How to parallel 2n3055 transistors?

I am working on an inverter that will be run off a car battery. It will use a ne556 timer to drive the 2n3055 transistors (50-60Hz) I will have in parallel leading to my centre tapped step up transformer which will be a modified microwave oven transformer.
My car battery is a 12V 65Ah battery that I will intend to draw about 200Amps from, the car battery will only be run through the transistor and transformer part of the circuit, the driver I will run off a much lesser power source. 
Altogether I will be using 50 2n3055 transistors, two groups of 25 for either side of the transformer this will have a potential of 2875 Watts.
How can I parallel these transistors to balance them to avoid cascade failure?

sort by: active | newest | oldest
Ah, it looks like you are making the mistake of assuming that the devices have to dissipate what the load does. In the case of the PWM driven Mosfet, that isn't true. Look at the figure "Rds(on)" - the resistance of the device fully turned on. For big devices thats <<0.1Ohms. Multiply THAT by the device current to get the dissipation.

BTW Potential is measured in volts, not watts.
The MadScientist (author)  steveastrouk6 years ago
Might you be able to simplify that I'm not to good with the specifics?
What do I have to do to figure out the minimum amount of MOSFET's I have to use with my power source?
I think I will have to scratch the 2n3055 idea so what MOSFET do you think is best the IRFP260 or the IRFP054 (I may not be able to get that one) I am unsure if the IRFP054 will work the pins seem to thin to handle to current?
My personal preference would be a nice classic TO3 can - they are simple to mount with substantial heatsinks. Most people will use TO3P, which is pretty good too.

You state the maximum current - I am assuming that's the peak, not mean current, I assume a worst case duty cycle of 50%, and the device SHOULD be able, in perfect conditions to handle 80 amps - you say 300A, so 300 / 80 = 4 devices. Personally, I'd look at 5.

You MUST HAVE GOOD, NO, EXCELLENT HEATSINKING TO DO THIS.

SQUARE wave driving of a transformer is very bad for a sine designed transformer. It WILL saturate if you are not very careful, causing the inputr curretn to tend towards infinity.... dead transistor territory.

Normally, transformers designed for driving like that have more core than you would expect.
The MadScientist (author)  steveastrouk6 years ago
What if I were to double the turns on both the secondary and primary?
Which MOSFET are you referring to neither of them have the continuous drain current of 80Amps?
I am assuming the IRFP054 if so will 5-6 of them be up to the job despite the 230W power dissipation leaving you with only 1150-1380Watts of power to dissipate?
How relevant is the power dissipation in watts when looking for how many you will require, the person who made this indestructible used the power dissipation to find out how many transistors to use not amps?
They don't NEED a CONTINUOUS rating of 80A, because, as I explained, I am assuming a worst case duty cycle of ~50%.

The instructable you note uses BJT not Mosfet. The power handling (NOT dissipation) is set by the transformer more than the transistors remember. Peak transformer current is one of the key parameters of the design, and the Instructable makes due allowance for that, but then doesn't add emitter degeneration to the comment to add a transistor for every - a sure fire way to kill the output stage.
The MadScientist (author)  steveastrouk6 years ago
I have found the IRFP064 MOSFET how many of these do you think I will need in total? 

I can't see the pins being able to handle any more than 30Amps, should this be a concern?

And with the transformer to avoid saturation could I take off the bases of two MOT's then connect the cores together (creating a really long transformer) with two original primary coils and double the new primary turns?
You should be OK with them. Make sure you start your design with a current limiting PSU powering everything.
The MadScientist (author)  steveastrouk6 years ago
How do you suggest I current limit it?

What do you think of that MOT idea?

I am thinking about 20 IRFP064's in total depending on the price I really don't want the pins to melt.
I suggest you start the thing off very very gently, and put some big -a resistors in a tank of water to feed it. Coils of resistance wire in water make a great load bank.

The pins WON'T melt, because a lot of the heat passes into the package.

Steve
The MadScientist (author)  steveastrouk6 years ago
Shall I just start it off with a different power source (I have 8 lantern batteries)?
You need to limit the current really to begin with - even lantern batteries can supply one heck of a short circuit current.

Steve
The MadScientist (author)  steveastrouk6 years ago
For current limiting will I always have to current limit it a bit or only in the experimental/ testing stage? Also how many ohms?

And what do I need to parallel 10 of these MOSFET's seeing as you seem to have found they require it?
No, just for the experiment. Keep it to a maximum current in your devices = to the contrinuous rating, or possibly s shade less.

You ONLY need a good gate drive to parallel mosfets.

Steve
The MadScientist (author)  steveastrouk6 years ago
I have checked my local electronic store and unfortunately the IRFP064 MOSFET's are $12 due to their uncommon use they are not bought in large amounts therefore expensive.

The 75N06 seems to be a cheap 70Amp MOSFET, it's has a 215W power dissipation but is in the TO-220AB case will in total 28 of these MOSFET's be able to handle the power passing through them?

Might a transistor be able to drive the MOSFET, if not I will probably get a driver?
iceng6 years ago
I have looked Till Blue In The Face for TO-3 high current FETs,
TO-3 devices do Not go as high as TO-220 styles now provide.

 Here is a real IR contender 30V 170A @ 100ºC

Sorry I didn't see if its available or the price.
But you only need 2 x 2 devices
Im exhausted someone else please dogleg this one.

