MOSFET parallel calculation

maicael · Jul 18, 2015

OK am confused all of a sudden.
I want to handle max of 1500w in push-pull configuration and I have decided to use irfp260n which has parameters
Vdss = 200v, Rds = 0.04ohms and Id= 50 amp. Power dissipation at 25°C = 300w.
Ok I am using a 24v supply input.
Now how many mosfets do I need on each side to effectively with minimal losses power up to 1500w full power.
Also I want to wind the center tapped transformer do wind a 2000w transformer or more for 1500w full power to give enough head room to ensure maximum performance.

crutschow · Jul 18, 2015

Can't determine that without knowing the load resistance.

alec_t · Jul 18, 2015

What is the transformer for? Are you building an inverter? If so, what output voltage?

audioguru · Jul 18, 2015

For an output of 1500W just think how big the battery and its wiring must be. If it draws 2000W from the 24V battery then its current is 83.3A! Its heat produced will be 500W.

The details of the Mosfet show that its can dissipate 300W only when its case is cooled to 25 degrees C with dry ice or something since with a huge heatsink and fan it will still get much hotter than 25 degrees C then it will melt.
Its thermal resistance is shown on its datasheet. You probably do not want to operate the Mosfets at their maximum allowed internal temperature anyway.

maicael · Jul 18, 2015

alec_t said:
What is the transformer for? Are you building an inverter? If so, what output voltage?

Yes am building an inverter and output voltage is 220v

maicael · Jul 18, 2015

audioguru said:
For an output of 1500W just think how big the battery and its wiring must be. If it draws 2000W from the 24V battery then its current is 83.3A! Its heat produced will be 500W.

The details of the Mosfet show that its can dissipate 300W only when its case is cooled to 25 degrees C with dry ice or something since with a huge heatsink and fan it will still get much hotter than 25 degrees C then it will melt.
Its thermal resistance is shown on its datasheet. You probably do not want to operate the Mosfets at their maximum allowed internal temperature anyway.

Hello, I am not drawing 2000w from the inverter I am only saying will it make sense to wind a transformer capable of delivering 2000w but using it for 1500w.
I want to leave enough room so that the transformer performs well.
And yes I do not want to operate the MOSFETs at anywhere near their maximum as I am trying to get efficiency as high as I can possibly get so i don't mind more MOSFETs connected but I am trying to figure out how many will be enough for optimum performance.

audioguru · Jul 18, 2015

If the power output is 1500W and the heat produced by the Mosfets and rest of the circuit is 500W then 2000W must be delivered by the battery.
Since this is your first high power inverter then the heat might be 1000W and then the battery must produce 2500W (2500W/24V= 104A!).

The more a Mosfet heats then the more resistance it has that makes it even hotter then its resistance goes even higher. Therefore efficiency is highest when the Mosfets each conduct the least amount of current. Use as many Mosfets as you can afford.

chemelec · Jul 18, 2015

If you can keep your Mosfets Relatively COOL, than you can probably get away with just 4 of them. ( 2 on each side )
But I would recommend 6 in total. ( 3 on each side )

It All depends on How good you can keep them Cool with a LARGE HEATSINK and FANS.
Actually One Large Heat Sink on EACH SIDE is Better, so you avoid using Insulating Washers on the Mosfets.

Also What is the Ambient Air Temperature in NIGERIA?
You Can't cool colder than the Ambient Temp.

I sure Don't want to be the one Winding that Transformer.
PRETTY BIG WIRE.

ronsimpson · Jul 19, 2015

maicael said:
I want to handle max of 1500w in push-pull configuration and I have decided to use irfp260n which has parameters
Vdss = 200v, Rds = 0.04ohms and Id= 50 amp. Power dissipation at 25°C = 300w.
Ok I am using a 24v supply input.

1500watts=24V x 60A if you use DC numbers.
>>You want AC out with a average of 1500W. There are times in a cycle where there is no power output. The peak output is 2100 watts.
2100w=24V x 87A
>>Usually there is margin designed in. So plan on 100A. (low battery, short on load, etc)
>>You need two MOSFETs in parallel, just for the "Id=50 amp"
Two MOSFETs in parallel = 0.02 ohms. 100A = 2V across the part. 2V x 100A = 200 watts peak.
>> 1700 watts in 1500 watts out (more or less) so about 22% of the energy is lost in the MOSFETs.

