   b , R 21   c , R 21   d , and R 21   e , and capacitor C 9  are all connected in parallel. The current output of diodes D 24   d  and D 24   a  are connected to the high side of resistors R 21 , R 21   a , R 21   b , R 21   c , R 21   d , and R 21   e , and capacitor C 9 . The low side of resistors R 21 , R 21   a , R 21   b , R 21   c , R 21   d , and R 21   e , and capacitor C 9  are connected to ground. This parallel resistor-capacitor circuit is used for the purpose of ensuring the voltage applied to CS+ (pin  4 ) of controller  18  is in the 1 volt range. 
     Output Circuit 
     The secondary-output side of transformer T 1  is connected to the DC load through a series of circuit elements making up the output rectifier and LC filter circuit  44 . The output circuit  44  includes capacitors C 19 , C 11 , C 13 A, C 12 , C 14 A, C 17 , and C 18 , schottky diodes D 11   a  and D 11   b , diode D 12 , resistor R 29 , inductor L 2 , fuses F 2 , F 3 , and F 4 , inductor beads L 3 , L 4 , L 10 , and L 11 , and heavy gauge wires  105 ,  106 , and  107 . The DC load is connected in parallel with capacitors C 11 , C 13 A, C 12 , C 14 A, C 17 , and C 18  (DC load capacitors), which are in series with inductor L 2 . The output circuit  44  is integral with the transformer secondary and includes two current loops with the current going in the same direction through inductor L 2 , the DC load capacitors, and the DC load ( FIG. 17 ). 
     As further illustrated by  FIG. 17  when transistor Q 2   a  is turned on (Q 2   b  is off) current I 1  will flow between pin  4  and pin  6  (primary-input side of transformer T 1 ) in a counter-clockwise direction. Current I 1  will cause current I 3  to flow between pin  8  and pin  1  (secondary side of transformer T 1 ) in a clockwise direction. Alternatively, when transistor Q 2   b  is turned on (Q 2   a  is off) current I 2  will flow between pin  4  and pin  5  (primary-input side of transformer T 1 ) in a clockwise direction. Current I 2  will cause current I 4  to flow between pin  8  and pin  2  (secondary side of transformer T 1 ) in a counter-clockwise direction. As illustrated the current (I 4  and I 3 ) applied to inductor L 2  is always going in the same direction. 
     Further explained, when transistor Q 2   a  is turned on, current flows in the secondary-output side of transformer T 1  from pin  1  through schottky diode D 11   a , through inductor L 2 . The DC load capacitors will be charged and current will be delivered to the DC load and back through pin  8  of the transformer. When transistor Q 2   b  is turned on, current flows in the secondary side of transformer T 1  from pin  2  through schottky diode D 11   b , through inductor L 2 , the DC load capacitors will be charged and current will be delivered to the DC load and then back through pin  8  of the transformer. 
     Resistor  29  and capacitor C 19  are connected in series between secondary-output pins  2  and  1  of transformer T 1  for the purpose of eliminating transient voltages. Inductor beads L 3 , L 4 , L 10 , L 11 , are connected between the secondary-output side of transformer T 1  and schottky diodes D 11   a  and D 11   b . Inductor beads L 3 , L 4 , L 10  and L 11  are placed on the leads of D 11 A and D 11 B, for the purpose of reducing transient noise. The DC load capacitors which are connected in parallel with the DC load are arranged as follows. Capacitor C 11  is the main output capacitor. The positive plate of capacitor C 11  is connected to the positive terminal of the DC load P 4  and the negative plate is connected to the negative terminal of the DC load P 1 . 
     The remaining capacitors are used for the purpose of reducing noise. Capacitor C 12  is connected in parallel with the DC load, where one plate of capacitor C 12  is connected to the positive terminal of the DC load P 4 , and the other plate of capacitor C 12  is connected to the negative terminal of the DC load P 1 . Capacitors C 13 A and C 14 A are connected in series, where one plate of capacitor C 13 A is connected to the positive terminal of the DC load P 4 , and one plate of capacitor C 14 A is connected to the negative terminal of the DC load P 1 . The remaining plates of capacitors C 13 A and C 14 A are connected to chassis ground. Capacitors C 17  and C 18  are also connected in series, where one plate of capacitor C 17  is connected to the positive terminal of the DC load P 4 , and one plate of capacitor C 18  is connected to the negative terminal of the DC load P 1 . The remaining plates of capacitors C 17  and C 18  are connected to chassis ground. Fuses F 2 , F 3 , and F 4  are connected in series with inductor L 2  and work in conjunction with diodes D 11   a  and D 11   b  to provide reverse battery protection. 
     The illustrated embodiment of this invention also includes the use of heavy gauge wires which supplement the copper laminations on the circuit board. Heavy gauge wires  105  are connected directly between the negative output (terminal  8 ) of transformer T 1  and the negative terminal of DC load P 1  (i.e. DC negative output  88 ) Heavy gauge wires  106  are connected directly between schottky diodes D 11 A and the input of inductor L 2 . Heavy gauge wires  106  are also connected directly between schottky diodes D 11 B and the input of inductor L 2 . Heavy gauge wires  107  are connected directly between the output of inductor L 2  and fuses F 2 , F 3 , and F 4 . The output of fuses F 2 , F 3 , and F 4  are connected to the positive terminal of DC load P 4  (i.e. DC positive output  90 ). 
     Waveforms 
