Soft-switching power converter

[0001] The present invention relates in general to a pulse width modulation power converter, and more particularly, to an improved pulse width modulation power converter using zero-voltage switching technique.

[0002] Power converters have been used to convert an unregulated power source to a regulated voltage or current source. The transformer that comprises a primary winding and a secondary winding is the heart of most power converters. Typically, a switching device is connected to the primary winding to control energy transferred from the primary winding to the secondary winding and output therefrom. Currently, under the control of the switching device, the pulse width modulated power converter can be operated at a high frequency with reduced size and weight. However, such a power converter suffers from switching loss, component stress, noise, and electromagnetic interference (EMI) issues.

[0003] To resolve the switching loss problem of the pulse width modulation power converter, a phase-shift scheme for soft switching has been proposed, particularly for the high-frequency power conversion. For example, the full-bridge (FB) quasi-resonant zero-voltage switching (ZVS) technique has been disclosed in U.S. Pat. No. 4,855,888, “Constant frequency resonant power converter with zero-voltage switching” issued to Christopher P. Henze, Ned Mohan and John G. Hayes on Aug. 8, 1989, U.S. Pat. No. 5,442,540, “Soft-switching PWM converters” issued to Guichao C Hua and Fred C. Lee on Aug. 15, 1995, and U.S. Pat. No. 6,356,462 “Soft-switched full-bridge converters” disclosed by Yungtaek Jang and Milan M. Jovanovic on Mar. 12, 2002. In U.S. Pat. No. 5,973,939, “Double forward converter with soft-PWM switching” issued to F. Don Tan on Oct. 26, 1999 and U.S. Pat. No. 6,191,960, “Active clamp for isolated power converter and method of operating thereof” issued to Simon Fraidlin and Anatoliy Polikarpov on Feb. 20, 2001. The active clamp technique has been employed in the forward zero-voltage switching power converters. In U.S. Pat. No. 6,069,798, “Asymmetrical power converter and method of operation thereof” issued to Rui Liu on May 30, 2000, an asymmetrical scheme has been developed for a half-bridge (HB) topology.

[0004] Among various zero-voltage switching power converters, a parasitic leakage inductor of the transformer or at least one additional magnetic component is used to be a resonant inductor or switch to generate a circulating current, so as to achieve the zero-voltage transition and switching. The parasitic leakage inductor of the transformer or the additional magnetic component, though aiding zero-voltage transition and switching, inevitably increases switching stress and noise. Further, in such an approach, power consumption caused by the circulating current is significantly high in the light load or zero-load condition.

[0005] The present invention provides a pulse width modulated soft-switching power converter for high frequency operation. The pulse width modulated soft-switching power converter is operated at a constant frequency with low switching loss, low stress, and low noise.

[0006] The present invention further provides a pulse width modulated soft-switching power converter that can generate zero-voltage transition and switching without using an additional magnetic device or leakage inductor of the transformer.

[0007] The present invention also provides a pulse width modulated soft-switching power converter that consumes relatively low power in light load and zero-load conditions.

[0008] Further, the present invention provides a control scheme to optimize soft switching of a power converter.

[0009] The pulse width modulated soft-switching power converter provided by the present invention comprises a transformer, a secondary circuit, a pair of main switches and a pair of auxiliary switches. The transformer has a primary winding coupled to the main and auxiliary switches and a secondary winding coupled to the secondary circuit. The main switches and auxiliary switches intermittently conduct an input voltage source to the primary winding of the transformer, such that the soft-switching power converter is operated in four operation stages in each switching cycle.

[0010] In the first operation stage, the connection of the transformer and the input voltage source is established by switching on main switches, such that power is delivered from the primary winding to the secondary winding. In the second operation stage, the connection between the input voltage source and the transformer established by main switches is cut off, such that energy stored in the transformer is reset and freewheeled back to the input voltage source through auxiliary switches. Meanwhile, energy stored in the secondary circuit is continuously output therefrom. In the third operation stage, the transformer operates as an inductor with the secondary winding thereof open circuited. Auxiliary switches are thus switched under a zero-current switching mode. In the fourth stage, energy stored in and magnetizing the transformer in the third operation stage flies back to the input voltage source through main switches to achieve a zero-voltage transition.

