Induction heating apparatus comprising a static converter

In an induction heating apparatus having an induction heating coil for heating metallic objects coupled thereto, a static converter fed by a d.c. voltage from a rectifier circuit and including:

• a resonant commutating circuit having a commutating capacitor and a commutating inductor connected in series, said commutating inductor being inductively coupled to said induction heating coil;
• a bidirectional controlled switch having a gate controlled first thyristor and a diode connected in parallel to conduct in opposite directions and connected in parallel to said commutating circuit;
• a charging inductor connecting the negative terminal of the rectifier circuit to the junction between said commutating circuit, the cathode of said first thyristor and the anode of said diode, the positive terminal of said rectifier circuit being connected to the junction between said commutating circuit, the anode of said first thyristor and the cathode of said diode, for providing said commutating capacitor with resonant charging to a voltage biasing said first thyristor positively and said diode negatively;
• a triggering circuit for recurrently gating said first thyristor to make said bidirectional switch conducting for a full cycle of resonant current through said commutating circuit, the first thyristor conducting during the first half-cycle and the diode during the second half-cycle of said resonant current, whereafter said bidirectional switch ceases to conduct;
• and
• an overshoot protection circuit for providing protection against spurious turning on of said thyristor due to overshoot voltages produced across said charging inductor when said bi-directional switch suddenly ceases to conduct, including a protection diode and a protection capacitor arranged in series and respectively connected by their free terminals to the terminals of said charging inductor, the anode of the protection diode being connected to the junction between the charging inductor and the negative rectifier output terminal, so that it conducts in a direction opposite the current flow through the charging inductor when the bidirectional switch is conducting, wherein, to utilize the energy stored in the protection capacitor, when said first thyristor is being turned on, said triggering circuit comprising:
• an auxiliary thyristor connected in series with a first, current-limiting resistor between the junction of the cathode of said protection diode and said protection capacitor and the gate of said first thyristor, said auxiliary thyristor having a gate fed by recurrent gating pulses for discharging said protection capacitor through itself and the gate-to-cathode junction of said first thyristor;
• and
• a second, starting resistor connected between the positive rectifier output terminal and the junction of said protection capacitor with the cathode of said protection diode and said current limiting resistor for positively charging said protection capacitor when said converter is being switched on.