Power supply providing constant D.C. output voltages from variable A.C. input voltages

Abstract: The novel power supply for generating a constant d.c. voltage from a variable a.c. voltage supply line, includes a plurality of switch circuits (10, 12) which monitor voltage levels on respective secondary windings (L,M,H) of a power transformer and as voltage levels change, one of the plurality of switch circuits is selected to charge a capacitor (C1) which provides a range of d.c. voltages which are processed to provide the constant d.c. voltage.

Description: The present invention relates to power supplies in general and more particularly to voltage regulating circuits suitable for processing variable a.c. line voltages to generate rectified d.c. output voltages. Most power supplies, especially the ones that are used in computers and other precision electronic devices are required to provide a fixed voltage at a given current. Usually, the allowed deviation at full load, is within the range of ± 5%. Preferably, the power supply should be low-cost and consume a relatively small amount of power. A typical power supply includes a power transformer with a primary winding which is connected to an a.c. input line voltage and one or more secondary windings disposed relative to the primary winding. Rectifying circuits are connected to the secondary windings. The rectifying circuits process the a.c. voltages appearing on the secondary winding to provide a desired d.c. output voltage across a bulk capacitor. One of the problems which a designer faces is that the input a.c. line voltage varies over a wide range. An obvious solution is to design a regulating circuit which dissipates a relatively large amount of power. Thus, as the input AC line voltage varies from minimum to maximum, more power is consumed within the regulating circuit to provide a desired voltage at a given current. Such a design is unacceptable because the regulating unit usually dissipates a relatively large amount of energy and its cost is also relatively high. It is believed that the high cost stems from the fact that the components which are used in such devices are specified to match the high energy that has to be dissipated rather than the power which the devices output. It is common knowledge that for most designs unit cost and power rating are closely related. Thus, as the power rating increases, the unit cost increases and vice versa. Therefore, it is desirable to design a device with components dissipating close to the output power rating rather than with components rated for higher power due to circuit inefficiency. The prior art has recognized the problem associated with the above-described design and sets forth alternative designs. US Patent 3,921,059 is an example of the prior art alternative design. In that patent multiple triac taps connected to secondary windings of a power transformer are switched to provide a range of output voltages. A control circuit including a shift register and optical isolator drivers are used to switch the triacs. US Patent 4,454,466 describes a power supply in which switched primary windings provide a variable voltage which is processed by a series regulator to output a fixed voltage to a load. An up/down counter circuit arrangement is used for driving switches that select the primary windings which are needed to provide a desired output voltage. IBM Technical Disclosure Bulletin (Vol. 13, No. 6, November 1970, pp. 1516-1517) and US Patent 4,090,234 describe a voltage regulating circuitry in which diodes and SCR taps are made selectively conductive to effectively vary the turn ratio of the secondary winding of a power transformer. Even though the above devices work well for their intended purpose, they are plagued by problems which adversely affect their use. Probably the most pressing problem is that the automatic tap settings have to be latched. This means that the tap switching to select coils cannot be done instantaneously, since the counters and/or latches which provide the latching function cannot be changed instantaneously. In other words, the rate at which the latching elements are changed is the rate at which the coils can be switched. Counters and other latching elements can only be changed on every clock cycle. Thus, the coils can only be switched on a clock cycle basis only. Any attempt to switch coils during a clock cycle is prohibited. However, there are several sensitive devices (such as computers, etc.) that require regulating circuits in which the coils must be switched instantaneously. As to those devices, the prior art regulating circuits and/or power supplies cannot be used. It is noted that the regulating circuits of the Technical Disclosure Bulletin and US-A-4,090,234 uses SCRs instead of latches. However, one of the inherent characteristics of an SCR is that it must remain in a desired state (i.e., conductive or non-conductive) for a complete clock cycle before it can be changed. Thus, in that regard, the SCR operates as a latch. In addition, a separate power supply is required for driving the SCR. This requirement makes the circuit more complicated and increases its cost. The increased cost also affects US-A-4,454,466 patent since switching is done on the primary side of the transformer and as a result the components are overdesigned in order to handle the high current and/or voltage in the primary windings. Also, some of the prior art references use opto-isolation and/or triacs in the switching circuits. These components are expensive and increase the overall cost of the power supply. It is therefore the general object of the present