[0011] In the above pulse width modulated soft-switching power converter, main switches and auxiliary switches are driven by a first switching signal and a second switching signal, respectively. The first and second switching signals are preferably in the form of pulse signals with a first pulse width and a second pulse width, respectively. Preferably, the first pulse width is broader than the second pulse width. In such manner, in each switching cycle, main switches are switched on only in the first operation stage, and auxiliary switches are switched on only in the third operation stage. In the second and fourth operation stages, both main switches and auxiliary switches are switched off.

[0012] In the above pulse width modulated soft-switching power converter, the duration of the second operation stage can be extended, allowing energy stored in the transformer to be completely released. The energy stored in the transformer in the third operation stage is equal to the multiplication of the square of the input voltage and the square of the duration of the third operation stage divided by 2 times inductance of the primary winding. In order to achieve zero-voltage transition in the fourth operation stage, energy stored in the transformer in the third stage is no less than energy needed for charging parasitic capacitors of main switches. Further, a minimum duration of the fourth operation stage is required to achieve zero-voltage transition. The minimum duration of the fourth operation stage, also referred as a minimum transfer time, is proportional to the inductance of the primary winding and inversely proportional to the parasitic capacitance of main switches. Moreover, the fourth operation stage may be delayed by a delay time for switching on parasitic diodes of main switches, allowing energy stored in the transformer to flow back to the input voltage source, so as to achieve zero-voltage transition. Therefore, energy stored in the third operation stage is no less than the sum of energy needed for charging the parasitic capacitors of main switches and the energy needed during the delay time.

[0013] The pulse width modulated soft-switching power converter provided by the present invention further comprises a controller to generate the first switching signal and the second switching signal for driving main switches and auxiliary switches, respectively. By the controller, a pulse width modulation switching frequency is determined. The pulse width modulated soft-switching power converter further comprises a first resistor coupled to the controller to adjust a pulse width modulation switching frequency. The pulse width modulated soft-switching power converter further comprises a second resistor coupled to the controller to adjust a pulse width of the second switching signal. The pulse width modulated soft-switching power converter further comprises a third resistor coupled to the controller to adjust a pulse width of the second switching signal as a function of a load of the power converter.

[0023] FIG. 1 shows the topology of a soft-switching power converter provided by the present invention. As shown in FIG. 1, the soft-switching power converter comprises a transformer 50, a pair of main switches 10 and 20, a pair of auxiliary switches 30 and 40, and a secondary circuit. The transformer 50 further comprises a primary winding Wp coupled to main switches 10, 20 and auxiliary switches 30, 40 and a secondary winding Ws coupled to the secondary circuit. More specifically, in this embodiment, the main switch 10 connects the primary winding Wp to an input voltage source V_IN at a first end A thereof, which is further connected to the auxiliary switch 40. The auxiliary switch 30 connects the input voltage source V_IN to a second end B of the primary winding Wp, and the second end B, is further connected to the main switch 20. The main switches 10, 20 and auxiliary switches 30, 40 can be metal-oxide semiconductor field effect transistors (MOSFET), insulated gate bipolar transistors (IGBT), or gate-turn-off transistors (GTO), for example. As shown in FIG. 1, the input voltage source V_IN is further connected to a capacitor 5.

[0024] The secondary circuit comprises a half-bridge rectifier, which is assembled of a diode 60 that is often referred as the rectifying diode and a freewheel diode 70, an inductor 80, an output capacitor 90, and an output terminal for outputting an output voltage V_O. An anode of the diode 60 is coupled to a first end of the secondary winding Ws, and an anode of the freewheel diode 70 is coupled to a second end of the second winding Ws. The inductor 80 is connected between a cathode of the diode 60 and a cathode of the freewheel diode 70 and the output terminal of the secondary circuit. The output capacitor 90 has a negative terminal connected to the anode of the freewheel diode 70 and a positive terminal connected to the inductor 80 and the output terminal of the secondary circuit.

