mosfet transistors

[transapax]

can u guys try and explain the fillowing to me. In choosing  a power mosfet for an inverter circuit;
1) what  are the minimum power mosfet vds one should choose in a 12volts as well as a 24volts battery application. someone said, for a 12v application, the vds is 30v and for 24v application, it is 60v.

2) if i connect a high power bridge rectifier across the drains of the transformer driving mosfets, and connect the positive side of the rectifier to the battery, will this help in greatly suppresing the the back emf that would have destroyed the mosfets. what are the disadvantages. Do you have a better way of doing this for a higher power inverter (say a 3kva type)?

transapax.

[indulis]

It depends on the topology that you are using... if the primary is push-pull, then the drain voltages on the MOSFET's will be 2x Vin plus any spikes from cross conduction or leakage inductance. How high these spike will be is hard to say, there are many factors to consider.

The "clamp" that your talking about is usually implemented with a zener and fast diode connected back-to-back... cathode of zener connected to the drain, the zeners anode would connect to the fast diodes anode and the fast diodes cathode would be connected to the "bulk input capacitor" / battery.

The zener value would depend on the FET's voltage rating. For a 12V system, you'd have a minimum of 24V plus spikes. A 30V FET isn't enough... the clamps/snubbers would take a beating. If it was me, I'd probably use a 60V FET and a 35V zener. I'd also use some RC/RDC snubbers. they can be connected from the drain to the "bulk input cap" or the source (gnd). It is very important to keep the snubber components very close to the FET and the connections VERY short!!

[transapax]

thanks indulis for your explanation,

  ante, can you please explain to me why the clamp consisting of a 62v zener and a 1n4002 diode for the 1.2kw power house inverter you posted was connected to the gate of the power mosfets rather than a bulk input capacitor/ battery as explained by indulis previously. how does the back emf suppression technique of connecting the  bridge rectifier straight to the battery rather than through the 4 mosfets affect the inverter effeciency?

secondly, since irf540 mosfets have  low current but high vds ratings, can anyone conversant with mosfets advice me on which popular high current high vds mosfets to use to replace the low current irf540. can irfz44n/ irf3205 be suitable for a 24v application?

thirdly, how do you the determine the value  of components for snubber circuit for different sizes of inverter circuits. say if you want to build a 3kva inverter.

lastly, if one wants to build  1kva continous power inverter with a surge power of say 2kva, what would be the safest wattage of the ouput transformer to use in this circumstace.