Abstract:
A power supply with a protection circuit that protects against over current, short circuit, output overvoltage, and input undervoltage with a minimal number of components. A pulse width modulated power supply has a controller using two operational amplifiers, namely an error amplifier and an overvoltage comparator, which are used to limit the duty cycle of the power supply. When the output voltage differs significantly from a reference, the overvoltage comparator triggers causing the supply to enter hiccup mode wherein it shuts off and automatically restarts, checking to see if the condition which caused shutdown is still present.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention is directed to power supplies, and more particularly, to protection circuits for use with a controller.  
           [0003]    2. Description of the Related Art  
           [0004]    In power supply design, a critical feature for many end users is the ability of a power module to self-protect both itself and the circuitry it powers during fault or trouble conditions. There are several common trouble modes that must be accounted for in the design of a power module. These include output overload protection, short circuit protection, output overvoltage protection, and input undervoltage protection.  
           [0005]    Output overload protection is required to protect the end user against excessive current and heating in either the power supply module or the end user&#39;s circuitry due to either a misapplication or a damaged device. Output short circuit protection is required to protect the end user from excessive current and heating in either the power supply or the end user&#39;s circuitry due to a failed device. Output overvoltage protection is required to protect the end user&#39;s circuitry from being powered by excessively high voltage. This failure mode is due to either a failure in the power module&#39;s control loop or a misapplication by the end user, e.g., adjusting an adjustable output voltage higher than the supply is rated for. Input undervoltage protection protects the end user against excessive input current and heating in either the power module or the end user&#39;s circuitry due to the power module running at an input voltage that is lower than the supply is rated for.  
           [0006]    It is a goal of the power supply designer to implement the required protection circuits in a manner that is both cost effective and space efficient. Various previous methods have attempted to address circuit protection in power supplies. In many designs, the power module continues to run using a different control mechanism, in others it enters a hiccup mode of operation, while in others the module shuts off until power is cycled. Regardless of the method of protection used, the protection circuits are implemented independently resulting in higher component count and cost, as well as higher space requirements.  
           [0007]    A typical prior art protection circuit is shown in FIG. 4. This diagram depicts an undervoltage lockout circuit. The circuit consists of a first resistor  410  coupled to a second resistor  420  that is in turn coupled in parallel with a capacitor  430 . The capacitor  430  is coupled to a first and second input of a universal voltage monitor (UVM)  440 , which monitors undervoltage conditions. A third resistor  450  is coupled between the first resistor  410  and the output of the UVM  440 . A fourth resistor  460  is coupled to a third input of the UVM  440  and a fifth resistor  470  is coupled between the fourth resistor  460  and the output of the UVM  440 . Finally, the output of the UVM  440  is coupled to a power supply  480 . An input voltage Vin is sensed and then compared to a reference voltage. The output of the UVM  440  will be zero when the input voltage is lower than a reference voltage. Once the output of the UVM  440  drops to zero, the power supply  480  will be disabled. Once the input voltage rises above the reference voltage, the power supply  480  will be reactivated. As can be seen, a significant number of components are used to implement this protection circuit.  
           [0008]    It is desirable to merge protection features to achieve low component count and low cost. Some art exists which combines an undervoltage and over current protection circuit. Despite this combination, the component count is still high, though lower than if the circuits were independently implemented. This type of solution often latches the power module off and therefore requires power cycling before restarting. For many users, the power cycling requirement is undesirable. There is therefore needed in the art a new, lower component count and low cost power supply protection circuit.  
         SUMMARY OF THE INVENTION  
         [0009]    The circuit demonstrating the present innovations is used to implement protection features with a minimal number of components. In a preferred embodiment, the innovative power module provides overcurrent (or overload), short circuit, output overvoltage, and input undervoltage protection. In a preferred embodiment, the protection circuit uses a hiccup mechanism where the module shuts off briefly and then attempts to auto-restart.  
           [0010]    In a preferred embodiment, an operational amplifier compares a reference voltage to a voltage which is proportional to the output voltage of the power supply. Under certain failure conditions, this produces an error signal which is compared to a second reference signal. Depending on the second comparison, a duty cycle exceeding a maximum or saturation of the control loop will cause the power supply to enter a hiccup mode wherein it repeatedly shuts itself down briefly then restarts to see if the failure condition is gone.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0012]    [0012]FIG. 1 is a schematic showing the combination of a control integrated circuit and other circuit components in accordance with the present invention.  
         [0013]    [0013]FIG. 2 is a schematic of circuitry for communicating output voltage information to an isolated opto-coupler in accordance with the present invention.  
         [0014]    [0014]FIG. 3 shows an alternate embodiment of a section of the innovative protection circuit.  
         [0015]    [0015]FIG. 4 shows a prior art protection circuit.  
