Electronics > Projects, Designs, and Technical Stuff
0-70V, 0-5A Lab Power Supply Design
H713:
For testing audio amplifier designs, it is useful to have a fairly beefy bench power supply. My current supply is capable of 10-34V at about 3 amps, and it has proven to be insufficient. I have decided that it is time to build a bigger and better linear bench power supply. Here are my minimum requirements. As a side note, this is by no means my first power supply design project.
1) 0-70V
2) 0-5A
3) Extremely now noise/ripple. I'd like to see less than 2 mV peak-to-peak.
4) Linear, which complicates #5
5) Dual power supply in a 2U rackmount enclosure.
6) Fan-cooled is acceptable
7) Adjustable crowbar circuit to avoid damage to low-voltage circuits (like mixing console channel strips) in the event of a catastrophic failure. This will be a manual control that will likely see seldom use, however, it is essential to have for this application.
Obviously it is impractical to dissipate 700W in the series pass elements when operating at low voltages and high currents. As such, some form of "pre-regulator", which will probably come in the form of a Triac "dimmer" circuit on the primary of the power transformer, will be necessary.
Even still, the pass elements present an issue because when the output is shorted while it is set to 70V, the main filter caps will take some time to bleed down after the pre-regulator adjusts for the reduced output voltage. Traditionally these would be a BJT in a TO-3 package. I have an alternative idea, however.
An application note from International Rectifier suggests that for linear operation, older high-voltage MOSFETs with a high Rdson will be less likely to suffer from secondary breakdown. Testing of the FQA8N90C and the 2SK3675 verified this. A single 2SK3675 or FQA8N90C on a substantial heatsink was successfully able to handle 40V at 6A in its linear region. None of the BJTs I tested came close. I was not successful in killing either of the MOSFETs that were tested. I do intend to test with the full 70 volts, however, I need to dig a larger transformer out of storage in order to do so.
I haven't seen any linear power supplies using MOSFETs in this way, so I'm curious about what I'm neglecting to consider here. I was planning to use six of them in parallel for each of the two power supplies, and they will not be seeing the full 70 volts dropped across them for more than a second.
Here's the basic idea I've been simulating for the regulator circuit. Nothing complicated, and I'm using the IRFP240 and 2N3019 SPICE models because they're fairly generic. This is a generic schematic, obviously there will need to be changes made so that it is stable into capacitive loads, etc. R7/R11 and R15/R16 will be replaced with pots. Yes, there will be limits imposed to ensure you can't set the thing to current limit at 20 amps.
moffy:
Best way to do the variable voltage while keeping noise down and power dissipation, is to have a multitap transformer, that changes taps in say 10v or 20v increments. You need a controller for the relays but it's fairly straight forward.
H713:
I thought about doing tap switching, but that would require a custom transformer. I agree that it would be the simple and robust way to do it and I may still see if it can be an option without having to wind a custom toroidal transformer.
moffy:
Yeah, a bit of a pain unless you use multiple say 5A/20v (secondary) transformers in series. But then packaging is a pain.
Kleinstein:
It is very common to get transformers with dual output windings, like 2 x 24 V. So a single transformer would already give 2 voltages.
With only 2 U hight one does not have the hight for a single large transformer anyway. Keep in mind that continuous 5 A DC need something like a 8 A AC rating, unless extra PFC is used.
With 2 transformers for each half one would have 4 taps (possibly more with different voltages). Even just 2 taps nearly cuts the worst case loss to halve. In addition it reduces the voltage seen by the transistors which helps with the SOA.
For the tap switching it is still the question if one wants relays on the AC side or an electronic version at the DC side (e.g. 2 transistors in series). Both have there pos and cons.
Normally an audio amplifier has to work with a normal unregulated supply, so there is no real need to have super low noise for the high power part. One may not need / want normal lab supply grade current limiting. There can be extra peak current, and once the voltage drops too much chances are one would prefer a full off than holding the peak current. This can reduce the worst case power loss quite a bit. I would consider the regulation more for getting adjustable voltages and to have some protection.
There are some HP supplies that use MOSFETs in linear supplies. It is not that unusual.
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