RU2313924C2 - Inductive cooking panel - Google Patents

Abstract

FIELD: inductive device for thermal processing of products, meant for use as home appliance.

SUBSTANCE: inductive system for thermal processing of products contains power inverter, microprocessor, protection circuit and circuit for detecting cooking vessels. Power management, synchronization and control of power inverter circuits are mainly performed by microprocessor. To protect the system from dangerous states, such as current overload and inappropriate gating signals, which may be a result of microprocessor failure, an additional analog protection circuit is included. One more circuit is included for detecting presence of applicable cooking vessels.

EFFECT: reduction of disadvantages of digital system, for example, random software errors and output signals resulting from power fluctuations.

7 cl, 16 dwg

Description

FIELD OF THE INVENTION

This invention relates to an induction heating system, and more particularly, to an induction device for heat treatment of products intended for use as a household appliance.

State of the art

Induction devices for heat treatment of products or hobs are more reliable and efficient, flameless and therefore safer than other devices for heat treatment of products. In induction hobs, high-frequency current is generated in a heating coil, which is usually connected to a resonant capacitor. The current in the heating coil creates a high-frequency magnetic flux, causing the effect of electromagnetic induction, as a result of which eddy currents are excited in a removable or non-removable cookware (in a pan, frying pan, etc.) made of magnetic material such as steel, iron, etc. d. As a result of these eddy currents, the cookware and products contained in it heat up.

Since the invention relates to a household device, this device should not introduce interference into the AC supply line, should operate at a unit power factor and should be protected from dangerous conditions, for example, protection against negative effects during load shedding, overcurrent, overvoltage, overheating. Another important problem associated with such devices is the cost reduction of the entire system.

Induction hobs basically have two circuit nodes: firstly, a power stage that generates energy for heat treatment, and secondly, a power control, synchronization and control circuit that drives the system and provides convenient control for the user.

There are two main types of power stages used in induction hobs: an inverter with a single transistor and a half-bridge inverter. An inverter with a single transistor is an inexpensive power stage with a single transistor, which facilitates control. On the other hand, hobs having a half-bridge inverter are two-transistor and are more expensive than when they have inverters on one transistor. But they can operate in wider power ranges and at higher frequencies, which can provide the device with the possibility of heating magnetic materials even with low resistance, such as aluminum.

There are various approaches to the implementation of control schemes for the induction system for heat treatment of products. Some of them are based solely on analog circuits designed to perform power control, monitoring, and synchronization operations, such as those disclosed in US Pat. No. 4,429,205. The main disadvantages of analog systems are that they cannot be easily modified to performance changes; Diagnosing faults in such systems can be difficult, and they are less stable and stable compared to digital systems. Other systems are based on both analog and digital circuits, such as, for example, disclosed in US Pat. No. 5,648,008. In this system, analog circuits are used only to control and drive a power inverter, but it is not preferable to fully digital systems, since it becomes very difficult to control power or adapt the excitation system in the case of a large number of coils. In addition, such a system pauses the hob on selected periods of the supply line, which creates a problem called “blinking light”.

Induction systems that are based solely on digital circuits are also known from the prior art. US Pat. No. 4,511,781 discloses a system in which a microprocessor is used to perform all control actions, but it attempts to perform all critical and fast synchronization operations using a microprocessor. Therefore, a very fast and expensive microprocessor is needed. Consequently, the cost of the system increases, and the system becomes less feasible.

In addition, the prior art claims that microprocessors are sensitive to fluctuations in the supply line, which can cause random software errors and output signals. Therefore, when exciting gate signals for a power stage, it is not recommended to rely solely on a microprocessor. However, in this invention, the power stage can be protected from damage using additional circuits. In the prior art there is no invention in which all control actions are carried out by means of a microprocessor, and it is stable, stable and competitive in terms of low cost in comparison with other systems.

Many methods are known for detecting the presence of suitable cookware on a hob. Some of these inventions use additional monitoring devices or special hardware to detect the presence or size of the cookware. In some of them, only methods were used to control various signals of power inverters in order to compensate for the load.

SUMMARY OF THE INVENTION

The main objective of the present invention is to create an induction heating plate that operates at a unit power factor, does not interfere with sound frequencies during operation, can continuously detect load changes and can operate in a wide power range, can control the temperature of cookware and protect itself from hazardous conditions, such as current overload situations and false gate signals.

