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Walter Harley
Guest
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 Posted:
Mon Dec 12, 2005 12:40 am Post subject:
250mA voltage clamp |
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I need to limit the output of an ordinary unregulated power supply. The
voltage ranges from 18V to 35V, taking load and line variation and ripple
into account, and the load draws up to 250mA. I'd like to limit the output
to 24V (give or take a volt); I don't care if it drops below that, though,
and in fact it will often be right around that. I don't want to introduce
bursts of noise or ringing when the voltage is right around the limit. I
would like to have the input-output voltage differential be no more than 2V,
and would prefer less than 1V. This is a one-off project, so using junkbox
parts is desirable, though not a requirement.
The best idea I've come up with is a power PNP or MOSFET pass transistor,
driven by an opamp that compares the output voltage to a zener reference.
Some potential problems are latchup in the opamp during power-on, and
stability of the opamp (since the feedback loop here includes things like
changes in the upstream voltage). Also, simply connecting the opamp between
the input rail and ground means it sees up to 35V supply, worryingly close
to the operating limit for a jellybean opamp.
Is there an even simpler solution that I'm not seeing?
Should I be worried about the stability of this solution, and if so, what
would be an appropriate way to control it?
Am I right to distrust an LM317 at and below dropout?
Thanks!
-walter
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Guest
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| Posted:
Mon Dec 12, 2005 1:24 am Post subject:
Re: 250mA voltage clamp |
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On Sun, 11 Dec 2005 10:40:13 -0800, "Walter Harley"
<walterh@cafewalterNOSPAM.com> wrote:
| Quote: | I need to limit the output of an ordinary unregulated power supply. The
voltage ranges from 18V to 35V, taking load and line variation and ripple
into account, and the load draws up to 250mA.
|
What's wrong with a 24 volt zener? Pick one that will handle
the dissipation when the supply is current limited by its internal
impedance. What is the current from the supply when it has a load
heavy enough to drop its output voltage to 24 volts?
Jim |
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Fred Bloggs
Guest
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| Posted:
Mon Dec 12, 2005 1:35 am Post subject:
Re: 250mA voltage clamp |
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| Quote: |
I need to limit the output of an ordinary unregulated power supply. The
voltage ranges from 18V to 35V, taking load and line variation and ripple
into account, and the load draws up to 250mA.
What's wrong with a 24 volt zener? Pick one that will handle
the dissipation when the supply is current limited by its internal
impedance. What is the current from the supply when it has a load
heavy enough to drop its output voltage to 24 volts?
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The interesting question is how he plans to limit the I/O differential
to 2V with 35V in and 24V out- looks like 12V to me, and at 1/4 amp
makes for 3W. Sounds like someone is confused, again.
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Walter Harley
Guest
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| Posted:
Mon Dec 12, 2005 1:35 am Post subject:
Re: 250mA voltage clamp |
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"Fred Bloggs" <nospam@nospam.com> wrote in message
news:439C89D3.9000006@nospam.com...
| Quote: | The interesting question is how he plans to limit the I/O differential to
2V with 35V in and 24V out- looks like 12V to me, and at 1/4 amp makes for
3W. Sounds like someone is confused, again.
|
It's a voltage clamp. When the input voltage is above 24V, the differential
will be Vi - 24V. When the input voltage is below (24V + Vdropout), the
differential will be Vdropout. I want Vdropout to be less than 2V,
preferably less than 1V. (Though there is no requirement that Vdropout be
precise or constant.) I'm sorry if that confused you.
I could simply use a LM317 as a preregulator. According to the datasheet
its dropout voltage (at room temperature or higher, at less than 500mA) is
around 1.7V. But I don't know whether to trust it to remain stable around
and below dropout, which is why I asked about that.
And in answer to Jim's question: it takes about 300mA draw for the internal
impedance of the supply to drop the voltage sufficiently. I'd rather avoid
a shunt regulator if I can, so as to avoid unnecessary heating. The
max-load condition is unusual and is expected (though not guaranteed) to
last only a short time; 99% of the time the load is expected to be only 30mA
or so. |
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Ken Smith
Guest
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| Posted:
Mon Dec 12, 2005 1:39 am Post subject:
Re: 250mA voltage clamp |
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In article <houdnZXKg_oe7wHeRVn-rQ@speakeasy.net>,
Walter Harley <walterh@cafewalterNOSPAM.com> wrote:
[...]
| Quote: | The best idea I've come up with is a power PNP or MOSFET pass transistor,
driven by an opamp that compares the output voltage to a zener reference.
|
If the supply is isolated, you can put the pass element in the (-) leg.
