DIY Induction Heating with pll

[sina_stuck]

Hi there,
I've been interested in this induction heating thing and would like to build one. Its quite a long time searching the net and experimenting to make it work. I've been stuck with the pll issue. My configuration is Hbridge which is fed from a simple power supply(not split) so the lowest point is ground. The two signals I try to maintain a phase lock of 90 degrees are the Inverter voltage and tank cap voltage. The inverter voltage is fed from pin 4(VCO out) to pin 3(COMP in) of 4046 as inverter voltage representative. But in order to get a square wave as a representative of cap voltage I'm trying to use a voltage comparator lm393.

My question is how to connect lm393 across the cap and meanwhile protecting its inputs from excessive voltage that appear across the cap?

I tried to put a shunt resistor in series with LC tank and use the voltage drop across it as an indicator of inverter current phase angle but the voltage across it is very odd in its shape and if converted to square wave does not show the phase angle of the inverter current.

Also I am very interested to use the phase comparator 2 of the 4046, this phase comparator tries to lock zero angle between its input signals which is alright if I was able to use the voltage across the shunt resistor. but in this case I should change the phase angle of the cap 90 degrees and then try to lock zero angle between it and the inverter voltage.

Any help, suggestions and schematics would be very welcomed. Please let me know if I should send schematics or if my explanations are not complete also I can be available for a chat at any time if this would be an easier way for anyone to help. Thanks in Advance.
Best Regards
Sina

 

[imsmooth]

If you go to http://inductionheatertutorial.com
you will find an extensive tutorial on what you are looking for. The unit uses a PLL circuit. Your question with regard to the capacitor voltage is also addressed. Look at the circuit in the middle (2kw unit).

 

[sina_stuck]

Thanks imsmoth for this address. I have already read it but the difference is two things between his and mine. first he uses a halfbridge while I use an hbridge. secondly he puts a matching inductor between inverter and tank while I do not plane to put one. what i seek now is how to capture cap voltage when there is no inductor in between assuming the same LC tank circuit.
thanks again
sina

 

[sina_stuck]

Hi,
I thought nothing would be better than visually express my point of confusion. Here I post a schematic and tried to clarify my question. please have a look an let me know if I should provide more details.
Thank you again
sina

 

[LudusRex]

I appreciate your interest and efforts in building your own induction heating system. It is a complex and challenging project, and I can understand your frustration with the PLL issue.

To address your question about connecting the lm393 across the capacitor and protecting its inputs from excessive voltage, I would suggest using a voltage divider circuit to limit the voltage across the inputs. This can be achieved by using two resistors in series, with the junction point connected to the positive input of the lm393 and the other end connected to ground. This will divide the voltage across the capacitor and protect the inputs of the comparator.

In terms of using the phase comparator 2 of the 4046, I would recommend using a phase shifter circuit to adjust the phase angle of the capacitor voltage. This can be achieved by using a resistor and capacitor in series, with the junction point connected to the negative input of the comparator. This will allow you to adjust the phase angle and lock it with the inverter voltage using the phase comparator.

I would also suggest checking the datasheet of the lm393 and 4046 to ensure you are using them correctly and to see if there are any application examples that can help you with your project.

I hope this helps and I wish you the best of luck with your DIY induction heating system. If you have any further questions or need any additional assistance, please feel free to reach out.