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

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H713:
I do not like to trust datasheets for this kind of thing- the intense competition in the power MOSFET market means that the numbers can be a somewhat idealistic.

Testing is easy enough to do. A big transformer, bridge rectifier and some decently sized caps, plus a power supply with a 10 turn pot to control the gate. I tested the 2SK3675 and the FQA8N90C and found them to be quite robust in this application, pushing them past 150 watts of dissipation. They don't handle HV linear operation too well, but they have seemed fairly tough in this application.

As an interesting aside, the little IRF640 actually held up pretty well- probably why BSS used them in the EPC780.

The HV PS design thread I started a while back had much discussion about the issue of MOSFET DC SOA. Discussion remained quite civilized.

schmitt trigger:
Just a thought:
because of the risks already mentioned with failing Mosfets, would a crowbar circuit be a good idea?

Of course the crowbar's threshold could not be a fixed value, but would have to track the actual voltage output setting.

Neomys Sapiens:

--- Quote from: H713 on April 09, 2020, 05:46:37 am ---
--- Quote from: Neomys Sapiens on April 08, 2020, 09:57:48 pm ---I would refrain from digital control other than providing the set values via D/A converters.

I think that those PS that use multiple MosFets in a linear mode must have had the benefit of a builder who could ask everything from a supplier, like very very tight selections. The only appliance which I have personal experience with was a custom unit which was used to weld hermetic cases for hybrids. If one of the Mosfets did break ranks even a bit, mass extinction ensued. While it is certainly feasible, you would have to do the selection yourself, which could be costly. If you want to try anyway, APT, IH (now Renesas) and IR had appnotes on the topic.

Where I see problems is the 2HE requirement. Even assuming that you use fans with tunnel-type heatsinks, the heatsink has to have some capacity for heat which means mass, and therefore size.

--- End quote ---

I'm hoping to mitigate the issues with the FETs in a few ways. For one, I'll be using 900V rated parts with a fairly high Rdson. Additionally, I will be using quite a few of them in parallel to have as much overhead as possible. The idea is that there will be enough beef in the series pass transistors that they will survive even if the tap-switching fails. They will also have some rather generous source resistors in order to help with the current sharing.


--- End quote ---
The voltage/current/power headroom was not what I had in mind. In the mentioned application, the MosFets died during parameter changes due to differences in transconductance. So you'll have to match for this and have ample beef in the driver stage to keep them in line. The worst case is when one is sticking out during a change in drive which coincides with a change in the load circuit.
Result: BANG zoing tingle crash Moo : literally!, as I saw TO3 caps leaving the base violently, when we tried to rebuild the thing with new MosFets.
Should not discourage you. The appliance I'm referring to was handling far higher currents and a load circuit which, depending of the tool condition and the cycle time, was varying apruptly.

David Hess:
A pair of more common transformers could be used with a toggle switch to wire the secondaries for series and parallel operation.  Unfortunately though for audio applications, higher current will be needed at higher voltage so this is of limited use.  But the same limitation works well with fold-back current limiting; in an audio application, full current is not needed at low voltages so build that into the power supply to considerably relax the power dissipation requirements.

Or use multiple small transformers with their secondaries in series to provide the taps.  Two transformers with center tapped or dual secondaries yield 4 output levels.

H713:
I expect that the 3 levels that would be available with the Antek transformer (two primaries, two secondaries) would be sufficient. Maybe not the best, but sufficient. As was mentioned, it's pretty easy to add windings as well, but we'll see if that's necessary.

Doing some back-of-the-envelope, worst-case-scenario napkin math, if the voltage rails were 90 volts (and never sagged under load), with 3 steps I the maximum power that would ever have to be dissipated would be 150 watts. That's a lot, but not unmanageable, and in practice it would never be that high because 1) the rails will be less than 90 volts, and 2) they will sag under load. Even if I only used 3 FETs per side, each FET would stay below 50 watts of dissipation. For the sake of having a safety factor, I'll probably use five or six.

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