     
    
     DETAILED DESCRIPTION  
       [0016]    The present innovations will be described with respect to the figures. FIG. 1 shows a circuit used to implement the protection features of the preferred embodiment with a minimal number of components. The figure does not show all components of the controller, only those necessary for the practice of the current innovations. The control chip used in this description has two comparators, or op amps, each of which is capable of comparing a reference voltage to another input. The requirements of the control chip are discussed first, followed by the additional circuitry required for implementing the current innovations.  
         [0017]    The operational amplifier, IC 1 A  102  is a component that is present internally to the majority of standard PWM controllers. It is typically used as an error amplifier to control the power supply and regulate the output voltage to the desired level. In the presently disclosed innovative system, this op amp serves as an “error amplifier” in that it measures the voltage difference between its two inputs, which include one 2.5V reference voltage and a voltage which is proportional to the output voltage of the power supply itself, as discussed further below.  
         [0018]    Operational amplifier IC 1 B  126 , also called the overvoltage (OV) comparator, is present internally in some PWM controllers. The OV comparator is traditionally used to sense over input voltage and turn off the PWM controller. In the presently disclosed innovative system, when the OV comparator is tripped, the PWM controller is disabled and the switching power supply stops switching.  
         [0019]    When the comparator  126  is tripped, the soft start capacitor C 5   130  is discharged. The switching resumes after soft-start capacitor C 5   130  is discharged below a threshold determined by the controller IC itself. The capacitor C 5   130  is referred to as a “soft start” capacitor because it allows increase of the duty cycle slowly, as the capacitor itself charges. In the preferred embodiment, this capacitor is attached to the soft start pin of the control chip.  
         [0020]    When the threshold is reached and switching resumes, the soft start capacitor C 5   130  charges again. If, after this autostart, the module is still experiencing a failure mode, then the overvoltage comparator  126  will again be tripped, discharging the soft start capacitor C 5   130 . The module will continue to auto-start in such a hiccup mode until the trouble condition is no longer present and the overvoltage comparator  126  is no longer tripped.  
         [0021]    Though in this description the two operational amplifiers are shown as integrated into the control chip, it is possible to instead use a control chip that does not have these exact components. For example, the error op amp or even the overvoltage shut down comparator may not be an integrated part of the control IC. In such an event, the missing components must be added as extra circuitry outside the control chip. Though such an implementation is possible, it is less preferred and requires more external components than a control chip with the comparators, etc., integrated therein.  
         [0022]    In the preferred embodiment, as shown in FIG. 1, the dashed line delineates between components which are part of the IC controller chip and those which are added individually to the power supply protection circuit. The operational amplifier IC 1 A  102  serves as an error amplifier and measures the difference between the reference voltage  104  (preferably 2-2.5 volts, depending on the control chip used) and the input from a voltage which is proportional to the output voltage of the power source  106 . As depicted, the reference voltage is internally available to op amp  102  while the other input is connected to a pin on the control chip which inputs to op amp  102 . Other external components, resistors R 3   108  and R 4   110  and opto-coupler U 1   112  are present for the purpose of regulating the output voltage of the power module. Although R 3  and R 4  are shown, other combinations of resistors and capacitors may be present around the error amplifier for the purpose of compensating the control loop. (An example of such alternate components is shown in FIG. 3, below.) Opto-coupler  112  is connected between the input voltage Vcc  114  (or another DC voltage) and node  116  which serves as input for op amp  102 . The voltage at node  116  is proportional to the output voltage. In a preferred embodiment, the opto-coupler feeds information about the controlled parameter (preferably the output voltage) across an input-to-output isolation boundary.  
         [0023]    [0023]FIG. 2 shows an implementation of a monitoring circuit for the isolated case that communicates to the opto-coupler. In FIG. 2, an op amp  202  receives input from the output voltage  204  to be monitored and a reference voltage  206 , preferably 1.2 volts. The output of the op amp  202  is connected to a light emitting diode  208  (LED), which is connected to the voltage Vc  210 . Depending on the current through the LED, an optical signal  212  is transmitted across the isolation boundary to the opto-coupler. It should be noted that the same idea could be implemented using a non-isolated power supply, except that the opto-coupler would not be required. The voltage output in such a case is fed directly to the error amp through a resistor.  
         [0024]    Referring once again to FIG. 1, also connected to node  116  are the two resistors  108  and  110 . Resistor R 4   110  connects to ground while R 3   108  connects to node  118 . Node  118  is coupled to the output of the op amp IC 1 A  102  and to resistor R 1   120 . R 1   120  then connects to a circuit comprising a resistor R 2   122  and a capacitor C 1   124 , which both, in parallel, connect to ground.  