The induction device for heat treatment of products according to this invention contains a power stage, which is a circuit of a quasi-resonant inverter with a single transistor (bipolar transistor with an insulated gate, IGBT). In accordance with a series of strobe pulses, the transistor continuously turns on and off. As a result of the action of these pulses, a high-frequency current arises in the heating coil and in the resonant capacitor. The current in the heating coil creates a high-frequency magnetic flux, which causes the effect of electromagnetic induction, due to which eddy currents are excited in a removable or non-removable cookware, which is placed on the hob and made of magnetic material such as steel, iron, etc. As a result of these eddy currents, the cookware and products contained in it heat up.

The induction hob is powered by an AC voltage source. A rectifier is connected between the AC voltage source and the power cascade of the induction hob to form a series of half-periods of the rectified alternating current. A heating coil is connected between the rectifier output and the semiconductor switch (insulated gate bipolar transistor). The resonant capacitor is connected in parallel to the heating coil, and the counter-connected diode is connected in parallel to the insulated gate bipolar transistor.

As mentioned in the previous section, in the prior art there is no practical system in which the microprocessor is used to energize the transistor and perform critical operations and fast synchronization operations, for example, for measuring feedback signals from a power stage. The idea of this system is to use all the advantages of the microprocessor to perform these critical operations, and using additional analog circuits to minimize the disadvantages of the digital system (for example, random software errors and output signals due to power fluctuations).

Brief Description of the Drawings

figure 1 is a functional diagram of the entire induction system for heat treatment of products that implements the invention;

figure 2 is a functional diagram in which the control units of figure 1 are replaced by a microprocessor;

figure 3 - diagram of the power stage and the input stage of the induction system for heat treatment of products;

figa-c are diagrams of the current waveform of an insulated gate bipolar transistor versus time, voltage V _{CE the} collector-emitter of an insulated gate bipolar transistor versus time and the gate signal of an insulated gate bipolar transistor versus time, respectively, for cases when the input power is relatively low;

figa-c are diagrams of the current waveform of an insulated gate bipolar transistor versus time, voltage V _CE collector-emitter of an insulated gate bipolar transistor versus time and the gate signal of an insulated gate bipolar transistor versus time, respectively, for cases when the input power is relatively high;

6a and 6b are block diagrams of a microprocessor software algorithm;

figa and 7b - scheme "subroutine 1: Obtaining user input" and "subroutine 2: Determining the duration of the off state" from the microprocessor software shown in figa and 6b;

Fig. 8 is a circuit diagram of a current transformer unit;

Fig.9 is a schematic diagram of a block of protection circuits;

figa and 10b is a diagram of a form of V _CE depending on the time after the receipt of the gate of a bipolar transistor with an isolated gate of one switching pulse and a timing diagram for the next switching pulse;

11a-d are diagrams of the current I _{L of the} inductor versus time, V _CE versus time, the gate signal V _GE versus time and the output signal of the detector of transitions through the zero level versus time;

12 is a schematic block diagram of a cooking utensil detection circuit;

13a and 13b are diagrams of the voltage V _{Cl of} the direct current line versus time and the output signal of the detector for detecting transitions through the zero level at 50 Hz for the case when suitable cooking utensils are present on the hob;

figa and 14b are diagrams of the voltage V _{DC of the DC} line versus time and the output signal of the detector of transitions through the zero level at 50 Hz as a function of time for the case when improper cooking utensils are present on the hob;

15 is a circuit diagram of a temperature to voltage converter unit;

Fig. 16 is a schematic diagram of a gate driving unit of an induction system for heat treating products.

Detailed Description of Preferred Embodiments

Figure 1, where the blocks are labeled in accordance with their functions, shows a functional diagram of the induction system for heat treatment of products. The power cascade block (10) is located where the energy is transferred from the system to the cookware (20) and where a high-frequency current and magnetic field are generated. The insulated gate bipolar transistor (IGBT) used in this unit is excited by gating pulses from the “gate exciter (11)” unit.

The “gate driver (11)” receives signals from the “controller (12) on-time duration” and “controller (13) off-time” blocks. The “on-time controller (12)” in accordance with the power level requested by the “power controller (14)” block determines the duration of the strobe pulses corresponding to the on-state. The gating pulse durations corresponding to the on state for each power level are set and different from each other. The level to which the inductor current (19) rises at this duration is also predetermined, therefore, the inductor current level (620) is also monitored. Block "controller (13) the duration of the off state" determines the corresponding duration of the off state of the strobe pulses (540). The durations corresponding to the off state do not change with a change in the power level, but depend on changes in the load. And the durations corresponding to the switched off state are set by transitions through the zero current level of the inductor or heating coil (19). Therefore, the circuit of the "detector through the zero level transitions" used in the block of "protection circuits (15)" detects the moments of transitions through the zero current level of the inductor (19), and at this moment a signal is transmitted to the controller (13) of the off state duration .

