IGBT/MOSFET Paralleling Calculation in Inverter Design

As a younger one in electronics, I want to know from the top engineers int his EDABoard, how I can calculate the capacity of my IGBT/MOSFET switch in my Inverter designs.

Please look at the attached picture below and tell me how many of this device I can parallel in an Inverter design of 6KW (6000W)

1. FGY75N60SMD



2. FGA60N65SMD



Please read my questions well and please answer me accordingly. I wouldn't have asked if I am sure of the calculations

Another thing is:


3. If I have 15A on the AC side (Secondary Winding) of the inverter at 240V is it true that the AC output of the Inverter, that part of winding in the transformer is working at 3600W!?

4. Then is it true that this 15A at the AC winding is approximately 150A at the DC,Input, switching side (Primary Winding)!?

5. If it is truly 150A, 48V at the primary winding, does it means that the primary winding of the inverter is working at 7200W!?

Thanks in advance for your replies.

mcmsat13 said:

As a younger one in electronics, I want to know from the top engineers int his EDABoard, how I can calculate the capacity of my IGBT/MOSFET switch in my Inverter designs.

Click to expand...

1 & 2 - Are the Mosfets for the primary or secondary? If the later, my experience tells me you should use an IGBT, but read further:

As you know BJT's are not known to have nice current sharing characteristics.
An IGBT is a transistor controlled with a mosfet, for convenience and simplicity, therefore the same principle applies. You can get around this using colector sharing resistors, but this is not desirable due to additional power losses. There are however, many decent "brick" power modules capable of several amperes. The actual rating will be not less that 2x your maximum output current. So if you expect 15A output the device must be rated 30Amp or higher.

Is this for a production run or one off unit?
If this is a one off get yourself an intelligent power modules. These devices have the IGBT's, drivers and fault protection circuitry already built in and therefore the extra you pay is assured as reliability. Unless you ignore the fault signals the device should be virtually indestructible. Snubber design is also greatly simplified. I assume you know and will implement them on your design.

If this is for the low side, mosfets are the best option, again for this outputs I would advise to use a brick module, rather than small individual devices. The later can be done, but experience tells me it is not worth the effort. You can either choose to trust my experience or learn by yourself. You also want to use 100V devices, due to the lower RDS(on).

3. No. Unless your load is purely restive. Reactive loads, loads without power factor correction, etc., they all reduce the output capability. Your output will be 3600VA, assuming no filtering, etc. If unsure, look at the ratings of any computer UPS and use them as reference.

4. Assuming 24V.
No. Your load will be nearer to 200A RMS. Power devices losses, transformer losses, battery sag, wiring drops, etc. All these account significant power losses. Hardly a DIY design can get better than 70% efficiency, with mainstream commercial devices on the low 80's and high quality custom designed (expensive) sine wave inverters in the low 90's. You also need to account no load losses on the transformer. Using a thyroidal transformer will largely reduce these, but prepare the wallet.
Another option is to use a small High frequency transformer. These are somewhere in between the above, however extra losses exist due to high switching frequencies and the need to reconvert the primary from HVAC to HVDC and modulate/filter this back into a 50Hz square/sine wave.

4. Read answer 4 and 5. With that in mind the output does increase , but a different transformer, rated to 48V (and not 24) must be used, otherwise core saturation will occur

At those power levels, it is far better to use a full bridge design, with a single primary winding.
Push-pull topologies suffer from staircase saturation. Google the term.
Additionally, since only half of the primary winding is being utilized at a given time, it has a poor copper window utilization.

Other things....never tap a battery series. Cells become easily unbalanced for you to add another load.
Use a proper 48v to 12v bias supply.

Schmitt, thank you for your suggestion. Half- Bridge and Full bridge has been what I am desiring but I am always confused especially in the boosting and filtering stages! Please can you give me a circuit diagram showing clearly the boosting and filtering stage of a Half-Bridge or Full Bridge using 48v battery bank and 50hz iron core transformer. Whenever I Google this I always get designs with small high frequency transformers. Please all engineers in this room my appeal also extend to you. You can also give me a link.

- - - Updated - - -

Schmitt, thank you for your suggestion. Half- Bridge and Full bridge has been what I am desiring but I am always confused especially in the boosting and filtering stages! Please can you give me a circuit diagram showing clearly the boosting and filtering stage of a Half-Bridge or Full Bridge using 48v battery bank and 50hz iron core transformer. Whenever I Google this I always get designs with small high frequency transformers. Please all engineers in this room my appeal also extend to you. You can also give me a link. If my circuit diagram above could be modified into half or full bridge, it will help much.

Thanks all in advance.

by mcmsat13
As a younger one in electronics, I want to know from the top engineers int his EDABoard, how I can calculate the capacity of my IGBT/MOSFET switch in my Inverter designs.

Please look at the attached picture below and tell me how many of this device I can parallel in an Inverter design of 6KW (6000W)

1. FGY75N60SMD

IF each device is well heat sunk for low Rja and devices have slight positive Tempco. Or PTC then you can gang as many as required. Check specs for Vce ESR and layout ESL values and Sensitivity with temp. Ideal is lowest ESR and ESL but VCE is usually NTC. for stable parallel sharing, change in voltage drop due to thermal Rise and Rja mismatch must be considered to prevent thermal runaway from current hogging. devices with PTC are easier to gang.

3. If I have 15A on the AC side (Secondary Winding) of the inverter at 240V is it true that the AC output of the Inverter, that part of winding in the transformer is working at 3600W!?
YES but Only if I and V are in phase

4. Then is it true that this 15A at the AC winding is approximately 150A at the DC,Input, switching side (Primary Winding)!?

depends on Transformer impedance ratio or current ratio and excitation losses

5. If it is truly 150A, 48V at the primary winding, does it means that the primary winding of the inverter is working at 7200W!?

show your choices in design. Proper Ferrite or CRGO laminated steel,Transformer ought to be 98% efficient.

Thanks in advance for your replies.[/QUOTE]

Texas Instruments, Intersil and others make excellent full bridge drivers, with lots of features and protection.
Go to their websites, and you can literally download dozens of proven and validated designs, along with the proper design techniques.

A full bridge driver that works for -say- 100Khz, will work for 50Hz, far easier.
Of course, you have to substitute the low frequency ferrite transformer with a proper steel-core one.
Therefore.....You can use any off-the-shelf 48 to 230 volt transformers, with only one caveat: the total volt-seconds product of a squarewave must match that of the sinewave, 11% less, to prevent core saturation.

schmitt trigger said:

Of course, you have to substitute the low frequency ferrite transformer with a proper steel-core one.

Click to expand...

Perhaps you mean the other way round?

I believe the more correct way is:

Filter the signal trough an inductor so that only the low frequency (50Hz) is present. This allows the use of small inductors. Then use this signal to drive a steel core transformer. This method requires a micro with some high PWM, such as used in motor control applications, but is simple and efficient and commonly used on expensive sine wave inverters.