PWM controller having off-time modulation for power converter

[0002] The present invention relates to switching mode power converter. More particularly, the present invention relates to the pulse width modulation (PWM) of the switching mode power converter.

[0003] The PWM is a traditional technology that is used in the switching mode power converter to control the output power and achieves the regulation. Most of the equipments, such as mobile phone, TV game, and computer etc. are using PWM power converters to supply power and charge battery. Based on the restriction of environmental pollution, the computer and other equipment manufactures have been striving to meet the power management and energy conservation requirements. The embodiment of power management is to manage the system only consuming the power during the operation. And only a little quantity of power is consumed during non-operation mode. With respect to the power supply in a power management application, saving power in the no load or light load conditions is a major requirement. According to the invention, the object of the off-time modulation for the PWM control is to reduce the power consumption in light load and no load conditions.

[0004] FIG. 1 shows a circuit schematic of the flyback power converter, in which a PWM controller 100 controls the power output and achieves the regulation. A transistor 510 switches a transformer 400. When the transistor 510 is turned off, the leakage inductance of transformer 400 keeps the current, which has been flowing in it constantly for some short time. The part that current continues to flow into the slowly off-switching transistor 510, and the rest of that current flows into a capacitor 560 through a diode 520. A resistor 620 dissipates the energy that is charged in the capacitor 620. The diode 520, resistor 620, and capacitor 560 form a snubber circuit to reduce the leakage inductance spike and avoid the transistor 510 breakdown. At the instance of transistor 510 is switched on and an output rectifier 530 is switched off, there is an exponentially decaying oscillation or a ‘ring’ will come out. The ring is at a frequency determined by the inherent capacity of the off-switching rectifier 530 and the secondary inductance of the transformer 400. The amplitude and duration of the ring are determined by the output current and the reverse recovery times of the rectifier 530. The ring will cause RFI problem and can easily be eliminated by a snubber resistor 630 and a snubber capacitor 570 across the output rectifier 530. The major issues for the loss of the power conversion in the light load are illustrated as follows:

[0005] (1) The switching loss of the transistor 510, P_Qcan be expressed (t_ol/T) (∫₀^tolV_Q×Ip dt), where T is switching period, and t_olis the duration of overlap of voltage V_Qand current Ip. Ip is the primary current of the transformer 400 and V_Qis the voltage across the transistor 510.

[0006] (2) The switching loss of output rectifier 530, P_Dcan be expressed (t_rr/T) (∫₀^trrVd×Id dt), where t_rris the reverse recovery time of the rectifier. The Vd is the voltage across the rectifier when it is switched off. The Id is limited by the secondary inductance of the transformer 400.

[0007] (3) The core loss of transformer 400, P_T, it is in direct proportion to flux density Bm, core volume Vv and the switching period T. P_T=K₀×Bm×Vv/T, where K₀is a constant that is determined by the material of the core and etc.

[0008] (4) The power loss of the snubber, P_Ris stated as P_R=(1/2)×C×Vd²/T, where C is the capacitance of the snubber, such as capacitor 570.

[0009] (5) The power loss of leakage inductance, P_Lcan be stated by P_L=(1/2)×Lt×Ip²/T, where the Lt is the primary leakage inductance of transformer 400. The resistor 620 dissipates the energy that is produced by the Lt.

[0010] We can find that all of the losses are indirectly proportional to the switching period T. Increase of the switching period T can reduce the power losses. However the power conversion is restricted to operate in a short switching period to shrink the size of power converter. To prevent the saturation of the transformer, the voltage-time ratio (Vin×Ton) has to be controlled to limit the flux density Bm of the transformer. It is given by

[0011] Bm=(Vin×Ton)/(Np×Ae)  (1)

[0012] where Vin is the input voltage of the power converter, Ton is the on-time of the switching period, Np is the primary turn number of the transformer, Ae is the cross area of the transformer. The value of (Np×Ae) represents the size of the transformer. A short switching period can earn a shorter Ton and a smaller transformer.

[0013] Take the flyback power converter as an example; the output power Po is equal to the [1/(2×T)]×Lp×Ip², where Lp is the primary inductance of the transformer 400. Due to Ip=(Vin/Lp)×Ton, it can be seen quantitatively as

[0014] Po=(Vin²×Ton²)/(2×Lp×T).  (2)

[0015] This is seen from that equation 2, in the light load condition, Ton is short that obviously allows the switching period T to be expanded. The power consumption of the power converter is dramatically reduced in response to the increase of switching period in the light load and no load condition. Nevertheless, it is unsafe to increase the switching period without limitation. According to the behavior of the transformer that is showed in Equation 1, the transformer may be saturated due to an expanded Ton. A dynamic loading may produce an instant expended Ton. The dynamic loading means the load is instantly changed between the light load and the high load. The saturation of magnetic components, such as inductors and transformer, causes a current surge. The current surge will generate a spike-noise in power converter and also cause an over-stress damage to the switching devices such as transistors and rectifiers.

