Induction Heating Temperature Detection via Constant Voltage

The patent applies periodic action by using pulse-width modulation with a constant carrier frequency. The excitation signal is switched on and off periodically at the resonant frequency, creating a square wave that can be effectively filtered. This periodic switching at a fixed frequency avoids the beat phenomenon that occurs when varying frequencies are used, thereby eliminating interference noise while maintaining controlled heating power output through duty cycle adjustment.

The patent changes the parameter of the excitation signal from variable frequency to constant frequency with variable duty cycle. By fixing the frequency at the resonant frequency and varying only the pulse width (duty cycle), the system maintains stable resonant conditions and avoids frequency-induced interference. This parameter change transforms the control mechanism from frequency modulation to temporal modulation, eliminating the harmful beats while preserving power control capability.

The patent applies preliminary action by pre-charging the resonant circuit capacitor before the main switching event. The capacitor is charged during the off-period when the switching elements are in the lower state, so that when switching occurs, the voltage is already built up. This preliminary charging action allows the resonant circuit to sustain oscillations without requiring excessively high switch-on currents, as the energy is already stored in the capacitor's electric field before the switching event.

The patent uses feedback by monitoring the resonant circuit's actual state and adjusting the switching timing accordingly. The control circuit detects the resonant frequency and phase, and synchronizes the switching events to match the natural oscillation pattern. This feedback mechanism ensures that switching occurs at optimal moments when the current and voltage are at appropriate levels, minimizing the peak switch-on and switch-off currents while maintaining effective heating power control.

The patent segments the operating cycle into distinct measurement intervals separated by the heating phases. During measurement intervals, the excitation signal is temporarily interrupted or reduced to minimal levels, creating quiet periods when temperature sensors can read without interference from the resonant circuit's electromagnetic fields. This temporal segmentation allows accurate temperature measurements to be taken during low-interference windows while maintaining effective heating during operational phases.

The patent introduces an intermediary measurement approach by using the resonant circuit's own oscillation characteristics as a proxy for temperature measurement. Instead of directly measuring temperature with sensors that are susceptible to electromagnetic interference, the system measures electrical parameters (such as resonant frequency shifts, Q-factor changes, or voltage amplitude variations) that correlate with temperature changes. These electrical intermediaries are less susceptible to direct electromagnetic interference and provide a more reliable temperature indicator during heating operation.