Is it difficult to do switching evaluation of “power semiconductors”? (Part 2)

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

In my previous blog, I explained the importance of measurement arrangement in the switching evaluation of power semiconductors. This time, I will continue to explain other points to be considered about measurement arrangement.

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

 

Wiring may be reconsidered to improve a measurement arrangement for switching evaluation of power semiconductors, but in reality it is difficult to make improvements unless the circuit inductance of the measurement system is quantitatively identified.
Therefore, in this article, I will talk about how to determine the circuit inductance.

Figure 1 shows an example of an IGBT module measurement circuit. The circuit inductance in this figure refers to the inductance components that exist in the following three locations.

  1. Stray inductance of capacitor C
  2. Wiring inductance
  3. Inductance appearing in IGBT module

 

♦ How to improve the circuit inductance

Of these inductance components, 1 and 2 are the ones we can deal with, so I will introduce improvement methods for these two.

First, for 1, select a capacitor C with a stray inductance as small as possible.
Next, for 2, in order to minimize the inductance, instead of twisted wires we use a bar-shaped metal wiring called bus bar with the shortest length.

By doing above, the intrinsic characteristics of the module can be obtained more accurately.

 

Fig. 1 Example for IGBT module measurement circuit

 

♦ How to measure the circuit inductance

In general, you can determine the circuit inductance from the turn-on and turn-off waveforms.
In this article, I will explain how to do it from the turn-on waveform.

The turn-on operation is checked at the timing when the rising current and falling voltage coincide. At that period, there is a point in the middle of the voltage drop where the voltage becomes constant.
The circuit inductance value can be obtained by measuring ΔVCE, the voltage decreased until the voltage becomes constant, Δt, the period of time during which the voltage becomes constant, and ΔIC, the current increased during Δt.
Following is how to do it.

First, the basic equation for current, inductor, voltage, and time is shown in equation (1).

 

 

Equation (2) for determining the circuit inductance L is derived from equation (1).

 

 

Putting the measured values ΔVCE, Δt, and ΔIc into equation (2), the circuit inductance value L can be obtained.

 

 

In the switching evaluation of power semiconductors, measuring the circuit inductance allows us to understand how it influences measured characteristics, etc. Taking this into account helps you obtain the exact characteristics of the power semiconductor.
In addition, you can obtain simulation results close to the actual measured waveform by taking the measured circuit inductance into consideration.

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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