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How to measure the on – state current of a MOSFET?

by lee•

Hey there! As a supplier of MOSFETs, I often get asked about how to measure the on – state current of a MOSFET. It’s a crucial aspect, especially for those who are using these components in their electronic projects. So, let’s dive right into it! MOSFETs

Why Measuring On – State Current Matters

First off, why do we even need to measure the on – state current of a MOSFET? Well, the on – state current, denoted as (I_{D(on)}), gives us an idea of how much current the MOSFET can handle when it’s in the conducting state. This is super important because if you exceed this current rating, the MOSFET can overheat and eventually fail.

In practical applications, like power supplies, motor drives, or LED lighting circuits, knowing the on – state current helps in designing a reliable and efficient circuit. You don’t want your circuit to break down because you’ve pushed the MOSFET beyond its limits.

Methods for Measuring On – State Current

Using a Multimeter

One of the simplest ways to measure the on – state current is by using a multimeter. Here’s how you can do it:

  1. Set up the circuit: First, you need to have your MOSFET in a circuit where it’s in the on – state. Connect the power supply, load, and the MOSFET properly. Make sure the MOSFET is being driven with the appropriate gate voltage to turn it on.
  2. Select the current measurement mode: On your multimeter, switch to the DC current measurement mode. You’ll usually have different ranges available, so start with a higher range and then adjust it as needed.
  3. Insert the multimeter: To measure the current, you need to break the circuit and insert the multimeter in series. This means that the current flowing through the MOSFET will also flow through the multimeter. Connect the positive lead of the multimeter to the side of the circuit that’s connected to the positive terminal of the power supply, and the negative lead to the other side.
  4. Take the reading: Once everything is connected, turn on the power supply. The multimeter will display the current flowing through the MOSFET. Make sure to note down the reading accurately.

However, there are some limitations to using a multimeter. For one, multimeters have a limited current – measuring range. If the on – state current of your MOSFET is very high, the multimeter might not be able to measure it accurately or at all. Also, inserting the multimeter in series can introduce some resistance into the circuit, which might affect the current flow slightly.

Using a Current Shunt Resistor

Another common method is to use a current shunt resistor. A current shunt resistor is a low – resistance resistor that’s placed in series with the MOSFET. Here’s how it works:

  1. Choose the right shunt resistor: The value of the shunt resistor depends on the expected on – state current of the MOSFET. You want to choose a resistor with a low enough resistance so that it doesn’t significantly affect the circuit operation, but high enough to generate a measurable voltage drop.
  2. Connect the shunt resistor: Place the shunt resistor in series with the MOSFET. Connect the power supply, load, and the MOSFET as usual.
  3. Measure the voltage drop: Use a voltmeter to measure the voltage drop across the shunt resistor. According to Ohm’s law ((V = IR)), where (V) is the voltage drop, (I) is the current, and (R) is the resistance of the shunt resistor. You can calculate the current flowing through the MOSFET by dividing the measured voltage drop by the resistance of the shunt resistor ((I=\frac{V}{R})).

This method has some advantages. It can handle higher currents compared to a multimeter, and it doesn’t introduce as much resistance into the circuit as inserting a multimeter in series. However, you need to make sure that the shunt resistor can handle the power dissipated ((P = I^{2}R)) without overheating.

Using a Current Probe

A current probe is a more advanced tool for measuring current. It’s especially useful for measuring high – frequency and high – current signals. Here’s how you use it:

  1. Select the appropriate current probe: There are different types of current probes available, each with its own current – measuring range and frequency response. Make sure to choose a probe that’s suitable for your application.
  2. Connect the current probe: Wrap the current probe around the wire carrying the current through the MOSFET. The probe will sense the magnetic field generated by the current and convert it into an electrical signal.
  3. Connect the probe to an oscilloscope or a data acquisition system: The output of the current probe can be connected to an oscilloscope or a data acquisition system. These devices will display the current waveform and allow you to measure the on – state current accurately.

The advantage of using a current probe is that it doesn’t require breaking the circuit, which makes it very convenient. It can also measure high – frequency currents, which is important in some applications like switching power supplies. However, current probes can be quite expensive, and they require some knowledge of how to use an oscilloscope or a data acquisition system.

Factors Affecting On – State Current Measurement

There are several factors that can affect the accuracy of on – state current measurement.

Temperature

The on – state resistance ((R_{DS(on)})) of a MOSFET is temperature – dependent. As the temperature increases, (R_{DS(on)}) also increases. This means that the on – state current will decrease for a given voltage. When measuring the on – state current, it’s important to take the temperature into account. You can use a temperature sensor to monitor the temperature of the MOSFET and make appropriate corrections to your measurements.

Gate Voltage

The gate voltage of a MOSFET determines whether it’s in the on – state or off – state. For a MOSFET to be in the on – state, the gate voltage needs to be above the threshold voltage ((V_{th})). The higher the gate voltage above (V_{th}), the lower the (R_{DS(on)}) and the higher the on – state current. When measuring the on – state current, make sure that the gate voltage is stable and within the recommended range.

Parasitic Inductance and Capacitance

In high – frequency circuits, parasitic inductance and capacitance can affect the on – state current measurement. Parasitic inductance can cause voltage spikes and ringing, which can make it difficult to measure the current accurately. Parasitic capacitance can also affect the switching behavior of the MOSFET, which can in turn affect the on – state current. To minimize the effects of parasitic inductance and capacitance, you can use proper layout techniques and add snubber circuits.

Tips for Accurate Measurement

Here are some tips to ensure accurate on – state current measurement:

  • Calibrate your measuring equipment: Before taking any measurements, make sure to calibrate your multimeter, voltmeter, current probe, or any other measuring equipment you’re using. This will ensure that your measurements are accurate.
  • Minimize the resistance in the measurement path: When using a multimeter or a shunt resistor, try to minimize the resistance in the measurement path. This will reduce the voltage drop and the power dissipation in the measurement circuit.
  • Take multiple measurements: To get a more accurate measurement, take multiple measurements and calculate the average. This will help to reduce the effects of noise and other measurement errors.

Conclusion

Measuring the on – state current of a MOSFET is an important step in designing and testing electronic circuits. There are several methods available, each with its own advantages and limitations. By choosing the right method and taking into account the factors that can affect the measurement, you can ensure accurate and reliable results.

Schottky Diode If you’re in the market for high – quality MOSFETs or need more information on how to measure their on – state current, feel free to reach out to us. We’re here to help you with all your MOSFET needs. Whether you’re working on a small hobby project or a large – scale industrial application, we’ve got the right MOSFETs for you. Contact us for a quote and let’s start a great partnership!

References

  • Horowitz, P., & Hill, W. (1989). The Art of Electronics. Cambridge University Press.
  • Mohan, N., Undeland, T. M., & Robbins, W. P. (2003). Power Electronics: Converters, Applications, and Design. John Wiley & Sons.

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