[Introduction]As the latest generation of power semiconductor devices, SiC and GaN have far superior characteristics than traditional Si devices, enabling power converters to achieve higher efficiency, higher power density and lower system cost. But at the same time, the extremely fast switching speed of SiC and GaN also brings challenges for engineers to use and measure, and it is impossible to obtain the correct waveform if one is not careful, which seriously affects the accuracy of device evaluation, the performance and safety of circuit design, The speed at which the project is completed.
In the measurement of dynamic characteristics of SiC and GaN, the most difficult part is to drive the voltage V of the upper-side device in the half-bridge circuit.GSThe measurement consists of two parts: switching process and Crosstalk. Measurements with passive probes are not possible at this time, causing hazards to equipment and personnel, and inaccurate results due to jumping common-mode voltages. Typically, we choose a high-voltage differential probe for measurement.
Let’s use the magic wand in the testing world, the optical isolation probe, to solve the problem of dynamic testing of SiC and GaN gates together.
Disadvantages of High Voltage Differential Probes
Using a high-voltage differential probe to switch the high-side device VGSThe measurements were made and the results were as follows:
V from aboveGSThe waveform can find the following problems in the measurement results:
1. VGSThe oscillation of the waveform is serious, and the oscillation peak exceeds the gate withstand voltage value of the device, which will have a negative impact on the gate life and safety of the device.
2. VGSThe noise of the waveform is very large and appears very coarse.
Seeing such a waveform creates the following confusion:
1. Such VGSWaveform oscillations are unacceptable in circuit applications, so are the measured oscillations measured correctly? Is it the problem of the device itself or the circuit design is wrong?
2. VGSThe waveform looks very thick. Is it caused by the excessive output ripple of the driving power supply?
Crosstalk Process V for High-Side Devices Using High Voltage Differential ProbesGSThe measurements were made and the results were as follows:
V from aboveGS The waveform can find the following problems in the measurement results:
1. During the forward Crosstalk process, VGSThe positive-going spike of the waveform significantly exceeds the V of the deviceGS (th), which is supposed to cause the device to be misconnected and cause the bridge arm to short-circuit, but it did not happen in the test.
2. During the forward Crosstalk process, VGSThe waveform has a large negative peak and also significantly exceeds the gate voltage of the device, which is inconsistent with the principle of Crosstalk.
3. During negative Crosstalk process, VGSThe negative-going peak of the waveform significantly exceeds the device gate withstand voltage value, which can affect the device gate life or cause its direct breakdown.
4. In a negative Crosstalk process, VGS The waveform has a large positive spike, which is inconsistent with the principle of Crosstalk.
5. VGSThe noise of the waveform is very large and appears very coarse.
Seeing such a waveform creates the following confusion:
1. According to the measurement results, the bridge arm should be short-circuited when the Crosstalk is forward, but it does not happen. Why? Should such a result continue to improve the circuit design or can it be accepted by the converter?
2. Positive Crosstalk has negative spikes that do not match the theory, and negative Crosstalk has positive spikes that do not match the theory. What’s going on? Is it the problem of the device itself or the circuit design is wrong?
3. VGS The waveform looks very thick. Is it caused by the excessive output ripple of the driving power supply?
Test Magic Wand – Optically Isolated Probe
Let’s put the above confusion aside and try another optical isolation probe.
As can be seen from the above figure, the switching process V after using the optical isolation probeGSThe oscillation of the waveform is significantly reduced, all within the gate withstand voltage range of the device, and the waveform is also thinner.
As can be seen from the above figure, the Crosstalk process V after using the optical isolation probeGSThe oscillation of the waveform is significantly reduced, and the positive and negative deductions are also within the acceptable range, and there is no situation that is inconsistent with the theory.
It can be seen that if we continue to struggle with the test results of using the high-voltage differential probe before, we are asking for trouble with the wrong waveform, and in the end, we can only waste time and energy. After using the optical isolation probe, all problems will be solved. So what magic does the optical isolation probe perform?
1. High CMRR
The Common Mode Rejection Ratio (CMRR) is a measure of the ability of the probe to not be affected by common mode signals. The unit is dB. The smaller the value, the stronger the common mode rejection. High-voltage differential probes also have common-mode rejection, but they drop sharply as the frequency of the signal under test increases. A typical high voltage differential probe has -50dB CMRR at 1MHz, but CMRR drops to -20dB at 1GHz. The extremely fast switching speeds of SiC and GaN lead to extremely fast common-mode voltage transitions, which requires the probe to have high CMRR at high frequencies. Optically isolated probes are capable of high CMRR over a wide frequency range, with -160dB at 1MHz and -90dB at 1GHz. This makes the optical isolation probe not affected by the high-speed jump common-mode voltage to produce non-existent waveform oscillations.
2. Minimum measurement loop
The front end of the high-voltage differential probe is two wires of more than ten centimeters, which will cause two problems: one is that the long wire can be regarded as an inductance in the measurement loop, which will cause oscillations that do not exist in the measured current; the other is the long wire. The enclosed loop can be thought of as an antenna that receives the magnetic field generated by the rapidly changing current of the device during switching, resulting in erroneous measurements. The optically isolated probe tip has a series of connectors and accessories that provide high performance and accessibility, which can make the measurement wiring distance as short as possible and the area enclosed by the measurement wiring to avoid the above problems.
3. High common mode range and low attenuation factor
When using a high-voltage differential probe, in order to cope with the high bus voltage of SiC and GaN, it is necessary to set the probe to a high attenuation ratio, and a high attenuation ratio will increase the measurement quantization error and the noise of the measurement system, which leads to the use of high-voltage The waveform measured by the differential probe appears coarse. The common mode range and attenuation ratio of the optical isolation probe are independent, that is, when it can withstand high common mode voltage, the measurement accuracy can also be improved by selecting a probe head with a small attenuation ratio, and the measured waveform appears finer. .
Through the above content, it can be seen that the optical isolation probe drives the voltage V of the upper bridge device in the half-bridge circuitGShas excellent performance in the measurement, in fact, for the low-side device driving voltage VGSThe measurement is also very powerful. As can be seen from the figure below, even if there is no common mode voltage with fast transition, the waveform measured by the optical isolation probe is significantly better than that of the high-voltage differential probe. It is indeed a magic wand for testing SiC and GaN.
Source: Public Account[Power Device Microscope]
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