650W test load for current source power supply

I must make a resistive SMPS test load which is to be 650W maximum.

The Load will have a 50KHz sinusoidal current in it which is 1.9 Amps RMS.
This is a constant , regulated current.

The voltage will be 342V rms.

The load needs to be comprised of about 13 individual resistors. We need the ability to be able to short out each of these resistors so as to be able to vary the load.
We are thinking of using relays to short out each resistor.

I am told that the load must “stand up” and must have fans.

The resistors should not be inductive, and indeed in the load, we wish the voltage and current to be in phase.
I have been advised to use planar resistors.

https://uk.farnell.com/h-s-marston/cf2-0816-0805-1500na/heat-sink-0-13-c-w/dp/936420

I am thinking of using something like the above to fasten the planar resistors to. Do you know the best way to fasten the resistors to the heatsink, and other details?

So 650W @325 V = 2A, R = 325/2 = 163 ohms, divide by 13 (why?) = 12.5 ohms/section@ 50W. Say 2 X 25 ohms @30W (or greater) in parallel. The Bird 1 kW RF load* needs no fans, though the 15kW Does - howls like a banshee!. The ones in the attached link :- **broken link removed** do say to have a maximum of .1 microhenries self inductance. So Xl = 10 ^ -7 X PI X 50 X 10 ^3 = 150 X 10 ^-4 = .015 ohms. so is .015/25 = ~.6 % However their power rating is only 3.5 W when mounted on a PCB, so I guess for their max power rating they need to be clamped to a heatsink (isolated case - helpful).
* **broken link removed**
Frank

Quite similar low inductance resistors in TO220 or TO247 case and respective better heatsink mounting capabilities are available from Caddock (MP800 and MP900 series).

I would probably prefer a massive parallel connection of standard air cooled wirewound resistors, which can achieve respective low total inductance with moderate inductance of individual resistor elements. If you don't care for very low reistor tolerances, you can accept high resistor element temperature and achieve small form factor. Touch guard for the hot surface (and possibly exposed high voltage) must be provided of course.

the problem is going to be in tapping all the screwholes to fasten all these resistors and there connectors , plus the shorting relays, to the heatsink, none of the available heatsinks appear to have "gutters" into which screws can be screwed.

If you can drill the required size hole then Taptite type screws can be used :- **broken link removed** These form a proper thread as they screwed in.
The arrangement I would use is one resistor per plate, every other plate has a relay on it, screw them together in pairs with spacers to give a free air gap of 10mm. Then use long studs and more spacers to assemble the plates into a bank. Stand the bank upright with legs on the outer corners to give 25mm air space underneath. If the plates are 300mm wide and high, just put a expanded metal cover over it.
Frank

My experience when dealing with high power heatsinks is that you'll have to construct them yourself. You'll have to start with heavy (at least 1/2 inch thick) aluminum or copper plate.

The plate must be planed at the spots where the resistors attach to, and after all the machining/drilling/taping is best if you black anodize them for protection and better heat radiation.

If you don't have the abilities/equipment to do such work, a local machine shop should be able to do it for you.

As with most high power projects, the challenges are often more mechanical than electrical. You should assess what sort of machining tools and materials are at your disposal, then design based on that.

If it were me I would go for an array of low inductance TO247 package resistors with insulated tabs, all on one common heatsink. I'd probably use a single large heat spreader with fins, and drill the holes myself, and use a lot of forced air cooling to keep size down.

If you use a fan then you need a thermostat and some other kit (another relay to open circuit the load? or air flow switch to put load into circuit? [ better idea, no PSU required]) to protect the resistors if some one puts power into it without powering up the fan, hence my idea of not using a fan.
This is not a high power heat sink application as the 650 W is distributed across 26 devices, rather is a lot of moderate power heat sinks. If you are running a couple of devices that are dissipating 100s of watts each, then thick heat sinks, loads of fins and a fan are in order, else KISS.
Frank