Im working on a power supply project to use with amateur radio equipment, im looking for 13.8v at 30A or so. Im working on a shoestring budget, so recycled components from sources such as junked PC power supplies and CRT TVs are the order of the day. I reckon the half bridge topology would be best for my requirements. For the time being, I have just been using the half bridge to drive a filament lamp so I know there is going to be no problem with inductive voltage spikes. TV horizontal output transistors seem to be a good choice, should be plenty about but I haven't yet found a cheap source. (Tried BU208 with success but its a bulky TO3 device.) I expect to be blowing a few up over the course of the project so I dont want to be spending much on them.
For the transformer I intend to use a 2 inch ferrite ring out of the 12V to +/- 36v converter section of a scrap car amplifier (good cheap source of parts, 3GBP from a car boot sale), the original amplifier transformer worked at 40kHz in a push pull arrangement and I expect the power rating is adequate. Should be easier to wind than an EDT transformer too.
Im using the base drive transformer out of a computer SMPS, so no rewinding to be done and the isolation should be up to scratch. I think they use a single turn to provide positive feedback to reduce drive requirements and make the supply self starting.
I have already had some exploding transistors (IRF730s) and would like to arrange protection on the primary side. If I used Mosfets I could use the voltage drop across a low value source resistor to short out the gate drive, possibly latching in this state (SCR made fron npn and pnp pair). Tried this with bipolars but it left them in a high dissipation state.
I do have some high power audio transistors rated at 230V and about 8A. I know the voltage rating doesnt give much headroom, im wondering about fitting a 200V TSD across each transistor as protection for the time being against spikes, probably wont be needed once a suitable snubber is in place. I am also considering the 2 switch forward convertor since no snubber is required with the clamping action, but this means changing to a gapped core.
Your comments please!
Regards, John
For the transformer I intend to use a 2 inch ferrite ring out of the 12V to +/- 36v converter section of a scrap car amplifier (good cheap source of parts, 3GBP from a car boot sale), the original amplifier transformer worked at 40kHz in a push pull arrangement and I expect the power rating is adequate. Should be easier to wind than an EDT transformer too.
Im using the base drive transformer out of a computer SMPS, so no rewinding to be done and the isolation should be up to scratch. I think they use a single turn to provide positive feedback to reduce drive requirements and make the supply self starting.
I have already had some exploding transistors (IRF730s) and would like to arrange protection on the primary side. If I used Mosfets I could use the voltage drop across a low value source resistor to short out the gate drive, possibly latching in this state (SCR made fron npn and pnp pair). Tried this with bipolars but it left them in a high dissipation state.
I do have some high power audio transistors rated at 230V and about 8A. I know the voltage rating doesnt give much headroom, im wondering about fitting a 200V TSD across each transistor as protection for the time being against spikes, probably wont be needed once a suitable snubber is in place. I am also considering the 2 switch forward convertor since no snubber is required with the clamping action, but this means changing to a gapped core.
Your comments please!
Regards, John