High Frequency Inverter for Microwave Oven Norikazu Tokunaga, Member, Yasuo Matsuda, Member, Kunio Isiyama, Non-member (Hitachi, Ltd.), Hisao Amano, Member (Hitachi Engineering, Co., Ltd.). Recently resonant inverter type power supplies for microwave ovens have been examined, but the stability of the filament temperature and the loss of the transformer have not been examined in detail. We studied the relation between circuit parameters and the characteristics of the power supply. As a result we conclude that putting a resonant reactor in the primary side of the transformer brings better stability of the magnetron filament temperature and lighter duty of the circuit elements than putting it in the secondary side. Newly-developed power supply can be continuously controlled in the range of 5 to 100%. Its efficiency is about 85% which is about 2% higher than that of the conventional one. Moreover the weight is reduced to about 1/4.
Fig. 1. Relation between magnetron anode current and anode-cathod voltage. Fig. 2. Inverter circuit for driving a magnetron. 448 T. IEE Japan, Vol. 11o-D, No. 5, '90
Fig. 4. Variable range of transistor conducting period. Fig. 3. Operation modes of inverter circuit.
Fig. 5. Transformer secondary voltage waveforms. 450 T. IEE Japan, Vol. 110-D, No. 5, '90
Table 1. Circuit parameters for calculation. Fig. 6. Maximum value of the tranformer secondary voltage. Fig. 7. Calculated transformer current waveforms.
Fig. 8. Cross section of transformer. Fig. 9. Relation between resistance of pri mary winding and operation frequency. Fig. 10. Loss of transformer.
Table 2. Voltage distributions of toransformer secondary windings. Fig. 11. Transformer winding model and its equivalent circuit. Fig. 12. Loss of the transformer Fig. 13. Equivalent circuit for voltage dis. primary winding. tribution calculation.
Fig. 14. Experimental waveforms of power supply input voltage, current and magnetron anode current. Fig. 15. Experimental waveforms of transistor collector-emitter voltage, transformer secondary voltage, primary current, secondary current and magnetron current. 454 T. IEE Japan, Vol. 110-D, No. 5,'90
Fig. 16. Relation between output power and temperature of filament in magnetron. Fig. 17. Relation between cutoff voltage of magnetron and controlable output power range. Fig. 18. Loss analysis on inverter type power supply.
(9) D. Bessyo, et al, "A new switching power supply for magnetron using a new device and a novel transformere", PCIM '88 Proceedings 456 T. IEE Japan, Vol. 110-D, No. 5,'90