Abstract:
A resonant gate driver circuit suitable for driving MOS-gated power switches in high-frequency applications recovers gate drive energy stored in the gate capacitance of the power switches, resulting in substantially lossless operation. The resonant gate driver circuit provides bi-polar gate control signals that are compatible with PWM operation.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to power converters and, more particularly, to a lossless gate driver circuit for MOS-gated power switching devices in high-frequency applications. 
     In a conventional gate driver circuit, energy stored in the gate capacitance of a switching device is dissipated in the internal and external gate resistors during turn-on and turn-off of the switching device. For example, for a 1200V/600A trench gate insulated gate bipolar transistor (IGBT) switching at 62 kHz, the power loss can be as high as 9 W and is mainly dissipated in the external gate resistor of the IGBT. High-power, low-inductance, high-cost resistors are thus needed to implement the external gate resistance, and additional heat sinking or cooling is typically required. 
     Accordingly, it is desirable to provide a substantially lossless gate driver circuit for power switching devices in high-frequency applications, while providing desirable turn-on and turn-off voltage levels and controllable gate voltage slew rate. It is further desirable that such gate driver circuit have the capability of recycling the gate charge and thereby achieve substantially lossless gate control. 
     BRIEF SUMMARY OF THE INVENTION 
     A resonant gate driver circuit suitable for driving power switches in high-frequency applications recovers gate drive energy stored in the gate capacitance of the power switches, resulting in substantially lossless operation. In a preferred embodiment, the resonant gate driver circuit provides bipolar gate control signals that are compatible with PWM operation. 
     In an exemplary resonant gate driver circuit for driving a power switching device, on-state and off-state voltage sources are coupled in series with each other, the series combination of voltage sources being coupled across a half-bridge configuration of on-state and off-state switching devices. A clamp diode is coupled across each on-state and off-state switching device, respectively. A resonant inductor is coupled between the junction joining the switching devices and the junction joining the clamp diodes. The gate of the power switching device is coupled to the resonant inductor at the junction joining the clamp diodes. The junction between the on-state and off-state voltage sources is connected to the emitter of the power switch. 
     In operation, energy stored in the gate capacitance is transferred to the resonant inductance during each switching event (i.e., turn on or turn off) of the power switch; and the stored energy is subsequently recovered by the on-state and off-state voltage sources in the same switching event of the power switching device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 schematically illustrates a conventional gate driver circuit for driving a power switch in high-frequency applications; 
     FIG. 2 graphically illustrates gate-capacitor charge versus gate voltage for the gate driver circuit of FIG. 1; 
     FIG. 3 schematically illustrates a resonant gate driver circuit in accordance with an exemplary embodiment of the present invention; 
     FIG. 4 graphically illustrates operation of the gate driver circuit of FIG. 3; 
     FIG. 5 schematically illustrates an alternative exemplary embodiment of a resonant gate driver circuit in accordance with the present invention; 
     FIG. 6 graphically illustrates timing of switching devices in the circuit of FIG. 5; 
     FIG. 7 schematically illustrates another alternative exemplary embodiment of a resonant gate driver circuit in accordance with the present invention; 
     FIG. 8 graphically illustrates exemplary gate voltage and gate current waveforms for a resonant gate driver in accordance with exemplary embodiments of the present invention and for a conventional (classic) gate driver circuit such as that of FIG. 1; 
     FIG. 9 graphically illustrates power switch internal gate resistor losses for a resonant gate driver in accordance with exemplary embodiments of the present invention and for a conventional (classic) gate driver circuit such as that of FIG. 1; and 
     FIGS. 10 and 11 graphically illustrate exemplary experimental results showing gate voltage Vge and resonant inductor current waveforms at turn-on (FIG. 10) and turn-off (FIG. 11) for a resonant gate driver circuit in accordance with preferred embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a conventional gate driver  10  for power switches in high power pulse width modulated (PWM) converters. Although the gate driver circuit according to preferred embodiments is described herein with respect to a PWM converter, it is to be understood that the present invention is not limited to PWM operation and is applicable to other types of converters using MOS-gated power switches, such as, for example, pulse density modulated converters, resonant converters, and quasi-resonant converters. 
     As illustrated in FIG. 1, a high-current, MOS-gated power switch  12  (S) normally comprises multiple low-current power switch cells in parallel. The power switch comprises an internal gate resistor  14  (R g—int ) that dampens potential gate signal parasitic ringing among the power switch cells. An external gate resistor  16  (R g—ext ) controls the turn-on and turn-off speed of the power switch. The power switch further comprises a gate capacitance C iss ,that is, the equivalent capacitance formed by the gate-to-collector capacitance  18  and the gate-to-emitter capacitance  19 . 
     In operation of the conventional gate driver circuit of FIG. 1, energy stored in the gate capacitance C iss  at each turn-on or turn-off occurrence, represented as E sw =½C iss (V on +V off ) 2 , is dissipated in the two gate resistors. In high-frequency PWM operation, for example, the total power loss P sw  can be estimated as follows: 
     
       
           P   sw   =C   iss ( V   on   +V   off ) 2   f   sw , 
       
     
     where f sw  is the switching frequency and            C   iss     =       Q   sw         V   on     +     V   off           ,                          
     where Q sw  is the charge stored in the gate capacitance. 
     Typically, for a 1200V/600A trench gate IGBT switching at 62 kHz, for example, the power loss can be as high as 9 W and is mainly dissipated in R g—ext  since normally R g—ext &gt;&gt;R g—int . Therefore, an expensive, high-power, low-inductance resistor is needed for R g—ext . Moreover, additional heat sinking or cooling is typically needed for R g—ext . 
