Suggestions for stabilising a TL431 + MOSFET current sink [Design]

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From: Grant on 3 Jul 2010 22:46

Hi there

I'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:

http://grrr.id.au/current-sink/

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:

http://grrr.id.au/current-sink/CurrentSink-circuit-750x736.jpg

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:

http://grrr.id.au/current-sink/CurrentSink-waveform-R9.jpg

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:

http://grrr.id.au/current-sink/CurrentSink-photo.jpg

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 24V.

What's needed are suggestions for what stabilising components
to use around the TL431B. Circuit frequency response is not
that critical as this current sink will be used to measure
the value of compound power resistors up to about ten Ohms,
while trimming their value.

The circuit is powered from a 12 cell 40AH or 100AH SLA battery.

Web page is here: http://grrr.id.au/current-sink/ where I'll add
further information as I play with the thing, and as ideas you
contribute are added.

Thanks,
Grant.

From: John Larkin on 3 Jul 2010 23:19

On Sun, 04 Jul 2010 12:46:36 +1000, Grant <omg(a)grrr.id.au> wrote:

>Hi there
>
>I'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:
>
> http://grrr.id.au/current-sink/
>
>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:
>
> http://grrr.id.au/current-sink/CurrentSink-circuit-750x736.jpg
>
>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:
>
> http://grrr.id.au/current-sink/CurrentSink-waveform-R9.jpg
>
>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:
>
> http://grrr.id.au/current-sink/CurrentSink-photo.jpg
>
>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 24V.
>
>
>What's needed are suggestions for what stabilising components
>to use around the TL431B.

Cap across it, from high side to ground.

John

From: John Larkin on 3 Jul 2010 23:27

From: Michael Robinson on 4 Jul 2010 00:06

"Grant" <omg(a)grrr.id.au> wrote in message
news:furv26hrvj3q0hfve2bkv6crkmh001ruso(a)4ax.com...
> Hi there
>
> I'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:
>
> http://grrr.id.au/current-sink/
>
> 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:
>
> http://grrr.id.au/current-sink/CurrentSink-circuit-750x736.jpg
>
> 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:
>
> http://grrr.id.au/current-sink/CurrentSink-waveform-R9.jpg
>
> 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:
>
> http://grrr.id.au/current-sink/CurrentSink-photo.jpg
>
> 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 24V.
>
>
> What's needed are suggestions for what stabilising components
> to use around the TL431B. Circuit frequency response is not
> that critical as this current sink will be used to measure
> the value of compound power resistors up to about ten Ohms,
> while trimming their value.
>
> The circuit is powered from a 12 cell 40AH or 100AH SLA battery.
>
> Web page is here: http://grrr.id.au/current-sink/ where I'll add
> further information as I play with the thing, and as ideas you
> contribute are added.
>
> Thanks,
> Grant.

I recommend a cap from K to R on the TL431. 10 nF is probably more than
enough.

From: Tim Williams on 4 Jul 2010 01:36

Your MOSFET is shown as a P type?? 33N10 certainly isn't..

Compensation across the '431 is the way to go. Besides 10nF, you may need series resistance to its input, to set the RC time constant. I don't know what R5 and R6 are; R7 is small enough at 150 ohms to be tough to drive. It's not like the input takes much current, 10k ohms is plenty.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

"Grant" <omg(a)grrr.id.au> wrote in message news:furv26hrvj3q0hfve2bkv6crkmh001ruso(a)4ax.com...
> Hi there
>
> I'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:
>
> http://grrr.id.au/current-sink/
>
> 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:
>
> http://grrr.id.au/current-sink/CurrentSink-circuit-750x736.jpg
>
> 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:
>
> http://grrr.id.au/current-sink/CurrentSink-waveform-R9.jpg
>
> 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:
>
> http://grrr.id.au/current-sink/CurrentSink-photo.jpg
>
> 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 24V.
>
>
> What's needed are suggestions for what stabilising components
> to use around the TL431B. Circuit frequency response is not
> that critical as this current sink will be used to measure
> the value of compound power resistors up to about ten Ohms,
> while trimming their value.
>
> The circuit is powered from a 12 cell 40AH or 100AH SLA battery.
>
> Web page is here: http://grrr.id.au/current-sink/ where I'll add
> further information as I play with the thing, and as ideas you
> contribute are added.
>
> Thanks,
> Grant.

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