[0025] As shown in FIG. 1, main switches 10 and 20 are driven by a switching signal S₁, while auxiliary switches 30 and 40 are driven by a switching signal S₂. Referring to FIG. 2, the switching signal S₁ preferably is in a pulse waveform with a pulse width of a duration T₁, while the switching signal S₂ is preferably in a pulse waveform with a pulse width of a duration T₃.

[0026] By controlling the on/off status of main switches 10, 20 and auxiliary switches 30, 40, the power converter as shown in FIG. 1 has four operation stages in each switching cycle as shown in FIGS. 3a to 3d. In order to operate the soft-switching power converter in four operation stages, the switching signals S₁ and S₂ are out of phase. That is, in the embodiment as shown in FIG. 1 and FIG. 2, main switches 10 and 20 are turned on when the switching signal S₁ is high within the duration T₁ for each switching cycle. During the duration T₁, auxiliary switches 30 and 40 are in an off state. When the switching signal S₁ is low for a duration T₂, as shown in FIG. 2, auxiliary switches 30 and 40 are turned on within duration T₃.

[0027] The four operation stages are further described as follows with reference to FIGS. 2 and 3a to 3d. At the beginning of each switching cycle, as shown in FIG. 2, main switches 10 and 20 are switched on within the duration T₁ of the switching signal S₁. As main switches 10 and 20 are turned on, as shown in FIG. 3a, the current I₁, flows from the input voltage source V_IN through main switches 10 and 20 across the primary winding Wp. Therefore, the input voltage source V_IN is applied to the primary winding Wp. The polarities of the primary winding Wp and the secondary winding Ws conduct the diode 60 by supplying a forward bias thereto. Meanwhile, the freewheel diode 70 is reversely biased. Therefore, a secondary current I₂ flows through the diode 60 and the inductor 80 along the arrow as shown in the secondary circuit of FIG. 3a. Consequently, energy is delivered to the output terminal of the pulse width modulated soft-switching power converter to generate an output voltage V_O.

[0028] After the duration T₁, the switching signal S₁ drops to zero or a lower voltage to switch off main switches 10 and 20 in the second operation stage as shown in FIG. 2. Referring to FIG. 2 and FIG. 3b, the current I₁ is cut off. However, before auxiliary switches 30 and 40 are switched on by the switching signal S₂ within the duration T₃, the energy that stored in the primary winding Wp produces a current that inverts the polarity of the primary winding Wp and the secondary winding Ws and induces a current I₃ flowing back to the input voltage source V_IN. As a result, the diode 60 connected to the secondary winding Ws is reversely biased, and the secondary winding Ws become an open circuit. Therefore, the energy stored in the transformer 50 (primarily generated by leakage inductance of the transformer 50) is reset and freewheeled back to the input voltage source V_IN with the current I₃ flowing through the parasitic diodes of auxiliary switches 30 and 40. Meanwhile, the freewheel diode 70 is forwardly biased and conducted to form a closed loop between the freewheel diode 70, the inductor 80, and the output capacitor 90 with a current I₄ circulating therethrough. Therefore, the energy stored in the inductor 80 and the output capacitor 90 is thus continuously delivered to the output terminal of the secondary circuit. Further, as shown in FIG. 2, the duration T₂ of the second operation stage is varied according to the amount of energy stored in the transformer 50. The variable duration of the second operation stage is denoted as T_R in FIG. 2.