     FIG. 2 illustrates a graph of gate capacitor charge Qg versus gate voltage Vge. The area under the graph of FIG. 2 represents the energy Esw that is dissipated in the power switch&#39;s internal gate resistor R g—int  and the external gate resistor R g—ext  in the conventional gate driver circuit of FIG.  1 . 
     FIG. 3 illustrates an exemplary resonant gate driver circuit  20  in accordance with preferred embodiments of the present invention. As illustrated, gate driver circuit  20  advantageously does not require an external resistor Rext, unlike the conventional circuit  10  of FIG.  1 . Gate driver circuit  20  comprises a voltage source, illustrated in FIG. 3 as comprising an on-state voltage source  22  in series with an off-state voltage source  24 . A half-bridge configuration of an on-state switching device  26  (Son) in series with an off-state switching device  28  (Soff) is connected across the series connection of Von and Voff. Each switching device  26  and  28 , respectively, has a clamp diode  30  (Don) and (Doff)  32 , respectively, coupled thereacross. A resonant inductance  34  (Lr) is situated between the junctions joining the switching devices and the clamp diodes. The junction between the voltage sources Von and Voff is connected to the emitter of power switch  12 , and the junction between the clamp diodes is connected to the gate of power switch  12 . 
     FIG. 4 graphically illustrates operation of the gate driver circuit of FIG.  3 . In particular, FIG. 4 shows the gate control signals on switches S, Son and Soff in addition to the gate-emitter voltage Vge and the resonant inductor current. Since the circuit of FIG. 3 is symmetrical for turn-on and turn-off, the ensuing description of FIG. 4 with respect to the operation of FIG. 3 describes turn-on only in detail. 
     During the interval [t 0 -t 1 ], assuming initially that the power switch is in an off-state, the gate voltage V ge =−V off  and the resonant inductor current is zero. At time t 0 , S on  turns on and a resonant tank is formed by Von, the resonant inductor Lr, and the equivalent gate capacitance C iss . The gate voltage V ge  rises in a resonant fashion from the value −V off . 
     During the interval [t 2 -t 3 ], once D on  conducts S on  is turned off. The freewheeling resonant inductor current L Lr  circulates through the integral, anti-parallel body diode (not shown) of S off  and clamp diode D on  to V on  and V off ; thus, the energy stored in the resonant inductor L r  is recovered back by the source. (As used herein and as understood in the art, the term anti-parallel refers to an opposite-polarity type connection, i.e., between a switching device and a diode such that the cathode of the diode is connected to the collector of the switching device.) This interval ends at time t 3  when I Lr  reduces to zero and the clamp diode across S off  turns off naturally. 
     During the interval [t 3 -t 4 ], the power switch is in the on-state. For high switching frequency operation, the gate voltage V ge  can hold at the voltage level V on  and keep the power switch in the on-state. 
     FIG. 5 illustrates an alternative exemplary embodiment of the gate driver circuit of FIG. 3 for increasing the circuit&#39;s noise immunity and to ensure the desired voltage levels at turn-on and turn-off. In particular, additional small clamp switches  36  and  38 , respectively, are coupled across the clamp diodes Don and Doff, respectively. Sc on  clamps V ge  at V on  at turn-on, while Sc off  clamps V ge  at−V off  at turn-off. Timing diagrams for the two clamp switches are shown in FIG.  6 . 
     FIG. 7 illustrates an alternative exemplary embodiment of the gate driver circuit of FIG. 3 for independently adjusting turn-on and turn-off times of power switch  12 . In particular, FIG. 7 shows the replacement of the resonant inductor  34  of FIG. 3 with an inductor-diode circuit  40  comprising a series combination of an on-state inductor  42  and a diode  44  and a series combination of an off-state inductor  46  and a diode  48 . When switch Son is conducting, the resonant inductor current flows through inductor  42  and diode  44 . When switch Soff is conducting, the resonant inductor current flows through inductor  46  and diode  48 . The additional inductor and diodes result in different resonant frequencies at turn-on and turn-off and, hence, different turn-on and turn-off times. 
     Once the power switch gate capacitance Ciss is known, the resonant inductance Lr can be designed. Selection of the resonant inductor is preferably based on two criteria: (1) satisfying the desired power switch turn-on and turn-off times; and (2) minimizing the power loss on the internal gate resistor. The rising (or falling) time of the gate voltage and the power loss on the R g—int  can be calculated. 
     For comparison and by way of example, FIG. 8 illustrates exemplary simulation waveforms of the gate driver of FIG.  1  and the resonant gate driver of FIG.  3 . Both the gate voltage and the gate current have different waveform shapes. 
     FIG. 9 graphically illustrates gate resistor losses for the conventional gate driver of FIG.  1  and the resonant gate driver of FIG. 3, each employing a Powerex 1200V/600A H Series IGBT power switch. As shown, the resonant gate driver circuit of FIG. 3 advantageously results in only 30% of the losses of those of the conventional gate driver. 
     FIGS. 10 and 11 graphically illustrate exemplary experimental results illustrating gate voltage Vge and resonant inductor current waveforms at turn-on (FIG. 10) and turn-off (FIG. 11) for a resonant gate driver circuit in accordance with preferred embodiments of the present invention. Advantageously, as shown, such a resonant gate provides bi-polar gate control signals Vge, i.e., positive voltage for turn-on and negative voltage for turn-off. 
     In accordance with the description hereinabove, resonant gate driver circuit in accordance with preferred embodiments of the present invention recovers gate drive energy stored in the gate capacitance of power switching devices, and this energy is recovered back by the source. This is particularly advantageous for power switching devices having high gate capacitance, such as, for example, power gate-trench IGBT&#39;s and power MOSFET&#39;s. 
     While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.