EP1629698B1

EP1629698B1 - Induction cooktop

Info

Publication number: EP1629698B1
Application number: EP03728210A
Authority: EP
Inventors: Baris Colak; H. Bulent Ertan
Original assignee: Tubitak-Bilten (Turkiye Bilimsel Ve Teknik Arastirma Kurumu-Bilgi Teknolojileri Ve Elektronik Arastirma Enstitusu); Tubitak Biltien
Current assignee: Tubitak Uzay
Priority date: 2003-05-28
Filing date: 2003-05-28
Publication date: 2006-12-27
Anticipated expiration: 2023-05-28
Also published as: WO2004107819A1; EP1629698A1; DE60310774D1; ATE349880T1; DE60310774T2; ES2279950T3; AU2003232875A1

Abstract

An induction cooking system including a power inverter (10), a microprocessor (18), a protection circuit (15) and a pan detection circuit (17) is provided. The power control, timing and a monitoring of the power inverter circuitry (10) is managed primarily by a microprocessor (18). Also an additional analog protection circuit (15) is provided to protect the system against hazardous conditions, such as over current and inappropriate gating signals that may be resulted by the malfunction of the microprocessor (18). So that main disadvantage of a system controlled primarily by a microprocessor is eliminated. Another circuit (17) for detection the presence of the suitable cookware is provided.

Description

Technical field

This invention relates to an induction heating system, more particularly induction cooking appliance for use as a home appliance.

Background of the invention

Induction cooking devices or cooktops are more reliable and efficient, flameless, and thus safer appliances when compared with other cooking appliances. In induction cooktops high frequency current is generated in the heating coil, which is usually coupled to a resonant capacitor. The current in the heating coil generates a high frequency magnetic flux that causes an electromagnetic induction action that generates eddy currents in a removable or non-removable cookware(pan, frying pan etc), which is made of a magnetic material such as steel, iron, etc. As a result of these eddy currents the cookware and the food contained therein is heated.
Since the invention is related to a home appliance, the appliance should emit no noise to AC power line, should operate at unity power factor, and should have protection against hazardous conditions, such as no load, over current, over voltage, over heat protections. Another important aspect of such appliances is the reduction of the cost of the overall system.
There are mainly two parts of the circuitry in the induction cooktops: First, a power stage produces power to perform the cooking and second, a power control, timing, and monitoring circuit operates the system and provides convenient control for a user.
There are two types of main power stages used in induction cooktops: Single Transistored Inverter and Half Bridge Inverter. Single transistored inverter is a low cost power stage with a single transistor that makes it easy to control. On the other hand cooktops having half-bridge inverter are two-transistored and have higher costs than those single-transistored inverters have. But they can operate in wider power ranges and in higher frequencies, which can allow the appliance to heat even magnetic materials with low resistance, such as aluminum.
There have been various approaches for implementing the control circuitry of an induction cooking system. Some of them rely solely on the analog circuitry for power control, monitoring and timing operations, such as that is disclosed in U.S. patent number 4429205. Main disadvantages of the analog systems are that they cannot be easily modified to change the operating characteristics, troubleshooting such systems can be difficult, and they are less stable and robust when compared with digital systems. Other circuits rely on both analog and digital circuits, such as that disclosed in U.S. 5648008. This system uses analog circuitry for only monitoring and driving power inverter, but this is not more advantageous than fully digital systems, since power control or adapting the drive system for multi-coil systems becomes very difficult. Besides, this system stops the operation of cooktop at selected powerline cycles that causes the problem called "light flicker".
Induction systems that rely on only digital circuits are also known in prior art. US 4,511,781 is a system that employs a microprocessor for all the control actions, but it tries to handle all the critical and fast timing operations with the microprocessor. So a very fast and expensive microprocessor is needed. Consequently, the cost of the system increases and the system becomes less feasible.
Moreover, in the prior art it is claimed that microprocessors are sensitive to powerline fluctuations, which can cause random program errors and outputs. So it is not advised to solely rely on the microprocessor to drive gating signals of the power stage. But in this invention, with the aid of additional circuitry power stage could be prevented from damaging. In the prior art, there is no invention that performs all control actions via a microprocessor, is stable and robust, and has a competitively low cost as other systems.
Many approaches have been made to detect the presence of a suitable cookware on the cooktop. Some of those inventions have used additional observers or special hardware to detect the presence or the dimension of the cookware. Some of them only used techniques to monitor various signals of the power inverters for load compensation.

