Things to note when measuring SiC MOSFET gate-source voltage

SiCMOSFET has excellent switching characteristics, but due to the very large changes in voltage and current during its switching process, such as Tech Web Basic Knowledge SiC Power Components "SiC MOSFET: Action of Gate-Source Voltage in a Bridge Structure - Introduction" The requirements described in require accurate measurement of the surge generated between the gate and source. Find components in stock at Weisang Mall. Here, we will introduce to you what you need to pay attention to when measuring the voltage between the gate and the source. We will take SiC MOSFET as an example to explain. In fact, the content explained is also applicable to various power components such as general MOSFET and IGBT. Feel free to refer to it.

Measuring SiC MOSFET Gate-Source Voltage: General Measurement Methods

Most of the power switching devices used in products such as power supply units are equipped with heat sinks for cooling. When measuring the voltage between device pins, it is usually not possible to directly install a voltage probe or the like on the device pins. Therefore, extension cables are sometimes soldered to the pins of the device and a voltage probe is connected to the outside of the product case for measurement.

Figure 1 shows an example when a heat sink is installed on the ROHM evaluation board (P02SCT3040KR-EVK-001) and the voltage probe is connected to an extension cable for measurement. Among them, the extension cable (about 12cm long) used to connect the voltage probe is welded to the pin of the device under test (DUT), and the extension cable is twisted to suppress the influence of radiated noise. Using this measurement method, implement the double-pulse test under the bridge structure shown in Figure 2 and observe the waveform.

Install ROHM's SiC MOSFET SCT3040KR on the high side (HS) and low side (LS) of the double pulse test circuit, and keep the HS switch and LS OFF (gate voltage = 0V). The extension cable shown in Figure 1 has been soldered directly to the gate and source pins of the HS.

Figure 3 shows the measured gate-source voltage waveform. When the external gate resistor RG_EXT is 10Ω, extending the cable does not have much impact, but when RG_EXT is set to 3.3Ω and the switching speed is increased, noise and high frequencies of the circuit will be induced due to voltage changes. work, resulting in significant changes in the measured waveform. In this example, due to the influence of the extension cable used for measurement, the frequency range displayed in the measuring instrument changes, causing the observed waveform to be completely different from the true original waveform due to the additional impedance.

Figure 3. Gate-source voltage waveform measured with extension cable installed. Completely different from the real original waveform.

It should be noted that one must always pay attention to whether the observed waveform is the true original waveform, or whether the observed waveform is different from the original waveform due to some influencing factors. To do this, it is necessary to know not only how to make accurate observations, but also the factors that influence them.

Figure 4 shows the equivalent circuit of the voltage differential probe used for this measurement (*1, *2). Typically, the frequency characteristic settings of a voltage probe include the head of the probe. However, if an extension cable is installed on the measurement pin of the DUT, when observing high-speed switching waveforms of tens of ns, a resonance phenomenon will be caused between the stray inductance LEXT and the input capacitance C of the voltage probe body, resulting in a superimposed High frequency voltage ringing on the raw voltage waveform, which may cause the observed surge to be larger than the actual surge.

Figure 4. Equivalent circuit of voltage differential probe