Method for regulating a voltage and circuit therefor

[0001] This invention relates, in general, to power supplies and, more particularly, to power factor correction systems.

[0002] Power Factor Correction (“PFC”) systems are typically used within power supply applications requiring ac/dc rectification. Rectifying arrangements for use in such an application may comprise a full wave voltage rectifier, typically in the form of a diode bridge, and a main Switch Mode Power Supply (“SMPS”) to provide regulation of the output waveform. PFC circuits are inserted between the line and the main SMPS to draw a sinusoidal current from the line and to provide Direct Current (“DC”) voltage to the main SMPS. For many systems to operate properly, it is desirable for the output voltage of the PFC circuit to be within a specified range. PFC circuits deliver a squared sinusoidal power that matches an average power demand of the load. Thus, when the power fed to the load is lower than the demand, the output capacitor compensates for the lack of energy by discharging and when the power fed to the load is greater than the demand, the capacitor stores the excess energy. As a consequence, a ripple appears in the output voltage that designers compensate for by integrating the output voltage. A drawback with the integration is that it degrades the dynamic performance of the PFC systems and makes them slow. For example, an abrupt decrease in the load results in high output voltage overshoot and an abrupt increase in the load results in a high output voltage undershoot.

[0003] Hence, there exists a need for a regulator circuit having an overload protection circuit and a method of improving the dynamic performance and speed of the regulator circuit. In addition, it is desirable for the regulator circuit to be cost and time efficient to manufacture.

[0008] FIG. 1 is a block diagram of a voltage regulator 10 comprising a feedback network 12, an undershoot limitation circuit 14, an overload protection circuit 16, a switching or Pulse Width Modulated (“PWM”) regulation portion 18, a Power Factor Correction (“PFC”) circuit 20, and a voltage rectifier 22. More particularly, an input of undershoot limitation circuit 14 is connected to an output of feedback network 12 and an output of undershoot limitation circuit 14 is connected to an input of regulation portion 18. The output of feedback network 12 is also connected to the input of overload protection circuit 16 and the output of overload protection circuit 16 is connected to the input of regulation portion 18. The output of regulation portion 18 is connected to PFC circuit 20 and the output of PFC circuit 20 is connected to an input to feedback network 12 and to a load 24. Regulation portions and PFC circuits such as portion 18 and circuit 20, respectively, are known to those skilled in the art.

[0009] In operation, voltage rectifier 22 provides a rectified signal to PFC circuit 20, which boosts the rectified voltage signal and generates an output voltage V_OUT at output 26. Output voltage V_OUT is fed back to feedback network 12 which generates a feedback signal V_FB at its output. If feedback voltage V_FB is less than a predetermined level, undershoot limitation circuit 14 transmits a boosted control voltage to regulation portion 18, which regulates the control voltage and transmits it to PFC circuit 20. In case of an overload condition, the detection of which is discussed hereinafter, then overload protection circuit 16 grounds the output signal of voltage regulator 10. In accordance with one embodiment, overload protection circuit 16 includes a delay element so that it is not triggered by transient voltages appearing in output voltage V_OUT.

[0010] Referring now to FIG. 2, a schematic diagram of undershoot limitation circuit 14 is illustrated. What is shown in FIG. 2 is a transconductance amplifier 30 and a comparator 32. Transconductance amplifier 30 has inputs 34 and 36 and an output 38, and comparator 32 has inputs 40 and 42 and an output 44. Inputs 36 and 42 are connected to each other and inputs 34 and 40 are coupled for receiving reference voltages V_REF1 and V_REF2, respectively. Commonly connected inputs 36 and 42 are connected to feedback network 12 (shown in FIG. 1). Output 38 of transconductance amplifier 30 is connected to an output 48 of a compensation network 46 and to an input 52 of a switching circuit 50. In accordance with one embodiment, switching circuit 50 comprises switches 56 and 58, a diode 60, and an inverter 62. Switch 56 has a terminal coupled to input 52 through diode 60 and a terminal connected to an output 64 and to one terminal of switch 58. The terminal of switch 56 that is coupled to input 52 is also coupled to output 48 of compensation network 46 through diode 60. Input 54 is operatively coupled for controlling switch 58 and for controlling switch 56 through inverter 62. In other words, a control signal is transmitted through input 54 to control the operating states of switches 56 and 58, i.e., to control whether switches 56 and 58 are open or closed. Output 64 of switching circuit 50 is connected to the input of regulation circuit 18 shown in FIG. 2.

[0011] By way of example, compensation network 46 is comprised of a capacitor 66 coupled in a parallel configuration with a Zener diode 68.

[0012] Feedback network 12 may be comprised of a voltage divider network comprising a pair of resistors configured so that one terminal of each resistor is connected together and to the commonly connected inputs 36 and 42. It should be understood that feedback network 12 is not limited to being a voltage divider network. Although feedback network 12 may be integrated in a semiconductor substrate along with undershoot limitation network 14, PWM regulator 42, and start-up circuit 162, it is preferably provided as an off-chip circuit network.

[0013] Referring now to FIG. 3, a schematic diagram of overload protection circuit 16 is shown. Overload protection portion 16 provides protection against an excessive power demand appearing on an output of a regulator circuit, e.g., output 26 of regulator circuit 10. What is shown in FIG. 3 is a transconductance amplifier 100 having an input coupled for receiving a feedback signal V_FB, an input coupled for receiving a reference voltage V_REF, and an output coupled to a current mirror 102, to a capacitor 104 that is connected to ground, and to an input 106 of a flag register 108. A clamping circuit 132 has an input connected to current mirror 102 and an output connected to an input 111 of flag register 108. Outputs 110 and 112 of flag register 108 are connected to inputs 114 and 116 of a delay circuit 118, respectively.

