Application Note

Using High Voltage TVS Diodes in

IGBT Active Clamp Applications

Insulated Gate Bipolar Transistors (IGBTs) are widely used in power inverters, industrial drives, electric vehicle chargers,

motor control, and induction heating in home appliances because of their ease of use and their high voltage and current

driving capabilities. Today, power semiconductor manufacturers are offering IGBT modules with ever-higher power

densities. The power density limit is determined by the maximum power loss that can be dissipated; optimization criteria

are the packaging technology, as well as the conduction and switching losses of the semiconductor chips. The high

current density of the modules, together with high switching speeds, place greater demands on the driving circuits,

both in normal switching operation and under overload conditions. Active clamping switching technology offers a solution

that illustrates how modern, high power IGBTs can be used with high reliability, especially in high speed railway and

automotive traction applications.

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 turn-

off 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.

Analysis of overvoltage during IGBT turn-off

DRIVER

Ls1

Ls3

Ls4

Ls2

Overshoot

Figure 1. Parasitic inductance Figure 2. IGBT shut-off overshoot

Application Note

The problems described in the previous paragraph have led to the development of two-stage turn-off, soft-switch-off, and slow turn-

off 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.

Soft turn-off

Rg (soft-off)

+15V

-10V

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.

Active clamp

DRIVER

Ls1

Ls3

Ls4

Ls2

Figure 3. Soft turn-off

Figure 4. Active clamp topology