Abstract:
A converter  10  has an IGBT power switch  34.  A resonant tank circuit  30  couples the IGBT to a voltage source. A gate controller turns the IGBT on and off by applying a suitable gate control voltage to the gate. The resonant tank circuit imposes a sinusoidal waveform on the emitter current. After the emitter current reverses direction, the gate signal is terminated and the IGBT is shut off. Minority carriers in the emitter are swept away by the tank circuit and are further deposited in a transformer that is coupled to the tank circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/198,692, filed Apr. 20, 2000. 
    
    
     BACKGROUND 
     This invention relates to DC-to-DC converters and, in particular, to quasi-resonant converters using Insulated Gate Bipolar Transistors (IGBT). 
     Quasi-resonant switching converters are used to convert one level of DC voltage and current to another level of DC level and current. They provide a similar function for direct current conversion that transformers provide for AC current and voltage converters use power semiconductor devices that turn off and on at relatively high rates. DC output is modulated by the time that the power devices are either on or off. This time is typically referred to as the “duty cycle”. The devices can be rapidly switched on or off and the resulting output is applied to an output filter for smoothing out the variations and switching waveforms. 
     Modern switching converters frequently use techniques that were pioneered by Edward J. Miller in his U.S. Pat. No. 4,138,715 issued Feb. 6, 1979. In that patent the basic problem of switching losses in DC converters is solved by applying a tank circuit to the switch. When power semi-conductor devices are switched at full current or full voltage, it takes a significant amount of time for the device to return to zero current or zero volts. In that time, the device and the circuit can experience short transients with relatively high peaks. These transients represent a waste of power and also a source of unwanted electromagnetic interference. In addition, rapid switching at high currents and high voltages creates thermal stresses on the power semi-conductors and thereby limits their useful life. 
     IGBTs are popular switching devices. One of their benefits is that they provide very low resistance in their on state. In addition, they can be rapidly switched on and off using their MOS controlled gate. In general, their on resistance is substantially less than power MOSFETs and they can handle higher currents for similar size devices. However, one drawback of the IGBT is that it includes minority carriers. If there is a high voltage or a high current in the collector when the IGBT is switched off, those carriers must be removed. Unfortunately, the minority carriers decay by recombination over a period of time. In effect, the presence of minority carriers in the collector resembles the discharge of a capacitor. When there are a large number of minority carriers, they can generate a substantial amount of heat until they are removed from the collector. This feature of IGBTs has limited their usefulness in DC-to-DC converters. 
     SUMMARY 
     The invention overcomes the defects of the prior art and solves the problem of the slow decay of minority carriers in IGBTs by providing a quasi-resonant circuit for an IGBT in a DC-to-DC converter. A resonant tank circuit is coupled to the collector of the IGBT. The resonant tank circuit generates a quasi-sinusoidal waveform for the collector voltage and for the emitter current. After the collector voltage falls below ground, the gate pulse is terminated and the IGBT turns off. Then, the minority carriers on the collector are swept away by the inductor of the tank circuit. Those minority carriers are stored in a capacitor that is coupled across the primary winding of the transformer. In this way, the minority carriers are rapidly removed from the collector thereby quickly discharging the IGBT and making it ready for its next cycle. 
    
    
     DRAWINGS 
     FIG. 1 is a schematic diagram of a basic prior art resonant converter. 
     FIG. 2 is a timing diagram of the current output of the converter shown in FIG.  1 . 
     FIG. 3 is a schematic diagram of the inventive quasi-resonant converter with an IGBT converter. 
     FIGS.  4 ( a )- 4 ( d ) are timing diagrams showing operations of the circuit of FIG.  3 . 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 2 are presented to show background information on the fundamental characteristics of resonant switching DC-to-DC converters. Turning to FIG. 1, a voltage source is connected across input terminals  1  and  2 . Two inductors,  3  and  4 , are connected in series between the input terminal  1  and a power switching device  5 . Capacitor  6  is connected between the junction of the conductors  3  and  4  and a reference potential such as ground. Input terminal  2  is also a referenced ground. A load is connected to output terminal  7  and output terminal  8  is connected to ground. The capacitor  6  is charged through the inductor  3  and, when switch  5  is closed, the capacitor  6  discharges through inductor  4 . 
     At a time T equal to zero, the voltage V c  across the capacitor  6   c  is positive relative to ground. Switch  5  is turned on at time T 0 . The values of the inductances L 1  and L 2  of the inductors  3  and  4 , respectively, are chosen t set the resonant frequency of L 2 C high as compared to the resonant frequency of L 1 C. Because of this relationship, the current inductor  3  has only a minor effect on the resonant behavior of the current in inductor  4 . So, beginning at time T 0 , the current I rises and falls sinusoidally, reaching 0 at a time T=T 1 . At that point the energy in the inductor  4  is zero. Control devices (not shown) sense the condition and turn switch  5  off at zero cross-over point of the current. Such control devices are well known in the art and are not illustrated herein. The switching losses at time T=0 and a time T=T 1  are zero. Following the time T=T 1 , there is a recovery period for the capacitor  6  to recharge through the inductor  3  before the switch  5  can again be turned on. 
     Those skilled in the art will appreciate that the switch  5  may be substituted by any power semi-conductor device including power bi-polar transistors, power MOSFETs, and IGBTs. However, IGBTs present unique problems in DC-to-DC converters. IGBTs employ minority carriers and those minority carriers may collect in the collector while the IGBT is operating. The presence of minority carriers lengthens the time for the IGBT to shut off and imposes power losses on the converter. It is highly desirable to use IGBTs in DC-to-DC converters. This is especially true for DC-to-DC converters used in connection with personal computers. Given the low on resistance of IGBTs, they represent a significant potential power saving as well as improved performance for personal computers, and other portable electronic devices. 