invention to provide a power supply and/or a regulating circuit that is more efficient than was previously possible. The improved power supply includes a linear regulator connected via a bulk capacitor charged by a plurality of switch circuit arrangements that monitor the voltage level on assigned taps of the secondary windings of a power transformer and as the voltage level changes on respective windings, different ones of the plurality of switching circuit arrangements are automatically selected so that the bulk capacitor is charged by different turn ratio of the secondary windings. An output capacitor is connected across the output terminals of the linear regulator. The voltage range across the bulk capacitor is controlled by the number of taps on the secondary windings and the number of switch circuit arrangements. Thus, as the number of taps and switch circuits increase, the voltage window across the bulk capacitor decreases and the power which is dissipated in the linear regulator also decreases. The taps are formed from a plurality of diodes connected to selected points on the secondary windings. A center tap conductor interconnects a center tap of the secondary windings to a ground potential. Each diode is connected to a switch circuit arrangement which includes a switch transistor connected in series with the diode. A differential amplifier means is connected to the switch transistor and a voltage reference generating means is connected to the differential amplifier. The foregoing and other objects, features and advantages of the invention are more fully described in the accompanying drawings. The sole figure shows a schematic of the improved power supply according to the teachings of the present invention. The structure and the theory of operation are as follows: a plurality of diode taps (CR1 through CR6) are placed on the secondary windings of a power transformer. The diode taps are connected to a plurality of tap selection circuit arrangements which select a different group of secondary coils to charge capacitor C1 as input a.c. line voltage varies across the primary winding of the power transformer. A linear circuit arrangement (LR1) processes the voltage generated across capacitor C1 to provide a fixed voltage Vout at a desired current across capacitor C2. Referring to the sole figure, the major sub-assemblies of the improved power supply include an output capacitor C2, linear regulator circuit arrangement LR1, window capacitor C1, power transformer including primary winding 1-2 and a plurality of secondary windings 3-4, 4-5, 5-6, 6-7, 7-8, 8-9, 9-N, a plurality of diode taps (CR1 through CR6) and a plurality of tap selection circuit arrangements 10, 12, 14 ... N. The output capacitor C2 provides more filtering for the output voltage. In the preferred embodiment of this invention the fixed output voltage is 5v &PM.5% at 0.5 amps maximum load. Of course, it is within the skill of the art to provide other output power ratings without deviating from the teachings of the present invention. The linear regulator circuit arrangement takes the voltage provided across C1, regulates it and outputs a desired voltage. The linear regulator is a conventional off-the-shelf module which contains the necessary circuit arrangement for regulating the input voltage. In the preferred embodiment of this invention a linear regulator module L7800 manufactured by SGS Corporation was used. Of course, other types of regulating circuit arrangements can be used without departing from the scope of the present invention. The tap selection circuit arrangements 10, 12, 14-N are identical. Their function is to monitor assigned taps or points on the secondary windings and to select a different set of secondary coils which charge capacitor C1 as the input a.c. line voltage across terminals ACH and ACN fluctuates within preassigned voltage range. The preassigned voltage range is separated into a plurality of different groups with each group covering an assigned voltage range. Likewise, the secondary windings are separated in a plurality of different groups of windings or coils. Preferably, the number of groupings for the input voltage range and the secondary coils should be identical. Thus, if the variable input voltage range is separated into n groups, the secondary windings should also be separated into n groups. Further, a different group of windings should be selected to charge C1 as the input a.c. line voltage varies within its assigned voltage range. In the preferred embodiment of this invention the input a.c. line voltage fluctuates between 70v a.c. and 259v a.c. The input a.c. line voltage is separated into three equal voltage ranges, namely: 70-107, 107-163 and 163-259v a.c. Also, the secondary coils are arranged into groups identified by alphabetical characters LL, MM and HH. For this embodiment, the a.c. line voltage and the windings are arranged into like groupings. A center tap conductor 16 interconnects the center tap of the secondary windings to a ground potential. Even though a three-tap winding was used to process the input a.c. voltage, this should not be construed as a limitation on the scope of the present invention since it is within the skill of the art to increase or decrease the number of secondary taps or division of the input line voltage without departing from the spirit and scope of the present invention. As will be explained subsequently, the higher the number of taps, the lower the voltage window on the bulk capacitor C1 and thus less energy will be required to be dissipated across the linear regulator in order to provide a fixed voltage at the