[0029] FIGS. 2 and 3c show the third operation stage in each switching cycle of the pulse width modulated soft-switching power converter. As shown in FIG. 2, before the next switching cycle, that is, before the main switches 10 and 20 are switched on by the pulse of the switching signal S₁ again, the switching signal S₂ switches on the auxiliary switches 30 and 40 within the duration T₃ thereof. As shown in FIG. 3c, by switching on the auxiliary switches 30 and 40, the input voltage source V_IN is connected to the second end B of the primary winding Wp to induce a current I₅ across the primary winding Wp and directed along at the side of the primary winding Wp, and energy is stored in the transformer 50. Similar to the second operation stage, polarity of the transformer 50 results in reverse bias of the diode 60, so that the secondary winding Ws becomes open circuited. The transformer 50 thus operates as an inductor in the third operation stage, such that the power converter is operated similar to a discontinuous mode flyback power converter. Switching on the auxiliary switches 30 and 40 under zero-current switching (ZCS) or zero-voltage switching (ZVS) can thus be realized. The energy stored in the transformer 50 in the third operation stage can be expressed as:
ε=Lp×Ip²/2,
where Lp is the inductance of the primary winding Wp, Ip is the current flowing through the primary winding Wp and can be expressed as:
Ip=V_IN×T₃/Lp,
where T₃ is the duration that auxiliary switches 30 and 40 are turned on, that is, the pulse width of the switching signal S₂. By substituting the equation of Ip into the equation of the energy ε,
ε=V_IN²×T₃²/(2×Lp).
Therefore, the energy stored in the transformer 50 in the third operation stage is proportional to the multiplication of the square of the input voltage V_IN and the square of the pulse width T₃ of the switching signal S₂, and inversely proportional to the inductance of the primary winding Wp.

[0030] In the fourth operation stage as shown in FIG. 2 and FIG. 3d, the switching signal S₂ drops to zero or lower to switch off auxiliary switches 30 and 40, while the switching signal S₁ stays zero or lower to keep the main switches 10 and 20 turned off. The current I₅ produced in the third operation stage flows through the primary winding Wp. Meanwhile, the energy stored in the transformer 50 during the period T₃ of the third operation stage flies back to the input voltage source V_IN through the parasitic diodes of main switches 10 and 20 to achieve zero-voltage transition.

[0031] To turn on the parasitic diodes of main switches 10 and 20, the parasitic capacitors of main switches 10 and 20 have to be discharged in advance. In other words, zero-voltage transition is achieved when the parasitic capacitors of the main switches 10 and 20 have been discharged. Therefore, to achieve zero-voltage transition, the energy stored in the transformer 50 in the third operation stage must be larger than the energy needed to discharge both of the parasitic capacitors of main switches 10 and 20. The relation can be expressed by the following inequality:
V_IN²×T₃²/(2×Lp)>2×(Cr×V_IN²/2)
Where Cr is the parasitic capacitance of the main switch 10 or 20. As the resonant frequency fr between the primary winding Wp and the parasitic capacitors of main switches 10 and 20 at the period T₃ can be expressed as:
fr=1/(2π×(Lp×Cr)^1/2),
a minimum transfer time T_Fto achieve phase shift for zero-voltage transition can be expressed as:
T_F=1/(4×fr)=π×(Lp×Cr)^1/2/2.
That is, the minimum time from the switching signal S₂ dropping to low to switch off auxiliary switches 30 and 40 to the time the main switches 10 and 20 being switched on again by the duration T₁ of switching signal S₁, namely, the minimum duration of the fourth stage can be calculated by the above equation of T_F. >From the above equation, it is known that the minimum time required for achieving zero-voltage transition is determined by the inductance of the primary winding Wp and the parasitic capacitance Cr.

[0032] The duration of the fourth stage may be delayed by a delay time T_Z after the parasitic diodes of main switches 10 and 20 are conducted and before the next switching cycle starts. Therefore, the total duration of the fourth stage is the sum of the minimum transfer time T_F and the delay time T_Z, that is, T₄=T_F+T_Z. However, in order to operate the inductor 80 in a continuous mode under the condition of zero-voltage transition, the energy stored in the transformer 50 in the duration T₃ of the third operation stage must satisfy the following inequality:
V_IN²×T₃²/(2×Lp)>{[Cr×V_IN²]+[V_IN×(Ts/Tp)×I_O×T_Z]+[T_Z×V_IN²×T₃/Lp]},
where Ts and Tp are turn numbers of the secondary and primary windings Ns and Np, respectively, and I_O is the output current of the power converter. That is, the energy stored in the transformer 50 in the duration T₃ must be large enough to discharge the parasitic capacitance 2Cr, and then provide the primary side backward freewheeling current and sustain the output current during the delay time T_Z.