This lets you use a low cost MOSFET.
The supply voltage to the op-amp can be regulated by zener. If you really
want, this zener could also serve as the reference.
You didn't rule out putting a large capacitor across the output terminals.
If you do this, you can make the common source MOSFET stage have its high
frequency gain determined mostly by the gm and the output capacitor.
BTW: A source resistor can make the MOSFET's gm easier to predict.
By limiting the supply voltage on the op-amp, you reduce how far it has to
swing as the supply rises past 24V. The op-amp should be a fairly fast
one. The output capacitor should be the important pole in the system.
--
--
kensmith@rahul.net forging knowledge |
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Guest
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| Posted:
Mon Dec 12, 2005 9:24 am Post subject:
Re: 250mA voltage clamp |
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On Sun, 11 Dec 2005 13:40:32 -0800, "Walter Harley"
<walterh@cafewalterNOSPAM.com> wrote:
| Quote: |
And in answer to Jim's question: it takes about 300mA draw for the internal
impedance of the supply to drop the voltage sufficiently. I'd rather avoid
a shunt regulator if I can, so as to avoid unnecessary heating. The
max-load condition is unusual and is expected (though not guaranteed) to
last only a short time; 99% of the time the load is expected to be only 30mA
or so.
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OK, a DC-DC converter, buck/boost type between your source and
load would solve all your problems at 80% efficiency. And be much
easier to implement than some op-amp and pass element scheme.
Jim |
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Fred Bloggs
Guest
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| Posted:
Mon Dec 12, 2005 9:35 am Post subject:
Re: 250mA voltage clamp |
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Walter Harley wrote:
| Quote: | "Fred Bloggs" <nospam@nospam.com> wrote in message
news:439C89D3.9000006@nospam.com...
The interesting question is how he plans to limit the I/O differential to
2V with 35V in and 24V out- looks like 12V to me, and at 1/4 amp makes for
3W. Sounds like someone is confused, again.
It's a voltage clamp. When the input voltage is above 24V, the differential
will be Vi - 24V. When the input voltage is below (24V + Vdropout), the
differential will be Vdropout. I want Vdropout to be less than 2V,
preferably less than 1V. (Though there is no requirement that Vdropout be
precise or constant.) I'm sorry if that confused you.
I could simply use a LM317 as a preregulator. According to the datasheet
its dropout voltage (at room temperature or higher, at less than 500mA) is
around 1.7V. But I don't know whether to trust it to remain stable around
and below dropout, which is why I asked about that.
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A voltage clamp is just a simple emitter follower with a zener from base
to GND. The TIP41 with a 24V zener will work well. The trick is to keep
the power transistor base drive low impedance and somewhat regulate the
zener current against your 18V to 35V input swing. You can use something
minimal like this:
View in a fixed-width font such as
Courier.
..
..
.. V+
.. |
.. .-------------+
.. | |
.. | |
.. [270] |
.. | |
.. | |
.. +------. |
.. | | |
.. | [47] |
.. >| | |/
.. 2N2907A |----+----| TIP41
.. /| | |>
.. | | |
.. | | |
.. | | + '-----> OUT
.. [1K/1W] Z
.. | A 24V [1N4749]
.. | | -
.. | |
.. '------+
.. |
.. |
.. ---
.. ///
.. |
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budgie
Guest
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| Posted:
Mon Dec 12, 2005 9:35 am Post subject:
Re: 250mA voltage clamp |
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On Mon, 12 Dec 2005 03:24:21 GMT, the.other.Jim@nowhere.net wrote:
| Quote: | On Sun, 11 Dec 2005 13:40:32 -0800, "Walter Harley"
walterh@cafewalterNOSPAM.com> wrote:
And in answer to Jim's question: it takes about 300mA draw for the internal
impedance of the supply to drop the voltage sufficiently. I'd rather avoid
a shunt regulator if I can, so as to avoid unnecessary heating. The
max-load condition is unusual and is expected (though not guaranteed) to
last only a short time; 99% of the time the load is expected to be only 30mA
or so.
OK, a DC-DC converter, buck/boost type between your source and
load would solve all your problems at 80% efficiency. And be much
easier to implement than some op-amp and pass element scheme.
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Surely replacing the highly unregulated power supply would be easier still? I
fail to see the point in spending time building a fix for a simple sub-assembly
that has a problem, when another PSU would solve it. |
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