       FIG. 5  illustrates waveforms found at various points in the circuit of  FIG. 17  under normal operating conditions.  FIG. 5A  shows the voltages across the two power transistors Q 2   a  and Q 2   b  during a complete cycle of operation. One voltage is the complement of the other.  FIG. 5B  shows the voltages across the primary windings of transformer T 1  during one complete cycle of switch operation.  FIG. 5C  illustrates the current waveforms  11  and  12  through the primary loops of  FIG. 17 . 
       FIG. 5D  shows the current through inductor L 2 . 
       FIG. 5F  shows the secondary current I 4  through diode D 11   b.    
       FIG. 5G  shows the current through C 11 . 
       FIG. 5H  shows the voltage at the top of the circuit of  FIG. 17 ; i.e., the top of L 2 . 
     Permanent Reverse Battery Indicator 
     The permanent reverse battery connection indicator  24  is diode D 12 . Diode D 12  and capacitor C 11  are connected in parallel. The cathode of diode D 12  is connected to the positive plate of capacitor C 11  which is connected to the positive terminal of the DC load P 4 . The anode of diode D 12  is connected to the negative plate of capacitor C 11  which is connected in to the negative terminal of the DC load P 1 . If a reverse battery connection is applied to the DC load output of the power converter, diode D 12  will blow before fuses F 2 -F 4  open circuit, permanently indicating that a reverse battery connection has occurred. If F 2 -F 4  blow, they may be replaced or reset and the converter  46  will be fully operational even if D 12  is not replaced. 
     Packaging a Commercial Device 
     Having described the preferred power conversion circuit, the packaging of a commercial embodiment will be described in detail with reference to  FIGS. 18-26 . 
     The commercial embodiment of converter  46  comprises a rectangular sheet metal housing  70  attached by screws to a finned aluminum extrusion  52  which forms the aforementioned heat sink for the FET&#39;s Q 2   a  and Q 2   b , diode D 11   a  and D 11   b , and the thermistor RT 1 . These components are held against a large flat surface  53  of heat sink  52  by spring clips  55  which are screwed into the heat sink extrusion in the manner shown in  FIG. 19 . The fan  32  is mounted by screws  57  onto an end of the heat sink extrusion  52  in which a relief  59  of circular design has been machined. The surfaces of the relief  59  lie below the end surfaces  61  of the fins  65  and the screw base  63  on which the fan  32  is mounted. This relief creates an air gap between the fan motor  50  and the heat sink which prevents heat from the sink reaching the fan motor. Numerous vents  58  are formed in the top and back plates of the housing  70 . 
     Flanges  84  are provided on both ends of housing  70  for mounting purposes. Fuses F 2 -F 4  are mounted outside the housing  70  for ease of replacement. Fuse Fx 1 , however, is inside the housing for reasons described above. The positive output terminals  90  and the negative output terminals  88  are mounted on the left side of housing  70  as shown in  FIG. 22 . A power cord  98  extends from housing  70  through aperture  100 . 
     The components in the circuit of  FIG. 2  are mounted on a conventional circuit board  102  which is secured by fasteners within housing  70 . The board  102  has conductive traces on both sides as shown in  FIGS. 25 and 26 . The inductor L 2  is mounted on board  102  as shown in  FIG. 25  along with the transformer T 1  (central in  FIG. 25 ). Two No. 12 gauge wires  104  run from the center tap of T 1  to a point  106  where they pass through a hole in board  102  and emerge on the other side as shown in  FIG. 26 . From there to the negative output terminal  88  the wires overlie a copper trace and are soldered to the trace to lower the resistance of this high current path and increase the robustness of it as well. The leads  108  from L 2  to the fuses F 2 -F 4  and the positive outputs  90  are similarly constructed. 
       FIG. 23  shows the converter  46  mounted within an RV  109  having a storage battery  114 . A power cord  112  brings 115 vac to the converter from a pedestal  111  of the type found in RV parks. The converter  46  is connected into the electrical system of the RV in a known manner. 
     Referring again to  FIG. 2  the circuit for the converter  46  is here equipped with a 4-wire terminal H 2  of which pin  4  is connected to the converter output fuses F 2 -F 4  via a 100 Ohm resistor R 57 . The terminal H 2  allows the converter to be connected to an external “management” system of the type described in U.S. Pat. No. 5,982,643 issued Nov. 9, 1999 to Thomas H. Phlipot and assigned to Progressive Dynamics, Inc. As is more completely described in the &#39;643 patent, the management system includes a microcontroller which gives the owner the option of various operating modes and various converter output voltages; e.g., 13.6 v for normal operation, 13.2 v for storage, and 14.4 v for boost. 
     Miscellaneous—Options 
       FIG. 2  also illustrates a terminal H 4  connected to ground via R 51 , R 31  and C 21 . Terminal H 4  is a two-contact terminal which is shorted out with a small bridge wire if a gel cell is used in place of the normal lead-acid liquid storage battery  114  in the RV. This lowers the operating voltages of the converter  46  by 0.4 v and is a convenient option for owners who wish to use gel cell storage batteries 
     While the present invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law. It is also to be understood that it is the inventor&#39;s intent to claim all novel subject matter contained within this disclosure. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 VALUES OF LISTED COMPONENTS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 C1 
                 Capacitor 
                 0.47 uF 
               
