JPH0679503B2 - Small load detection device for induction heating cooker - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a small load detection device for an induction heating cooker that detects a small load such as a spoon or a fork.

2. Description of the Related Art Conventionally, in an induction heating cooker that induction-heats a metal pot with high-frequency magnetic flux, if you accidentally place small metals such as spoons and forks in a power-on state, these small metals will heat up to high temperatures, There was a risk of burns when touched by people. Therefore, so as not to be heated by small metal,
Further, a small load detection device is provided so that the switching element is not destroyed by an abnormal voltage when there is no load. As the small load detection device, an input current or an output heating coil current of a frequency conversion circuit, which is a high frequency power source for induction heating cooking, is detected, and it is determined whether or not it is a small load based on the detected current value.

Problems to be Solved by the Invention However, the conventional small object load detection has a problem in that a current detection device such as a current transformer is required for current detection, which makes the device complicated and expensive. It was

The present invention has been made in view of the above points, and provides a small load detection device for an induction heating cooker with a simple configuration and at a low cost.

Means for Solving the Problems To solve the above problems, the present invention provides an inverter circuit for supplying high frequency power to a heating coil and a resonance capacitor by turning on and off a reverse blocking switching element, and the reverse blocking switching element. A control circuit for controlling on / off of the reverse blocking type switching device, a peak voltage detecting circuit for detecting a peak voltage across the reverse blocking switching element, a voltage change rate detecting circuit for detecting a change rate of the peak voltage, and A start control circuit for limiting the conduction time of the reverse blocking switching element, the control circuit limiting the on-time of the reverse blocking switching element by the start control circuit, and the peak from the voltage change rate detection circuit. When the voltage change rate is equal to or more than a predetermined value, it is detected as a small load.

Operation With the above configuration, an induction heating cooker using a voltage-type single-ended resonant inverter that uses a reverse blocking switching element can prevent the heating coil
Since the resonance attenuation rate of the resonance loop including the resonance capacitor is low, the voltage across the reverse blocking type switching element greatly increases when the conduction time is kept substantially constant at the time of startup. Therefore, the conduction time of the reverse blocking switching element is made substantially constant at the time of startup, the rate of change of the peak voltage across the reverse blocking switching element is detected, and the small object load is detected depending on whether the level is equal to or higher than a preset level. It is a thing.

Embodiment One embodiment of the present invention will be described below with reference to the accompanying drawings.

In FIG. 1, a DC power supply 11 is connected to a parallel circuit of a heating coil 13 and a resonance capacitor 14, and a reverse blocking switching element 12. The reverse blocking type switching element 12 has a structure in which a reverse breakdown voltage can be set in a practical operation region.
There is a thyristor. . Reverse blocking switching element 12
The control circuit 15 for controlling the on / off of the voltage change rate detection circuit 17 for detecting the rate of change of the peak voltage from the peak voltage detection circuit 16 for detecting the peak voltage across the reverse blocking switching element 12, It is connected to a start control circuit 18 that limits the conduction time of the reverse blocking switching element 12 at the time of start.

The start-up control circuit 18 causes the control circuit 15 to turn on / off the reverse blocking switching element 12 at the time of start-up so that the on-time is shorter than the off-time as shown in FIG. 2A. That is, when a small load or no load is applied, a large current easily flows through the reverse-blocking switching element 12, so the on-time is shortened and damage to the switching element 12 is prevented. As shown in FIG. 2B, the peak voltage of the peak voltage detection circuit 16 with no load or with a small object load at the time of start-up increases with time. Therefore, the wavy line shown in the figure, that is, the rate of change of the peak voltage is also increasing. On the other hand, the waveform of the voltage (peak voltage of the peak voltage detection circuit 16) across the reverse blocking switching element 12 under normal load is as shown in FIG. 2 (C),
The wavy line in the figure shows the rate of change of the peak voltage, and the rate of change is almost zero. Therefore, when no load is applied or when a small object is loaded, the rate of change in peak voltage across the reverse blocking switching element 12 is very large as compared with a normal load. Therefore, it is possible to detect the small load depending on whether or not the peak voltage change rate of the reverse blocking switching element 12 at the time of startup is equal to or higher than a preset level.

In this embodiment, since the reverse blocking type switching element 12 that does not require a damper diode antiparallel to the switching element and has a sufficient reverse breakdown voltage in the practical operating region is used to form the voltage single-ended resonant inverter, the heating coil 13, the resonance attenuation rate of the resonance loop including the resonance capacitor 14 is reduced, the rate of change in peak voltage is also large, and small object load detection can be performed reliably. In another embodiment according to the present invention, as shown in FIG. 3, a series circuit of a current limiting inductor 19, a heating coil 13 and a resonance capacitor is connected to a DC power source 11, and reverse blocking is performed between the current limiting inductor 19 and the DC power source 11. The switching element 12 and the second resonance capacitor 20 are connected. Also in this embodiment, the reverse blocking switching element 12
The waveform of the peak voltage across both ends of the curve shows the same tendency as the voltage waveform shown in FIG. 2, and small object load detection is possible.

Although the two types of voltage source single-ended resonant inverters are shown in this embodiment, the peak voltage change rate at both ends of the reverse blocking switching element is the same even when other types of voltage source single-ended resonant inverters are used. Therefore, similar small object load detection is possible.

EFFECTS OF THE INVENTION As described above, according to the small load detection device for an induction heating cooker of the present invention, the peak voltage detection circuit for detecting the peak voltage across the reverse blocking type switching element and the rate of change of the peak voltage are detected. It is possible to detect small loads with a simple configuration that includes a voltage change rate detection circuit and a startup control circuit that limits the on-time of the reverse blocking switching element at startup, and is an inexpensive small load for induction cookers. A detection device can be provided.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a small load detecting device for an induction heating cooker according to an embodiment of the present invention, FIG. 2 is a waveform diagram showing an operation of the small load detecting device for an induction heating cooker, and FIG. It is a circuit diagram of the small load detection apparatus for induction heating cooking appliances in other Examples. 12 …… Reverse blocking type switching element, 13 …… Heating coil,
14 …… Resonance capacitor, 15 …… Control circuit, 16 …… Peak voltage detection circuit, 17 …… Voltage change rate detection circuit, 18 …… Startup control circuit.

Claims (2)

[Claims] 1. An inverter circuit for supplying high-frequency power to a resonance capacitor and a heating coil by turning on / off a reverse blocking switching element, a control circuit for controlling on / off of the reverse blocking switching element, and the reverse blocking. Voltage detection circuit that detects the peak voltage across the switching element, a voltage change rate detection circuit that detects the rate of change of the peak voltage, and a startup control circuit that limits the on-time of the reverse blocking switching element at startup. The control circuit limits the ON time of the reverse blocking switching element by the start control circuit, and detects as a small load when the peak voltage change rate from the voltage change rate detection circuit is equal to or more than a predetermined value. Small load detection device for induction heating cookers. 2. The small load detection device for an induction heating cooker according to claim 1, wherein the starting control circuit makes the conduction time of the reverse blocking type switching element substantially constant at the time of starting.