DRIVE CIRCUITS FOR POWER MOSFETs AND IGBTs
by B. Maurice, L. Wuidart
1. INTRODUCTION Unlike the bipolar transistor, which is current driven, Power MOSFETs, with their insulated gates, are voltage driven. A basic knowledge of the principles of driving the gates of these devices will allow the designer to speed up or slow down the switching speeds according to therequirements of the application. It is often helpful to consider the gate as a simple capacitor when discussing drive circuits. 2. IGBT / MOSFET DRIVE BASICS 2.1 Gate vs Base Power MOSFETs and IGBTs are simply voltage driven switches, because their insulated gate behaves like a capacitor. Conversely, switches such as triacs, thyristors and bipolar transistors are “current” controlled, in the same way as aPN diode. 2.2 Driving a gate As shown in figure 2, driving a gate consists of applying different voltages: 15V to turn on the device through S1, and 0V to turn off the device through S2. Figure 1. Nature of power semiconductor inputs
A remarkable effect can be seen in both the turn-on and turn-off switching waveforms; the gate voltage exhibits a “step”, remaining at a constant level while thedrain voltage rises or falls during switching. The voltage at which the gate voltage remains during switching is known as the Miller voltage, Vgm. In most applications, this voltage is around 4 to 6V, depending on the level of current being switched. This feature can be used to control the switching waveforms from the gate drive. 2.3 MOSFET and IGBT turn-on / turn-off. When turned on under the sameconditions, IGBTs and MOSFETs behave in exactly the same way, and have very similar current rise and voltage fall times - see figure 3. However, at turn-off, the waveforms of the switched current are different, as shown in figure 4. At the end of the switching event, the IGBT has a “tail current” which does not exist for the MOSFET. This tail is caused by minority carriers trapped in the “base” ofthe bipolar output section of the IGBT causing the device to remain turned on. Unlike a
Figure 2. Driving MOSFET / IGBT gates
VCC S1 R
VCC S1 R
Figure 3. MOSFET / IGBT turn-on
Figure 4. MOSFET / IGBT turn-of
bipolar transistor, it is not possible to extract these carriers to speed upswitching, as there is no external connection to the base section, and so the device remains turned on until the carriers recombine naturally. Hence the gate drive circuit has no effect on the tail current level and profile. The tail current does however increase significantly with temperature. 2.4 IGBT turn-off losses The turn-off of an IGBT can be separated into two distinct periods, as shown in figure5. In the first period, its behaviour is similar to that of a MOSFET. The increase in drain voltage (dV/dt) is followed by a very fast fall of the switched current. Losses in this “dV/dt” period depend mainly on the speed of the voltage increase, which can be controlled by a gate drive resistor. The second “tail current” period is specific to the IGBT. As this period occurs while there is alreadya large voltage across the device, it causes losses at each turn-off. The total turn-off losses are shown in figure 5 by the shaded area. 3. FROM GATE DRIVE TO SWITCHING 3.1 Speeding up turn-off The power involved in these two types of switching
losses is linked to the switching frequency. Turn-off losses become critical when operating at high frequencies. In this case, the dV/dt can beincreased (and hence losses reduced) by decreasing the size of the gate drive resistor Rg, which will allow the gate to charge more quickly. The turn-off losses are proportional to the size of the gate resistor - for example decreasing the gate resistor from 100 to 10 Figure 5. IGBT turn-off losses
VD ID dV dt
will reduce the...
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