I think you need to use 4 MOSFETs in parallel not one. 100 watts lost in the MOSFETs.
The numbers are not good because I don't know many things. What type of inverter, etc.

maicael · Jul 19, 2015

chemelec said:
Also What is the Ambient Air Temperature in NIGERIA?
You Can't cool colder than the Ambient Temperature

Just checked and where I stay its about 25°C to 30°C. Could be slightly higher but those were the figures yesterday

Diver300 · Jul 19, 2015

A sinewave has a peak voltage that is √2 time larger than the RMS voltage. With most loads, that means that the peak power is twice the average power. If you have a 1500 W load, the peak current is 125 A from 24 V even at 100% efficiency.

maicael · Jul 19, 2015

ronsimpson said:
1500watts=24V x 60A if you use DC numbers.
>>You want AC out with a average of 1500W. There are times in a cycle where there is no power output. The peak output is 2100 watts.
2100w=24V x 87A
>>Usually there is margin designed in. So plan on 100A. (low battery, short on load, etc)
>>You need two MOSFETs in parallel, just for the "Id=50 amp"
Two MOSFETs in parallel = 0.02 ohms. 100A = 2V across the part. 2V x 100A = 200 watts peak.
>> 1700 watts in 1500 watts out (more or less) so about 22% of the energy is lost in the MOSFETs.

I think you need to use 4 MOSFETs in parallel not one. 100 watts lost in the MOSFETs.
The numbers are not good because I don't know many things. What type of inverter, etc.

Its a Modified sine wave type.
Been learning some electronics of late.
A sine wave is hard to build whatever topology used.
Come to think of it if i pulsed the transformer with PWM pulses at high frequency can I get a sine wave using same push-pull topology.
Like I said have been experimenting a lot. So apart from wanting to have a good inverter to back me up during power outage I intend to try experenting on a sine wave using this same topology(push pull) just and other topologies later for learning and experience as am not as good as you guys.

Les Jones · Jul 19, 2015

Most commercial inverters I have seen do not use a large 50/60 hZ transformer. They start with a high frequency inverter and step the voltage up through a high frequency transformer (Which is much smaller, lighter and cheaper than one operating at 50/60 hZ) This is then rectified to give about 340 volts DC (For 240 V AC output 240 x the square root of 2.) This is then converted to the modified sine wave.

Les.

chemelec · Jul 19, 2015

Les Jones said:
Most commercial inverters I have seen do not use a large 50/60 hZ transformer. They start with a high frequency inverter and step the voltage up through a high frequency transformer (Which is much smaller, lighter and cheaper than one operating at 50/60 hZ) This is then rectified to give about 340 volts DC (For 240 V AC output 240 x the square root of 2.) This is then converted to the modified sine wave.

Les.

TRUE, But Unfortunately these types are NOT very practical for Home Construction.

Personally I would just Recommend that Maicael just BUY one of these Commercial made Inverters.

audioguru · Jul 19, 2015

But will Canadian Tire or Walmart ship a cheap inverter to Nigeria?

Before the National Holiday, in each parking lot there was placed a huge bin labelled, "FIREWORKS!". The bin had electric lights, a cash register and maybe an air conditioner. They did not use a huge battery and inverter, instead they used a gasoline fuelled generator.

Electric cars do not use a huge and heavy 12V battery delivering hundreds of Amps. Instead they use a huge and heavy high voltage battery delivering 10 or 20 Amps. Then the wiring does not need to be huge and heavy.

chemelec · Jul 19, 2015

audioguru said:
But will Canadian Tire or Walmart ship a cheap inverter to Nigeria?

MANY Companies all over the world Sell these Inverters.
Just Google it on the Internet.

Or go to EBAY, and see ones like This:
**broken link removed**

maicael · Jul 22, 2015

OK I know this may sound Like a silly question but I just need to satisfy curiosity.
The pics below the one with caps C1 and C2, does it make any sense? Assuming the feta are pulsed with either low or high frequency.( curiosity).
The other pic with diodes,
Now I know the auto transformer action stuff about swinging double the supply voltage but some folks argue that the diodes protect from surges and back emf but I also know they prevent double the supply voltage full swing so what does one do here?
Does the diodes get replaced and with what?
I mean ultimately we want to protect our switching devices and treat them like babies cos they are getting more expensive even if it is by a single digit increase.

maicael · Jul 22, 2015

Also pls what is magnetic bias correction circuit.I hear that in push pull topology I think?.

Tony Stewart · Jul 23, 2015

I think the MOSFET chosen would have a Zener protection > 2x Vout rather than using Diode clamps to both rails.

analog1 · Jul 24, 2015

You better reconsider any devices in parallel at hi amp rating is a really bad idea. Specs are written for typical device there is not golden rule or assurance. It just like putting batteries in parallel if one is weak or stronger they will share the load no way the good battery will try to stabilize the others in other words the good will overwork until it dies. Same idea with tranx, fets, whatever in parallel. There ways to equalize them but at a power loss. Your signal better have space of inactivity or blow up. You need some zener across the gates, When the magnetic collapses there is a lot of un wanted spikes all over.

MOSFET parallel calculation

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