120v to 12v 3.5+A switching converter ciruitry
Bobb Fwed
Posts: 1,119
I am looking for help and/or existing circuitry that could help me create a 120VAC to 12VDC switching power supply that can output at least 3.5A.
I have already successfully created a circuit board for our latest project that can supply 12VDC at 2A. It uses a transformer to take the 120VAC to 18-20VAC, then it gets switched down to 12VDC. but the transformer generates too much heat. I've tested it for extended periods of time (hundreds of hours) at above 150°F, and it doesn't fail, but the power supply is in an closed system (no access to fresh air to circulate). So the heat in our system will cause increased degradation in parts (especially electrolytic caps and the like). Plus the added 18 watts of power being transmitted through a larger transformer would increase the total heat generated by the system.
So I am looking for a cooler solution using a circuit to directly switch 120VAC to 12VDC (more precisely: 13VDC ±15%).
I have a few ideas that I've SPICE'd with referenced comparators and some MOSFETs, and a fairly normal Schottky/inductor/capacitor filter circuit. It appears it would work, but to keep the MOSFET happy, it would have to quite large, and switch at low frequencies because the 108V difference will generate a ton of excess heat during the switching (the exact numbers don't SPICE well), but I know it would be high, a large part of the circuit is supplying enough voltage and power to switch the MOSFET as quickly as possible.
I've tried doing a google search for existing circuits, with no luck. Any help or advice would be helpful.
I have already successfully created a circuit board for our latest project that can supply 12VDC at 2A. It uses a transformer to take the 120VAC to 18-20VAC, then it gets switched down to 12VDC. but the transformer generates too much heat. I've tested it for extended periods of time (hundreds of hours) at above 150°F, and it doesn't fail, but the power supply is in an closed system (no access to fresh air to circulate). So the heat in our system will cause increased degradation in parts (especially electrolytic caps and the like). Plus the added 18 watts of power being transmitted through a larger transformer would increase the total heat generated by the system.
So I am looking for a cooler solution using a circuit to directly switch 120VAC to 12VDC (more precisely: 13VDC ±15%).
I have a few ideas that I've SPICE'd with referenced comparators and some MOSFETs, and a fairly normal Schottky/inductor/capacitor filter circuit. It appears it would work, but to keep the MOSFET happy, it would have to quite large, and switch at low frequencies because the 108V difference will generate a ton of excess heat during the switching (the exact numbers don't SPICE well), but I know it would be high, a large part of the circuit is supplying enough voltage and power to switch the MOSFET as quickly as possible.
I've tried doing a google search for existing circuits, with no luck. Any help or advice would be helpful.
Comments
look on ebay for some surplus units. I bought some a year ago
that were meant to power external hard drives. They are small
units that output a few amps at 12v and they do not generate much heat.
I bet you could find some similar units on ebay today. You might
be able to open up such a supply and use just the board inside
if you did not want the supply to be external.
Even if you did design one, how much would it cost to get agency approvals? To conform to FCC EMI regs? To conform to power factor requirements? This is one case where "build vs. buy" is a no-brainer.
-Phil
If you really need to design your own (quality over cost), consider a torroidal transformer. 50VA torroid would be about 3.5" diameter and 1.25" high, but they are generally more efficient. They are also used a lot in 1U rack mount gear which would have similar cooling constraints.