Protection and heat removal
A transistor that dissipates 200 W in 1 ms pulses with a 10 % duty cycle peaks at 71 °C; with 100 ms pulses and the same average, at 128 °C. The average power is not enough for sizing.
01Overvoltages
When a switch carrying current opens, the parasitic inductance of the circuit —traces, cables, package— generates an overvoltage \( L\,di/dt \) that adds to that of the bus. With the edges of tens of nanoseconds of modern devices, a few nanohenries are enough for tens of volts. It is tackled on three fronts:
- Layout: laminated busbars and decoupling capacitors right next to the switches, to shrink the switching loop.
- Damping networks (snubbers): an RC or RCD network in parallel with the switch absorbs the energy of the parasitic inductance and slows the rise of the voltage.
- Clamps: TVS suppressors or an active clamp that turns the switch back on if the voltage exceeds a threshold.
Overvoltages that arrive from the grid, from lightning or switching operations, are limited with varistors and surge arresters, as seen in other devices.
02Overcurrents and short circuits
A power semiconductor has very little thermal capacity: an overcurrent destroys it in microseconds or milliseconds. The protections act at different speeds:
| Protection | Response time | How it works |
|---|---|---|
| Desaturation detection | A few microseconds | If the collector-emitter voltage of a turned-on IGBT rises, there is a short circuit: the driver turns it off softly. IGBTs typically withstand a short circuit of up to about 10 μs |
| Cycle-by-cycle current limiting | One switching cycle | Current-mode control cuts the pulse when a threshold is exceeded |
| Ultrafast fuse | Milliseconds | It is chosen with a melting \( I^2 t \) lower than the one the semiconductor can withstand |
| Thermal protection | Seconds | A sensor on the heat sink or in the module reduces the power or shuts down |
03Heat removal
In steady state, heat follows a path of thermal resistances in series, like a current through resistors:
But each part also has thermal capacity: in a short pulse, the junction heats up before the heat reaches the heat sink. Datasheets give the transient thermal impedance \( Z_{th}(t) \), which is modeled with a Foster network, a sum of exponentials:
A transistor with Rjc = 0.4 K/W in three Foster stages (0.05, 0.15 and 0.2 K/W with 0.5 ms, 5 ms and 50 ms), an interface of 0.1 K/W and 1 s, and a heat sink of 0.5 K/W and 60 s, at an ambient temperature of 40 °C. The curves are the effective impedance as a function of pulse width for several duty cycles; the blue one is the one chosen.
04In the laboratory
Measure the turn-off overvoltage of a MOSFET with an inductive load, without an RC network and with several values, and calculate the power dissipated by the resistor.
Apply constant power to a transistor mounted on a heat sink, measure the case and heat sink temperatures, and deduce the thermal resistances.
Use the voltage of a diode as a thermometer to record the cooling of the junction and fit a Foster network.
05Common mistakes
- Sizing with the average power when there are long pulses. The temperature peak can be much higher.
- Forgetting the thermal interface. A dry or poorly tightened mica insulator can double the case-to-heat-sink resistance.
- Protecting a semiconductor with an ordinary fuse. It is too slow: the semiconductor fails first.
- Adding a snubber without counting its loss. Its resistor dissipates \( C V^2 f \), which at high frequency is a lot.
06Self-assessment
What junction temperature does the lab transistor reach with 100 W continuous?
\( 40 + 100 \cdot 1 = 140 \) °C.
And with 200 W pulses, 1 ms, duty cycle 0.1?
About 71 °C at the peak: the effective impedance is 0.157 K/W.
How long does an IGBT typically withstand a short circuit?
Up to about 10 μs, according to its datasheet.
What is an RC network in parallel with a switch for?
To absorb the energy of the parasitic inductance and limit the overvoltage and the rate of rise at turn-off.
07Further reading
- Ned Mohan, Tore M. Undeland and William P. Robbins. Power Electronics: Converters, Applications, and Design. 3rd ed., Wiley, 2003. Snubbers, protected drivers and thermal design.
- Andreas Volke and Michael Hornkamp. IGBT Modules: Technologies, Driver and Application. 2nd ed., Infineon Technologies, 2012. Short circuit, desaturation and the thermal impedance of modules.
- Frank P. Incropera and David P. DeWitt. Fundamentals of Heat and Mass Transfer. Wiley. (Spanish edition: “Fundamentos de transferencia de calor”, Pearson.) The conduction, convection and radiation behind thermal resistances.