The current of the inductor (19) is measured and converted into a low-level voltage signal by the current transformer (16) unit. A “power controller (14)" receives user input signals, such as a predetermined power level or a temperature of a cookware; and accordingly sets the duration of the on state for the gating pulse (540). It also controls the peak level (610) of the inductor current and the temperature of the cookware (20) to adjust the power level and temperature of the cookware (20). In addition, it monitors the outputs (570A, 570B, 590) of warning signals and, in a certain case, suspends operation.

The block of "protection circuits (15)" is intended to protect the system from current overload conditions and from erroneous gate signals (520). This unit continuously monitors the voltage V _CE collector-emitter of an insulated-gate bipolar transistor and the inductor current level (620); and forces the power control unit (14) to stop the system when a dangerous condition occurs. The cooking utensil detection circuit (17) monitors the voltage (500) of the DC line of the power stage (10) and forces the power controller unit (14) to stop operation if it detects unsuitable cookware.

Figure 2 shows the same structural diagram at the functional level as in figure 1, for the case when the system uses a microprocessor (18) to communicate with the user, regulate the power and temperature of the dishes (20) for cooking and monitoring for dangerous conditions.

The “power cascade” circuit (10) creates a high-frequency (about 20–25 kHz) electromagnetic field for heating magnetic cookware (20). Figure 3 shows a detailed circuit diagram of the power stage (10). The power circuit is connected to the network through a “half-wave rectifier” (24), D1, D2, D3 and D4. The output voltage (500) of a half-wave rectifier (24) is supplied to the power stage (10) from a high-frequency shunt capacitor C1. The voltage at C1, V _C1 or V _DC is also called the voltage (500) of the DC line. The C1 capacitance is not large enough to make V _{DC a} smoothed _DC signal; therefore, V _DC represents a plurality of half-cycles of the rectified mains voltage. Since the switching frequency (about 20 kHz) of an insulated gate bipolar transistor is much higher than the mains frequency (50 Hz), the voltage V _DC (500) of the direct current line can be considered constant during the switching period (about 50 μs).

The transistor (insulated gate bipolar transistor) turns on and off under the influence of pulsed signals (520) from the excitation circuit to bring the heating coil (19), L, and the capacitor C _RES parallel to it into a resonance state. In accordance with this, the heating coil (19) creates a magnetic flux, which causes the effect of electromagnetic induction, as a result of which eddy current is excited in the magnetic cookware (20). When the transistor turns on, the closed-circuit current I _Cres passes through the resonant capacitor and the insulated-gate bipolar transistor for a very short period of time (within a few microseconds). During the remaining duration of the on state, the current I _L flows through the heating coil (19) and the insulated gate bipolar transistor. The sum of these two currents is the current of the insulated gate bipolar transistor and is shown in FIG. 4a. The level I _Cres depends on the voltage level V _CE collector-emitter of a bipolar transistor (510) with an insulated gate at the moment of switching to the on state. Form V _CE is presented in fig.4b. As you can see, the transistor turns on at the moment when the collector voltage is not equal to zero. Therefore, the voltage at the resonant capacitor is forced to change to become equal to the value of V _CE (510) at the time of inclusion. Therefore, the following equation can be written:

C _RES H _Cres / _fi = C _RES HV _CE / _fi = I _Cres ,

where t _fi shown in fig.4b, is defined as the duration of the decline V _CE (510) to zero, that is, the time to switch to the on state. In addition, V _Cres is a voltage change at C _RES .

As the on-state durations increase, the power taken from the supply line also increases. The current of an insulated gate bipolar transistor for the case of high power output is shown in FIG. Since V _CE (510) at the moment of switching on is 0 V, V _{Cres is} also equal to zero. Therefore, using the above equation, we find that I _Cres must be equal to zero. It follows from FIG. 5a that only the inductor current (19) flows through a bipolar transistor with an insulated gate. The shape of the collector voltage (510) is shown in Fig.5b. Due to the properties of the resonant circuit, V _CE tends to reach negative values, but the on-off diode D _P prevents these negative cycles. Therefore, when D _P is in a conductive state, then, as shown in FIG. 5b, V _{CE is} limited to 0 V.

