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Induction Heater driver circuit, 555 timer, 2 solid state relays, 2 MOSFETS

gary350

Well-Known Member
I need to turn 2 N channel MOSFETs on/off at 200Khz. One MOSFET has to be ON when the other one is OFF. They have to cycle on/off oposite each other.

I am thing about using a 555 time to drive 2 solid state relays and use the 2 relays to drive the 2 MOSFETs?

One solid state relay will be Normally Open, the other solid state relay will Normally Closed. The 555 timer turns both relays on/off at the same time and the MOSFETs should both go on/off oposite each other at the same time.

I am worried both MOSFETs will be ON for a microsecond at the same time then it will be a short circuit. I think it needs some type of safety device so both MOSFETs can never be on at the same time?

I think some type of delay ON for each MOSFET will work.
 
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shortbus=

Well-Known Member
Most Helpful Member
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gary350

Well-Known Member
Came out of this thread http://www.electro-tech-online.com/threads/complementary-square-waves-creation.93765/ Post #12.

The relays aren't a good idea or even needed. If you use something like this half-bridge driver - http://www.electro-tech-online.com/custompdfs/2012/02/irs2103.pdf And wire it to your 555 output it will turn on the upper mosfet when 555 is high and then turn on lower mosfet when 555 is low. And you need the driver any way for the high side mosfet.
That looks good. I can probably use a battery to power the IRS2103 and 170 VDC for the rest of the circuit. The circuit is finally coming together. Its about time to order parts.
 

johansen

Member
it is just about impossible to make an induction heater without an oscope, and even if you have a scope, you will still need a floating power supply (unless you have a differential oscope probe, and they are expensive.)

why do you need to go as high as 200khz?
at that frequency, switching losses aren't negligible anymore, even for mosfets.
 

gary350

Well-Known Member
it is just about impossible to make an induction heater without an oscope, and even if you have a scope, you will still need a floating power supply (unless you have a differential oscope probe, and they are expensive.)

why do you need to go as high as 200khz?
at that frequency, switching losses aren't negligible anymore, even for mosfets.

Several induction heaters I see online are in the 200 KHz range. I don't care what frequency it is as long as it works. One guy did a very good artical about induction heats, it says the frequency needs to be adjusted to find the sweet spot, if the frequency is too low or too high it does not work as well. He was saying his induction heater is 2??KHz. I don't remember his exact freq. If it will run on 60Hz that will work for me.

What is a floating power supply?
 
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johansen

Member
A floating power supply lets you connect the ground lead of your oscope to the 300+ volt dc rail of your induction heater without blowing anything up.
You can float the oscope by cutting off the ground lead, but most people say don't do that :)
 

ronv

Well-Known Member
Most Helpful Member
You need the high frequency so the coil and capacitors are resonant.

Since both have tolerances you have to search out the exact resonant frequency by varing the frequency of the 555 to find the perfect point.

This driver would work best. The other one I posted only works at 12 volts.
 

johansen

Member
if you do throw together a 555 controlled half bridge, you will find that you need a 10 or 100 ohm pot in series with the timing resistor.
the 'Q' can be that high.

It doesn't take much change in frequency to swing from above resonance and virtually no power consumption to below resonance and high switching losses..
to in the middle where the current flowing is high enough to fry something.

the "easy" way to do it is build a pll to keep the system at resonance, and you change the current by varying the supply voltage.
 

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