In this article we will explain how you can use the Micsig SigOFIT optical probes to measure interference-free on SiC and GaN technology in, for example, DC/DC converters, inverters and motor drives.
What is SiC and GaN?
Silicon Carbide (SiC) and Gallium Nitride (GaN) are modern semiconductor materials that are used as an energy-efficient alternative to traditional silicon. In general, SiC is popular at high voltages and power levels (think electric cars and solar/home batteries), while GaN is more commonly found in high-frequency compact applications (such as a powerful USB-C chargers). SiC and GaN transistors have very low switching losses, which makes a design much more efficient.
What are the challenges?
These low switching losses are due to a reduced gate charge (Qg), which allows SiC/GaN to switch on and off very quickly. This results in a huge dv/dt when a voltage of more than 1000V is brought down to 0V within a few nanoseconds (and vice versa). This combination of high voltage and high frequency generates a lot of high-order harmonic energy. This energy manifests itself as common mode (CM) interference. Although this also brings challenges in the product design, in this article we will focus on measuring such a circuit.
Measurement techniques
When we want to determine the switching losses of a MOSFET, such as in a Double Pulse Test or in the actual application, we typically want to observe the gate-source voltage with an oscilloscope. Let’s look at the example below:
When measuring at the gate of LS1 with respect to GND, you can simply use a cheap, standard passive probe (up to about 500 MHz) and get an excellent measurement result.
However, when we want to measure at the gate of HS1 with respect to M1 (the midpoint), it’s a different story. M1 moves up and down between +V and GND and therefore cannot be connected to GND (which a passive probe would do). To still be able to measure the gate of HS1, one of the following methods is often used:
1. Connect a passive probe between GND and M1, and a second passive probe between GND and HS1, and use the MATH function in the oscilloscope to calculate the difference.
2. Connect a high voltage differential probe between HS1 and M1.
Both measurement methods will be unusable with SiC and GaN technology and will lead to significant disturbances. We will explain why.
CMRR
Earlier we saw that SiC and GaN cause large common mode (CM) interference, a disturbance that is present simultaneously on both the GND and the probe input. As long as it remains CM interference, and it does not translate into DM interference (differential mode, i.e. affecting the actual signal), there is no problem.
In the case of passive probes, the entire voltage level must be displayed (up to +V) to then look at a relatively small difference between the two signals. The noise floor at this V/div setting, and the differences between channels and probes, will easily distort the signal. In addition, relatively long connections to GND are usually needed to connect the probe, which also leads to coupling of interference.
In the case of a high voltage differential probe, the electronics inside the probe will try to suppress the CM as much as possible, so that only the signal of interest will be shown. The extent to which the probe (or the differential amplifier inside) can do this is called CMRR: Common Mode Rejection Ratio. The higher the CMRR, the less impact from CM disturbances.
A typical CMRR value of a high voltage differential probe at a bandwidth of 20 MHz is -40 dB (Micsig DP series). At higher frequencies the CMRR decreases, e.g. it is only -26 dB at 120 MHz. To measure the high side FET (HS1) with SiC and GaN, both a high (isolation) voltage with respect to GND and a high bandwidth are required, due to the fast switching behavior.
Optically isolated probe
The best way to measure a relatively small signal in an environment with strong interference is by using galvanic isolation. This is a well-known concept in the communications world, but it has only existed for oscilloscope probes for a few years. The concept is that the measurement is performed as close as possible to the source to minimize interference coupling, and then the measurement is transmitted via an optical link to the oscilloscope. This way an enormous CMRR is possible, up to 180dB at DC and still 108dB at 1 GHz with
Comparison
The table below shows the difference between -26dB and -122dB CMRR, or in other words the difference between a measurement at 120 MHz with a Micsig high voltage differential probe, or a :
|
CMRR @ 120 MHz |
CM example: 600Vpk |
|
|
Standard differential probe |
-26dB |
30Vpk disturbance |
|
Optically isolated probe |
-122dB |
0.5mVpk disturbance |
The disturbance as a result of CM will therefore be practically eliminated when using optically isolated probes. In practice, there can always still be interference due to coupling on the connection between probe and PCB. For this reason, it pays off – if possible – to provide a coaxial connection on the PCB.
The photos below show a comparison between the SigOFIT probe (red trace) and the DP probe (white trace). These are connected with a so-called ‘pigtail’ – a non-coaxial connection, which makes coupling easy. But of course this is a real-world scenario. Even in this situation, the optically isolated probe shows a significantly cleaner signal.
The optically isolated probe has been around longer and was introduced in 2016 by Tektronix (IsoVu™). Although these are of course excellent probes, they can only be used with Tektronix oscilloscopes and come with a hefty price tag. The popular Micsig SigOFIT probes have been offered since 2022, are very competitively priced, and can be used with any brand and type of oscilloscope.
Questions?
If you have any questions regarding this article about measuring power electronics, please feel free to contact one of our specialists.
