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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
This is a circuit by Tonskulus:
How does the 1:40 transformer connected to the Schmitt trigger control the circuit. This looks like a feedback to cotrol the frequency?
If I built a small 500w half-bridge unit with IRFP460 Fets how necessary is the feedback? I have seen designs such as this one without feedback. This one is not a resonant circuit.
Is it that resonant circuits need a feedback loop to prevent the resonant current from getting out of control as the workpiece affects the natural frequency of the work coil?
Registered Member #1223
Joined: Thu Jan 10 2008, 04:32PM
Location:
Posts: 133
Feedback is used because resonant frequency of tank circuit is not uniform, it always varies when different work pieces or work coils are used. And it also varies while workpiece heats up. So it is necessary to use feedback circuit to keep working or firing frequency matched with tank resonant frequency to get ZVS or ZCS operation. Thus, inverter stays cool and power losses are minimized. If fixed frequency is used, there is risk for mosfet/igbt failure if inverter is undertuned (firing freq lower than resonant freq of tank circuit).
My circuit is actually self resonant or self excited. Small part of tank current is fed through the primary of current transformer. Schmitt trigger contains relaxant oscillator to start operatian and keeps system running but as soon as feedback signal is applied, it overtakes relaxant oscillation and now system is running in resonance (ZCS operation).
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
No, it's exactly the opposite. If you don't drive the induction heater at its resonant frequency, you won't get the high current needed to melt stuff. The resonant frequency changes as you insert the workpiece, and the circuit needs to track that. A simple feedback oscillator is the easiest way to achieve the tracking.
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Feedback control for Induction Heater
