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Fet heating up unexpectedly

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Enzy

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What I did was I built a 12vdc power supply and used it to power the arduino and I have a truck battery, the connections other wise are the same but the fets get hot in seconds and I release that the bigger the transformer the more current they pull.

Even if I don't connect the gate of the fets and just apply dc power to center tap and source the fets get hot.
My last attempt was today I bought a 1/2amp transformer and without gate connections it was pulling 1.4 amps for the battery. When I apply oscillation to the gate I get the correct voltage and frequency and duty cycle. If I decide to step up the transformer size to 3 amps they start pulling like 5 amps right away and I tried a 10 amp transformer and they started pulling like 15amps.

I have hundreds of Fets mostly irfz44n and all had the same issue, I tried irf510 and same issue, I tried irf3205 and same issue.

I tried tip122, tup41c and bc547 but those didn't turn on.

Any ideas on what could be causing this issue.

1505025886612-47138196.jpg
 
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Leaving the gates disconnected is asking for trouble. They consume such small current that they will 'float' to ambient static levels and quite possibly turn full on. I'm not surprised they got hot. If you want to run it with the gates disconnected from the Arduino, wire a resistor of about 10K between each gate and source pin.

The Arduino also probably can't provide enough current to drive the MOSFETS on and off quickly enough. To ensure fast switching you need to be able to drive and sink enough current to charge and discharge the gate to source capacitance without delay. This is why most designs use purpose made gate driver ICs (IR2112 for example) which can supply and sink hundreds of mA. If you can't supply/sink the current, there will be a gradual rise and fall of current in the MOSFETs and during the cross over period one will try to drive the other and cause a serious loss of efficiency.

Note that even in normal operation, you will need considerable heat sinking on those devices. It is essentially a square wave generator and relies on the losses in the transformer and the filter capacitor to 'round' it to be nearer a sine wave.

In the future, please post images directly to Edaboard rather than use an external site.

Brian.
 

Sorry about the pic, I seem to be unable to upload the way I used to so I sent a link of the pic from my cloud storage.

What if I don't have any gate driver ic, is it the same as using a buffer stage with bjt transistors and using the battery to power them to drive fets?
 

I doubt that the two FETs are really turned on hard- The gate must be driven sufficiently high to fully turn on the FET so that the voltage drop will be less. If the gate voltage is not high enough the FET will be partly on when the dissipation will be highest.

Even then you will need some form of heat sink because the FETs will dissipate some heat. BUT you should not be leaving the gate hanging: it may open or close depending on the slightest wind here or there...
 

What if I don't have any gate driver ic, is it the same as using a buffer stage with bjt transistors and using the battery to power them to drive fets?
Basically, yes, but as c_mitra points out, you need to increase the voltage as well as the current. If you can drive the gate close to 12V it will ensure it becomes fully conductive and if you provide enough current to rapidly charge the gate capacitance to 12V and discharge it back to 0V it will minimize switching losses. You should be able to find examples of gate driver circuits if you search.

Brian.
 

Hi,

You don't tell about switching frequency...
You need very close match of the duty cycles.
Add overvoltage protection very close on both Mosfets between drain and source. At least 30V working voltage.

Klaus
 

Basically, yes, but as c_mitra points out, you need to increase the voltage as well as the current. If you can drive the gate close to 12V it will ensure it becomes fully conductive

Brian.

OK so if I am using a bjt stage with bc547 and bc557 I would use the 12 to 14v from the battery and that would drive the gate of the fets while the 5v from the arduino would be driving the bjt stage.

- - - Updated - - -

Hi,

You don't tell about switching frequency...
You need very close match of the duty cycles.
Add overvoltage protection very close on both Mosfets between drain and source. At least 30V working voltage.

Klaus

The switching frequency is 50hz and the duty cycle is 50% I have never added a over voltage protection to a circuit I'll have to check up on how to do that.
 

I guess your switch freq is rather low; the transformer is getting saturated and drawing excess current. Do you have any idea about the currents drawn by the mosfets?

The simplest test is to replace the transformer primary with two 12V automotive lamps and note whether the heating (at the FET) is more or less (qualitative test).
 

Yes the frequency is low since I only need mains 50hz at the output but why would that be an issue, isnt it a norm for circuits to built with 50hz switching?

I know that you can use high frequency then use a low pass filter at the output but the signal I'm sending from the arduino is a spwm signal after filtering it should be fine to run my loads don't you think.

I'll have to source 2 automotive lights and give it a try.

- - - Updated - - -

Using a .5amp transformer it's pulling about 1.3amps from the battery so I'm assuming that the same is passing the fets. If I use a 10 amp transformer it pulls about 15amps from the battery.
 

The primary inductance needs to be high impedance at switching rate while low impedance at line f. Saturation current must be known and avoided with some margin otherwise, primary L drops , excitation current rises and core saturation gets worse.
 

What I suspect is that your original transformer was made for 220V input and 12-0-12 output (sometimes they are called 24V with CT but sometimes that are also called simply a 12V transformer) and in principle your logic is correct. Just check the original power rating of the transformer and the power it is taking now. Many of these small transformers use the DC resistance of the primary coil (the 220V side) to limit current and prevent saturation. IF THAT IS CORRECT, then you need to reduce the applied voltage (which may not be easy). With an iron core transformer, you cannot go much above 500Hz but I will be checking the transformer first.
 

I didn't mention that the 10 amp transformer was a 12-0-12v and the 1/2 amp transformer was 6-0-6v and both has a 110/220v output
 

OK so if I am using a bjt stage with bc547 and bc557 I would use the 12 to 14v from the battery and that would drive the gate of the fets while the 5v from the arduino would be driving the bjt stage...

Did you try this? Did you see any improvement?

Also try with some load on the 220V side. Does that make any difference?
 

I got a short circuit, I did it in a rush, I'll try again in the next half hour. I'll try a load also.

Do I need to sign up to upload images here these days?

- - - Updated - - -

I made a mistake the first time when I added the buffer stage, I tried again and it worked, atleast I'm getting a high frequency of 5khz and 144v and 55% duty.

- - - Updated - - -

Fets doesn't warm up as power is applied like they normally do, they stay cool
 

Hi,

Do I need to sign up to upload images here these days?
Yes. But you also need to be signed up to write a post.
--> use the icon "add image" and follow the informations

and 55% duty.
What do you mean by 55%?
One complete cycle:
* 55% ON positive
* 45% OFF
* 55% ON negative
* 45% OFF?

Klaus
 

I made a mistake the first time when I added the buffer stage, I tried again and it worked, atleast I'm getting a high frequency of 5khz and 144v and 55% duty.
Fets doesn't warm up as power is applied like they normally do, they stay cool

The mystery deepens; At 5kHz, your transformer will be getting warmed up. Just because of its mass, it may appear cool.

Can you reduce the frequency in steps (say in steps of log2) and tell us more?
 

I used my multimeter to check the output of the transformer, when its checking duty cycle I don't know if it's showing on percentage or off, my objective is to get it to 50% on and 50% off.

I would assume that my meter tests 55% per cycle.

- - - Updated - - -

The mystery deepens; At 5kHz, your transformer will be getting warmed up. Just because of its mass, it may appear cool.

Can you reduce the frequency in steps (say in steps of log2) and tell us more?

I had a code that I was working on which I'm sure produced 50hz but them I was seeking different codes because of the heating issue. So I'll go back to my code and see if there is a difference with the voltage and frequency.
 

Do not forget to keep a small dead time between the on and off states; yes, it is not going to short the power supply but you will be driving both the half-coils in the opposite way...
 

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