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0-70V, 0-5A Lab Power Supply Design

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Neomys Sapiens:
@H731: would you possibly post the LTSpice-file for the schematic above?

H713:
Sorry, I meant to post this earlier.

b_force:
For just audio purposes I would make a much simpler design.
Which is a VARIAC followed by a isolation transformer with symmetrical outputs.
Just make a simple bridge rectifier followed by a MOSFET regulator (capacitance multiplier) where the voltage is set by resistors.

For repair purposes this is also a nice approach, because quite often you want to turn up the voltage very slowly.
Having a VARIAC in general is nice to have anyway, so you kill two birds with one stone (poor birds :( )

Pawelr98:

--- Quote from: Kleinstein on July 19, 2020, 10:03:50 am ---The usual topology for a lab supply (especially with more than some 25 V) is the floating regulator. Here current regulation is naturally with an OP.
The main positive aspect of the simple Vbe type current limit is the simplicity and speed - so it could still be used as an additional fast emergency limit.

The lm317 is not really helping for lab supply and the lm723 is not much more than a possibly (some versions) good reference and a poor OP with access to the compensation. Using OPs and a separate reference is not more complicated. One has to think about the compensation anyway.

--- End quote ---

Traditional Vbe current limiter was, in my experiments when designing a fairly fast digital I-V curve tracer, a much more stable approach compared to differential opamp circuit.

If one can get a hand on a NPN Germanium transistor then this approach is very efficient and simple.
Not very thermally stable (as Ge parts have bigger thermal drift) but still an interesting addon.
I have used a GT404 russian NPN Ge transistor for testing my design.
Required just 50-55mV Vbe drop to activate the current limiter.
Collector was connected to COMP pin on the 723 chip.
Compared to usual 650mV drop this is pretty much nothing.
Had to limit the current to around 2A. Voltage wise (leakage currents, gain) it was boucing around in the +- 50mA region which is OK.
When enclosed in a small space with hot elements it can and most likely will have a very noticeable thermal drift but I have not tested it in my design as it was not meant for long term operation, just few seconds at most.

--- Quote from: H713 on July 19, 2020, 05:12:19 am ---
A need for higher supplies would not have been an issue. An easier solution to the charge pump is to simply spec a slightly higher voltage power transformer. One of the goals here is to try and minimize how much noise and ripple the regulator circuit has to reject.

--- End quote ---

Also a thing to note.
With bipolar transistors it makes more sense to sense the current at the emitter.
Base currents with a few big power transistors is not really negligible. It could be as much as 100mA (beta of 50).

Charge pump is a simple design.
Two diodes, two capacitors.
There is no switching intererence/ripple as it operates strictly on 50/60Hz voltage on the main rectifier bridge.
Components cost pennies and make the control circuitry supply "independent" of the main supply.
That is you can drop 1V at the pass transistors instead of say 3V because of control circuitry requiring higher voltage just to go through pass transistors Vbe drops.
Of course one may simply use an extra winding (with toroid transformers just add turns of any wire around the core) or something like that to provide that extra voltage for the control circuitry.

To give an example of why such design improves efficiency.
Main supply provides 72V under load.
If you use the main supply for both control circuitry and pass elements then the maximum output voltage will be 69V as there's 1V missing to overcome the pass element base-emitter junction voltage drop.
To get around this, you bump the supply voltage to 73V. At 5A it means extra 5W which has to be dissapated.

With a charge pump (or extra winding, doesn't matter) the main supply voltage is still 72V but the control circuitry has 80V available.
Now the maximum output voltage can be 71V. At 70V you only drop 2V*5A=10W instead of 15W like in previous example.

And when choosing a off-the-shelf transformer it's often not possible to say "oh, I would like an 0.5Vac extra on the main winding to get over the regulator Vdrop". So the 5W difference can evolve into 10W or more.

Also if done right then the control circuitry will get a ripple-free voltage for controlling the pass elements.
Another diode+capacitor connected to the main filter capacitor means almost no ripple as the current draw of the control circuitry is pretty much nothing compared to the main pass elements.

H713:
@b_force, the problem is a lack of adjustable current limiting. When doing initial tests on a circuit that I don't necessarily trust, either because it is new or intermittently failing. Once I am confident that it is behaving well,  I may turn that maximum current up to something like 3A- just so I don't vaporize PCB traces if something stupid happens.

Again, the charge pump is an interesting idea but I don't think it's necessary. The heatsinks being used here are out of a Crest CA9, and they will be used with forced air cooling. I'm aware that I can improve efficiency by running the control circuitry off a higher voltage, but in this case it is not necessarily a priority.

I may play some more with the Vbe current limiter. The speed of this type of current limiting circuit meant that it worked very well in my HV power supply, which has suffered countless output shorts without issue. I may also be able to get around the thermal drift issues through physical design to make sure that the transistor used stays at a relatively constant temperature. If I can come up with a way to allow variation between about 100 mA and the full current (6A), without a stupidly twitchy control, then that will work. I may be able to get around the issue with a switch for high and low ranges.


Also, what is not shown in the schematic yet is a diode-capacitor network to feed the control circuitry, which drastically improves the ripple rejection.

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