I am repairing a HP 6023A lab power supply for a friend. The problem was that the power fets had failed. This is what the relevant schematic looks like:
The incoming 120VAC or 240VAC is rectified and the resulting approx 340V (they use a doubler for 120V) feeds the switcher. Here Q3 and Q4 had developed shorts and F1, F2 had blown.
Problem is that IRF440 is a High-Reliability part. MIL std and radiation hardened. The minimum quantity from the distributor is 100 pcs and none of the regular distributors, say Mouser, Newark carry it. It is available from eBay, but I am a bit wary as there are lots of counterfeits around.
The IRF440 is a 500V, 8A N-Channel FET with an Rdson of 0.85ohms. The IRF840A from Vishay has the same parameters or better, with one exception: Eas, maximum pulse avalanche energy. Rest, like diode recovery time is better (5.0 vs 3.5), Qg (38 vs 68) etc and lastly, they are both max 1C/W junction to case, which is surprising as a TO3 part has so much more metal than a TO220. The FETs sit on separate cooling fins and are cooled by a fan.
So, I am tempted to go with the IRF840A as is is cheaper and also readily available. This PSU will be used by a hobbyist and never launched into orbit :)
Is my reasoning sound?
PS. Should F1, F2 be ultrafast, fast or slow-blow? The manual does not say. They are small axial fuses with leads.
The HP6023A has a very endearing feature: A swith on the motherboard isolates the HV supply from the logic supply. You flick that switch and feed mains to two spade connectors. That lets you run the logic without the power part. You then connect a DC lab power supply to the mains input to test in a controlled fashion.
I soldered in two IRF 540 to test and fed in 60VDC. I could verfy that the unit operated properly, ie CC/CV, current limit etc. I am pretty certain that it is just the two FETs and fuses that are faulty.
The service manual also states that they are prone to blowing :)