Short-circuit Testing of Power Devices

Hello everyone, nice to meet you.
I’m Nakamatsu from the Power Device Design Section.

I am mainly involved in the evaluation of power devices (discrete power MOSFETs and IGBTs and their modules).

In this article, I would like to talk about short circuit tests of power devices.
(Click here for our “Power Module and Power Device” Evaluation Services)

 

A shorted power device flows a large current inside under a high voltage when the device turns on and becomes conductive.

Ideally, short-circuit faults do not happen in inverters and other power electronics devices, but they may be shorted due to malfunctions or failures.

When a power device is shorted, it generates heat due to the high current, resulting in destruction generally within a few µs to several tens of µs, depending on the short-circuit conditions and the rated current of the device. Once the device is destroyed, it often remains in a short-circuit state, which can cause failure of the power electronics equipment, fire, etc. Therefore, power devices are usually protected from short circuits by the short circuit protection circuit of the power electronics equipment to prevent destruction.

However, since it takes time for the short-circuit protection circuit to detect the short-circuit condition and complete the short-circuit interruption of the power device, the power device must withstand the short-circuit condition for at least a few µs.

Therefore, in order to design a short-circuit protection circuit, it is necessary to determine the amount of time a power device can withstand a short circuit without being destroyed (short-circuit withstand capability). This test to determine the short-circuit withstand capability is called a short-circuit test.

 

From here, the details of the short circuit test will be explained.

One of the items to be measured in the short circuit test is the short-circuit withstand capability time, which refers to the time from when the power device starts to receive a short-circuit current until it is destroyed. The short-circuit withstand capability time is one of the characteristics that indicate the endurance and reliability of power devices.

Figure 1 shows the circuit diagram for the short circuit test. As an example, an IGBT is used for the DUT (Device Under Test), but the same test circuit can also be used for a MOSFET.

 

Fig. 1 Short-circuit test circuit diagram

 

In many cases the short-circuit test is performed by gradually extending the on-time of the DUT and repeatedly measuring the short-circuit withstand capability time until it is finally destroyed.

Figure 2 shows the short-circuit test waveforms, where the left side is the case of no breakdown in the short-circuit test and the right side is the case of breakdown during it.

 

Fig. 2 Waveform of short-circuit test

 

When the DUT is turned on, a short circuit occurs and the charge stored in the capacitor flows into the DUT, causing the collector current Ic to increase rapidly. At the moment of turning on, the stray inductance temporarily bears the power supply voltage Vcc, so the collector-emitter voltage Vce starts to drop, but soon after Ic saturates, Vce becomes Vcc. After that, the DUT heats up because it is energized with high voltage and high current, and Ic gradually decreases.

As shown in the left side of Fig. 2, no short-circuit fault occurs as the DUT turns off normally at the end of the preset ON time. When the ON time is extended, however, the DUT will eventually be destroyed and a short circuit failure will occur as shown in the right side of Fig. 2. When a short circuit failure occurs, Ic increases rapidly and Vce decreases. The short-circuit withstand capability is determined by monitoring this change.

These are the details of the short circuit test.

 

However, this is not the whole picture of short-circuit tests of power devices.

Can you guess what actually happens in short-circuit tests?

What really happens is that when the DUT breaks down and leads to a short circuit failure, there is no longer anything to block the discharge of the charge stored in the capacitor shown in Figure 1.

You might think, “Isn’t it about a capacitor just discharging?”

The energy stored in the capacitor is consumed instantly, so the power can reach the order of megawatts instantaneously, which is the level of a small solar power plant. This makes the test I would say “thrilling”, because if the DUT is destroyed, the DUT will scatter into pieces with flashing lights and explosive sounds, or the gate driver and measuring instruments will fail, etc.

Furthermore, the short-circuit withstand capability varies and it is impossible to know exactly when the DUT will destroy. Regardless of whether it destroys or not, my hands sweat every time I press the starting switch.

I also often hear that short-circuit tests are conducted in a dedicated, remote laboratory or outdoors where there are no people around, since loud noises can disturb the neighborhood.

 

As such, short-circuit testing is a dangerous test. Therefore we secure a safe environment for short-circuit test by conducting it in a thick acrylic box so that there is no safety problem even if the DUT is scattered into pieces.

We are also building a system to shut down the capacitor from the circuit to prevent it from drawing excessive current in case the DUT is destroyed.

 

WTI has a special test environment for power devices and engineers with rich expertise for special tests, so please contact us if you need evaluation of various power devices, including avalanche capability tests.

 

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