Sizing mosfet for smps (220v to 40-30v)

[nandoanalog]

 I need a power supply capable of delivering at least 20 amps at 30-40v continuously. When stepping down 220v dc to my desired voltage, how do I rate the switching mosfet amperage wise?

[Ammar]

First thing is to choose a topology and then look at the design equations for it.

[rob77]

 I need a power supply capable of delivering at least 20 amps at 30-40v continuously. When stepping down 220v dc to my desired voltage, how do I rate the switching mosfet amperage wise?

stepping down 220V DC ? what's the AC input ?

[joseph nicholas]

I would start with a bigger package size that a to220.  Next, consider the size of the heat sink.  Think about how fast it must switch to produce the desired output.  You must consider if you need more than one of these.  Think if it needs dropping resistors on the input to limit the current if using more than one.  Last, do a practical test to see if it meets you needs before building and manufacture.

[JS]

Right now I'm using a welder as a transformer to drop 230ac to 32 ac. Then it's being rectified by a 400v 50 amp bridge converter, feeding a 100v 10000uf capacitor. Signal quality isn't a priority as the load doesn't seem to care. Needless to say, the welder eats up 50% of the power, and is pretty noisy.

 I want to rectify the 220-230 ac, then step down via switching.

Thats over 310VDC once rectified and properly filtered, even if you don't filter it properly you should be able to handle those 310V peaks.

The voltage rating of the components doesn't mean much more than, "it's less than this" but you already knew it for both since you started with 32VAC to feed them.

How do you know and what do you mean by the welder eats up 505 of the power? where does the other 50% goes?

JS

[poorchava]

If the OP is asking questions like that he or she should not try to design an 800W+ mains psu as they obviously lack the basic knowledge of the matter.

Sent from my HTC One M8s using Tapatalk

[T3sl4co1l]

http://www.mouser.com/Power/Power-Supplies/Switching-Power-Supplies/_/N-axgkh?P=1yxt68iZ1yxt6euZ1yxt778Z1yxt6q2Z1yxt76mZ1yxt7fgZ1yxt6evZ1yxt64rZ1yxt6f8Z1yxt619Z1yxt656Z1yaq8r3&Rl=axgkhZerbiZ1z0wc7vZ1yot873SGT&Keyword=meanwell&FS=True

Even if you're an expert, you won't beat them on price.  Or reliability.  Or regulatory approval.  Or...

I say this as an expert at switching supply design.

Tim

[poorchava]

No matter do you need isolation or not, I will strongly recommend a transformer coupled topology since 300V->30V is 10:1 ratio, which means your switch must be able to handle both high current and high voltage. By using a transformer, it handles the current boosting, hence your switch only has to handle low current high voltage.
For 800W, I would say it is better to use a full bridge or at least half bridge topology if you need output voltage regulation. If regulation is not needed, a push pull is also not a bad idea.
For bridge topology, you need a pair or two pairs of 400V or higher, 5A or higher switches. For push pull, you need a pair of 800V or higher, 5A or higher switches.
I personally will go with 600V 8A+ CoolMOS or 900V 8A+ CoolMOS from Infeneon, or if budget allows, 650V 80mR SiC from Wolfspeed or 1200V 80mR SiC from Wolfspeed.

I guess that depends on requirements such as cost effectivity, design time and efficiency. IMO Full bridge requires more time to design and is more expensive in production but more efficient. I think in most cases here a half-bridge would be more optimal.