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.