If we assume that for 20 ms, which correspond to the input signal period (V _ac ) of an alternating current of 50 Hz, the corresponding on states and the corresponding off states of the duration of the strobe pulses (520) are unchanged, and the current I _Cres is negligible, then we can take that the average current of an insulated gate bipolar transistor, that is, the input current, is proportional to the voltage (500) of the direct current line and, therefore, V _ac . As a result, the system draws current from the network at a unit power factor, i.e., the input current and V _ac are in phase.

The digital device (18) is responsible for starting and stopping the power stage (10). The microprocessor (18) starts the power stage (10) when power is supplied to the system and input data is received from the user. The microprocessor (18) suspends operation when the temperature of the cookware (20) reaches a value previously set by the user. In addition, it blocks the operation of the inverter when unsuitable cookware is detected, and at the same time restarts the system. Upon receipt of a fault signal (570A, 570B, 590) from analog peripheral circuits (15, 17), the microprocessor (18) processes this signal and blocks the strobe signals (540) for a certain period of time, after which it restarts after a period of shutdown.

The digital device (18) adjusts the power level by adjusting the duration of the strobe pulses (540) corresponding to the on states; to control the power level does not require operation in intermittent mode. When starting, the hob starts to heat up at minimum power, which means the shortest strobe signals corresponding to the on state, and then the power rises until the power level set by the user is reached. Power is controlled by monitoring the peak current value (610) of the inductor; this value is directly related to the power output.

The microprocessor program algorithm (18) will be described using the flowchart shown in FIGS. 6a and 6b. When power is supplied to the system (100), the microprocessor (18) reads the user input, as shown by subprogram 1 (110): OBTAINING USER DATA, which is shown in Fig. 7a. When executing subroutine 1 (110), the microprocessor (18) reads the MODE and LEVEL (111) variables set by the user using the user panel, which is not shown in the drawing due to necessity. The MODE can be TEMPERATURE or POWER (112). If it is TEMPERATURE, this means that the induction hob will work as a temperature controlled system (114). Therefore, it will operate at maximum power, designated as maximum_power, until the temperature reaches the value specified by the user, designated as final_temperature, which is equal to the variable LEVEL. If the MODE is POWER (113), this means that the hob will operate at a power set by the user, which is equal to the variable LEVEL. In addition, operation will be suspended for safety reasons if the temperature reaches the maximum allowable value designated as maximum_temperature. Maximum_temperature and maximum_power are constant parameters of the system and cannot be changed by the user.

After receiving user input, the current power level, the power_ level at which the hob operates, is set at the minimum_ power level (120). Durations of the on state are predetermined values that vary according to the current power level. Each power level has its own predetermined on-time durations (120). Minimum_power is a constant parameter of the system and cannot be changed by the user.

The duration of the off state depends on the resonant frequency of the power stage (10), namely, on changes in the load; therefore, they must be updated dynamically. During start-up, in order to determine the duration of the off state, one gating pulse is first generated (130). "Gating" is inherent in the modulated in duration (PWM) microprocessor output signal. The first parameter of the "gating" function is the duration of the gating signal (520) corresponding to the on state, and the second parameter is the duration of the off state. To generate one pulse, a predetermined on-state duration is transmitted as the first parameter, and a sufficiently large duration of the order of 1 s (this value is not mandatory, only preferred) is entered (130) as the off-state duration, so that at the gate output (520) microprocessor (18) will be formed only one pulse.

After the formation of one gate pulse, subroutine 2 is called: DETERMINATION OF THE DURATION OF THE OFF STATE (140) shown in Fig. 7b. In this subroutine, at the time (141) the gating signal is turned off (520), the timer is turned on (142) and the time interval (143) is counted up until the start of the pulse cutoff of the zero-crossing sensor. This duration is stored as the duration (144) of the off state.

After determining the durations of the gating signal corresponding to the on and off states, the gating signals can be triggered (150) by using the "gating" function.

After every 1 s, the user input is controlled if the user has made any update, and every 100 ms (this value is not mandatory, only preferred), the level (610) of the peak current of the inductor is controlled; power_level is updated so that the final power_level can be reached; and finally, the temperature of the cookware is controlled (20). Two timers are used to count these time periods, timer 1 and timer 2. These timers are turned on (160, 170) only after the start of the strobe pulses (540). When the DC link voltage reaches a peak value, the off-state duration is adjusted by using routine 2 (140) described above. This operation is repeated in a cycle every 10 ms (this value is not required, only preferred).