Summary of the Invention

The main objects of the present invention are to provide an induction heating cooker that operates at unity power factor, causes no audible noise during operation, can continuously detect to the load variations and can operate in a wide power range, can control the temperature of the cookware, and protect itself against hazardous conditions, such as over-current situations and false gate signals.
The induction cooking apparatus in this invention comprises of a power stage, which is a quasi-resonant inverter circuit with a single transistor (Insulated Gate Bipolar Transistor, IGBT). According to the series of the gating pulses the transistor is turned on and off continuously. As a result of these pulses a high frequency current is generated in the heating coil and the resonant capacitor. The current in the heating coil generates a high frequency magnetic flux that causes an electromagnetic induction action that generates eddy currents in a removable or non-removable cookware, which is placed on the cooktop and made of a magnetic material such as steel, iron, etc. As a result of these eddy currents the cookware and the food contained therein is heated up.
The induction cooktop system is fed from a source of AC voltage. A rectifier is connected between the AC voltage source and power stage of induction cooktop for generating series of rectified AC half cycles. The heating coil is connected between the output of the rectifier and the semiconductor switch (IGBT). The resonant capacitor is connected parallel to the heating coil and an anti-parallel diode is connected parallel to IGBT.
As mentioned in the previous section, in the prior art there is no feasible system, which employs a microprocessor for driving the transistor and critical and fast timing operations, such as measuring feedback signals from power stage. The aim of this system is to exploit all the advantages of a microprocessor for these critical operations and with the aid of additional analog circuitry to minimize the disadvantages of a digital system (e.g. random program errors and outputs due to power fluctuations).

Brief description of the drawings

Figure-1 shows the functional block diagram of the whole induction cooking system embodying the invention.
Figure-2 shows the block diagram where the controlling blocks in Figure-1 are replaced by the microprocessor.
Figure-3 shows the power stage and input stage of the induction cooking system.
Figure-4a and 4c show the IGBT current waveform vs. time, Collector-Emitter voltage, V_CE, of the IGBT vs. time, and IGBT gate signal vs. time, respectively; when the input power is relatively low.
Figure-5a and 5c show the IGBT current waveform vs. time, Collector-Emitter voltage, V_CE, of the IGBT vs. time, and IGBT gate signal vs. time, respectively; when the input power is relatively high.
Figure-6a and 6b show the flow diagram of the microprocessor software.
Figure-7a and 7b show the "Routine 1: Get User Inputs" and "Routine 2: Determine Turn-off Duration" of the microprocessor software described in Figure-6a and 6b.
Figure-8 shows the circuit diagram for the current transformer block.
Figure-9 shows the circuit diagram for the protection circuits block.
Figure-10a and 10b show V_CE waveform vs. time, after the gate of IGBT receives a single turn-on pulse, and the timing of the next turn-on pulse.
Figures 11a-11d show inductor current, I_L, vs. time, V_CE vs. time, gate signal, V_GE, vs. time, and Zero Cross Output versus time.
Figure-12 shows the circuit diagram of the pan detection circuit.
Figure-13a and 13b show DC link voltage, V_C1, vs. time, and output of 50 Hz zero cross detector vs. time; when a suitable pan is present on the cooktop.
Figure-14a and 14b shows DC link voltage, V_DC, vs. time, and output of 50 Hz zero cross detector vs. time; when no suitable pan is present on the cooktop
Figure-15 shows the circuit diagram of Temperature to Voltage Converter block.
Figure-16 shows circuit diagram of gate drive block of the induction cooking system.

