Electric heating – Inductive heating – With power supply system
Patent
1996-11-05
1999-06-22
Leung, Philip H.
Electric heating
Inductive heating
With power supply system
219625, 219663, 363 98, H05B 606
Patent
active
059140666
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a circuit for the control of the energy supply in a resonance converter of the type which comprises a rectifier for the mains voltage, an inverter and a resonant circuit which delivers energy by inductive coupling to a cooking vessel for food, with a view to increasing the safety.
A resonance converter for use in connection with induction heating of foodstuffs must deliver a high frequency current, e.g. in the bottom of a cooking vessel It is essential that the efficiency is high which means that current losses in electronic switches must be low. It is furthermore important that the semiconductor components, such as switches, which form part of the construction are not subjected to even brief loads to their specified limits because this decreases their lifetime considerably. A resonance converter of the type described utililizes a resonant circuit where the self-inductance participates, both for determining resonant frequency of the resonant circuit and for inductive energy transfer. It is the intention that the only losses shall be the desired losses in the bottom of the cooking vessel. It has been found advantageous to use a so-called hybrid resonance converter which utilizes the advantages of a parallel resonant circuit which are that no resonant current passes through switches, while this is series coupled to a further self-inductance which together with the components of the parallel resonant circuit define a series resonant circuit, the advantage of which is that the current may be interrupted at a zero crossing of the series resonant current.
Many converters are based on the rectification of the mains voltage in a double rectifier (in single-phase and between phases in a three-phase mains), a charging capacitor in a DC circuit and a subsequent inverter. Most conventional inverters load the DC circuit by a rectangular current which gives high internal losses and which reacts back to the mains where it gives high peak currents, even though the effective value of the current only corresponds to the power taken up by the inverter to which losses in the circuits must be added.
Traditional parallel resonant circuits in an inverter present serious problems in connection with closing or breaking of the current to the resonant circuit due to the placement of the self-inductance. An inverter which is built as a full bridge or a half bridge must invert the current through the resonant circuit and this occurs by letting one set of switches break a current essentially simultaneously with another set having to close a current. A full bridge will have four switches which take turns conducting two and two, and in a halt bridge two of the switches have been replaced by capacitors. Due to the self-inductance a break which is too late will mean that a constant current flows for period of time which is in effect a short circuit, and too early breaking will create a strong over-voltage. Both phenomena are harmful to the semiconductor components which are used for the switches. Due to the series connected capacitor a series resonant circuit does not display these disadvantages, but the resonant current must always run through switches, and thus undesired losses are created.
It is the purpose of the invention to provide a circuit for the control of the feed of energy in a circuit of the type described in the preamble which reduces the losses, reduces the load on the switches and which permits a control of the power supply, e.g. to zero in the case of a missing load, such as a cooking vessel or another inductively heated heating element.
This is obtained in that the current fed to the parallel resonant circuit has a pulse width which is less than or equal to half the period of oscillation of the parallel resonant circuit. Hence the case is that current is fed once per half period of the parallel resonance current by means of a pulse, the temporal development of which being predetermined, whereby it does not become a rectangular current, and it is switched off efficiently without the generation of o
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patent: 3737757 (1973-06-01), Fowler et al.
patent: 4426564 (1984-01-01), Steigerwald et al.
patent: 4833584 (1989-05-01), Divan
patent: 5486752 (1996-01-01), Hua et al.
patent: 5486993 (1996-01-01), Sakurai et al.
Aktiebolaget Electrolux
Leung Philip H.
Safran David S.
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