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Evaluation of a 40V/5A DIY lab power supply

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profdc9:
The MC34071 can have up to 44 volt supply.    This may be useful if you want to push the output up to 40 volts.  For example, I have these schematic in my archive I got off of another forum:

You could probably push it up to 40 V and 5 A with the right component subsitutions.

David Hess:

--- Quote from: xavier60 on August 22, 2019, 12:28:49 am ---The floating type design makes possible the best performance such as very low regulation dropout voltage and fast control of the output stage.
--- End quote ---

Even when they make more sense, amateurs often avoid floating designs because they require an extra low voltage transformer or secondary winding.  In practice however, the extra complexity makes the rest of the design much easier.


--- Quote from: TheJC on August 22, 2019, 02:06:45 pm ---
--- Quote from: Kleinstein on August 21, 2019, 02:58:58 pm ---To check the design it is a good idea to use a simulation, before actually solder.
--- End quote ---

Oh yeah I do that all the time, but sometimes you can't go around soldering or at least building it up on a breadboard.
--- End quote ---

I tend to simulate it *after* I get it functionally working.   I have had too many problems with the SPICE modeling power pass elements accurately and at least for me, working out the frequency compensation for a first cut is faster with graph paper and pencil if the control loop is kept simple for good performance and ease of design.  If I cannot work it out manually, then it is likely to have stability problems anyway.


--- Quote from: profdc9 on August 22, 2019, 04:10:14 pm ---The MC34071 can have up to 44 volt supply.    This may be useful if you want to push the output up to 40 volts.  For example, I have these schematic in my archive I got off of another forum:
--- End quote ---

There are a bunch of old but still available non-specialist operational amplifiers which support 44 volt operation including the LF355/6/7, LF412, LF441, LM201A, some 741 varieties, LT1001 series, LT1006 series, OP-07 series, etc.

The supply voltages can be bootstrapped to allow the use of a lower voltage operational amplifier however this is tricky.

rstofer:
The problem with these DIY power supplies isn't with delivering 5A at 40V - as long as the input voltage is on the order of, say, 45V.  No, the problem is with delivering 1V at 5A and dropping 44V*5A or 220W.  It's the low output voltage that eats them alive.

Almost all commercial supplies will have transformers with tapped secondaries and relays to select the tap based on the voltage setpoint.  It's easy for manufacturers to get transformers made any way they want.  Not so for the one-off hobby project.

There are plenty of high side current sense amplifiers that are design for this kind of thing.  It's probably better to buy one than try to recreate it.

David Hess:

--- Quote from: rstofer on August 22, 2019, 06:49:53 pm ---The problem with these DIY power supplies isn't with delivering 5A at 40V - as long as the input voltage is on the order of, say, 45V.  No, the problem is with delivering 1V at 5A and dropping 44V*5A or 220W.  It's the low output voltage that eats them alive.
--- End quote ---

The power issue is real but can be handled with brute force if necessary rather than design finesse.  I am a believer in designing big linear power supplies by starting with what big inexpensive power transistors, heat sinks, and transformers are available and working backwards from there.

TheJC:

--- Quote from: rstofer on August 22, 2019, 06:49:53 pm ---The problem with these DIY power supplies isn't with delivering 5A at 40V - as long as the input voltage is on the order of, say, 45V.  No, the problem is with delivering 1V at 5A and dropping 44V*5A or 220W.  It's the low output voltage that eats them alive.

Almost all commercial supplies will have transformers with tapped secondaries and relays to select the tap based on the voltage setpoint.  It's easy for manufacturers to get transformers made any way they want.  Not so for the one-off hobby project.

There are plenty of high side current sense amplifiers that are design for this kind of thing.  It's probably better to buy one than try to recreate it.


--- End quote ---
I know this is why I later want to add a pre-regulator to keep the power loss manageable. I first wanted to get the series-regulator part up and running and afterward the pre-regulator.

It's definitely a better idea to use a premade high side current sense amp to avoid the problem of the higher voltages and also the problem of matching resistors when using a standard opamp.

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