Magnetron with frequency control for power regulation

A power control system for microwave heating apparatus comprising:

• an inverter having a plurality of switches and being powered from an AC input line, each of the plurality of switches being a controlled switch;
• a power transformer having a primary connected to receive power from said inverter and a secondary;
• a heating magnetron connected to receive power from said secondary;
• a control circuit for controlling power flow from said inverter to said magnetron, said control circuit having an oscillator for supplying gate pulses to switch said plurality of switches of said inverter at a switching frequency;
• sensing means for sensing variations in line voltage of said AC input line;
• and
• frequency determining means connected to said oscillator for determining the switching frequency, said frequency determining means being connected and responsive to said sensing means to change the switching frequency an amount which is dependent upon the variation in line voltage of said AC input line;
• and
• wherein said secondary of said power transformer is part of a tuned circuit having a resonant frequency and wherein said frequency determining means changes a difference between the switching frequency and the resonant frequency to minimize fluctuations in the power output of said cooking magnetron, which fluctuations would otherwise be caused by variations in line voltage of said AC input line and wherein said secondary of said power transformer is connected to a voltage doubling circuit having a capacitor and diode and connected to supply high voltage to said magnetron, and wherein said capacitor is part of said tuned circuit.

A power control system for a microwave heating apparatus comprising;

• a full wave full bridge inverter powered from an AC input line and including four switches, each switch being a controlled switch, a first switch and a second switch of said four switches being connected in series, a third switch and a fourth switch of said four switches being connected in series;
• a power transformer having a primary connected to receive power from said inverter and a secondary, said primary being connected between a node common to said first and second switches and a node common to said third and fourth switches;
• a heating magnetron connected to receive power from said secondary;
• a control circuit for controlling power flow from said inverter to said magnetron, said control circuit having an oscillator for supplying gate pulses to switch said four switches of said inverter at a switching frequency;
• sensing means for sensing variations in line voltage of said AC input line;
• and
• frequency determining means connected to said oscillator for determining the switching frequency, said frequency determining means being connected and responsive to said sensing means to change the switching frequency an amount which is dependent upon the variation in line voltage of said AC input line such that the power output of said cooking magnetron is maintained relatively stable and independent of variations in line voltage of said AC input line and;
• wherein said frequency means determines the switching frequency over a continuous range between a lower switching frequency and an upper switching frequency, and wherein said secondary of said power transformer is part of a tuned circuit having a resonant frequency not greater than said lower switching frequency and wherein said frequency determining means changes a difference between the switching frequency and the resonant frequency to minimize fluctuations in the power output of said cooking magnetron, which fluctuations would otherwise be caused by variations in line voltage of said AC input line.

A method of stabilizing a system with a heating magnetron against variations in power output of the heating magnetron due to variations in line voltage of an AC input line, the steps comprising:

• using power from the AC input line to power an inverter such that the inverter operates at a switching frequency;
• supplying power to a primary of a power transformer from the inverter;
• supplying power to a heating magnetron from a secondary of the power transformer;
• sensing variations in line voltage of the AC input line;
• and
• changing the switching frequency in an amount determined by the sensed variations such that the power output of the cooking magnetron remains relative constant as a result of changes in the difference between the switching frequency and a resonant frequency of a resonant circuit including said secondary.