The discussed induction cooker circuit is truly simple and uses just a few active and passive ordinary components for the required actions.
Learn how to design your own customized induction heater cooktop:
Designing an Induction Heater Circuit – Tutorial
Induction Heater LC Resonance Frequency and Current
According to the involved principle when a change in magnetic field is forced around a metal, electrons inside the metals get agitated and begins flowing across the metal, this is termed as eddy current. This flow of current in the metal or the introduced conductor causes a heat to be generated in the metal making it warmer.
The generated heat is proportional to (current)^2 x resistance of the metal.
The above heat is also directly proportional to the induced frequency and that’s why ordinary iron stamped transformers are not used in high frequency switching applications, instead ferrite materials are used as cores.
However here the above drawback is exploited for acquiring heat from high frequency magnetic induction.
Referring to the proposed induction heater circuit below, we find the concept utilizing the ZVS or zero voltage switching technology for the required triggering of the mosfets. The technology ensures minimum heating of the devices making the operation very efficient and effective.
Further to add, the circuit being self resonant by nature automatically gets sets at the resonant frequency of the attached coil and capacitor quite identical to a tank circuit.
The circuit fundamentally makes use of a Royer oscillator which is marked by simplicity and self resonant operating principle. However the main downside of the design is that it employs a center tapped coil as the transformer, which makes the winding implementation a bit trickier. However the center tap allows an efficient push pull effect over the coil through just a couple of active devices such as mosfets.
As can be seen, there are fast recovery or high speed switching diodes connected across the gate/source of each mosfet. These diodes perform the important function of discharging the gate capacitance of the respective mosfets during their non-conducting states thereby making the switching operation snappy and quick.
You can use IRF540 as the mosfets which are rated at good 110V, 33amps. Heatsinks could be used for them, although the heat generated is not to any worrying level, yet still it’s better to reinforce them on heat absorbing metals.
The inductor L2 terminating from center of the main induction coil is a kind of choke for eliminating any possible entry of the high frequency content into the power supply and also for restricting the current to safe limits.
Relatively the value of L2 should be high enough, a 2mH will do the job well. However it must be built using high gauge wires for enabling high current usage through it safely.
C1 and L1 constitute the tank circuit here for the interned high resonant frequency latching. Again these too musts be rated to withstand high magnitudes of current and heat.
Here we can see the incorporation of a 330nF/400V metalized PP capacitors.
Now comes L1, which is the most crucial element of the whole circuit. It must be built using extremely thick copper wires so that it sustains the high temperatures during the induction operations.
The capacitor as discussed above must be ideally connected as close as possible to the L1 terminals. his is important for sustaining the resonant frequency at the specified 200kHz frequency.
For the induction heater coil L1, many 1mm copper wire may be wound in parallel or in bifilar manner in order to dissipate current more effectively causing lower heat generation in the coil. Even after this the coil could be subjected to extreme heats, and could get deformed due to it therefore an alternative method of winding it may be tried.
In this method we wind it in the form of two separate coils joined at the center for acquiring the required center tap.
In this method lesser turns may be tried for reducing the impedance of the coil and in turn increase its current handling capability. The capacitance for this arrangement may be in contrast increased in order to pull down the resonant frequency proportionately.
In all 330nF x 6 could be used for acquiring a net 2uF capacitance approximately.
Parts list for the above induction heater circuit or induction hot plate circuit
R1, R2 = 330 ohms 1/2 watt
D1, D2 = FR107 or BA159
T1, T2 = IRF540
C1 = 10,000uF/25V
C2 = 2uF/400V made by attaching 6nos 330nF/400V caps in parallel
D3—-D6 = 25 amp diodes
IC1 = 7812
L1 = 2mm brass pipe wound as shown in the following pics, the diameter can be anywhere near 30mm (internal diameter of the coils)
L2 = 2mH choke made by winding 2mm magnet wire on any suitable ferrite rod
TR1 = 0-15V/20amps
POWER SUPPLY: Use regulated 15V 20 amp DC power supply.
Using BC547 transistors in place of high speed diodes
In the above induction heater circuit diagram we can see the mosfets gates consisting of fast recovery diodes, which might be difficult to obtain in some parts of the country.
A simple alternative to this may be in the form of BC547 transistors connected instead of the diodes as shown in the following diagarm.
The transistors would perform the same function as the diodes since the BC547 can operate well around 1Mhz frequencies.