[0033] FIG. 4 shows a circuit diagram of the controller, which generates switching signals S₁ and S₂. As shown in FIG. 4, the controller includes an oscillator 200, an inverter 370, comparators 320 and 330, a programmable current source 310, D-type flip-flops 340, 350 and 360, and AND gates 380 and 390. The oscillator 200 is coupled to an input of the inverter 370, the negative inputs of the comparators 320 and 330, and a reference resistor 515. An output of the inverter 370 is coupled to the D-type flip-flops 340, 350, 360, and the inputs of the AND gates 380 and 390. The D-type flip-flop 340 is further coupled to a voltage source Vcc and the output of the comparator 320, while an output thereof is coupled to the AND gate 380. Signals S_A and S_B output by the D-type flip-flop 350 are inverted from each other and fed into the AND gates 380 and 390, respectively. Signal S_B is fed to the D-type flip-flop 350. The D-type flip-flop 360 is further coupled to the output of the comparator 330 and the voltage source Vcc, while an output thereof is coupled to the input of the AND gate 390. From the AND gates 380 and 390, the switching signals S₁ and S₂ are output to drive the main switches 10, 20 and the auxiliary switches 30, 40, respectively.

[0034] As shown in FIG. 4, the D-type flip-flop 350 provides signals S_A and S_B to the AND gates 380 and 390, respectively. The main switches 10, 20 and the auxiliary switches 30 and 40 as shown in FIG. 1 are driven out of phase with slightly less than 50% of the maximum duty cycle. The oscillator 200 is operative to generate a clock signal 210, a ramp signal 220 and a saw signal 230. The clock signal 210 is input to the inverter 370 to determine the switching frequency. A feedback voltage V_FB reflecting the output voltage of the power converter is compared to the ramp signal 220 in the comparator 320. When the feedback voltage V_FB is high, the duration T₁ of the switching signal S₁ is broadened, and more power is forwarded to the output of the power converter. Therefore, the feedback voltage V_FB sourced from the output voltage V_O of the power converter is used to regulate the output voltage V_O. The oscillator 200 further generates a saw signal 230 that is synchronized with the ramp signal 220. The amplitude of the saw signal 230 is inversely proportional to that of the ramp signal 220. The programmable current source 310 generates a programmable current Im as a function of the feedback voltage V_FB. The programmable current Im flows through a resistor 315, and thus results in a programmable voltage across the resistor 315. The saw signal 230 is compared to the programmable voltage in the comparator 330. By adjusting the programmable current Im, the programmable voltage across the resistor 315 is programmed, such that the duration T₃ of the switching signal S₂ can be programmed or adjusted. When the programmable current Im is increased, the duration T₃ of the switching signal S₂ is broadened and zero voltage switching can be achieved.

[0035] FIG. 5 shows the circuit of the programmable current source 310. As shown in FIG. 5, the programmable current source 310 includes a current source 490, a pair of mirrored transistors 460 and 470, another transistor 450, op-amplifiers 410 and 420 and a programmable resistor 415. The current source 490 is connected to the voltage source V_CC to provide a constant source current. In the programmable current source 310, the programmable current Im can be expressed as:
Im=K×(V_FB−V_TH)/Rm,
where
0≦Im≦Imax.
In the above equation and inequality of the programmable current Im, Rm is the resistance of the programmable resistor 415, K is the mirror ratio of the mirrored transistor 460 and 470, Imax=Ic−Ib, where Ic is the current of the constant current source 490, and Ib is the current flowing through the transistor 450. The programmable resistor 415 determines the variation range programmed by the feedback voltage V_FB. The duration T₃ of the switching signal S₂ becomes narrower or even turned off when the feedback V_FB is reduced, that is, when the load coupled to the output of the power converter is decreased. Therefore, no circulated power is consumed, and power consumption in the light load condition is reduced.

[0036] In the topology of the pulse width modulated soft-switching power converter provided by the present invention, the main switches 10, 20 and the auxiliary switches 30, 40 are activated with zero voltage switching and zero current switching operations, respectively. Compared to the conventional pulse width modulation power converter, the switching loss is greatly reduced. Further, the present invention does not require an additional magnetic device or leakage inductance of the transformer, such that the switching loss, stress and noise are reduced. In addition, the power consumption under light load condition is reduced.

[0037] While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.