Thus, there are 3 nested cycles (each has its own timer: timer 1, timer 2, timer 3), which are continuously repeated until the system is interrupted. The envelope of the inductor current value (610) is monitored every 100 ms, if the peak value (610) of the current is not within ± 20% (this value is not mandatory, only preferred) of the expected value, then the operation is suspended (200) for 3 s, and the whole procedure starts over, otherwise the software continues its work (190).

The current power_level is compared with the final power_level, which is the power level specified by the user (190). If the power_level is greater than the final power_level, then the power_level is reduced (210). If they are equal, no adjustment is made (220); otherwise, power_level rises (230). The temperature of the cookware (20) comes from the analog peripheral circuit (21) and is stored as a variable TEMPERATURE. TEMPERATURE is compared with the value, with the final_temperature set by the user (240). If the temperature of the cookware (20) reaches the set value, the system stops for 10 s (this value is not mandatory, only preferred) and the whole procedure resumes (250) from the beginning. Otherwise, the software continues (260) to work.

The shortest cycle ends at the peak of the signal (500) of the DC link; this moment occurs 5 ms after the output signal level (580) of the detector (23) of transitions through the zero level at 50 Hz becomes high (if the network frequency is taken equal to 50 Hz). If this output signal level (580) is not high, then the output signals (570A, 570B) of the protection circuits (15) and the output signal (590) of the cookware detection circuit (17) are monitored (280). If any of them (570A, 570V, 590) is a high-level signal, this means that the wrong cookware is on the hob or a dangerous situation has occurred, so the strobe signals (540) are interrupted (200) for 3 s. After a lapse of 3 s, the entire procedure starts (100) again. If they (570A, 570B, 590) do not have a high level, the output signal (580) of the detector (23) transitions through the zero level at 50 Hz is again monitored (260) cyclically.

When the level of the output signal (580) of the detector (23) transitions through the zero level at 50 Hz becomes high, the microprocessor (18) resets "timer 3", and begins (270) 5 ms countdown. After counting this period of time, he (18) also controls the output signals (570A, 570B) of the protection circuits (15) and the output signal (590) of the cooking utensil detection circuit (17). If any of them (570A, 570B, 590) has (320) a high level, the gate signals (540) are interrupted (200) for 3 s. Otherwise, after 3 ms has elapsed (330), timer 3 is monitored. If this is not done, then danger signals (570A, 570B, 590) are monitored cyclically (320). In addition, the gating pulses (540) corresponding to the off state are corrected by using subroutine 2 (140) and, accordingly, the gating signals corresponding to the off state are corrected (290).

Then, if the cycle times are excessive (300, 310), timers are monitored. If the duration exceeds (310) 100 ms, then timer 2 is reset (170), and the procedure described above is repeated. If a duration of 1 s is exceeded (300), then using the subroutine 1 (110) to update, the user input is controlled and then timer 1 is reset (160), and the cycle repeats.

The current transformer unit (16) converts the inductor current (19) into a voltage value, which is indicated as the inductor current level (620). On Fig shows a schematic diagram of the block (16) of the current transformer. On the primary side of the transformer there is a heating coil (19), and the secondary side is the one on which the converted voltage is released on the resistor R10. The output voltage V _OUT of the current transformer is determined by a transformation ratio of 1: N as

V _OUT = R10CHI _L / N,

where: I _L is the current flowing through the heating coil (19).

In addition, by using D10 and C10, the peak value of V _OUT and, as a result, the peak value (610) of the inductor current is stored and fed to the microprocessor (18).

The protection circuits (15) and the cooking utensil detection circuit (17) are designed to prevent unexpected dangerous states of the power circuit (10) arising, for example, due to microprocessor failure (18).

An analog protection circuit (15) monitors the inductor current level (620) and the collector voltage (510) of the semiconductor switch. When the level (620) of the inductor current exceeds the maximum permissible value (V _{reference, current} ), the output level of the comparator 1 (COMP1) becomes high. This output signal turns on the bipolar transistor T3 (530); therefore, the output of the gate signal (520) is locked in a low state, so that the power inverter (10) is blocked. Consequently, the power circuit (10) and the insulated gate bipolar transistor are protected against current overloads. In addition, a malfunction signal (570) is supplied to the microprocessor (18) to block the output gate signals (540) of the digital circuits (18). Similarly, when the collector voltage (510) of the transistor is higher than a predetermined value (V _{reference voltage} ), the output of the gate signals (520) is fixed in a low state by T3 (530), so that false gate signals that may be caused by a microprocessor failure are excluded. If the gating pulse (540) arrives at the moment when the output signal level of the comparator 2 (COMP2) is high, then this gating signal cannot turn on the insulated gate bipolar transistor. This is because the gate voltage (520) with collector voltage T3 (530) is already reduced to a low state. So the power stage (10) and the insulated gate bipolar transistor can be protected. But the output signal (570V) of the element And (1) will change to a low state, indicating a dangerous state. Therefore, the formation of gating pulses (540) by the microprocessor (18) will stop.

