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pll for induction heater

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emperor110

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dear all
hello
I have a 1kw induction heater which uses half bridge inverter with 80khz switching frequency.I want to design a pll for it but I have no idea how to do. could you please guide me?
 

Why do you want a PLL? Need to command a precise frequency?

Seems like generating the 80kHz could be done well enough
with a much simpler oscillator.
 
dick_freebird said:
Why do you want a PLL? Need to command a precise frequency?

Seems like generating the 80kHz could be done well enough
with a much simpler oscillator.
Click to expand...


during the process of heating the magnetic properties of workpiece will change causing changes in the inductance of work coil and then the resonant frequency work coil and capacitive tank will change too. so i need a pll to tune the switching frequency with resonant frequency.
 
Strictly spoken, PLL involves locking of a variable oscillator frequency to an external reference. Obviously this is not the case in your application. But possibly the phase detector of a usual PLL design can be "abused" to tune an oscillator to achieve a resonant condition in the induction heater load circuit. At first you have to think about a suitable resonance criterion for your circuit.
 

Hi emperor110,
You can do in 2 ways
1. Change your Oscillator to be a VCO. Or
2. Change your resonant circuit to be a tuning circuit to match the resonant point by changing the capacitor value (connecting some more or disconnect some capacitor according to the steps of the process).
 

Dear sir
i am trying to make induction heater using cd4046,but i have a question regarding the cutoff freq,how to determine it? for example if my fmax is 70KHZ and center freq is 60KHZ,and min freq is 50KHZ,what will be the cutoff frequency according to it, so I can design my low pass filter, any excel sheet any one did to calculate it automatically? or any formula,plz i need ur help in this issue
i appreciate ur answer.
Best Regards
Alloush
 

Cutoff freq is determined from the speed of turning. (and phase noise you need)
Span of the PLL freq is determined from span of the VCO and the voltage span of the controller.
If the cutoff freq of LPF is low, step responce of the freq turning is slow.
 

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