     Turning to FIG. 3, there is shown a schematic diagram for a DC-to-DC converter  10  with an IGBT power switch. A conventional full wave bridge (not shown) or other suitable AC to DC converter converts at an alternating current into a DC voltage source  12 . The DC voltage source  12  may vary between 127 to 374 volts. A capacitor  14  is connected in parallel with the voltage source  12 . The capacitor  14  is typically rated as high as 400 volts and may be 150 micro farad capacitor. A clamp circuit  20  is connected between the capacitor  14  and IGBT  34 . Clamp circuit  20  includes a diode  19  whose anode is connected to the collector of the IGBT  34 . The cathode of diode  19  is connected to a parallel network that includes in one leg a resistor  16  of approximately 33k ohms and in its other legs a series connection of a 10 ohm resistor  17  and a 0.0022 micro farad capacitor  18 . The clamp circuit operates to clamp the voltage to 600 volts or less. Also connected to the collector of IGBT  34  is tank circuit  30 . That circuit includes tank capacitor  31  which is approximately 0.0022 micro farads in series with tank inductor  32 , 70 micro henries. Tank capacitor  31  is also connected across the primary winding  33  of transformer  60 . The secondary winding  38  of transformer  60  is coupled across output capacitor  40  and a load resistor  56 . The output capacitor  40  is approximately 1800 micro farads and the output resistor  56  is approximately 5.43 ohms. 
     The IGBT  34  is driven by a conventional gate driver circuit  36 . It provides a pulse-type output to the gate G of the IGBT  34 . It is connected to the gate G by a 10 ohm resistor. The emitter of the IGBT is connected to ground. The IGBT is turned on by the gate pulse from gate control circuit  36 . The output of the converter circuit  10  is switched off not when the collector of the IGBT is at zero volts, but only after the collected has been driven slightly negative. This feature is contrary to the accepted teachings of the prior art that shut the power switch off at either zero current or zero voltage. As will become apparent by the following description, the IGBT is not turned off until its collector voltage has gone slightly negative. 
     The operation of the circuit is best understood with reference to FIGS. 4 a - 4   d.  Consider the time just prior to T 0 . At that time the IGBT is off and the capacitors  14 ,  31  are charged and no current is flowing through the inductor  32 . At time T 0 , the gate driver circuit  36  turns the IGBT  34  on and connects the collector to ground. At that time, the voltage on the collector falls to zero and the emitter current begins to rise in a sinusoidal manner. Likewise the voltage across the tank capacitor  31  begins to fall in a corresponding sinusoidal manner. Given the sizings of the tank capacitor  31  and tank inductor  32 , the emitter current changes quicker than the tank voltage. With reference to FIG. 4 b,  note that the collector voltage will slightly rise and then fall below zero. This phenomenon is due to the presence of minority carriers in the collector. The inductor  32  limits the rise of the emitter current. Inductors  32 , primary winding  33  and capacitor  31  set the frequency of the tank circuit  30 . The tank capacitor  31  and the primary winding  33  store energy from inductor  32 . As a result, the emitter current gradually decreases and, by design, reverses direction. The tank circuit  30  generates the quasi-sinusoidal waveforms for the collector voltage as shown in FIG. 4 b  and for the emitter current as shown in FIG. 4 c.  At a time T 1  after the collector voltage has gone below ground, the gate pulse terminates. 
     When the gate pulse is removed, there is a slight negative voltage on the collector due to the reversal in direction of the current flowing through the inductor  32  and the removal of minority carriers. Those minority carriers are quickly swept away by the inductor  32  which pulls the minority carriers out of the IGBT&#39;s collector and puts the minority carriers into the capacitor  31  that is across the primary winding  33  of transformer  35 . Since the IGBT  34  is off, the energy stored in the capacitor  31  is transferred to inductor  33  in a resonant manner and from there into secondary coil  38  and load resistor  50 . 
     It is important to note that the voltage V TANK  of the tank circuit  30  (FIG. 4 d ) goes below ground. Also note that the gate pulse of the IGBT  34  does not terminate before the tank voltage goes negative. Thus, by delaying termination of the gate pulse on the IGBT until the emitter current has reversed direction, the inventive circuit pulls the minority carriers out of the collector. By reducing the number of minority carriers in the collector, the lifetime of the remaining minority carriers is likewise reduced. This cycle is repeated if the energy stored in the inductor  32  is dissipated. If the cycle is repeated too soon, the collector voltage will not go negative and the minority carriers will not be swept away from the collector. 
     The invention effectively changes the minority carrier modulation of the IGBT  34  and thus reduces switching losses. Those losses occur when there is a high voltage and high current in the collector when the IGBT is switched off. The collector current at turn-off is due to the presence of minority carriers in the collectors. The minority carriers in the collector will decay by recombination over time, if they are not swept out as described above. In effect, the IGBT at turn-off acts like a capacitor that is discharging. As a result, the more charge that is removed, the lower the losses. When a large number of minority carriers recombine at turn-off they generate heat. With the present invention, the minority carriers in the collector are removed by the tank circuit and the losses are reduced during turn-off. 
     Those skilled in the art understand that other embodiments of the circuit may be made without departing from the spirit and scope of the appended claims. For example, the circuit may be modified by omitting the transformer and using just a second inductor. It could also work with only one inductor if the tank capacitor is coupled to ground. Likewise, the relative locations of the IGBT and the tank circuit could be reversed so that the IGBT is coupled at its collector to the DC voltage source and the tank circuit and the transformer are coupled to the emitter. Such juxtapositions of circuit elements easily implemented by those skilled in the art.