Output MOSFETs getting hot.

[Ashleyzn]

Hello guys. I found this forum to be very informative so I decided to join.
I'm in need of some help, I'm currently working on a star sound amplifier, I replaced all the output MOSFETs together with the drivers ir21844s.
The MOSFETs are getting very hot without any load. Can someone please help on what to check for. Thanks in advance.

 

[JonSnell Electronic]

If class D, check the dead time width. If class AB or B check the bias current.

 

[Ashleyzn]

It is class D . You mean check dead time width on pin 4 of ir21844s ,if so what should it be and if it's not correct what could be the cause of the.

 

[Perry Babin]

Where did you buy the ICs and the FETs from?

What FETs are they using in this amp?

Did you use exact replacements?

Post a photo of the driver board and the entire output section. Use the 'go advanced' and 'manage attachments' feature of the forum to post the photos.

 

[Ashleyzn]

The MOSFETs are irfp360lc they are original replacements bought them from mantech electronics.

 

[Perry Babin]

Did you buy the ICs from them as well?

Are they an autorized distributor?

Did the ICs arrive sealed in a pack with desiccant and a moisture monitor card?

Are all of the FETs heating equally?

Do you have any experience repairing amps?

Have you repaired any of the 21844 based amps before?

 

[Ashleyzn]

All the parts came in antistatic packets, Always buy from this company they sell genuine parts never had a problem before. I repaired one amplifier like this before with blown power supply fets and output fets replaced drivers ICs and the amp works perfect.
All the fets are not heating equally.

 

[Ashleyzn]

Do you think I need to replace the tl072 opamp and the lm311comperator.

 

[Perry Babin]

Each IC drives two groups of FETs. Are the FETs that are heating all on one IC or on one of the two halves of any one IC?

Do all of the output FETs have the same date code?

Anti-static packaging isn't the same as a sealed package. See attached.

 

[Ashleyzn]

It did not come in sealed packaging like that.

I'm gonna have to re visit the heating and get back to you.
All the fets do have the same code.

 

[Ashleyzn]

From what I noticed they all get hot , some get hot faster than the others.
I will check again and tell you exactly which ones get hot.

 

[Ashleyzn]

Hi Perry. I checked again there are two sets of 6 which are getting extremely hot and the others get warm. Are you suspecting It could be faulty ICs.

 

[Perry Babin]

I'm trying to find some pattern that could lead t othe fault, if there is one. How fast do they get too hot to touch?

Do they heat up at idle? (I don't know how you're testing)

Mark the ones getting hot in a photo.

 

[Ashleyzn]

One IC drives 6 fets three high and three low. I'm tempted to remove the ICs that drives that two sets and see if the others are still getting warm.
What do you advise.

 

[Ashleyzn]

The ones that are getting to hot to touch within a minute just by powering the amplifier are the ones in the third picture that I sent previously are bottom right 6 and top left 6.

 

[Perry Babin]

OK. That's likely 2 of the 4 ICs. Compare the readings on all of the ICs. Do the two driving the heating FETs read differently than the other two. Use the attached diagram.

Discharge the rail caps or let them discharge before testing.

 

[Ashleyzn]

Thanks Perry will check when I get a chance. That's some good information you sent there I really appreciate it.

 

[Perry Babin]

Something that may help is to make a probe similar to the one attached. IT's a dental probe, insulated to very near the tip and so thin that you can't short between pins if you hold it perpendicular to the body of the IC.

 

[Ashleyzn]

Hi Perry. I tested all the ICs they all read similar. They don't seem to be faulty.

 

[Perry Babin]

The readings should have been very nearly identical.