Metal base cookware induction heating apparatus having improved control circuit using infra-red temperature sensor

Abstract

Abstract A metal based cookware induction heating apparatus having an improved power supply and control circuit comprised of a chopper inverter including a gate controlled power thyristor coupled to and supplying an induction heating coil with periodic energization currents. The gating circuit controlling operation of the chopper inverter is controlled by a temperature sensing arrangement that includes an infra-red heat sensor device arranged to view the metal-based cookware or other object whose temperature is to be controlled. A constant current amplifier is connected in series circuit relationship with the infra-red heat sensor device across a source of low voltage direct current excitation potential and means are provided for periodically interrupting the view of the infra-red heat sensor device to produce an output alternating current signal component whose amplitude is indicative of the temperature of the metal-based cookware or other object being heated. An alternating current amplifier including an adjustable temperature set resistor is coupled to the output from the infrared heat sensor device with the setting of the adjustable temperature set resistor serving to control the gain and establish the temperature level at which the output from the alternating current amplifier attains an operative level of magnitude. An output on-off control amplifier either in the form of a Schmitt trigger type of circuit or reed relay is coupled to and controlled by the output from the alternating current amplifier for producing an amplified output on-off control signal representative of the temperature of the metal-based cookware or other object whose temperture is being controlled in accordance with the temperature setting of the adjustable temperature set resistor. The on-off output from the temperature sensing arrangement control an inhibit circuit connected to and controlling a zero point sensing and control circuit that in turn controls the operation of a soft starting zero point switching device connected intermediate the rectified ac supply terminals and the thyristor switching device employed in the chopper inverter whereby both radio frequency interference effects and audible noise production by the inductive heating coil during energization periods from the chopper inverter is substantially reduced due to the fact that energization of the coil under the control of the temperature sensing arrangement is inhibited until only the beginning or near the beginning of each half cycle of the rectified full wave potential appearing at the output from the high voltage rectifier. To further reduce RFI effects, a smoothing inductor having an inductance L3 and a smoothing capacitor having a capacitance C3 is connected in series circuit relationship across at least one of the commutating components of the chopper inverter power supply circuit. The combined impedance of the commutating capacitor, the smoothing inductor L3 and the smoOthing capacitor C3 is capactive in nature and is series resonant with the communtating inductor L1 to establish the commutating period t1. The smoothing inductor and the capacitor serve to shape the output current flowing through the smoothing inductor which is used as the inductive heating coil to substantially a sinusoidal waveshape having little or no radio frequency interference emission effects. Additionally, a capacitor switching arrangement is provided for the commutating capacitor C1 which is formed from a plurality of parallel connected capacitors and power rate controlling switching means for switching selected ones of the parallel connected capacitors into effective circuit relationship in the chopper inverter circuit in accordance with a desired rate of power production. Where the improved induction heating apparatus is employed in connection with metal-based cookware, the inductive heating coil preferably is in the form of a helically-shaped planar inductive heating coil and is disposed under a heat transparent member such as an infra red transmissive pyroceramic that can form a smooth continuous tabletop over the induction heating unit and itself does not become heated except by conduction and convection from the metal-based cookware. The inductive heating coil excites and heats the metal-based cookware directly so that all heat is generated only by the metal-based cookware and no excess waste heat is produced in the manner of known resistance heating units. Since the temperature sensing units directly views the bottom of the metal-based cookware through the heat transparent member and controls the application of power to the induction heating coil, temperature control is directly related to the actual temperature of the pan or other object being heated by the induction heating apparatus. As a consequence, a considerable improvement in efficiency and reduction of waste heat results. Temperature control is accurate, rapid in response and there is no overshout in temperature such as is characteristic of temperature control systems which control the temperature of a heat source which is external to the object being heated. Improved arrangements for the heat transparent member, infra-red heat detector, chopper device, and cooling fan together with preferred designs for the inductive heating coil with ferrite core of without to maximize coupling to the metal-base cookware or other object being heated, are described whereby further improvements in efficiency of operation, economy of components and minimum size are achieved.

Inventors

• 👤 Jr Philip H Peters

Bibliographic Details