[0014] In accordance with one embodiment, current mirror 102 comprises a pair of bipolar junction transistors 120 and 122 having commonly connected bases. The collectors of bipolar junction transistors 120 and 122 are coupled for receiving a source of operating potential V_DD through resistors 124 and 126, respectively. The base of bipolar junction transistor 122 is connected to its collector. The emitter of bipolar junction transistor 122 is coupled to a source of operating potential such as, for example, ground through a diode 130 and the emitter of bipolar junction transistor 120 is coupled to the input of clamping circuit 132 through diode 128. Although transistors 120 and 122 are shown and described as being bipolar junction transistors, they can also be field effect transistors.

[0015] Clamping circuit 132 comprises a Zener diode 134 coupled in parallel across a pair of resistors 136 and 138. More particularly, the cathode of Zener diode 134 and one terminal of resistor 136 are connected to the cathode of diode 128 and the anode of Zener diode 134 and one terminal of resistor 138 are coupled for receiving a source of operating potential such as, for example, ground. The other terminals of resistors 136 and 138 are connected together and to a regulation portion such as regulation portion 18 (shown in FIG. 1) at output 141.

[0016] Delay circuit 118 comprises a two-input AND gate 140 having an input that serves as input 114 of delay circuit 118 and an input coupled for receiving a control signal PFC_OK. An output of two-input AND gate 140 is connected to an input of a 2-input OR gate 142. The other input of 2-input OR gate 142 serves as input 116 of delay circuit 118. An output of 2-input OR gate 142 is connected to an input of a delay element 144. The output of 2-input OR gate 142 is coupled for resetting delay element 144 through an inverter 146. An output of delay circuit 118 serves as a control input for a switch 150, which has a terminal connected to output 141 and a terminal coupled for receiving a source of operating potential such as, for example, ground.

[0017] In operation, when the load demand exceeds the maximum power that PFC circuit 20 (shown in FIG. 1) can provide (i.e., when an overload condition exists), the output signal of the PFC stage decreases. In response to this decrease, feedback signal V_FB also decreases causing transconductance amplifier 100 to charge capacitor 104 in order to increase the power delivery. In this condition, PFC circuit 20 cannot maintain the bulk voltage at its nominal level, hence transconductance amplifier 100 continues to charge capacitor 104. In response to the overload condition, transconductance amplifier 100 generates a current substantially equal to its maximum current, which causes a flag in flag register 108 to be asserted that transmits a logic high voltage level from output 110 of flag register 108 to input 114 of delay circuit 118. The logic high voltage level serves as a control signal. If the voltage regulator is no longer in the start-up phase, a control signal PFC_OK is at a logic high voltage level, which logic high voltage level is transmitted to the second input of 2-input AND gate 140. In response to the logic high voltage signals at its inputs, AND gate 140 transmits a logic high voltage level to 2-input OR gate 142, which generates a control signal for turning on delay element 144, which delay element delays the transmission of an overload signal V_OVL for a predetermined amount of time. By way of example, the delay is 100 milliseconds (msec). In other words, the control signal at input 114 indicates that an overload condition exists and control signal PFC_OK indicates that the overload condition is not occurring during start-up. Thus, control signal PFC_OK serves as an indicator signal. If the overload condition continues for more than the predetermined time, overload signal V_OVL disables the voltage regulator.

[0018] Alternatively, when an overload occurs at the output of the voltage regulator, transconductance amplifier 100 injects a current through clamping circuit 132 which increases the voltage across Zener diode 134 to a high enough level to clamp the voltage. The voltage across Zener diode 134, when activated, sets a flag of flag register 108. Flag register 108 then transmits a logic high voltage level to input 116 of delay circuit 118, i.e., it transmits a logical high voltage to an input of 2-input OR gate 142. Like the condition in which a logic high voltage levels appear at the inputs of 2-input AND gate 140, the logic high voltage level at the input of 2-input OR gate 142 generates a control signal for turning on delay element 144, which delay element delays the transmission of an overload signal V_OVL for a predetermined amount of time. Delay element 144 is also referred to as a timer. In this case, the output voltage of the regulator is at its maximum level, thus delay element 144 is activated even if the regulator circuit is in the start-up phase. An advantage of including clamping circuit 132 is that it allows detecting conditions in which PFC circuit 20 does not start-up. Including the delay circuit allows distinguishing an overload condition from other transient conditions that may occur. These events are not considered overload conditions and the timer immediately resets as soon as control signal V_OVL falls to a logic low voltage level.

[0019] By now it should be appreciated that a regulator having undershoot and overload protection and method for regulating output voltage have been provided. In accordance with an embodiment of the present invention, the overload protection circuit includes a delay element to verify that the overload condition is not a transient condition, thereby decreasing the risk of short transient events triggering the overload protection circuit.

[0020] Although certain preferred embodiments and methods have been disclosed herein, it will be apparent from the foregoing disclosure to those skilled in the art that variations and modifications of such embodiments and methods may be made without departing from the spirit and scope of the invention. It is intended that the invention shall be limited only to the extent required by the appended claims and the rules and principles of applicable law.