A
The MadScientist (author)  iceng6 years ago
I am sorry but the local electronic store I go to I know doesn't have any MOSFET's above the rating of 75Amps. Also I am not to confident that the pins can actually take the current it's rated for. However I have found that they have the IRFP064 which looks to be a level up to the IRFP054 MOSFET you suggested it looks a lot more promising than the IRFP260.
And even more promising is the IRL3713 30V, 170A @ 100ºC

So I Buy parts and have them shipped to my lab all the time.

The 400LB Gorilla is only two of the IRL3713 are used per  leg.

This your best option.

A
The MadScientist (author)  iceng6 years ago
I din't think I will be able to get that one but I'll check tomorrow.
Nothing can overcome the mind !
Paralleling BJTs is done by adding a low value resistor in the emitter lead, known as an "Emitter degeneration resistor", its effect is to reduce Vbe as the current through the device increases. In a parallel system, it prevents "current stealing", where one transistor taking slightly more current than another gets warmer than the other, so the current it takes increases, so it gets warmer......

The value of the resistor is usally less than an ohm.

In MOSFETs, many people say they are "easy to parallel", yes, but only in full gate saturation - ie in PWM mode. In a linear mode, they ALSO need paralleling resistors !!! I found that out expensively.
The MadScientist (author)  steveastrouk6 years ago
I have looked at IRFP260 MOSFET's and will use them if I must but they are a lot more expensive if I were to use them I would have a total of 20 with 10 in parallel either side giving me a potential of 2800 Watts. If I do chose this option how would I parallel them?

Does the ne556 timer part sound okay?
As a switching driver ? Well, it will generate a square wave I suppose, but not a very good one, and CERTAINLY not a device with an output stage to drive the Mosfets properly. YOu have to really whack the Mosfet gate with a high volt, low resistance source to saturate the gate quickly so that it turns from hard off to on very quickly - if you don't you will kill your mosfet.

46 A is a continuous rating. If you switch at a 50% duty cycle, you can probably run nearer 90 A.

The absolute maximum, in perfect conditions is over 180 A., but thats with a very very narrow on pulse.

IF you do it right and saturate/desaturate the gate properly, you just parallel the devices. You don't need source degeneration resistors.

Steve
The MadScientist (author)  steveastrouk6 years ago
When you say whack the gate with a high volt do you mean just a higher voltage say 24V if so I can just use a regulator in the circuit?
I can't condense years of experience in power electronics into a couple of sentences, but Mosfet gates are capacitive, so you have to charge and discharge that capacitor, that means you need gate drives with high current, and low resistance characteristics. Without a doubt, the BEST way to drive a gate is with a specialist drive circuit - International Rectifier, Linear Technology et al specialise in those chips. They are not that expensive, and for the best results I suggest you use them,
The MadScientist (author)  steveastrouk6 years ago
I just read a bit on that but with the MOSFET's in parallel the capacitance increases to a point where the driver won't work properly how can you resolve this?
You do realize 2800 Watts from 12VDC means you will draw over 230 amps
from your battery at full load.
That's less then a half hour on a fully charged battery.

A NE556 is a dual NE555 in one package.

The IRFP260 can deliver continuous 46 A at room temp
or 29 A at a case temp of 100ºC
If you heat sink them only need 8 x 2 IRFP260 devices

Or IRFP054 is 60V 64A @ 100ºC for $7 only need 4 x 2 devices.

A
Someones transformer is going to be saturated methinks.
The MadScientist (author)  steveastrouk6 years ago
Even a MOT?
What if I put it in oil?
Saturation is a magnetic effect in the core. It happens when you have too many ampere turns on the transformer.

Steve
The MadScientist (author)  iceng6 years ago
A potential of 2800 Watts I don't want to underestimate my battery and fry all of them.
I am suggesting 10 IRFP260's because of the above reason.
Thanks for the MOSFET I will see if I can get that one, but the maximum power dissipation is only 230W which is 50W under the IRFP260 will it physically be able to handle the current passing through it?
iceng6 years ago
Hung up on 2N3055 BJTs you are.
The MadScientist (author)  iceng6 years ago
Not really I will use the IRFP260 MOSFET's if the 2n3055's are a bad idea.
76543216 years ago
3kW... That sounds dangerous... lol
I wouldn';t use 2N3055s. For one thing, 50 of them in a TO-3 case will be huge!
Also, each one woudl require a decent amount of base current to drive it to saturation. I think using MOSFETS would be a better approach.

MOSFETS don't have the need for the balancing resistors in series, as when one part carries more current, it will heat up more and its on resistance will increase which will in turn limit the current. They don't have the same thermal runaway issue that BJTs do.

If I were you, I would research MOSFETS for this application. I doubt that any modern 12V to 120VAC inverters use BJTs in the power electronics portion.

Also, why are you using a microwave oven transformer? I guess I assumed that you were needing at 120VAC output, but are you actually wanting a high voltage output?
I have looked at the IRFP260 MOSFET but they will cost more than the 2n3055 if I have to I will use the MOSFET's. How has this guy paralleled his transistors.
My microwave transformer is modified I will leave its primary winding and wind on a new coil for a new centre tapped primary to step up to 240-260V.
Re-design6 years ago
Like this.

But... putting 25 in parallel to get the current needed is not a good idea.  Any weak trans or failure may cause a collapse of the entire string even with the resistors.

Did you look for a more robust part?
The MadScientist (author)  Re-design6 years ago
Yes something like that. I have looked at MOSFET's and I found the best one I could get to be a IRFP260 but the cash to amp ratio is a lot better with 2n3055's. What has this guy done.