In industrial control systems, modern electric vehicles, power inverters and other high-voltage applications, Insulated Gate Bipolar Transistors (IGBT) are used often for high-efficiency switching. However, given the constantly increasing power densities of today’s electronics, greater demands are placed on the system — necessitating new methods for control. In this article, learn how Littelfuse TVS diodes are an excellent choice for protecting circuits against overvoltage during IGBT turnoff.
Analysis of overvoltage during IGBT turn-off
IGBT modules and converter circuits have parasitic inductances that can’t be completely eliminated; their influence on system behavior also can’t be ignored. Figure 1 illustrates the parasitic inductances contained in a commutation circuit. The current change caused by turning off the IGBT produces an overshoot voltage at its collector terminal, as shown in Figure 2. The commutation speed (and therefore, the turn-off overvoltage) at an IGBT can, in principle, be affected by the turnoff gate resistance Rg(off). This technique is used particularly at lower power levels. However, the Rg(off) must then be matched for overload conditions, such as turn-off of the double-rated current, short circuit, and a temporarily increased link circuit voltage. In normal operation, this results in increased switching losses and turn-off delays, which reduces the usability or efficiency of the modules. As a result, this simple technique is unsuitable for modern high power modules.
Soft turn-off
The problems described in the previous paragraph have led to the development of two-stage turn-off, soft-switch-off, and slow turnoff driver circuits operating with a reversible gate resistance. In normal operation, a low ohmic gate resistor is used to turn the IGBT off in order to minimize the switching losses; a high ohmic one is used when a short circuit or surge current is detected (see Figure 3). However, the problem lies in detecting these conditions reliably: desaturation monitoring always involves a delay until a fault is detected (typically 4-10 μs) known as the response time. When IGBTs are driven with a pulse that is shorter than the response time in the event of a short circuit, the fault is not detected and the driver turns off too quickly. The resulting overvoltage destroys the IGBT. Moreover, coverage of limit cases (between overcurrent/non-overcurrent) presents a problem; for instance, a higher overvoltage may well occur when the double-rated current is turned off than at a short-circuit turn-off. These kinds of driver circuits must be considered dangerous; users should be advised not to use them in higher power equipment and in systems from which high reliability is expected.
Active clamp
Active clamping is traditionally used only to protect the semiconductor in the event of a transient overload. Consequently, the clamping elements are never subjected to recurrent pulse operation. The problem of repetitive operation is limited by the IGBT and driver power; during active clamping, both the IGBT and the driver will absorb energy. Active clamping means the direct feedback of the collector potential to the gate via an element with an avalanche characteristic. Figure 4 illustrates this principle using an IGBT switch.