Four-quadrant power supply

[Christoph Loew]

Hi everyone,

I've been looking for a circuit for a 15 V, 5 A
four-quadrant power supply powered from 110 V AC.

So far I've found :
http://www.reed-electronics.com/ednmag/article/CA371090
the schematic :
http://www.reed-electronics.com/ednmag/contents/images/371090f1.pdf
which looks OK to me except for the current rating (fixable)
and that weird LT1970 for which I can't find the Spice model.

Any thought regarding this circuit ?
Any other ideas ?

Since for me the educational effect is more important than
e.g commercial viability or efficiency I'd prefer a design based on
off-the-shelf (more likely out-of-the-parts-bin-except-for-
the-power-transistors) op-amps and tansistors.

Any pointers appreciated.

(And no, this is *not* a homework assignment)

Chris

 

[Tim Shoppa]

Hi everyone,

I've been looking for a circuit for a 15 V, 5 A
four-quadrant power supply powered from 110 V AC.

So far I've found :
http://www.reed-electronics.com/ednmag/article/CA371090
the schematic :
http://www.reed-electronics.com/ednmag/contents/images/371090f1.pdf
which looks OK to me except for the current rating (fixable)
and that weird LT1970 for which I can't find the Spice model.

And you need the Spice model for.... ???

Since for me the educational effect is more important than
e.g commercial viability or efficiency I'd prefer a design based on
off-the-shelf (more likely out-of-the-parts-bin-except-for-
the-power-transistors) op-amps and tansistors.

Not a big deal. The voltage regulator part is an op-amp with a power
booster (for low or medium power you may prefer bipolar transistors
in the booster stage). The current regulator is where it gets tricky,
you have several choices for where to put the sense resistor and
you have to budget for the voltage drop across the resistor somewhere
(depends on how much headroom you have in the unregulated supply and
whether input and output have to share a common ground and op-amps
with inputs operated near a rail... or not, depending on how you do it.)

If you haven't done two-quadrant power supplies before and are
really gung-ho about SPICE, start simple (op-amp with power booster) and
build up the current regulation on top of that with a sense resistor
and comparator for the limit current.

Most folks don't model their linear power supplies in SPICE, and in fact I've
never seen a LM723 SPICE model, but you could build one up based on the
internal schematic I'm sure.

Tim.

 

[CFoley1064]

Subject: Four-quadrant power supply

From: Christoph Loew [email protected]
Date: 9/17/2004 1:46 AM Central Daylight Time
Message-id: <[email protected]>

Hi everyone,

I've been looking for a circuit for a 15 V, 5 A
four-quadrant power supply powered from 110 V AC.

So far I've found :
http://www.reed-electronics.com/ednmag/article/CA371090
the schematic :
http://www.reed-electronics.com/ednmag/contents/images/371090f1.pdf
which looks OK to me except for the current rating (fixable)
and that weird LT1970 for which I can't find the Spice model.

Any thought regarding this circuit ?
Any other ideas ?

Since for me the educational effect is more important than
e.g commercial viability or efficiency I'd prefer a design based on
off-the-shelf (more likely out-of-the-parts-bin-except-for-
the-power-transistors) op-amps and tansistors.

Any pointers appreciated.

(And no, this is *not* a homework assignment)

Chris

Hi, Chris. Building your own tools is the best education.

Generally, what you're referring to is called an "operational" power supply, as
in op (operational) amp. The easiest way to make one of these is with a power
op amp. I had to make one of these once, and chose the National Semiconductor
LM12.

http://cache.national.com/ds/LM/LM12CL.pdf

Look at the circuit on p. 9 of the datasheet. That circuit has independent
current limit controls for + and - current. The operational power supply I
cobbled together was similar to the one in the datasheet. (I made a couple of
mods, but nothing critical - the circuit on the datasheet works.)

The LM12 has a bw of 700kHz, and the LT1970 has a bw of 3.6MHz. Your supply
will be faster, and that's one of the rubs of the situation. Layout on both
circuits is _extremely_ important. These things want to be bad if you let
them. A tight layout and good power supply bypassing is a must on either one.
Since the LT1970 is a 20-pin tssop with copper bottom, you're going to
_require_ a PCB for your circuit, where I was able to do the LM12 circuit on a
perfboard next to the heatsink, since it's a TO3-type package. Also, your
article suggests the power input is a regulated bulk +/-17V power supply. If
you use an unregulated supply as input, you're going to have an increased power
dissipation issue with the LT1970. Lay out your copper heat sink carefully,
and use at least 1 oz and preferrably 2 oz. copper on the board.

If this is a "home use" lab supply, you also might want to consider that an
LM12 will be easier to replace than an LT1970, meaning your power supply will
be easier to service. However, I don't think you're going to be able to get
quite to 5A because of power dissipation considerations. If you're willing to
settle for 4A and a lower bandwidth, the LM12 circuit might be an alternative
choice.

Good luck
Chris

 

[Christoph Loew]

Thanks for the help so far.

I was unfamiliar with the term "operational power supply"
and the LM12CL pointed in the right direction. I'm now
starting on a design with general-purpose op-amps and
bipolar power transistors based on the LM12CL application
schematic.

Again, thanks for the quick replies.

Chris

 

[legg]

Hi everyone,

I've been looking for a circuit for a 15 V, 5 A
four-quadrant power supply powered from 110 V AC.

So far I've found :
http://www.reed-electronics.com/ednmag/article/CA371090
the schematic :
http://www.reed-electronics.com/ednmag/contents/images/371090f1.pdf
which looks OK to me except for the current rating (fixable)
and that weird LT1970 for which I can't find the Spice model.

Any thought regarding this circuit ?
Any other ideas ?

One of the important features of a 4-quadrant supply is it's ability
to absorb power.

The magazine schematic is of a 2-quadrant supply with absorption
capability limited to that of its internal dc supply.

RL