Choosing the Right Probe for Double-Pulse Testing

  Double-pulse testing is an important means of evaluating the dynamic characteristics of power semiconductor devices, including MOSFETs, IGBTs, etc.

Owning a double-pulse testing platform is the first step to obtaining correct evaluation results. In addition to basic equipment such as the test board, power supply, signal generator, inductor, oscilloscope, etc.,

This article focuses on the probe part; after reading it, you will have a clear understanding of probe selection.

I. Importance of Probes in Double-Pulse Testing

      Most engineers underestimate the importance of probes. Without probes, an oscilloscope cannot perform accurately; probes enable you to observe the more realistic condition of the device under test.

There are hundreds of probe types available on the market today, but selecting the right probe for your specific measurement to obtain accurate observation results is crucial.

Many engineers believe that the passive probe supplied as standard with an oscilloscope is good enough, but this is not the case. If your application uses the wrong oscilloscope probe,

at best, it leads to incorrect measurement results; at worst, it may cause an accident and damage instruments and equipment.

      For example, in double-pulse experiments, there were users who used a passive probe for the gate probe together with a coaxial shunt and had the shunt burned out by the negative current formed by the common-ground loop,

There are quite a few such users. Probe Home has also conducted dedicated experiments on this issue; for details, please click the link to read this article:Negative Current Issue in Double-Pulse Experiments with GaN Devices

II. Which Types of Probes Are Needed in Double-Pulse Testing

Double-pulse dynamic testing circuit diagram

In general double-pulse experiments, the three signals Vgs, Vds, and Ids are mainly measured,

Both Vgs and Vds can use optically isolated probes or differential probes; considering equipment and personnel safety, passive probes are generally not recommended.

Ids generally uses a non-inductive coaxial shunt; if conditions permit, a Rogowski coil can be added for corroboration.

Test board and probe demonstration figure

1. High precision

      a.Voltage probesIn this respect,First, optically isolated probes are recommended. An optically isolated probe is a probe that uses light to isolate and transmit signals.

Because the medium is light, it is more convenient and flexible in applications and is suitable for situations with relatively high requirements on size and weight.

Relatively speaking, differential probes more often use differential signal processing technology to improve signal stability and reliability.

In this respect, the characteristics of optically isolated probes become even more important; they can ensure accurate transmission and reception of information,

avoiding errors and interference in signal transmission. Optically isolated probes are the final arbiter for determining the authenticity of the signals measured by other voltage probes.

      b.Current probesIn this respect, the most widely used is the CVR high-frequency non-inductive coaxial shunt from the U.S. company T&M, with 0.2% ultra-high resistance accuracy and extremely low inductance.

Example of non-inductive coaxial shunt & test board

2. High bandwidth

      Some customers use high-end oscilloscopes, but the waveforms measured are still distorted to an unreasonable degree; the problem usually lies in the signal sampling front end: the probe.

If the probe bandwidth is insufficient, no matter how high the oscilloscope bandwidth is, it is useless; this is the bottleneck effect.

In double-pulse testing, be sure to select a probe with sufficient bandwidth. Generally, for accurate amplitude measurement, the bandwidth of the oscilloscope and probe

should be five times the frequency of the waveform under test. Insufficient bandwidth will distort the signal, making it difficult for you to make sound engineering test or design decisions. Higher bandwidth allows more details to be seen.

      The favorable bandwidth characteristics of optically isolated probes and coaxial shunts are very prominent; currently available optically isolated probes,

For example, PMK's maximum bandwidth can reach 1.3 and 1.5 GHz; Tektronix, LeCroy, and Micsig can reach up to 1 GHz; Cybertek can achieve 500 MHz.

T&M coaxial shunts have selectable bandwidth from 400 MHz to 2 GHz.

Very few high-voltage differential probes generally reach 200 MHz; PMK's BUMBLEBEE can reach 400/500 MHz, well ahead;

Emerging domestic brandMicsig's MDP series now reaches 300 to 500 MHz; the domestic Cybertek DP6000 series has also been upgraded to 500 MHz;

There are also some that can reach 200 MHz, such as Tektronix's THDP0200, Germany's TESTEC TT-S18052, and the domestic Tronovo HDP1520; all are high-voltage differential probes with very good performance.

