Thread: Power MOSFETs
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Old 14th May 2019, 6:09 am   #44
Radio Wrangler
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Location: Fife, Scotland, UK.
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Default Re: Power MOSFETs

National power grids regulate the output of their turbo-alternators by varting the exciter drave to their fields to control the output voltage at the machine, and to balance the load current between machines. Voltage control around the network to handle usage variations and network losses is done in multiple places by transformers and tap changers. The limitation of a feedback loop is that it can control the amplitude in only one place, while a power distribution system has many places, so voltage controllers have to be distributed around the network, and in view of the power levels, have to be very efficient.

Back to aircraft.... The system needing to be substituted is a variable speed engine, running with about a 2:1 rev range, an alternator and a carbon pile regulator.

The carbon pile regulator uses a solenoid versus a spring to apply pressure to a stack of carbon discs operating as a variable resistor. The spring applies pressure to the discs. The output voltage creates current in the solenoid, and that current creates force opposing the spring force, letting the disc stack resistance rise to reduce the output voltage. It's a series-pass regulator done electro-mechanically. It isn't efficient by any stretch of the imagination, particularly in view of its limited ability in the low resistance direction.

With the change in revs, the EMF of an alternator varies. I don't know the system in the aircraft involved, but I suspect the carbon pile reg wasn't in series with the alternator output, but would have been in series with the field current feed from a metal rectifier. This is much more efficient, and the solenoid can still sense the alternator output voltage. The carbon pile can be a lot smaller, lighter and need less cooling. Valuable factors on an aircraft.

If you have a search around, Hugo has designed a number of solid state substitutes for the old magnetic (vibrating contact) controllers for battery-charging dynamos for period vehicles. Back in their day those things had a limited life. One RB340 seemed to have thelife of two sets of dynamo brushes.

Now back to the problem in hand...

There is another possible approach, a resonant inverter.

Neville Mapham at General Electric (USA) Wrote a paper on SCRs in a resonant power converter. One of his diagrams was picked up by SW Amos and published in an early Illiffe book on semiconductor applications - can't remember the tiitle but I had a copy in about '73. It had a red cover and was by Amos.

Mapham's resonant bridge circuit was used By HP for power distribution within a modular frame... but at tens of kHz. Interestingly for the TV people reading this, they used the fast SCRs that Philips developed for the Syclops line output/power supply system. In this case it gave 240W of rated output, but there was about 1.2kW circulating in the resonator.

They stopped building Syclops TVs, Philips took the SCRs out of production and HP bought a lifetime buy of the things for their Mapham inverters. These modular instruments had a rather long production life being used in military and three-letter government agencies. Life-buys ran low, as life-buys do. Muggins here got landed with the job of redesigning the thing. "But I'm an RF engineer!" got met with "Think of it as a longwave transmitter!"

Siemens had been a second source of fast SCRs, but they too had pulled them long ago. SCRs were out as an approach. Mapham's resonant bridge circuit provided a reverse voltage period to turn off the scrs. I analysed its operation in more detail but without the assumption of using a latching device like an SCR. If I had to turn off my devices myself, when would I have to do it? Well, it turned out to be surprisingly non-critical. I didn't believe my luck. I could drop IGBTs into the Mapham circuit and transformer drive their gates with opposing phases of a square wave. Simples! it was efficient as well, and left behind all the reprehensible habits of the SCRs (self-triggering on dv/dt worse if hot and causing a damaging failure).
So my modified converter was capable of higher power and we upped the rating for the frame distribution to 320W. Oh, the AC output was regulated by controlling the DC supply voltage to the inverter from an SMPS.

I think a 1600Hz inverter could be made to handle several hundred watts of load. The output could be regulated and the whole thing could get into the 80% efficiency stakes. The resonator inductors would need to be specially made and finding reliable resonator capacitors is not going to be easy. The wrong ones glow red and flash into flame.

There may be more development work in this than the OP would want, but I thought it might be interesting to show that others have been down this path before... only I had contracts from US admirals and spooks to contend with, so no pressure, then Trapped between Popeye and 'Get Smart'

If I had to do it myself right now for the OP, I'd phone Stewart of Reading and ask if they happened to have an old 'Elgar' AC supply amongst their stock (BIG audio power amp good for a kilowatt)

David
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