               
                 C10 
                 Capacitor 
                 220 uF 
               
               
                 C11 
                 Capacitor 
                 16 V 
               
               
                 C12 
                 Capacitor 
                 0.01 uF 
               
               
                 C13A 
                 Capacitor 
                 0.01 uF 
               
               
                 C14A 
                 Capacitor 
                 0.01 uF 
               
               
                 C15 
                 Capacitor 
                 2.2 nF 
               
               
                 C16 
                 Capacitor 
                 2.2 nF 
               
               
                 C17 
                 capacitor 
                 0.047 uF 
               
               
                 C18 
                 capacitor 
                 0.047 uF 
               
               
                 C19 
                 capacitor 
                 0.001 uF 
               
               
                 C2 
                 capacitor 
                 2.2 nF 
               
               
                 C20 
                 capacitor 
                 0.015 uF 
               
               
                 C22 
                 capacitor 
                 0.47 uF 
               
               
                 C24 
                 capacitor 
                 270 pF 
               
               
                 C25 
                 capacitor 
                 0.12 uF, 2% 
               
               
                 C26 
                 capacitor 
                 0.47 uF 
               
               
                 C27 
                 capacitor 
                 1000 pF 
               
               
                 C28 
                 capacitor 
                 0.1 uF 
               
               
                 C29 
                 capacitor 
                 2.2 nF 
               
               
                 C3 
                 capacitor 
                 2.2 nF 
               
               
                 C30 
                 capacitor 
                 0.47 uF 
               
               
                 C31 
                 capacitor 
                 2.2 nF 
               
               
                 C4a 
                 capacitor 
                 820 uF, 250 V 
               
               
                 C4b 
                 capacitor 
                 820 uF, 250 V 
               
               
                 C4c 
                 capacitor 
                 820 uF, 250 V 
               
               
                 C5 
                 capacitor 
                 100 uF, 35 V 
               
               
                 C7 
                 capacitor 
                 0.1 uF 
               
               
                 C8 
                 capacitor 
                 0.1 uF 
               
               
                 C9 
                 capacitor 
                 0.01 uF 
               
               
                 D10 
                 zener diode 
                 13 V 
               
               
                 D11a 
                 schottky diode 
                 40 A, 100 V 
               
               
                 D11b 
                 schottky diode 
                 40 A, 100 V 
               
               
                 D12 
                 diode 
               
               
                 D15a 
                 schottky diode 
                 1 A, 20 V 
               
               
                 D15b 
                 schottky diode 
                 1 A, 20 V 
               
               
                 D16a 
                 schottky diode 
                 1 A, 20 V 
               
               
                 D16b 
                 schottky diode 
                 1 A, 20 V 
               
               
                 D1a 
                 schottky diode 
                 1 A, 100 V 
               
               
                 D23 
                 zener diode 
                 220 V, 5 W, 5% 
               
               
                 D24a 
                 diode 
                 75 V, 150 mA 
               
               
                 D24b 
                 diode 
                 75 V, 150 mA 
               
               
                 D24c 
                 diode 
                 75 V, 150 mA 
               
               
                 D24d 
                 diode 
                 75 V, 150 mA 
               
               
                 D27 
                 schottky diode 
                 1 A, 20 V 
               
               
                 D3 
                 schottky diode 
                 1 A, 20 V 
               
               
                 D4 
                 diode 
                 75 V, 150 mA 
               
               
                 D9 
                 zener diode 
                 15 V, 2 W 
               
               
                 DB1 
                 diode bridge 
                 20 A, 400 V Bridge 
               
               
                 F1 
                 fuse 
                 15 A 
               
               
                 F2 
                 fuse 
                 30 A 
               
               
                 F3 
                 fuse 
                 30 A 
               
               
                 F4 
                 fuse 
                 30 A 
               
               
                 Fx1 
                 fuse 
                 0.5 A 
               
               