    Among the probes used in double-pulse testing, the Rogowski coil has somewhat lower bandwidth; most current applications only reach 30 MHz,

This is because it must pass through narrow device pins; its physical dimensions have reached the limit, and its design and construction limit its bandwidth performance. Some closed-loop versions can reach 50/100 MHz.

Double-pulse probe configuration example

3. High common-mode rejection ratio

     The absolute value of the ratio of the voltage gain of the amplifier circuit for the differential-mode signal to the voltage gain for the common-mode signal, because we need to suppress zero drift,

Therefore, the smaller the common-mode voltage gain, the better, and the larger the differential-mode voltage gain, the better.The higher the CMRR specification, the better the performance of the isolated probe.

Some high-performance high-voltage differential probes achieving DC:80 dB are already very good, and this is where the ultra-high common-mode performance of optically isolated probes becomes evident,

Currently, PMK and Micsig can reach DC:180 dB, while Tektronix and LeCroy reach DC:160 dB.

Example of measured gate waveform for double-pulse testing

4. Safe floating measurement

Whether it is a differential probe or an optically isolated probe, both are for isolation. Safe floating measurement ensures personnel and equipment safety.

Sometimes the ground you connect to is not necessarily true ground; once again, we remind everyone to use passive probes with caution.

PMK high-voltage differential probe BUMBLEBEE (figure)

5. Appropriate measurement range

Selecting an appropriate range can maximize measurement accuracy; for both current and voltage probes, data measured close to the probe's range value is more accurate.

This is also why Rogowski coils are made in so many range variants. Differential probes generally also have 2 range settings; for example, PMK's BUMBLEBEE (Bumblebee) has 4 range settings.

In this respect, optically isolated probes are more flexible; the same main unit can be equipped with ratio settings for multiple different voltage ranges to meet voltage measurement needs of different magnitudes.

Example diagram of an optically isolated probe attenuator

6. Flexible connection

 This connection generally refers to matching the oscilloscope. For example, probes from brands such as Tektronix not only can be connected only to Tektronix oscilloscopes of the same brand, but also must be paired with 4-series or 5-series high-end oscilloscopes.

Brands such as PMK impose no restriction on oscilloscope brand, but it is generally recommended to use a high-performance, high-bandwidth oscilloscope to ensure acquisition of more realistic and credible measurement data.

7. High temperature, voltage withstand, and other considerations

In many current applications, device heating and other conditions need to be considered, which makes it necessary to consider using high-temperature-resistant probes. At present, in the high-temperature fieldthere are only 2 types of probes:

Voltage probe: PMK optically isolated probe optional components offer ultra-high temperature resistance up to 155 ℃

Current probe: IWATSU Rogowski coil SS-280A-H series, high-temperature resistance up to 150 ℃

In addition, when selecting a Rogowski coil, the coil's insulation withstand voltage must be considered; different series have different withstand voltage values.

4. How should probes be configured in double-pulse testing

After reading the above points, you should now have a clear understanding of probe specifications and most brands, and can select suitable probe products according to your own testing needs.

Selecting high-performance probes can provide more reliable, more accurate, more professional, safer, and more comprehensive measurement data.PMK&IWATSU and T&M as

leading representatives of high bandwidth, high precision, and high performance, having been widely used by major manufacturers at home and abroad and validated and favorably received by customers.

Some imported products have relatively high costs. If they exceed the budget range, you can consider models from domestic brands with adequate specifications, such as the Micsig optically isolated probe OIP series and

the high-voltage differential probe MDP series; after multiple experimental verifications by 探头之家, its performance meets the requirements and it is recommended.

3. If you have difficulty making choices, you can leave it to a professional team

As a quality supplier in the field of power semiconductor testing, 探头之家 distributes mid-to-high-end probes from first-tier global brands and provides a variety of advanced current and voltage probe products for double-pulse testing,

We can customize a complete set of double-pulse dynamic and static testing equipment according to your testing requirements and budget.

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