AN-936 (v.Int)
4. BEWARE OF DRAIN OR COLLECTOR VOLTAGE SPIKES INDUCED BY
SWITCHING
The uninitiated designer is often not aware that self-inflicted overvoltage transients can be produced when the device is switched
OFF, even though the DC supply voltage for the drain circuit is well below the VDS rating of the transistor.
Figure 3 shows how a voltage spike is produced when switching the device OFF, as a result of inductance in the circuit. The
faster the device is switched, the higher the overvoltage will be.
Inductance is always present to some extent in a practical circuit, and therefore, there is always danger of inducing overvoltage
transients when switching OFF. Usually, of course, the main inductive component of the load will be "clamped", as shown in
Figure 4. Stray circuit inductance still exists, however, and overvoltage transients will still be produced as a result—to say
nothing of the fact that the clamping diode may not provide an instantaneous clamping action, due to its "forward recovery"
characteristic.
The first approach to this problem is to minimize stray circuit inductance, by means of careful attention to circuit layout, to the
point that whatever residual inductance is left in the circuit can be tolerated. HEXFET®s have an inductive energy rating that
makes capable of withstanding these inductive spikes, assuming that the data sheet limits for energy and temperature are not
violated. IGBTs, however, do not have an avalanche rating, and a clamping device should be connected, physically as close as
possible to the drain and source terminals, as shown in Figure 5. A conventional zener diode, or a "transorb" clamping device,
are satisfactory for this purpose. An alternative clamping circuit is shown in Figure 6, depending on the voltage and current
rating of the circuit.
The capacitor C is a reservoir capacitor and charges to a substantially constant voltage, while the resistor R is sized to dissipate
the "clamping energy" while maintaining the desired voltage across the capacitor. The diode D must be chosen so that its forward
recovery characteristic does not significantly spoil the transient clamping action of the circuit. A simple RC snubber can also be
used, as shown in Figure 7. Note, however, that an RC snubber not only limits the peak voltage, it also slows down the effective
switching speed. In so doing, it absorbs energy during the whole of the switching period, not just at the end of it, as does a
voltage clamp. A snubber is therefore less efficient than a true voltage clamping device.
Note that the highest voltage transient occurs when switching the highest level of current. The waveform of the voltage across the
device should be checked with a high-speed oscilloscope at the full load condition to ensure that switching voltage transients are
within safe limits.
5. DO NOT EXCEED THE PEAK
CURRENT RATING
All power transistors have a specified maximum peak
current rating. This is conservatively set at a level that
guarantees reliable operation and it should not be
exceeded. It is often overlooked that, in a practical
circuit, peak transient currents can be obtained that are
well in excess of the expected normal operating current,
unless proper precautions are taken. Heating, lighting
and motor loads, for example, consume high in-rush
currents if not properly controlled. A technique that
ensures that the peak current does not exceed the
capability of the device is to use a current sensing control
that switches it OFF whenever the current
instantaneously reaches a preset limit.
Unexpectedly high transient current can also be obtained
as a result of rectifier reverse recovery, when a transistor
is switched ON rapidly into a conducting rectifier. This is
illustrated in Figure 8. The solution is to use a faster
rectifier, or to slow down the switching of the transistor
to limit the peak reverse recovery current of the rectifier.
6. STAY WITHIN THE THERMAL LIMITS
+E
L
S
L
S
R
L
L
S
V
DS
D
SCLAMPING
ZENER
OVERVOLTAGE
TRANSIENT
CLAMPED
BY ZENER
(c) CLAMPED
INDUCTIVE LOAD
WITH LOCAL D-S
ZENER CLAMP
Figure 5.
Overvoltage Transient at Switch-Off Clamped by
Local Drain-Source Zener