output. It should also be noted that N as shown in the sole figure signifies that additional secondary coils with associated diode taps and selection circuit arrangements can be used. Still referring to the sole figure, the voltage which is provided across each group of coils is rectified and is switched by one of the tap selection circuit arrangements for charging C1. Thus, the voltage appearing across L6 is rectified by components CR1, CR6 and C3. Likewise, the voltage appearing across terminal M6 is rectified by diodes CR2, CR5 and capacitor C4. Finally, the voltage appearing across terminal H6 is rectified by diodes CR3 and CR4 and C1. It should be noted that when the input a.c. voltage is within its low range (say, 70v a.c. to 107v a.c.) the voltage across terminal L6 is selected to charge C1. Similarly, when the input voltage is in its mid-range, say, between 107v a.c. to 163v a.c. the voltage across terminal M6 is selected for charging capacitor C1. Finally, when the input a.c. line voltage is within its upper range, say, 163v a.c. to 259v a.c., the voltage across terminal H6 is used for charging C1. As stated above, the tap selection circuit arrangement which selects the set of coils which is used for charging C1 is identical. Thus, only one of these circuits identified by numeral 10 will be described in detail, it being understood that the other circuits (including the one identified by numeral 12) are identical in structure and function in like manner as the detailed circuit. Still referring to the sole figure, each of the tap selection circuits is comprised of a switching transistor such as Q1 which is coupled to a differential amplifier formed by transistors Q3 and Q4. For brevity, different numerals Q2, Q5, etc. are used to identify components in circuit arrangement 12 that are similar to components in circuit arrangement 10. The emitter terminals of the differential amplifier transistors are connected through resistor R4 to ground. A zener diode CR11 is coupled to a constant current source formed by circuit arrangement 18. The zener diode and its associated current source provide a reference voltage of approximately 5v to the base of Q3 and the bases of similar situated transistors of differential amplifiers which are attached to node 20. Still referring to the sole figure, a more detailed description of the improved power system is given. Component CR11 is a 5v zener diode which provides a 5v reference signal on node 20. Components CR9, CR10, R1, R2 and Q7 constitute a 6 milliamp constant current source which limits power dissipation in zener diode CR11. The high transformer output voltage which is provided across terminal L-6 is rectified by components CR1, CR6 and C3. Components Q3, Q4 and R4 form a differential amplifier. R5 and R6 form a voltage divider and are selected such that the base of Q4 reaches 5.0v when VC3 reaches 11.5v. The emitter-collector voltage of Q1 (VQ1CE) is one volt. For VC3 less than 11.5v, Q4 is off and Q3 is on. For VC3 higher than 11.5v, Q4 is on and Q3 is off. It should be noted that Q1 is on whenever Q3 is on and as a result C1 is charged. CR7 is a 7.4v zener diode and is needed to prevent emitter to base breakdown of Q3. It should be noted that without CR7, VQ4 base would go up to approximately 17v when VIN reaches approximately 259v a.c. The medium transformer voltage across terminal M-6 is rectified by devices CR2, CR5 and C4. A differential amplifier is formed by devices Q5, Q6 and R3. R7 and R8 form a voltage divider. The values of the resistors are selected such that the voltage on the base of Q6(VQ6) reaches 5v when VC4 reaches 11.5v. Simultaneously, the voltage across VQ2CE is 1v. If VC4 is lower than 11.5v, Q6 is off and Q5 is on. If VC4 is higher than 11.5v, Q6 is on and Q5 is off. With Q5 on, Q2 is also on and charges up C1. CR8 is a zener diode which prevents Q5 emitter to base breakdown. Absent the zener diode, VQ5 base would go up to approximately 10.5v when VIN is approximately 259v a.c. For a.c. input voltage greater than 163v a.c., both VC3 and VC4 are higher than 11.5v. Thus, Q1 and Q2 are off and C1 gets charged through diodes CR3 and CR4. In other words, the voltage across terminals H-6 charges up C1. In operation, zener diode CR11 and its associated current source 18 set a reference voltage approximately 5v on the base of devices Q5 and R3, respectively. This forces both devices to conduct simultaneously. As a result, Q1 and Q2 also conduct. When the voltage of the primary winding is within a low range, say, between 70v a.c. to 107v a.c., a high voltage is reflected across the coils in terminal L-6. Switching transistor Q1 of coil selection circuit means 10 conducts and charges up C1. The diodes CR2-CR5 are reversed biased and, as a result, the voltage across terminals M-6 and H-6 do not charge capacitor C1. When the voltage on the base of Q4 exceeds 5v, Q1 and Q3 are turned off and Q4 conducts. Similarly, when the input voltage VIN is within the range of 107v a.c. to 163v a.c., the voltage generated across terminals M-6 causes current to flow through Q2 and charges up C1. Finally, when the input voltage is within its maximum range, 163v a.c. to 259v a.c., the voltage across terminals H-6 charges up capacitor C1. It should be noted that by arranging the output coils into a plurality of electronically switched center tap outputs and providing tap selection circuit arrangement which selects appropriate taps and group of coils as the a.c. input line voltage varies a power supply is provided which dissipates minimum amount of power across the linear regulator circuit arrangement LR1. It should