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 theirinsulated 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 simplyvoltage 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 aredifferent, 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”...