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Old 13th March 2014, 04:43 PM   #1
jetbat is offline jetbat  United States
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Default Tl431, MOSFET shunt voltage regulator

I was wondering if the transistor in this circuit could be replaced with a P channel MOSFET like the IRF9610?
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Old 13th March 2014, 04:50 PM   #2
Mooly is offline Mooly  United Kingdom
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Probably, as long as you remember that the FET needs around 4 volts across its G-S to turn on compared to the 0.6 of a bjt. So that places a lower limit on your regulated voltage.
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Old 13th March 2014, 06:16 PM   #3
jetbat is offline jetbat  United States
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Thanks. I just wasn't sure if there was something I over looked.

If it works the regulated voltage should stay the same. The IRF9610 has a gate-source threshold voltage of -2 to -4V. *The TL431 would pull the gate voltage down till the MOSFET turns on. The reg voltage should still be set by the voltage divider at the TL431 reference input.
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Old 13th March 2014, 08:24 PM   #4
Mooly is offline Mooly  United Kingdom
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As long as you have the voltage to play with it should be OK. No good if you only want say a regulated 3.3 volt rail or whatever.
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Old 14th March 2014, 12:37 AM   #5
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I've used that format - very simple and sharp knee.

The 431 cathode resistor can be split in series if you want the 431 cathode voltage at regulation point to be say around 2-5V if the supply input was say 12-24V. The 431 regulation current needs to be above a certain level - see datasheet.

Also, the dissipation in the FET can be mainly moved in to a power resistor in the drain, which is very practical for larger power applications where the FET junction temperature management becomes high priority - but the FET heatsinking still needs to cope with half the supply voltage dissipation condition.
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Old 14th March 2014, 04:19 PM   #6
jetbat is offline jetbat  United States
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I understand.
Thanks.
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