Since losses of an insulated-gate bipolar transistor when switching off and in a conducting state are inevitable, only switching-on losses can be minimized. To solve this problem, it is necessary to analyze V _CE (510), since it is the most important parameter that affects the magnitude of the power-on losses. As can be seen from Fig. 10a, after the transistor is turned off, oscillations V _CE (510) occur at the resonant frequency of the heating coil (19) and the resonant capacitor C _RES . Therefore, at constant load, heating coil (19) and C _{RES, the} minimum V _CE (510) is always observed at the same moment after switching off.

The duration of the off state is constant for half the period of the alternating current and is directly related to the resonant frequency of the power inverter (10). But due to changes in load, the resonant frequency may vary. To compensate for these changes, the current of the inductor (19) is controlled by a current transformer (16), which is also used to protect against overcurrent. Theoretically, the minimum voltage V _CE (510) exists when passing through the zero current level of the inductor (19). 11 a and 11 b show typical current forms of the inductor (19) and V _CE (510), respectively. Thus, by monitoring the inductor current level (620), off-time durations can be obtained with minimal energy loss. A controlled level (620) of the inductor current is inverted by an inverting amplifier (22) and fed to the microprocessor as the output signal (560) of the detector for transitions through the zero level. Controlling this signal, the microprocessor (18) corrects (290) the duration of the signals corresponding to the off state every 10 ms. It (18) calculates the time elapsed between the moment of transition of the gating signal (140) to the state corresponding to switching off and the cut-off of the output pulse (560) of the detector (22) of transitions through the zero level. On fig.11d shows the output signal (560) of the detector (22) transitions through the zero level.

The system also includes a circuit called a cookware detection circuit (17) for detecting whether a suitable cookware (20) is installed on the hob. The circuit (17) controls the signal (500) of the DC line voltage. In accordance with the monitored signal, through the detection of the cooking utensils for cooking (17), a decision is made as to whether or not the cookware (20) is on the hob. If there is no suitable cookware, this circuit (17) sends a malfunction signal (590) to the microprocessor (18) in order to block the operation of the power inverter (10) for a predetermined period of time.

If no suitable cooking utensils are available on the hob, no energy will be transferred from the power stage (10) to the cooking utensils (20). Detecting the presence of cookware (20) is based on a change in the shape of the voltage (500) of the DC line. On figa this voltage (500) is shown for the case when there is a suitable cookware (20) for cooking. For the case where there is no suitable cooking utensil, the waveform (500) takes the form of a signal in figa. Due to the fact that there is no energy transfer when there is no suitable cooking utensil, the energy will be stored in a high-frequency shunt capacitor C1. Therefore, the presence of cookware (20) can be detected by evaluating the shape (500) of this signal.

The detector (23) of transitions through the zero level at 50 Hz during the transition through the zero level of the signal (500) of the direct current line generates a high level signal (580) with a duration of approximately 400 μs. This output signal (580) is also supplied to the microprocessor (18) to detect the moment corresponding to the peak voltage of the DC line. The voltage (500) of the DC line is controlled by a resistor divider at the inverting input of the comparator 3 (COMP3) shown in Fig. 12. The level at the non-inverting input, that is, the reference value, is a small part of the peak voltage value (500) of the DC line. Therefore, when the voltage (500) of the DC line drops below a certain value, then, as shown in FIG. 13b, the output signal (580) of the comparator goes into a high level state.

As can be seen in FIG. 14b, in the case when there is no suitable cookware, transitions through the zero level do not occur. Therefore, when there is no suitable cooking utensil, the detector (23) of transitions through the zero level at 50 Hz does not generate a high level output signal. The cooking utensil detection circuit (17) monitors these output signals and, if for a period of 400 ms there is no high level output signal (580) generated by the detector (23) transitions through the zero level at 50 Hz, it sends a lock signal (590) to the microprocessor (18) to stop the power stage (10). After 3 s after the termination of work, it (17) starts up the system again to detect whether the user has placed suitable cookware (20) for cooking on the hob. Then, for 400 ms, it monitors the output signals (580) of the detector (23) transitions through the zero level at 50 Hz and blocks the system or leaves it in standby mode in accordance with the level of this signal, that is, until the kitchen utensils appear (20) .

