Tuesday, May 12, 2009
The circuit behind induction heating
It is a "quasi resonant converter" design, where the induction coil (connected to the screw terminal X1) is the inductance of the LC-tank and C1-C5 are forming the capacitance. The principle of induction heating is explained very good in this Fairchild-document: http://www.fairchildsemi.com/an/AN/AN-9012.pdf#page=1
Here the description of my circuit:
The electrolytic capacitors C6 and C7 together with the inductance L2 are smoothing the current seen by the used PC power supply. Therefore a high peak current can flow through the induction coil without burdening the power supply with this peak value. C6 and C7 have to be low-ESR (Equivalent Series Resistance) in order to reduce the losses and to prevent overheating caused by the high AC-current flowing through these capacitances. For the same reason two capacitors are switched parallel instead of using only one with the double capacitance. The induction coil is connected to the screw terminal X1. If a cable is used the wires should have a high cross section (minimum: 1.5mm²) and it should not be longer than one meter in order to reduce the losses. C1 to C5 are forming the resonant capacitance, again several capacitances with low ESR are used in parallel to reduce the losses. R5 is a current sensing resistor, the current through the induction coil can be measured with an oscilloscope connected to JP2. The Power-FET Q1 is driven by the MOSFET-driver IC1. Depending on the signal "SWITCH_ON" at JP1 (which comes from the microcontroller board) a inverting or non-inverting driver can be selected through the solder jumpers SJ1 and SJ2. The driver is necessary to increase the TTL-high-level of the switching signal at JP1 to +12V, because Q1 requires a minimum level of +10V to be switched on properly. Furthermore the driver has an output current up to 1.5A which loads the gate-capacitance of Q1 fast at switching on. Therefore a very short rise time of the gate voltage is achieved which helps to reduce the switching losses. The circuit consisting of D1, R1, D2, D3, R7 and C12 cuts the drain voltage of Q1 seen at JP3 to a maximum of about 4V (Z-diodes D2 and D3) and a minimum of 0.3V (Schottky-diode D1). Two parallel Z-diodes are used to make sure that the signal at JP3 does not exceed 4V even in the case that one Z-diode fails. The resulting signal "ZERO_VOLT_SENSE", which is connected to the microcontroller board, is used for zero voltage detection and makes zero voltage switching (ZVS) possible. ZVS means switching on the FET when the drain-source voltage becomes zero. This reduces the switching losses, because no power is generated in the FET at this time. As it can be seen in the priviously published photo of the PCB there is no heatsink for the FET necessary because of the low losses.
Switching at zero crossing will also greatly reduce RFI providing the power switch is placed close to the coil along with the resonant capacitance and reservoir capacitors. With the leads kept short as well.
There is almost a case for spliting part of the switching circuitry off then and placing it on the coil and former.
Very innovative altogether. off for a read of the doc link you left.
For a metal extruder the heater chamber could be ceramic or something else and the metal itself will be heated.
Induction certainly does open up possibilities!
P.s. When I shorted a wire in my extruder somewhere, surprisingly, I got back sound from a ferrite core that I placed over the wires going to the extruder. Is that a piezzo effect or what is it? (BTW: No extruders were harmed during that experiment).
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