# Diy Induction heater- mosfet getting hot

#### rahul411

Joined Feb 19, 2018
260
Im making a diy induction heater but the problem Is despite making things hot on its business end coil mosfets are getting heated up with a huge voltage drop. I have checked for any short in the circuit haven't found any.

Circuit is

Waveform at the gate terminal ( both similar)

I' using 12v and the voltage drops to about 4.6v with a current of 0.7A.

This circuit uses zero voltage switching to oscillate, i've read that. Also author says that the gate resistor should be small but not to small that when mosfet is on and other off by pulling down one resistor it should not create a short in power supply. I think that is what happening here.

#### mvas

Joined Jun 19, 2017
539
Is it possible that Q1 and Q2 are ON at the same time?
The MOSFETs may be in Linear mode - Gate turn off is slow.
What is the peak positive voltage and peak negative voltage on the Gates?

I do not understand ...
"... I'm using 12v and the voltage drops to about 4.6v with a current of 0.7A .."

Are you saying ...
a) Vcc = 12 Volts
b) the Output Voltage across pins P3 and P4 is 4.6 Volts at 0.7 amps?

Why are you using this circuit, and not a standard PWM controlled circuit ?

Last edited:

#### rahul411

Joined Feb 19, 2018
260
Is it possible that Q1 and Q2 are ON at the same time?
The MOSFETs may be in Linear mode - Gate turn off is slow.
Yes may be i haven't probed both the gates at a same time. So there could be a possibility.

I do not understand ...
"... I'm using 12v and the voltage drops to about 4.6v with a current of 0.7A .."

Are you saying ...
a) Vcc = 12 Volts
b) the Output Voltage across pins P3 and P4 is 4.6 Volts at 0.7 amps?
Yes, vcc 12v. And when i connect circuit to supply the 12v vcc drops to 4.6v.

Why are you using this circuit, and not a standard PWM circuit
Because it works for many and is easy.

#### Alec_t

Joined Sep 17, 2013
12,456
With Vcc=12V your FET gates are biased at 6V (and the zener diodes ensure they would be clamped at 6.8V anyway). However, the IR540 is not a logic level type and needs about 10V Vgs to turn on fully. Not surprising it gets hot! .

#### rahul411

Joined Feb 19, 2018
260
With Vcc=12V your FET gates are biased at 6V (and the zener diodes ensure they would be clamped at 6.8V anyway). However, the IR540 is not a logic level type and needs about 10V Vgs to turn on fully. Not surprising it gets hot! .
So what you suggest about getting this on track?
Increasing vcc will help?
And zener used are 10v zener.
And what about the voltage drop, why that's happening? Because of fet's?

#### DickCappels

Joined Aug 21, 2008
8,157
What are 1N4007 diodes doing in the position of switching diodes in that circuit?? They are very, very slow to recover.

#### rahul411

Joined Feb 19, 2018
260
Little bit of update in the circuit i posted.
Gate pull downs are 10k not 220ohms.

#### rahul411

Joined Feb 19, 2018
260
What are 1N4007 diodes doing in the position of switching diodes in that circuit?? They are very, very slow to recover.
Yes i will be replacing that with 1n5819 or 1n4148 tommorow in college.

#### BobaMosfet

Joined Jul 1, 2009
1,917
I would think, if the MosFETs are handling a significant amount of current (being an induction heater), that they would get 'warm'. In electronics it isn't really 'hot' until it's above 100C - Look at the data sheet regarding thermal temperature based on current through drain-source. Plus there is a spike on the inductor if power is removed (for a pulse), so I'd put snubber diodes around the inductors, as well.

#### rahul411

Joined Feb 19, 2018
260
Have replaced the 1n4007 with 1n4148.
Added the flyback diodes across inductors.
Still there is a significant voltage drop and mosfets getting heated.
Any word on this?

#### rahul411

Joined Feb 19, 2018
260
I would think, if the MosFETs are handling a significant amount of current (being an induction heater), that they would get 'warm'. In electronics it isn't really 'hot' until it's above 100C - Look at the data sheet regarding thermal temperature based on current through drain-source.
Yes, when load is connected mosfet heating is normal but without load the temperature is so high that i can barely touch for 2 seconds and all this happens instantaneously as the supply is connected.

#### BR-549

Joined Sep 22, 2013
4,938
Can your power supply, supply the needed current?

#### rahul411

Joined Feb 19, 2018
260
Can your power supply, supply the needed current?
The power supply im using us rated to output 3A per channel and i have connected both channel in series so maximum of 6A... Still there is considerable voltage drop and no heating on business end.

#### rahul411

Joined Feb 19, 2018
260
Also the OL (overload) indicator also turns on after connecting. So i dont understand how much current this thing even require. Like 6Amps isn't enough?

#### BR-549

Joined Sep 22, 2013
4,938
What are the heating elements power rated at?

#### BR-549

Joined Sep 22, 2013
4,938
Try it with a car battery........AFTER you are sure that you are cleanly switching.

#### DickCappels

Joined Aug 21, 2008
8,157
Also the OL (overload) indicator also turns on after connecting. So i dont understand how much current this thing even require. Like 6Amps isn't enough?
In the prior post you said that the two channels are in series. If that is true then the maximum current is the most a single channel can supply (3A). If they are in parallel then you could get twice the current if they shared the load nicely (without shutting down or oscillating).

#### rahul411

Joined Feb 19, 2018
260
Try it with a car battery........AFTER you are sure that you are cleanly switching.
I will try to get hands on car battery.

#### rahul411

Joined Feb 19, 2018
260
In the prior post you said that the two channels are in series. If that is true then the maximum current is the most a single channel can supply (3A). If they are in parallel then you could get twice the current if they shared the load nicely (without shutting down or oscillating).
I thought current would also add up after connecting in series.

#### rahul411

Joined Feb 19, 2018
260
Results after connecting both channels in parallel

Same voltage drop and now only one fet is heating... I think other one is got blew up.

Thought this would be a easy project but everything went south