[0016] FIG. 2 shows the circuit schematic of the PWM-controller. FIG. 3 displays the waveform of the circuit in FIG. 2. When the switch 25 is turned on by a charge signal IVp, a charge current I_Ccharges a capacitor C_Tand once the voltage across the capacitor C_Treaches the high trip-point V_Hof the comparator 10, the comparator 10 and the NAND gates 17,18 generate a discharge signal Vp to turn on the switch 26 that discharges the capacitor C_Tvia a discharge current I_D. The charge current I_Cand the discharge current I_Dare correlated normally. The phenomenal of the discharge is continuous until the voltage of capacitor C_Tis lower than the low trip-point voltage V_L, in which a comparator 11 is enabled. The charge current I_C, the discharge current I_D, capacitor C_T, comparators 10, 11 and the switches 25, 26 and NAND gates 17, 18 form a saw-tooth-signal generator and produce a clock signal to clock-on the flip-flop 20. The comparator 12 resets the flip-flop 20 once the voltage in the pin Vs is higher than the signal 35, which is attenuated from the feedback signal V_FBby the resistor R_Aand R_B. The resistor 610 converts the current information of the transformer 400 to a voltage signal 37, which is a ramp signal. The input voltage Vin and the inductance of transformer 400 determine the slope of the ramp signal. V_R610=R₆₁₀×(Vin×Ton)/Lp. The voltage in resistor 610 is the voltage signal 37, which drives the pin Vs of PWM controller. The feedback signal V_FBis derived from the output of an optical-coupler 200 via a level shift diode 21. An error amplifier 300 drives the input of the optical-coupler 200. The input of the error amplifier 300 is connected to the output of the power converter Vo to develop the voltage feedback loop. Through the control of voltage feedback loop, the voltage of the V_FBdominantly decides the output powers. The discharge time of capacitor C_Tdetermines the dead time of the PWM signal 39 that decides the maximum duty cycle of PWM controller 100. The PWM signal 39 is turned off in response to the high of the discharge signal Vp.

[0017] According to that observation, the expanding T can be achieved by expanding Toff, where Toff is the off-time of the switching period T. T=Ton+Toff. While the Toff is expanded in response to a low feedback voltage V_FBin light load and no load conditions and the power consumption can be reduced.

[0018] The invention provides an off-time modulation for the PWM controller to increase the switching period in the light load and no load conditions. The off-time modulation is accomplished by moderating the discharge current of the saw-tooth-signal generator in the PWM controller. Reducing the discharge current increases the switching period, in which the off-time of the switching period is expanded. The charge current of the saw-tooth-signal generator is kept as a constant, in which the charge current decides the maximum on-time of the switching period. The feedback voltage, which is derived from the voltage feedback loop, is taken as an index. The discharge current is modulated to be a function of the feedback voltage. A threshold voltage is a constant that defines the level of the light load. The differential of the feedback voltage and the threshold voltage is converted to a current, which is then amplified by a current mirror amplifier and turned into the discharge current. A limiter clamps the maximum discharge current to set up the switching period in normal load and full load conditions and determines the dead time of PWM signal. Once the decrease of the feedback voltage is close to the level of the threshold voltage, the discharge current will be reduced and the switching period will be increased continuously. When the feedback voltage is lower than the threshold voltage, a minimum discharge current decides a maximum switching period. The gain of the current mirror amplifier decides the slope of the decrement of the discharge current in response to the feedback voltage decrement. The charge current associated with the discharge current decides the switching period of the PWM signal. The minimum discharge current determines the maximum off-time and the maximum switching period. Keeping the maximum on-time as the constant and increasing the switching period by increasing the off-time can prevent the magnetic components, such as inductors and transformer, from being saturated.

[0019] Advantageously, the off-time modulation in the PWM controller can reduce the power consumption of the power converter in light load and no load conditions. And the operation of power conversion in normal load and full load conditions are not affected by the off-time modulation.

[0020] It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.

[0021] The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings,

[0022] FIG. 1 shows a circuit schematic of the flyback power converter having a PWM controller.

[0023] FIG. 2 shows the circuit schematic of the PWM-controller.

[0024] FIG. 3 displays the waveform for the circuit as shown in FIG. 2.

[0025] FIG. 4 schematically shows the block schematic of the off-time modulation according to a preferred embodiment of the present invention.