         [0025]    Resistor R 1   120  also connects to an input of operational amplifier or comparator IC 1 B  126 , also called the overvoltage comparator, preferably located on the control chip itself. The second input of op amp  126  is connected to a second reference voltage  128 . In a preferred embodiment, the reference voltage  128  is about 1.5 volts, depending on the particular chip which is used.  
         [0026]    In the current invention, the output of the error amplifier  102  is sensed by resistor divider R 1   120  and R 2   122  which converts the output of the error amplifier  102  to a level which can be used by the comparator IC 1 B  126 . Capacitor C 1   124  provides filtering to the signal at the input of the op amp  126  to prevent false triggering of the op amp  126 . The capacitor C 1   124  also provides a delay between output of the error amplifier  102  rising and the overvoltage comparator  126  being tripped. This delay prevents the overvoltage comparator  126  from being tripped immediately during transient conditions.  
         [0027]    The output of the error amplifier  102  serves indirectly as an input for comparator  126 . As can be seen, the output voltage from error amp  102  will not be exactly the same as the input of comparator  126  because of intervening components, namely resistors R 1   120 , R 2   122 , and capacitor C 1   124 . The resistor R 1   120  causes a voltage drop relative to the output of error amp  102 . The resistors R 1   120  and R 2   122  can therefore be sized to provide a maximum duty cycle limit for the power module. As the output voltage of the power supply changes due to an error condition and moves away from the reference voltage value of 2.5 volts (which is input to the error amplifier  102 ), the error amplifier outputs greater and greater voltages while the PWM control chip attempts to provide added duty cycle to increase the load, operate with a lower input supply voltage, or increase maximum output voltage as determined by the type of error condition. This increasing voltage input (as decreased by resistors R 1   120  and R 2   122 ) to the comparator  126  causes it to trip once it exceeds the reference voltage  128  for comparator  126 . Under normal operating conditions, the voltage input to comparator  126  must not be very close to the reference voltage (1.5 volts in a preferred embodiment) or the output signal will saturate high, causing the switching power supply to stop switching, or shut off. The comparator  126  is only triggered if the adjusted error voltage is higher than the reference voltage Vref 2   128 .  
         [0028]    [0028]FIG. 3 shows a possible alternative circuit configuration for compensating the control loop. The op amp  102  is as shown in FIG. 1 part of the control chip, the border of which is shown by the dashed line. Op amp  102  has a reference voltage  104  and is connected to R 3   108  and R 4   110 . This figure differs from the previous example by the addition of a resistor  302  and capacitor  304  in parallel with R 3   108 .  
         [0029]    The innovative protection system can be combined with a current limit circuit (not shown) which senses the current in one or more devices to limit the peak and/or average current. When the current reaches a preset level, the current in the device is prevented from increasing further, effectively enforcing a maximum duty cycle on the device. This in turn causes the output voltage of the power module to decrease below the nominal output voltage setpoint of the power module control loop. When the output voltage drops below this nominal output voltage setpoint, the control loop saturates and the error amplifier, op amp  102 , output increases as high as it can go. When the output of the error amplifier  102  saturates, the voltage at the overvoltage comparator, comparator  126 , increases depending on the values of R 1   120 , R 2   122 , and C 1   124 . When the overvoltage comparator  126  hits the threshold, i.e., the input voltage from the error amplifier overcomes the value of the reference voltage of 1.5 volts, then the power module enters hiccup mode.  
         [0030]    The innovative protection circuit offers circuit protection in a variety of conditions with minimal circuit components. In overload and short circuit conditions, the actual output voltage is lower than the nominal output voltage. The output voltage control loop will tend to saturate and the error voltage, or the output of the error amplifier (IC 1 A), will increase as the voltage control loop attempts to increase duty cycle to raise the power module&#39;s output voltage. In the cases of an input undervoltage condition, as the input voltage decreases additional duty cycle is required to maintain the output voltage. As the duty cycle attempts to increase, the error voltage increases. In the case of output overvoltage, as the output voltage increases, additional duty cycle is required, which causes the error voltage to increase. In all the above cases, the error voltage will increase, eventually causing the overvoltage comparator to trip. Thus the innovative system allows protection against the above mentioned failure modes with a minimal addition of circuit components. It should be noted that although the innovative circuit can protect against all the failure modes described herein, in some cases a tighter tolerance protection circuit is required or present for one or more of the features.  
         [0031]    The description of the present invention has been presented for purposes of illustration and description, but is not limited to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention the practical application to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.  
         [0032]    The following references provide additional background and understanding of the current state of the art, and are hereby incorporated by reference: Modern DC-to-DC Switchmode Power Converter Circuits, by Rudolph P. Severns and Gordon Bloom, Van Nostrand Reinhold Company, New York, N.Y. (1985); and Principles of Power Electronics, by John G. Kassakian, Martin F. Schlect and George C. Verghese, Addison-Wesley Publishing Company, Reading, Mass. (1991).