How to calculate amount of fets for inverter output

[joelyoung]

Hello everyone I need to know how to calculate the amount of fets I need to build a specific wattage inverter at a specific Ac volt output. Eg I want to build 500 watt inverter using h Bridge topology I want to use IRFZ 44 how much would I need and how do I calculate this . Thank u

 

[D.A.(Tony)Stewart]

One needs to know;

1. Input DC voltage range
2. Output avg. AC voltage range
3. Power Factor of load and peak current demand of load ( e.g. if nonlinear rectified AC to DC, peak current can be 3x~10x Avg. ) or motor surge can be 5~8x Average )
4. Thermal resistance of your mechanical heatsink design Rca=1 to 3 deg/w if forced air cooled which limits the peak current and duty cycle of bridge.
5. After you compute Iavg and I peak* duty cycle to can estimate Irms since heat rise is instantaneous I²RdsOn per device and heatsink will share power from all devices.
6. Don't forget to add Rjc=1'C/W for this part number for each device to heatsink) to compute Rja*I²*Rds*%d.c=temperature rise from each part acting alone
7. Then compute total temperature rise and resulting junction temp Tj of all parts on same heatsink. 85'C max Tj worst case for good design. 105'C max worst case Tj for poor design)
8. You might also consider a safe thermal current limiter and safety factor of >2 for conduction loss budget as overload on ferrite can cause saturation , low L and fast rising I which can burn up ferrite and bridge.

 

[chuckey]

You do not say what sort of inverter you are going to build. The IRFZ 44, seems to have a 20V and 49->35 A current rating dependent on temperature. So if your supply is 12 V, and you allow 10% loss in the transformer, you need to switch 550W @12V ~45 A, which the FET will do if kept cool. If you can get your inverter to work at 18V, then the current will be only 30A which would be well within its ratings. If you are stuck with 12V then it would be better to use a FET with a higher current rating, so it would work with a "flat" battery of 10.8V.
Frank

 

[joelyoung]

Thank u for ur support I apprecite this very much because I have been asking around for some time now but not getting an answer. I dont fully understand all ur terms used but as I go long with the post I hope to understand better

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thank u my friend for ur answer which I apprecite much.I just gve a suggestion on using IRFZ44 as a example for my question . If I got the concept on this I would apply to my design. I am really using like 8 IRFP150 in h bridge configuration and supplying with 48v battery bank and having a step up transformer to give me 120AC. It is working but I dont know how much watts it can handle. Ok for example we are building an inverter using 4 IRFZ44 in bridge u are saying it would be better or safer to run the fets at about just over half of the max current rating. In an h bridge it says two opposite fets work at the same time so would it still be sa 45amps it can handle at each cycle or 90 amps. Ok in my inverter the fets are IRFP150n which are rated at 44amps and I am using 8 in the h bridge please calculate and tell me how much watts would be safe for this inverter to handle using 48v . The fets are raated at 100v I await ur reply Thank u

 

[joelyoung]

Hello I sent u a reply did u receive it could someone help me further on understanding this concept thank you