High Voltage Switching with MOSFETs

Using a MOSFET as a switch is generally pretty simple. Make the gate voltage sufficient with respect to the source and current flows through the channel. However, if you are switching higher voltages, you may need some additional circuitry to protect the device’s gate and possibly the microcontroller driving the whole thing, too. [Lewis] discusses high voltage switching in the latest in his series of videos dealing with MOSFETs. You can see the video below.

You’ll see in the video a breadboard setup driving a 50 V load and also a higher-voltage H-bridge. There are three major topics covered: Using an optoisolator, using a gate bleeder resistor, and using a zener diode to limit gate voltage.

Of course, an optoisolator isn’t necessary with or without the high voltage, as long as everything works well. It can, however, prevent high-frequency noise from conducting from the FET channel through the gate acting as a capacitor. It can also be useful for saving the controller if there is a failure that shorts the channel to the gate.

The bleeder resistor isn’t specific to high voltages, either. Because the gate is practically an open circuit at DC, it is a good idea to add a resistor like this so that when the gate drive is off, you don’t have to wait for the charge on the gate to dissipate. In addition, the resistor offers some protection against electrostatic discharge (ESD) damage, since the ESD will tend to flow through the resistor instead of punching through the gate oxide on the device.

The zener sees service in two ways. At first, the circuit uses it to derive 12 V from the 50 V supply. However, in a later incarnation, the design uses it as a clamp to keep a P-channel’s gate at 12 V. These are both important because the maximum rating of the gates is 20 V.

Although the devices in the video are IRF630s and IRF9630s, the principles apply to lots of different kind of FETs. Just remember what you kind you have and understand the datasheet before you finalize your design.

If you want something more basic, there are plenty of tutorials. We’ve even looked at high-side switches in detail.

7 thoughts on “High Voltage Switching with MOSFETs

    1. Common misconception. Bulbs are basically a pure resistive load.
      For small, lightly inductive loads you can rely on the body diode of the (power) FET. That being said, diodes are cheap; put one across the FET or load for paranoia sake.

        1. … until it doesn’t anymore and you chase the magic smoke out :(

          Stray inductance in the leads can be enough to blow up a driver FET by exceeding Vdsmax, especially if you’re using it in chopper/PWM mode. The freewheel diode *must* be present.

          (if you have a full H-bridge, the body diodes in the pair-of-devices-that-didn’t-just-turn-off can save you, but the energy will be pumped into your DC bus so watch that voltage climb if the “load” is a motor going downhill)

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