Suggestions for stabilising a TL431 + MOSFET current sink

Grant · 2010-07-04T02:46:36+00:00

Hi thereI've built a one amp current sink and at present it is uncompensated and it is performing quite nicely as a 200kHz power oscillator, with about 2V p-p across the 2.579 Ohm compound current sense resistor.Following text is based a screen scraping of a web page I just put up, along with some pictures to describe this issue at: I've not done much with MOSFETs in linear amplifier mode, so some suggestions for stabilising this one Amp current sink circuit are needed. The circuit uses an N-channel MOSFET for the pass element, a 0.5% accurate TL431B reference and a couple dozen 0.1% resistors provide for accurate current sensing with a 2.579 Ohm shunt.The circuit 'works' in the sense that the magic smoke stays where it belongs and no components get overly warm. The circuit needs stabilising as the thing is at present uncompensated and oscillating around 200kHz.The circuit diagram: Circuit description: Fixed 1.000 Amp current sink. Of that one Amp, 983mA goes through the current sense resistor, the remaining 17mA current goes via a trimpot scaling circuit to feed the TL431B's Ref pin. A 13V zener limits gate drive voltage during circuit saturation, and a 110 Ohm gate resistor is placed close to the MOSFET.A bank of 24 by 61R9, 250mW, 0.1% resistors to form R9, a 2.579 Ohm, six Watt current sense resistor. Calibrating the current sink to 1.000A is done by adjusting the trimpot to so that 2.535V is measured across the current sense resistor, with a 4 1/2 digit Fluke multimeter.Photo of the oscillation waveform: The oscillation waveform shows about 2V p-p across the 2.579 Ohm current sense resistor. The MOSFET is happy enough on its large heatsink, it gets very warm to the touch but it's not too hot for continuous operation.Photo of the built circuit: Photo of the thing shows placement of the MOSFET gate resistor up close to the MOSFET gate where it belongs. The large heatsink allows continuous operation, as the input voltage may go up to 30V, from the nominal...

Jul 04, 2010 24 Replies

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Grant 16 years ago

Yeah, right -- and let the gate drive go up to 24 to 30V on no load? No thanks.

Grant.

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Grant 16 years ago

Might try that once I can do some dynamic load testing, thanks.

Grant.

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Grant 16 years ago

I use something like what I see in the data books, at the risk of being confused about the arrow direction as I was this time ;)

Tim's explanation that it shows the reverse biased junction diode at least makes sense, rather than confusing with IG whatevers.

Grant.

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Grant 16 years ago

Twice you've suggested slugging a large cap directly across the TL431, I'm wary of doing that because the MOSFET is inside the feedback loop.

What am I missing here?

Grant.

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Jim Thompson 16 years ago

[snip]

The "R" terminal itself is relatively hi-Z, just like an OpAmp input

...Jim Thompson

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