Author Topic: Are power supplies based on opamp+transistor linear or switching?  (Read 6794 times)

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Offline bson

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Re: Are power supplies based on opamp+transistor linear or switching?
« Reply #25 on: June 22, 2017, 08:27:34 pm »
The circuit on page 11 has a problem in that at low voltages, below the B-E threshold of the power BJT, the op amp is in open loop.  The fix is to put a resistor across B-E, e.g. 2.2k, so the loop is always closed.  This would also allow it to regulate outputs less than a diode drop.

I quickly hooked this up, with the LF411C being pin compatible with the LM741; the MJE15030 is just what I happened to have a bunch of.



It happily regulates 3.3V or 5.1V off a zener to the point of overheating of the BJT.  (Tested with a B&K 8600 DC load.)
I replaced the resistor-zener with a pot and it happily regulates down to 0.1V 3A (for a few seconds, before the heat sink got very hot).
This was off a 12V transformer seconday.

D1-C1 improves ripple rejection by not having it dip into the zener or opamp supply as sharply if the secondary is overloaded/underfiltered.

RV1 isn't needed; the output can't exceed the rail anyway, so positive gain is pointless.  Just make it a buffer and set the voltage on the positive input; skip this adjustable gain hack.  But a trimmer is okay in case a zener is used for reference, but in that case the zener needs to be less and the desired voltage and the trimmer used to adjust it upwards.  It can't ever adjust it downwards, obviously.

The voltage could be controlled by a DAC very easily, just replace D2 with it and make R1 bigger (for pull-up pull-down to COM).  This will also greatly improve PSRR since there's no zener current to drain C1.

Size C1 for ripple rejection.

Remove the GND reference to float off the transformer secondary.
« Last Edit: June 22, 2017, 08:55:21 pm by bson »
 
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Offline Zero999

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Re: Are power supplies based on opamp+transistor linear or switching?
« Reply #26 on: June 23, 2017, 08:47:31 am »
The circuit on page 11 has a problem in that at low voltages, below the B-E threshold of the power BJT, the op amp is in open loop.  The fix is to put a resistor across B-E, e.g. 2.2k, so the loop is always closed.  This would also allow it to regulate outputs less than a diode drop.
I don't see how that makes any difference. The op-amp you've chosen common mode range doesn't extent to 0V and even if it did, the B E resistor isn't required. Try it with the LM358 and you'll see it works all the way down to 0V, with no B E resistor.

Quote
D1-C1 improves ripple rejection by not having it dip into the zener or opamp supply as sharply if the secondary is overloaded/underfiltered.
Note that it will also increase the drop-out voltage by another diode drop.
 

Online T3sl4co1l

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Re: Are power supplies based on opamp+transistor linear or switching?
« Reply #27 on: June 23, 2017, 08:57:44 am »
I don't see how that makes any difference. The op-amp you've chosen common mode range doesn't extent to 0V and even if it did, the B E resistor isn't required. Try it with the LM358 and you'll see it works all the way down to 0V, with no B E resistor.

Rbe helps maintain a linear condition (setting minimum gain and bandwidth, over more of the operating range, both static and dynamic).  It's a good idea in many situations.

It can also make things worse, by increasing the bias current, and therefore Gm and fT, of the driving stage, or reducing turn-off time of the driven stage (see, for example, Darlington-wired transistors and using Rbe's around them).  But that's a system compensation problem (it's "too good", so merely needs RLC values adjusted), not a linearity problem.

This is especially relevant to the LM358, which sucks so bad, it's marginal even for control applications.  Biasing the output (with a load resistor to GND or +V) helps alleviate this.

Tim
Seven Transistor Labs, LLC
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Offline Zero999

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Re: Are power supplies based on opamp+transistor linear or switching?
« Reply #28 on: June 23, 2017, 09:33:22 am »
I don't see how that makes any difference. The op-amp you've chosen common mode range doesn't extent to 0V and even if it did, the B E resistor isn't required. Try it with the LM358 and you'll see it works all the way down to 0V, with no B E resistor.

Rbe helps maintain a linear condition (setting minimum gain and bandwidth, over more of the operating range, both static and dynamic).  It's a good idea in many situations.

It can also make things worse, by increasing the bias current, and therefore Gm and fT, of the driving stage, or reducing turn-off time of the driven stage (see, for example, Darlington-wired transistors and using Rbe's around them).  But that's a system compensation problem (it's "too good", so merely needs RLC values adjusted), not a linearity problem.

This is especially relevant to the LM358, which sucks so bad, it's marginal even for control applications.  Biasing the output (with a load resistor to GND or +V) helps alleviate this.

Tim
Oh I didn't mean do not use a base-emitter resistor, just that it isn't needed to close the loop. If the output voltage is set below 0.6V, then the op-amp's input will just sit at VOUT+VBE, assuming the common mode range isn't exceeded.

Another reason for adding a base-emitter resistor is to help the op-amp's output stage discharge the base, when the output voltage is low.
 
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