The output signal (580) of the detector (23) transitions through the zero level at 50 Hz is supplied to the input of the element NAND (A) in Fig.13. The output signal of the element NAND (A) charges the capacitor C23, and if its input signal is low, this means that transitions through the zero level do not occur. The time spent on the C23 charge to a value at which a low state is established at the output of the NE-I (B) element is approximately 400 ms. If the transition through the zero level does not occur, C23 is charged to a threshold value, the output signal of the NAND element (B) becomes low, and the output signal (590) of the element NAND (C) becomes high, and it goes to the microprocessor ( eighteen).

On Fig shows a schematic diagram of a block (21) of the temperature to voltage Converter. A negative temperature coefficient thermistor (OTC) is placed under the hob of the hob, and it responds to the temperature of the cookware (20). The NTC thermistor and R11 form a resistor divider, and the voltage of the node (600) changes when the temperature changes. Therefore, by monitoring the voltage of the assembly (600), the microprocessor can obtain data on the temperature of the cookware (20).

The composition of the invention includes a gate driver (11) with one cascade output, intended for direct excitation of an insulated gate bipolar transistor. This block is necessary because the strobe pulses (540) of the microprocessor (18) vary from 0 to 5 V, and to obtain the best performance in the case of an insulated gate bipolar transistor, it is necessary to have from 0 to 15 V. When the strobe signal (540) corresponding to the on state comes from the microprocessor (18), the lower transistor T1 closes, and the upper transistor T2 opens. Therefore, the gate of the bipolar transistor with an insulated gate through the node (520) and the upper transistor T2 will receive a signal of 15 V. When the gate signal corresponding to the off state (540) comes from the microprocessor (18), the lower transistor T1 opens and the upper transistor T2 closes. Therefore, the gate of the insulated gate bipolar transistor will be grounded through the node (520) and the lower transistor T1. Through the node (530), the gate exciter output can be grounded by protection circuits (15), regardless of the signal from the microprocessor (18). This prevents the failure of a powerful transistor with false gating signals (540) due to a microprocessor failure.

Claims (7)

