Is it difficult to do switching evaluation of “power semiconductors”? (Part 1)

Hello, everyone.
I’m Zhang from Engineering Department 2.

In this article, I will talk about switching evaluation of power semiconductors such as power devices and power modules.

(Click here for our  “Power Module and Power Device” Evaluation Services)

 

What is the purpose of switching evaluation of power semiconductors?
Switching evaluation literally means to check the characteristics of power semiconductors such as switching time (rise and fall), surge voltage, and power loss, etc.
As you know, in the ideal switching operation there is no delay time either in current or voltage when turning on and off, in other words, no power loss.

But there is inevitably a delay time during the switching operation of power devices.
In addition, surge voltage due to wiring and other factors can destroy the device if it exceeds its absolute rated voltage.
This is why the switching evaluation of power devices needs to check characteristics such as switching time (rise and fall), surge voltage, and power loss, etc.

Then, how should the switching evaluation be done?
I will explain it using a circuit example and a measurement environment of an IGBT module shown in the figure below.

 

 

  • Connect a capacitor in parallel to the DC power supply. This is because some power modules have a current rating of several hundred amperes, and it is not easy to find a DC power supply capable of several hundred amperes.
  • The evaluation is performed in a half-bridge circuit configuration where the elements are connected in series in the up and down direction.
    There are a variety of power modules, such as 2-in-1 products with a single half-bridge circuit and 6-in-1 products with three half-bridge circuits.
  • For the connection between the collector of the high-side IGBT and the capacitor, the emitter of the low-side IGBT and the capacitor, a bus bar (copper plate) is used to reduce the stray inductance.
  • Use the L load. This is because the recovery characteristics of the diode (recovery time, power loss, etc.) cannot be confirmed by using R load.
  • Connect the gate driver to the gate terminal of the power module.
  • Connect an oscillator to the input terminal of the gate driver, and input square wave voltage pulses (two or more) into the gate driver.
  • Measure the switching time and power loss by observing the gate-to-emitter voltage VGE, collector-to-emitter voltage VCE, and emitter current with an oscilloscope.

Now, you are finally ready for the switching evaluation.
Let’s begin the evaluation!
When two or more pulses of a predetermined voltage are input to observe the waveform, the surge voltage in the turn-off waveform may show ringing significantly.
In such a case, it is difficult to distinguish whether the ringing comes from the product’s innate characteristics or the effect of the measurement arrangement.
For example, when long wire materials are used for connection, the inductance becomes large and the surge voltage of the turn-off waveform becomes large.
To avoid this trouble, the bus bars are used to connect them to minimize the inductance.

As you can see above, the measurement arrangement is important for switching evaluation, and simply connecting the devices as per the circuit diagram will not work.

WTI’s engineers with rich switching evaluation expertise certainly can help you with evaluation of various power devices, so please contact us.

 

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