I'm building a inductive coupling wireless charger. Since for the induction its required a time varying magnetic field I'm using IRF 3205 mosfets driven by a function generator. The supplied voltage is 24V. Current is 4A. I connected 10 mosfets series and parallel as well. But the problem is those are getting heat and burning. I cant figure out why. Can you help me....

 

Schematic please.
What is the drive voltage to the FET gates?

 

Schematic please.
What is the drive voltage to the FET gates?

0-5Vp-p

 

That is not enough voltage to fully turn on the FETs. See the extract from the datasheet below. RDSon is specified at a gate voltage of 10V and with 4V on the gate the FET might be just beginning to turn on. You needd to aim for at least 10V on the gate.

 

That is not enough voltage to fully turn on the FETs. See the extract from the datasheet below. RDSon is specified at a gate voltage of 10V and with 4V on the gate the FET might be just beginning to turn on. You needd to aim for at least 10V on the gate.
View attachment 155645

The function generator is capable of supplying 20V. I tried upto 20V. But still the issue is there.

 

Hello,

I connected 10 mosfets series and parallel as well.

Can you provide a schematic, as I do not see the parallel / series connections?

Bertus

 

Hello,


Can you provide a schematic, as I do not see the parallel / series connections?

Bertus

Of course. I attached here with.

 

Hello,

In your posts you are talking about 10 mosfets.
In the posted schematic you are using 4 mosfets.
Can you post a COMPLETE schematic?
Also how the mosfets are driven.

Bertus

 

A function generator may well not be able to source enough current for driving one FET gate directly, yet alone several gates in parallel, at a speed sufficient to prevent FET heating. Gate-driver ICs are available.

 

When it's about an induction, I lack information about a free running diode over the coil.

 

What waveform and frequency?

Bob

 

One of those FETs is entirely adequate for 4 amperes, even without a heatsink, provided the switching losses are not too great, it is being driven properly and is not having the energy of the coil dumped back into it. Connecting a bunch in parallel is just making things more difficult and can result in high frequency oscillations that greatly increase losses. It also places huge demand on the gate driver, as has been mentioned by others.

What is the switching frequency and duty cycle?
How is the coil constructed? What is it's inductance?
How do you know the current required is 4 A? Average? Peak? RMS?
How much of the energy put into the coil is being transferred to the secondary?

Since there is no freewheeling diode, according to the schematic, any energy that is not transferred will be dumped back into the body diode of the FET when the FET switches off. This cause the body diode to avalanche, which is permissible if the energy is within limits, but will greatly heat the FET if the frequency is sufficiently high.