Inverter Control Circuit for High-Frequency Induction Heating
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Solution Overview
Problem
High-frequency dielectric heating apparatuses face issues with overheating of semiconductor switching elements and potential overvoltage when transitioning from low to normal output after power down control, leading to moding phenomena and reliability concerns.
Innovation Solution
The implementation of an output increase suppressing unit and output increase delay section, which include capacitors for charge storage and voltage fluctuation, to control the resonance voltage and prevent abrupt output rises, ensuring smooth transitions and preventing overheating.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If power down control is implemented to prevent IGBT overheating, then reliability of the IGBT is improved, but the output restoration after power down causes moding phenomena and overvoltage issues
Solution Approach 1:
The patent applies preliminary action by implementing output increase suppressing control that activates before output restoration can cause harmful effects. The control circuit detects when output is being restored after power down control and preemptively suppresses the output increase, preventing moding phenomena and overvoltage from occurring in the first place.
Solution Approach 2:
The patent uses feedback control by having the control circuit continuously monitor the output state and IGBT temperature, and dynamically adjust the output based on detected conditions. When output restoration is detected after power down control, the feedback mechanism triggers output increase suppressing control to prevent harmful effects, creating a closed-loop system that adapts to prevent moding phenomena.
2Productivity
If output is restored quickly after power down control, then productivity is improved, but thermal destruction of IGBT may occur
Solution Approach 1:
The patent applies dynamics by making the output control adaptive and time-dependent. The control circuit dynamically adjusts output based on the operational state: allowing rapid restoration when safe, but implementing suppressing control when restoration could cause thermal destruction. This dynamic control optimizes productivity while preventing IGBT damage.
Solution Approach 2:
The patent implements beforehand cushioning by preparing the output increase suppressing control mechanism in advance to cushion against potential thermal destruction. When output restoration is detected, the control circuit activates this cushioning mechanism to gradually limit output increase, preventing sudden power surges that could thermally destroy the IGBT.
3Productivity
If pulse width is widened to increase output power, then productivity is improved, but IGBT temperature rises causing thermal destruction
Solution Approach 1:
The patent uses feedback control where the control circuit monitors IGBT temperature and output power levels, and adjusts the pulse width accordingly. When temperature approaches dangerous levels, the feedback mechanism reduces pulse width to prevent thermal destruction, while allowing wider pulse widths when temperatures are safe, thus optimizing productivity within thermal constraints.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enhances the reliability and usability of high-frequency dielectric heating apparatuses by preventing overheating and moding phenomena, ensuring stable operation and maintaining output control during transitions.
Implementation Method 1
The output increase suppressing unit includes a capacitor for charge storage and voltage fluctuation control
Implementation Method 2
The direct current is converted into any desired high frequency (20 to 40 kHz) by turning on/off a semiconductor switching element, IGBT (Insulated Gate Bipolar Transistor) in the inverter 16
Implementation Method 3
In the boosting transformer 18, high-frequency voltage output by the inverter 16 is applied to a primary winding 181 and a high voltage responsive to the turns ratio is obtained at a secondary winding 182
Implementation Method 4
A thermistor 9 for detecting the temperature of the IGBT 16a is attached directly to a leg part of the IGBT 16a
Implementation Method 5
The heating part of the IGBT 8 for generating high heat is fixed to the radiation fin 7
Data Source
Figure 1
Figure 2
Figure 3(a)~3(j)
AI summary
Occurrence of a defect of the moding phenomenon, etc., accompanying rise in output of a magnetron after power down control is prevented and the reliability of a high-voltage dielectric heating apparatus is further improved. After power down control, when the temperature of a thermistor T1 falls again, the resistance value of the thermistor T1 increases and potential Vpc at a point PC also rises and when the potential Vpc becomes higher than Vc3 of another input potential of a comparator C3 (Vpc > Vc3), the comparator C3 outputs "0." In association with this, a switch S2 is turned off and PD (power down) 2 of a resistor R7 is turned off and thus the input voltage to a positive terminal A and a negative terminal B of a comparator C1 is again restored to 3 V. When the temperature of the thermistor T1 further falls, the resistance value of the thermistor T1 increases and the potential Vpc at the point PC also starts to rise. When the potential Vpc becomes higher than Vc2 of another input potential of a comparator C2, the comparator C2 outputs "0." Then, a switch S5 turned on in the power down control is turned off through a switch S4 and an OR gate circuit and the partial pressure action of a resistor R11 does not work. The potential at a point SS starts to rise with charge storage in an electrolytic capacitor CO1. When the potential at the point SS continues to rise and exceeds 3 V, a comparator C4 outputs "1," a NAND gate circuit outputs "0," and a moving contact K1 is switched to the side of a second fixed terminal b. The input voltage to the positive terminal A, the negative terminal B of the comparator C1 starts to rise.