Inverter power source control for high frequency heater
View Patent ↗At least one of a start abnormality detector ( 31 ), a temperature abnormality detector ( 32 ) and a voltage abnormality detector ( 33 ) has a latch inhibiting circuit for inhibiting retention of a detecting state when one of the abnormality detectors detects an abnormality, so that a current abnormality detector ( 34 ) can be inhibited from detecting an abnormal input current caused when the oscillation is halted by the abnormality detector. Accordingly, after the abnormality is removed, automatic resetting can be easily accomplished without causing the oscillation to halt as a result of the current abnormality detector detecting the presence of the abnormality. Also, since a power source abnormality detecting circuit ( 30 ) is designed in the form of an analog circuit, a low cost can be achieved and the monitoring level can be modified. In addition, the monitoring level associated only with the temperature abnormality occurring in an IGBT element ( 7 ) can be modified.
1. An inverter power source control circuit for a high frequency heating apparatus, which is an inverter power source circuit for supplying an electric power to the high frequency heating apparatus by rectifying an alternating current output from an alternating current source to a direct current output and rectifying the direct current output to a desired alternating current output by applying a drive signal, based on a start signal, to a switching element, which circuit comprises:
an electric power source abnormality detecting circuit including a start abnormality detector for detecting an abnormal start signal, a temperature abnormality detector for detecting an abnormal temperature, a voltage abnormality detector for detecting an abnormal voltage present in the drive signal to be applied to the switching element, and a current abnormality detector for detecting an abnormal input current while retaining a detecting state;
an inverter oscillation control circuit operable in response to a abnormality detection signal fed from at least one of those abnormality detectors to output an abnormality control signal to the switching element to halt oscillation of the inverter power source, thereby protecting from an abnormality occurring in the power source; and
a latch inhibiting circuit provided in at least one of the start abnormality detector, the temperature abnormality detector and the voltage abnormality detector for inhibiting retention of the detecting state of the abnormality detector whereby the current abnormality detector is inhibited from detecting the abnormal input current based on a halt of the oscillation brought about by the detection of the abnormality, thereby allowing an automatic resetting to be accomplished upon removal of the abnormality without causing the oscillation to halt as a result of the current abnormality detector detecting the presence of the abnormality.
2. The inverter power source control circuit for the high frequency heating apparatus as claimed in claim 1 , further comprising:
a reference voltage setting circuit for setting a reference voltage for detection of an abnormality;
wherein the start abnormality detector detects the abnormal start signal by comparing, with a first voltage comparator, an output voltage of an PWM control signal, which is the start signal, with a first reference voltage set by the reference voltage setting circuit;
wherein the temperature abnormality detector includes a heat sensitive element disposed in a vicinity of an insulated gate bipolar transistor element, which is the switching element, and operable to detect the abnormal temperature by comparing, with a second voltage comparator, a voltage generated in correspondence with the detected temperature, with a second reference voltage set by the reference voltage setting circuit;
wherein the voltage abnormality detector detects the abnormal voltage by detecting the drive signal to be fed to a gate of the IGBT element corresponding to an input voltage from the alternating current power source and the comparing, with a third voltage comparator, the voltage of the detected drive signal with a third reference voltage set by the reference voltage setting circuit; and
wherein the current abnormality detector includes a current transformer for detecting an input current fed from the alternating current power source and detects the abnormal current by retaining a detected voltage generated in correspondence with the detected input current and by comparing, with a fourth voltage comparator, the detected voltage, so retained, with a fourth reference voltage set by the reference voltage setting circuit.
3. The inverter power source control circuit for the high frequency heating apparatus as claimed in claim 2 , wherein the first to fourth voltage comparators are provided in the form of a single voltage comparator capable of performing a corresponding number of comparing functions.
4. The inverter power source control circuit for the high frequency heating apparatus as claimed in claim 1 , wherein the high frequency heating apparatus is a microwave oven.