also be noted that in the preferred embodiment the input a.c. line voltage is separated into three identical voltage ranges, high, medium and low. However, it should be noted that this is only for purposes of explaining applicants' invention and does not limit the invention in any way. Also, the invention may be implemented in a fullwave or halfwave rectifier circuitry. With the particular arrangement of the circuit in the sole figure, the voltage across capacitor C1 is between 5.7v to 10.5v for each voltage range of the input a.c. line voltage. The voltage across C1 is achieved by proper selection and arrangement of the transformer windings. Thus, for an input line voltage which ranges between 70v a.c. to 259v a.c., when the line voltage is between 70v a.c. and 107v a.c., the voltage across capacitor C1 fluctuates between 5.7 and 10.5v. Similarly, for line voltage between 107v a.c. and 163v a.c. the voltage across C1 fluctuates between 5.7 and 10.5v and when the line voltage is between 163v a.c. and 259v a.c., the voltage across C1 is between 5.7v and 10.5v. Thus, since the voltage swing across C1 is relatively low (5.7v to 10.5v), the maximum power consumed in the linear regulator LR1 is relatively low. This can be shown mathematically as follows: P = VI, where P = power V = voltage I = current With reference to the sole figure, PLR1 = Io(VC1 - Vo) (Equation 1) where PLR1 represents power consumed by the linear regulator, VC₁ represents voltage on C1; Vo represents Vout and Io represents output current. Thus, when VC1 = 5.7 volts, Vo = 5 volts and Io = 1 amp, only 0.7 watt is dissipated in the linear regulator. Likewise, if 10.5 volts are generated on C1, only 5.5 watts are dissipated in the linear regulator. It should be noted that if additional coils and coil selection circuitry are added to the sole figure the power which is consumed in the linear regulator module will be even less. In contrast with no switching of transformer taps, the voltage across C1 varies between 5.7 volts and 20.1 volts. When the voltage across C1 is 20.1 volts, 15.1 watts are dissipated in the linear regulator. The wattage is obtained by substituting 20.1 volts in Equation 1 above. Likewise, by comparing the power consumption with tap switching and the power consumption without tap switching, it can be shown that tap switching provides a saving of 9.6 watts. Stated another way, a significant amount of energy is saved when one uses the tap switching topology disclosed above. For the specific example described above, the savings are 9.6 watts. Several advantages inure to one who uses the teachings of the present invention. Among the advantages are an improved power supply with the following characteristics: 1. Low power dissipation. 2. Low cost due to the fact that the power rating of the components is relatively low. 3. Low failure rate of the components used 4. No EMC filter or shielding is required because no high frequency switching is needed. 5. No tap selections latching, thus the switching circuits can respond to instantaneous changes occurring at the a.c. input line voltage.

Claims: A power supply for providing a constant voltage at a constant current comprising a linear regulator circuit (LR1), a bulk capacitor (C1) for providing a voltage window connected to said linear regulator circuit and a power transformer comprising a primary winding for coupling to a variable input supply voltage source and a plurality of secondary windings (L,M,H) coupled in series and disposed relative to the primary windings; said power supply being characterized in that it comprises: a plurality of coil selection circuit (10, 12) connected to the secondary windings with each one including a switching transistor (Q1, Q2) with a first electrode coupled to sense voltages on a selected set of windings within said plurality of secondary windings, a second electrode coupled to said bulk capacitor, and a differential amplifier (Q3, Q4) connected to said switching transistor with said differential amplifier causing said switching transistor to convey current from the selected set of windings to charge said bulk capacitor if the voltage on said set of windings falls within a selected voltage range and to deactivate the switching transistor that is currently charging said bulk capacitor if the voltage on said selected set of windings falls outside of the selected voltage range whereby another one of said plurality of switching circuits is activated to charge said bulk capacitor. The power supply of claim 1 further including at least a rectifier (CR1) between each secondary winding (L) and the first electrode of the switching transistor of the corresponding coil selection circuit, for rectifying current flowing from said secondary winding The power supply of claim 2 further including a conductive terminal interconnecting a center tap on said secondary windings to a ground potential; and a zener diode (CR 7) having an anode electrode connected to said conductive terminal and a cathode electrode connected to said differential amplifier (Q3, Q4). The power supply of claim 1 to 3 wherein differential amplifier includes a first transistor (Q3) and a second transistor (Q4) arranged in a parallel configuration with an electrode from each transistor connected to a first resistor (R4) connecting the ground potential, and a pair of series connected resistors (R5, R6) connected to another electrode of said first transistor:

Inventors: CELENZA NICHOLAS, NAJM ELIE MICHEL, CELENZA, NICHOLAS, NAJM, ELIE MICHEL
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1989
Main classification: G05F1
IPC codes: H02M7, G05F1