[0026] FIG. 5 displays the waveform of the circuit shown in FIG. 2, in which an off-time modulation is included.

[0027] FIG. 6 shows an exemplary circuit of the off-time modulation shown in FIG. 4.

[0028] FIG. 4 schematically shows the block diagram of the off-time modulation according to one embodiment of the present invention.

[0029] The feedback voltage V_FBderived from the voltage feedback loop subtracts a threshold voltage V_Avia an adder 51. The threshold voltage V_Ais a constant that defines the level of the light load. A V/I converter 53 converts the output of the adder 51 from voltage to a V/I current. A mirror amplifier 55 magnifies the V/I current to an amplified current. A limiter 57 generates a limited current Ilimit, in which the amplitude of the amplified current is clamped between the range of zero and an utmost current Iut. An adder 59 adds the limited current Ilimit and a constant current I_A, wherein the constant current I_Adetermines the minimum discharge current and decides the maximum off-time. A transistor 61 and a transistor 63 form an echo current mirror. Through the echo current mirror, the output of the adder 59 turns into the discharge current I_D. The gain G of the mirror amplifier 55 decides the slope of the decrement of the discharge current I_Din response to the feedback voltage V_FBdecrement. Once the decrease of the feedback voltage V_FBis close to the level of the threshold voltage V_A, the discharge current I_Dwill be reduced and the switching period T will be increased continuously. When the feedback voltage V_FBis higher than the threshold voltage V_A, the maximum discharge current I_Dis determined by the utmost current Iut, which decides the switching period T in the normal load and full load conditions. The minimum discharge current determines the maximum off-time and the maximum switching period. The charge current I_Cis a constant, which decides the maximum on-time of the switching period. Keeping the maximum on-time as the constant and increasing the switching period by increasing the off-time prevent the magnetic components from being saturated. FIG. 5 shows the waveform of the circuit in FIG. 2, in which an off-time modulation is included. The limited current Ilimit can be stated as

[0030] Ilimit=[(V_FB−V_A)/Rs]×G;

[0031] 0≦Ilimit≦Iut

[0032] Ilimit=0Vif Ilimit≦0V;

[0033] and

[0034] Ilimit=Iutif Ilimit≦Iut;

[0035] Then

[0036] I_C=K1;

[0037] I_D=(Ilimit+I_A)×Gm;

[0038] And

[0039] I_A≦I_D≦(Iut+I_A)

[0040] Where the Rs, the transfer function of V/I converter 53, is a resistor; the Gm is the gain of the echo current mirror, Gm=1; and the K1 is a constant.

[0041] FIG. 6 shows an exemplary circuit of the off-time modulation shown in FIG. 4. An operational amplifier (op amp) 81, a transistor 65 and a resistor Rs form the V/I converter, which outputs the V/I current. Via an op amp 83, the threshold voltage V_Ais connected to the resistor Rs. The V/I current is then can be stated as (V_FB−V_A)/Rs; a transistor 67 associated with a transistor 69 develops the mirror amplifier and output san amplified current, in which the gates of the transistors 67, 69 and the drain of transistor 67 are tied together. The geometrical size of the transistors 67, 69 decides the gain G of the mirror amplifier. The V/I current is linked to the drain of transistor 67. The sourced of the transistors 67, 69 are tied together and then driven by a constant current I_B. Since the constant current I_Blimits the total current of the mirror amplifier, the utmost current Iut is equal to the current that the constant current I_Bsubtracts the V/I current. Through the drain of the transistor 69, the mirror amplifier outputs the limited current [(V_FB−V_A)/Rs]×G, which is limited between zero and the utmost current Iut. The transistors 61 and 63 form an echo current mirror, in which the gates of the transistors 61, 63 and the drain of the transistor 61 are tied together. The sources of the transistors 61 and 63 are grounded. The amplified current added to the constant current I_Ais connected to, the drain of the transistor 61, which is the input of the echo current mirror. The drain of the transistor 63 outputs the discharge current I_D. Because the geometrical size of the transistors 61 and 63 is identical, the gain of the echo current mirror Gm is equal to 1. Thus

[0042] I_D=[(V_FB−V_A)/RS]×G+I_A;

[0043] And

[0044] I_A≦I_D≦(Iut+I_A)

[0045] The charge current I_Cis a constant, which decides the maximum on-time of the switching period. The maximum on-time remains constant and increases the switching period by increasing the off-time.

[0046] As described above, the PWM controller including an off-time modulation of the present invention can reduce the power consumption for the power converter in light load and no load conditions. Furthermore the off-time modulation will not cause magnetic saturation.

[0047] It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.