1. An induction device for heat treatment of products containing a circuit (10) of a power inverter, receiving from the network a current with a substantially unit power factor supplied to an induction heating coil (19), which creates a high-frequency magnetic field that induces removable or non-removable magnetic cookware (20), a resonance capacitor (C _res), parallel to the heating coil (19), insulated-gate bipolar transistor as a switching element connected in series to the parallel in the contour of the heating coil (19) and the resonance capacitor (C _res) and a power diode (D _P), included in antiparallel insulated-gate bipolar transistor; a microprocessor (18), receiving from the user panel the set temperature of the cookware or the set power level of the device for heat treatment of products, controlling the power transmitted to the cookware (20) by adjusting the duration of the strobe pulses (520, 540) corresponding to the on state by exciting the bipolar an insulated gate transistor through a gate driver (11) that controls the temperature of the cookware (20) by interrupting the operation of the system when the temperature of the cookware (20) reaches the set value and the temperature data comes from the unit (21) converting the temperature to voltage, or by restarting the system after waiting for a predetermined period of time after it was interrupted, regulating the duration of the strobe pulses corresponding to the off states by observing the moments transitions through the zero level of the current flowing through the heating coil, using the detector means (22) transitions through the zero level and interrupts the work a power inverter circuit (10) for a predetermined period of time when a warning signal (570A, 570B, 590) is received from the cooking utensil detection circuit (17) or from the protection circuit unit (15); a cooking utensil detection circuit (17) connected between the microprocessor (18) and the power inverter circuit (10) and detecting whether suitable cookware (20) is placed on the hob by monitoring the voltage (500) of the DC line and comprising a detector means (23) of transitions through a zero level at 50 Hz for detecting transitions through a zero level of an AC input voltage signal (V _ac ); a block of analog protection circuits (15), connected between the microprocessor (18), the gate driver (11), and the power inverter circuit (10), which protects the power inverter circuit (10) from current overloads by controlling the current flowing through the heating coil (19) ) by means of a current transformer unit (16), and at the same time from inappropriate gate signals by monitoring the collector voltage of an insulated gate bipolar transistor (510) and gate signals (540). 2. The induction device according to claim 1, in which the microprocessor (18) in power mode receives a power input signal from the user panel, compares the set temperature of the cookware (20) with a predetermined value of the maximum allowable temperature of the cookware (20) and performs circuit operation (10) power inverter with selected input power signal; in temperature mode, receives the temperature input from the user panel, operates the power inverter circuit (10) at the maximum power level, stops the system when the temperature of the cookware (20) reaches the set value and restarts after waiting for a predetermined period of time . 3. The induction device according to claim 1, in which the microprocessor (18) starts the power inverter circuit (10) with a minimum power level of the device for heat treatment of products, compares the current power level with the power level that the user entered using the user panel, increases or reduces the current power level to equalize both levels; generates gating pulses (540), which have durations corresponding to the on states, corresponding to a power level at this moment; compares the peak level (610) of the inductor current with the expected value and pauses the system for 3 s if it is not within ± 20% of the expected value. 4. The induction device according to claim 1, in which the microprocessor (18) monitors the first output signal (580) of the detector (23) transitions through the zero level at 50 Hz and waits for 5 ms in the case when the output signal (580) getting high; calculates the time elapsed between the moment of switching to the off state of the current gate pulse (540) and the moment when the output signal (560) of the detector (22) transitions through the zero level changes from high to low; uses this calculated time as the off state of the duration of the strobe signals (540) until the next calculation. 5. The induction device according to claim 1, in which the microprocessor (18) monitors the output signal (590) of the cookware detection circuit (17) and the output signal (570A, 570B) of the protection circuit block (15); pauses the strobe signals (540) for 3 s and restarts the system after the end of this period of time if any of these output signals (570A, 570B, 590) is high. 6. The induction device according to claim 1, in which the cooking utensil detection circuit (17) comprises a detector means (23) of transitions through a zero level at 50 Hz, comparing the voltage (500) of the direct current line with a reference value by means of a comparator 3 (COMR3) and generating a high level output signal (580) if the DC link voltage (500) is lower than this reference value, or a low level output signal (580) otherwise; monitors the inversion of this output signal (580) using the element NAND (A); charges the capacitor (C23) with the output signal of the element NAND (A) through R56 (with the time constant R56 × C23), if the output signal (580) of the detector (23) transitions through the zero level at 50 Hz is low, discharges it through (R55 // R56) (with a time constant (R55 // R56) × C23) otherwise; sends a high level signal to the microprocessor (18) if C23 is charged to a value above the input threshold value NAND (B), a low level otherwise; charges the capacitor (C22) with the output signal NAND (C) through R59 and R52 (with a time constant (R52 + R59) × C22), if the output signal (590) of the cooking utensil detection circuit (17) is high, discharges it through R59 (with a time constant of R59 × C22) otherwise; discharges C23 via R58 by setting the output signal of the comparator 4 (СОМР4) to a low level, if C22 is charged above a predetermined reference value in the time interval determined by the time constant ((R52 + R59) × C22), the discharge is not performed otherwise; resets the output signal NAND (C) to a low state when C23 discharges below the input threshold NAND (B). 7. The induction device according to claim 1, in which the block (15) of the protection circuits comprises a detector means (22) of transitions through the zero level, inverting the level (620) of the inductor current obtained from the block (16) of the current transformer, and sending the specified level to microprocessor (18) as an output signal (560); contains a comparator 1 (COMP1) comparing the indicated level (620) of the inductor current with a reference value (V _{reference, current} ), which corresponds to the maximum allowable current value, sending a high level signal (570) to the microprocessor (18) if the specified level (620 ) the inductor current is higher (V _{reference, current} ), or the signal (570) is low otherwise; turns on the bipolar junction transistor (T3) and brings the gate signals (520) to a low level using the node 530 if the output signal of the comparator 1 (COMP1) is high, or does not affect the strobe pulses otherwise; contains a comparator 2 (СОМР2) comparing the collector voltage of the bipolar transistor (510) with an insulated gate with a reference value that corresponds to the maximum permissible value (510) of the collector voltage before turning on the bipolar transistor with an insulated gate, and generating a high level output signal if the collector voltage (510) above the reference, or a low signal otherwise; sends a high level signal (570) to the microprocessor (18) if the output signal of the comparator 2 (COMR2) is high and at the same time the output gate signal (540) is high, or a low level signal otherwise; turns on the bipolar junction transistor (T3) and lowers the gating signals (520) using node 530 if the output signal of the comparator 2 (COMP2) is high, or does not affect the gating pulses otherwise.

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