ESD and overvoltage protection for MOSFET driving off-board RGB LED (5V system)
I’m using an N-channel MOSFET (SSM3K15AFU,LF) to drive a 5V RGB LED (Common Anode) that’s mounted off-board and connected via a 12-inch cable. The LED has an internal resistor and draws about 20 mA. The LED is often touched by the human because the led panel has push button also.
Here’s the current circuit setup:
I previously had failures where the MOSFET (Q1) got damaged. After reading more, I suspect ESD or hot-plug transients from the long LED cable might be causing it. The 5V rail comes from an onboard buck converter. I have now added D5 and D6 for gate and drain protection. Do we need gate protection ?
Questions:
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Is using a PESD18VS1UA (18V TVS) on the drain appropriate for a 5V LED line, or should I switch to a 5V bidirectional TVS instead?
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Do I also need a 5V to GND TVS on the power rail near the connector, or is the drain clamp sufficient?
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Any suggestions for a more reliable ESD/overvoltage protection scheme for this setup?
1 answer
I previously had failures where the MOSFET (Q1) got damaged.
ESD can damage MOSFETs with an exposed drain. I've seen that before.
- Is using a PESD18VS1UA (18V TVS) on the drain appropriate for a 5V LED line, or should I switch to a 5V bidirectional TVS instead?
Not finding the datasheet for your 18V part. For the purposes of the answer, let's go with the 12V equivalent PESD12VS1UA (datasheet). It clamps at 27V, and the MOSFET is rated for 30V max VDS. There's not a lot of margin left. I'd look for a TVS which clamps at 20V or less.
You should use a unidirectional TVS. The PESD12VS1UA can clamp negative ESD pulses. See fig. 8 in the datasheet.
[Please get into a habit of posting datasheet links for the lazy volunteers. ]
- Do I also need a 5V to GND TVS on the power rail near the connector, or is the drain clamp sufficient?
Maybe you can survive without a TVS for the 5V rail at the connector. But such TVS wouldn't hurt.
Capacitors can arrest ESD pulses too. There are ESD-rated ceramic capacitors (some discussion here). The capacitor would have to be placed next to the connector. Otherwise the ESD may jump from the power supply rail somewhere more sensitive.
[D6 for gate protection] Do we need gate protection?
In this case you don't require ESD protection for the gate. The gate isn't eposed. Drain protection will catch the ESD.
The LED [which is powered by the MOSFET] is often touched by the human because the led panel has push button also.
I haven't seen ESD jump from the case of an LED to the pins. Consider grounding the panel to which the LED is attached. A grounded panel will divert the ESD away from the circuit board.
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