Gilly wrote:Thanks Steve
Just a question ..... Will be using a carrier voltage of 30VDC and have 2 Power Transformers to choise from for the HV...
One has 60VAC with a CT and the other 45VAC with a CT...
For 30VDC Carrier which would be the better choice ?
Also for Q1 and Q2 was going to use IRF840 FETS.......they should be fine do you think...?
Wayne
Ok, the transformer will work very well, I believe, for 30 VDC. Use the 60 VCT winding in the center tapped mode, which will give you around 40 volts (maybe a little less, but close). This will be the carrier supply. Then use the 45 volt winding in bridge mode which will give you about 60 volts DC. Put these in series, and the high voltage output will be the positive peak supply.
On the IRF840, I would not use that small device unless you are going to run fairly low power. The power dissipation (and current) is small for analog service. Consider this: If you run 30 volts at 3.5 amperes (about the max you can run with 2 board mounted banks and without "outboarding" more MOSFETs) with the power supply voltages proposed, the average dissipation (under full modulation) in the top bank of MOSFETs will be around 165 watts. But the peak dissipation will be about twice this value - exceeding the device dissipation of 2 devices.
You are also getting close to the maximum current rating at full modulation - about 12 to 13 amperes - split between 2 devices is getting close to 8 amperes (each - the max DC current).
Unfortunately, class H is still analog service, and as such, there is a lot of power dissipation under modulation. If you use 4 of them in each bank (and run 30 volts at 3.5 amperes), it will probably work.
Anyway, just some thoughts. I picked the IRFP260n because it has such a high current rating that we will never exceed it, and the dissipation is high enough to withstand the riggors of analog service

There are probably other high dissipation devices that will work as well !