Selecting T&M Non-Inductive Coaxial Shunts for Double-Pulse Ids Measurement

To help more users correctly select the model suitable for their application, this article provides a clear analysis of how to select a coaxial shunt, with the aim of providing a reference for coaxial shunt selection.

A coaxial shunt is essentially a very small resistor that measures current via Ohm's law and is generally used to measure high pulsed currents. The appropriate coaxial shunt must be selected for different measurement scenarios. Selection mainly considers the following aspects:

1. Power Dissipation

The primary parameter to consider in selection is Joule heating; the heating of the coaxial shunt must not exceed the maximum allowable Joule heating. Joule heating formula:

Q=I²*R*t

【Joule=current squared * resistance * pulse width″】            

The current here must be calculated using the maximum peak value.For double-pulse testing, the high-level durations of 2 pulses must be calculated; for multiple pulses, the high-level durations of all pulses must be calculated.

For general selection, a margin should be reserved for Joule heating1/2of up to 1/3. It can also be seen from the formula that the larger the time t, the larger the Joule heating Q; therefore, coaxial shunts are mainly used to measure pulsed current.

It is worth noting here that,If the Joule heating is exceeded, the coaxial shunt may burn out.。

If the commonSDNseries cannot meet the Joule heating requirement, medium-power M series or A series can be considered, with Joule heating up to several tens to more than 2,000 J.

The figure shows an actual photo of a medium-power M series coaxial shunt.

II. Resistance Value

The magnitude of the resistance value is related to the signal strength and the impact of insertion into the circuit.The smaller the resistance, the smaller the impact of insertion into the circuit; the larger the resistance, the stronger the voltage signal received by the oscilloscope.。

When measuring SiC current (generally 500 A to 2000 A), coaxial shunts are mostly used to measure the current in this case. Here, the shunt model should be selected according to the magnitude of the current under test; generally, when measuring such large currents, models with relatively small resistance values are selected, such as SSDN-414-01, etc.

The figure shows a coaxial shunt & double-pulse test waveform.

When measuringGaNcurrent (generally not exceeding 100 A), a signal with a slightly larger internal resistance can be selected to ensure measurement accuracy. For example, when the internal resistance of the selected model is about 0.01 Ω, then when the measured current is 50 A, U=I*R=0.5V can still be displayed on the oscilloscope; if the measured current is 1 A or even smaller, a model with a slightly larger resistance is required, such as a model with a resistance of 0.1 Ω, so as to ensure the accuracy of the reading. This choice is made to ensure that the waveform signal displayed by the oscilloscope has sufficient fidelity, because the oscilloscope can capture very small signals but is greatly affected by the noise floor, which will cause signal distortion and make the measurement data invalid.

As for why an ordinary resistor is not used for direct measurement in the latter case, it is because direct measurement with an ordinary resistor makes it difficult to ensure measurement accuracy and is susceptible to interference from various factors, which is unacceptable for high-precision measurement requirements.

Since a model with a slightly larger resistance is accurate for measuring small currents, would it not be even more accurate for measuring large currents? This is not the case. When we use a shunt to measure current, it is essentially connected in series with the on-resistance in the circuit. If a model with a large resistance is also used to measure a large current, according to the formula, this will increase the overall resistance, which will also increase measurement interference and affect measurement accuracy.

III. Bandwidth

The choice of bandwidth is then quite simple and direct, provided the two points above are met,the higher the bandwidth, the better。

The following are commonly used coaxial shunt models:

4. Isolation

A coaxial shunt is a purely resistive structure; its greatest advantage is high bandwidth and near-zero (approaching zero) inductance, so it is only slightly affected when measuring high-frequency currents, making it well suited for high-frequency current measurement, such as double-pulse dynamic testing of SiC/GaN wide-bandgap power devices. The disadvantage of the coaxial shunt is that it provides no isolation: the oscilloscope and the circuit under test must share a common ground. Therefore, it is generallyonly used to measure non-floating currents and cannot directly measure floating currents。

To avoid this drawback, we recommend a solution that combines a coaxial shunt with an optically isolated probe to achieve isolation, as shown below.

If you still have questions about coaxial shunts after reading this, you can contact us directly for consultation.