Re: Op-amp for power supply
- From: hondgm@xxxxxxxxx
- Date: 29 Apr 2006 11:03:06 -0700
Ban wrote:
There are several problems with your design. They require understanding how
a transistor works. On the net you will mainly find the outcome of others in
the same situation, so I wouldn't rely on this.
First, that diode will have a huge diffusion capacitance when conducting, it
is only dependent on the forward current and the reverse recovery time,
because when it is supposed to shut off, the charge carriers have to be
removed. This is a multiple of the depletion capacitance and can be several
nF. Luckily the very low ohmic junction resistance is parallel to it. The
same is true for the transistor.
But the opamp is *much* faster and sees this big capacitance. It will start
oscillating and turning the diode+transistor on again.
So for the voltage control output this is bad. Better to use a resistor
instead of the diode.
How is this done? If I replace the diode with a resistor, then you
have a different circuit alltogether. The diode scheme allows either
the current OA or voltage OA alone to control the output. Whether this
is a good design, however, is questionable.
Slow down the response time with a capacitor from the Vreg opamp O/P to its
neg. input, maybe 100p. This will greatly improve stability. What you were
doing was the opposite and aggravates the problem.
I believe I've tried this. It does help some.
Now this "pull-up" resistor will draw a lot of current when the voltage is
low and not enough to pull the darlington up, since the opamp with the diode
cannot source current. So use a current source made from a resistor and
current mirror between the rails and set it to say 5mA when output is zero.
The new resistor on the voltage regulator has now to pull the darlington
base down to 0.9V or less and has to sink those 5mA, so maybe 0.7V/5mA=
120R. If that is not enough put a high current diode from output gnd to the
negative rail to improve this margin.
The pull-up isn't much of a problem. Actually, I have a 47K on there
and it has no problem supplying enough current to the transistor. And
the OA has no problem sinking this current.
Ok, so I think you're saying to build a plain old voltage regulator
Now the current limiter can not work any more, since when it kicks in, the
voltage opamp output goes high and delivers another 20mA until the current
limit. There is a possibility: you pull down its *reference voltage* through
a Schottky diode and also a 120R resistor. But now the regulation goes thru
2 opamps so the current one needs to have an even slower response, maybe 1n
in the same position as
before with the voltage opamp. Now your voltage opamp will never be out of
regulation.
Don't forget a resistor on the DAC voltage O/P since it gets pulled down by
the
current limiter.
with an OA and output transistor, like I have here, minus the diodes.
But, pull down the voltage reference into the op-amp with my current
limiting OA. I still end up using diodes.
I'm confused now.
--
ciao Ban
Apricale, Italy
.
- References:
- Op-amp for power supply
- From: hondgm
- Re: Op-amp for power supply
- From: Ban
- Op-amp for power supply
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