                 L2 
                 inductor 
                 20 uH 
               
               
                 Q1 
                 transistor 
                 5 A, 40 V 
               
               
                 Q2a 
                 transistor 
                 24 A, 500 V, .20 on resistance 
               
               
                 Q2b 
                 transistor 
                 24 A, 500 V, .20 on resistance 
               
               
                 R1 
                 resistor 
                 390 Ohm, 5% 
               
               
                 R13 
                 resistor 
                 1.82K 
               
               
                 R14 
                 resistor 
                 16.2K 
               
               
                 R15 
                 resistor 
                 35.7K 
               
               
                 R16 
                 resistor 
                 1.8K, 5% 
               
               
                 R17 
                 resistor 
                 5.49K 
               
               
                 R18 
                 resistor 
                 15.4K 
               
               
                 R19a 
                 resistor 
                 12.1K 
               
               
                 R19b 
                 resistor 
                 12.1K 
               
               
                 R20 
                 resistor 
                 50 Ohm, 5%, 3 W 
               
               
                 R21 
                 resistor 
                 18.7 Ohm 
               
               
                 R21A 
                 resistor 
                 422 Ohm 
               
               
                 R21B 
                 resistor 
                 422 Ohm 
               
               
                 R21C 
                 resistor 
                 845 Ohm 
               
               
                 R21D 
                 resistor 
                 845 Ohm 
               
               
                 R21E 
                 resistor 
                 1690 Ohm 
               
               
                 R23a 
                 resistor 
                 15 Ohm, 5% 
               
               
                 R23b 
                 resistor 
                 15 Ohm, 5% 
               
               
                 R24a 
                 resistor 
                 1.5K, 5%, 10 W 
               
               
                 R24b 
                 resistor 
                 1.5K, 5%, 10 W 
               
               
                 R25 
                 resistor 
                 1K, 5%, ½ W 
               
               
                 R26 
                 resistor 
                 31.2K or 30.1K 
               
               
                 R28 
                 resistor 
                 6.98K, ¼% 
               
               
                 R29 
                 resistor 
                 10 Ohm, 5% 
               
               
                 R2a 
                 resistor 
                 453K 
               
               
                 R30 
                 resistor 
                 4.7K 
               
               
                 R33 
                 resistor 
                 3.24K 
               
               
                 R34 
                 resistor 
                 3.24K 
               
               
                 R37 
                 resistor 
                 100 Ohm, 5%, 10 W 
               
               
                 R38 
                 resistor 
                 84.5K, 0.5 W 
               
               
                 R39 
                 resistor 
                 866 Ohm 
               
               
                 R4 
                 resistor 
                 22.6K 
               
               
                 R40 
                 resistor 
                 97.6K 
               
               
                 R7 
                 resistor 
                 32.4K 
               
               
                 R8 
                 resistor 
                 499K 
               
               
                 RN1A 
                 resistor 
                 16.2K 
               
               
                 RN1B 
                 resistor 
                 47.5K 
               
               
                 RN1C 
                 resistor 
                 9.53K 
               
               
                 RT1 
                 thermistor 
                 100K 
               
               
                 RT2 
                 thermistor 
                 1 Ohm 
               
               
                 T1 
                 transformer 
                 2:13:13:2:2 
               
               
                 T2 
                 CMC transformer 
                 custom 
               
               
                 T3 
                 CMC transformer 
                 custom 
               
               
                 T4 
                 transformer 
                 80 MH 
               
               
                 U2 
                 optically isolated amplifier 
                 FOD2741 
               
               
                 U3A 
                 operational amplifier 
                 LM2902 
               
               
                 U3B 
                 operational amplifier 
                 LM2902 
               
               
                 U3C 
                 operational amplifier 
                 LM2902 
               
               
                 U3D 
                 operational amplifier 
                 LM2902