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Old 18th December 2015, 02:22 PM   #1
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Default Unexpected behaviour of linear regulator

Wasn't sure whether to post this under power supplies as it's not strictly intended for audio.

I made the following simple voltage regulator ( see attachment ) With a tl431 shunt regulator fed from a jfet CCS and an npn buffer. The odd thing is with the measured output resistance.

I used Rout = ∆V/∆I, the strange thing is that bellow 125mA the output resistance is negative! The output voltage actually increases under load until over 125mA where it drops with increasing load as would be expected. Anyway I'm concerned that this negative resistance could lead to instability, would an output capacitor help with this? Can anyone explain the mechanism of action of the negative resistance zone, my guess is that its something to do with variations in the gain of the npn transistor.
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Old 18th December 2015, 02:31 PM   #2
Mooly is offline Mooly  United Kingdom
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Is it oscillating ? Have you tried a small cap between the base of the pass transistor and the adj terminal of the 431.
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Old 18th December 2015, 02:32 PM   #3
djoffe is offline djoffe  United States
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Do you know that the current thru the JFET is enough to put the TL431 in its proper operating region? The JFET must supply the sum of the TL431 required current plus the base current. Measuring the current into the TL431 would be a good place to start. (I couldn't find a data sheet on the JFET.)
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Old 18th December 2015, 02:46 PM   #4
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Hi Gasboss,

The 2N5858 is a regular BJT, not a JFET, this could be a problem...

If you would use a JFET, your outputresistance would be depending on the unregulated inputvoltage with this setup.

Better to use a seperate diode from the unregulated voltage to the reference JFET and TL431, and use a capacitor to get a "clean" voltage.

Finally use a small capacitor (< 1nF) close to the output to limit the regulating behaviour.
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Old 18th December 2015, 02:54 PM   #5
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Plus the details of the TL431 model may be inaccurate
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Old 18th December 2015, 02:57 PM   #6
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I'm going to 'scope it to see if there's any sign of oscillation.

The Idss of the jfet measured at 5.8mA, the TL431 needs a minimum of 1mA, the bd139 has hFE = 151. My reckoning was that the CCS current was adequate for outputs up to around 250mA where the hFE drops to about 85, hence Ib = approx 2.5mA

I will try the capacitor.
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Old 18th December 2015, 03:03 PM   #7
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Quote:
Originally Posted by audiowizzard View Post
Hi Gasboss,

The 2N5858 is a regular BJT, not a JFET, this could be a problem...

If you would use a JFET, your outputresistance would be depending on the unregulated inputvoltage with this setup.

Better to use a seperate diode from the unregulated voltage to the reference JFET and TL431, and use a capacitor to get a "clean" voltage.

Finally use a small capacitor (< 1nF) close to the output to limit the regulating behaviour.
Thanks. It was actually 2N5458, there was a mistake on the schematic. What is the purpose of your proposed diode?
Should the <1nF Capacitor be connected in parallel with the output?
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Old 18th December 2015, 03:35 PM   #8
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It was oscillating at about 17 MHz! Was fixed with a 220pF cap between the cathode and adjust terminal of the tl431.

Thanks for everyone's input.
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Old 18th December 2015, 05:36 PM   #9
Mooly is offline Mooly  United Kingdom
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that's good.
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Old 18th December 2015, 06:09 PM   #10
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I decided to try an nmosfet instead of the npn transistor which of course required a higher voltage. This was provided by a 555 based voltage doubler. The performance of the circuit was far superior in most respects.

As an LDO, it was able to regulate with as little as 12mV input output differential. The output resistance was 14 milliohms at 800mA output current and load regulation was 0.22%

See attached schematic. The only downside was a bit of extra noise from the voltage doubler (which could be filtered if necessary)
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