Detailed Description of the preferred embodiments

Figure-1, where the blocks are labeled according to their functions, shows the block diagram representation of the induction cooking system. "Power Stage" (10) block is where the energy transfer from the system to the cookware (20) occurs and high frequency current and magnetic field are generated. The Insulated Gate Bipolar Transistor (IGBT) employed in this block is driven by the gate pulses sent by the "gate drive" (11) block.
"Gate drive" (11) receives signals from "turn-on duration controller" (12) and "turn-off duration controller" (13) blocks. "Turn-on duration controller" (12) determines the turn-on duration of gate pulses according to the power level required by the "power controller" block (14). Turn-on durations of gate signals of each power level are defined and different from each other. The level that the inductor (19) current rise in that duration is also defined, therefore the inductor current level (620) is also monitored. "Turn-off duration controller" (13) block determines the turn-off durations of gate pulses (540). Turn-off durations do not vary with the power level, but they are sensitive to load variations. And turn-off durations are defined by the zero-crosses of the inductor or heating coil (19) current. So "Zero-Cross Detector" (22) circuit employed in "Protection Circuits" (15) block detects the zero-cross instants of the inductor (19) current and sends a signal to turn-off duration controller (13) at this instant.
The inductor (19) current is measured and transformed into a low-level voltage signal by "current transformer" (16) block. "Power controller" (14) gets the user inputs, such as desired power level or pan temperature; and defines gate pulse (540) turn-on durations accordingly. It also monitors the inductor current peak level (610) and pan (20) temperature to control power level and pan (20) temperature. Furthermore, it monitors the warning signal outputs (570A, 570B, 590) and stops the operation in such a case.
"Protection circuits" (15) block is designed to protect the system against over-current conditions and erroneous gate signals (520). This block continuously monitors the collector-emitter voltage, V_CE, of IGBT and the inductor current level (620); and makes power control block (14) terminate the operation of the system, when a hazardous condition occurs. Pan detection circuit (17) observes the DC link voltage (500) of the power stage (10) and makes power controller block (14) stop the operation, if it detects an improper cookware.
Figure-2 shows the same functional block diagram as Figure-1, when a microprocessor (18) is employed in the system for user interface, power level and pan (20) temperature controlling, and observation of hazardous conditions.
"Power stage" (10) circuit generates a high frequency (around 20-25 KHz) electromagnetic field for heating magnetic cookware (20). Figure-3 shows the detailed circuit diagram of the power stage (10). The power circuit is connected to the mains through a "full-wave rectifier" (24), D1, D2, D3, and D4. The output (500) of the full wave rectifier (24) is fed to the power stage (10) through a high-frequency bypass capacitor, C1. The voltage across C1, V_CI or V_DC, is also called DC link voltage (500). C1 is not big enough to make V_DC a smooth DC signal; so V_DC is the plurality of rectified powerline half cycles. Since the switching frequency (about 20 kHz) of the IGBT is much higher than the mains frequency (50 Hz), DC link voltage (500), V_DC, could be assumed to be constant during a switching period (about 50 µsec).
The transistor (IGBT) is turned on and off in response to pulse signals (520) from the driving circuit to put a heating coil (19), L, and a capacitor parallel to it, C_RES, into a resonant state. Accordingly, the heating coil (19) generates a magnetic flux, which causes an electromagnetic induction action to generate an eddy current in a magnetic cookware (20). When the transistor is turned on, a short circuit current, I_Cres, passes through the resonant capacitor and IGBT for a very short duration (a few microseconds). During the rest of the on-time duration the current, I_L, flows through the heating coil (19) and IGBT. The sum of these two currents is the IGBT current and drawn in Figure-4a. The level of I_Cres is dependent to the level of collector emitter voltage of IGBT (510), V_CE, at the turn-on switching instant. The waveform of V_CE (510) is drawn in Figure-4b. As it can be seen the transistor is turned on at the instant where the collector voltage is not zero. So the voltage across the resonant capacitor is forced to a change equal to value of V_CE (510) at the turn-on instant. Thus, following equation can be written: $C_{RES} * {ΔV}_{Cres} / {Δt}_{fi} = C_{RES} * V_{CE} / {Δt}_{fi} = I_{Cres}$
Where Δt_fi, shown in Figure-4b, is defined as the duration that V_CE (510) falls to zero, namely turn-on switching time. Also ΔV_Cres is the change of the voltage across C_RES.
When turn-on durations are increased, power drawn from the powerline increases, too. The IGBT current for the high power output case is drawn in Figure-5a. Since V_CE (510) at the instant of turn-on is zero volts, ΔV_Cres is equal to zero, too. So using previous equation, I_Cres is found to be zero. Hence, it is obvious in Figure-5a that only inductor (19) current flows through IGBT. The collector voltage (510) waveform is drawn in Figure-5b. Due to the nature of the resonant circuit V_CE tends to go to negative values, but the anti-parallel diode, D_P, prevents these negative cycles. Therefore, V_CE (510) is limited to zero volts when D_P is conducting as indicated in Figure-5b.
If for 20 milliseconds, which is the period of 50 Hz input AC signal (V_ac), turn-on and turn-off durations of gating pulses (520) are assumed to be constant and I_CRES current is neglected, then it could be accepted that the average of IGBT current, namely input current, is proportional to DC link voltage (500), hence to V_ac. Consequently, the system draws current from mains at unity power factor, that is the input current and V_ac are in the same phase.
The digital device (18) is responsible for starting and stopping the operation of the power stage (10). The microprocessor (18) starts the power stage (10) when the system is energized and a user input is received. The microprocessor (18) stops the operation when the temperature of the cookware (20) reaches to a value predetermined by the user. Also it disables the operation of the inverter when a non-suitable cookware is detected and restarts the system after a while. When an alarm signal (570A, 570B, 590) is received from the analog peripheral circuits (15, 17), the microprocessor (18) processes this signal and disables the gating signals (540) for a determined duration, then restarts when the silence period is over.
The digital device (18) adjusts the power level by adjusting the turn-on durations of gate pulses (540); no intermittent operation is required to adjust the power level. At the startup the cooktop begins to heat at the minimum power, which means the shortest turn-on gating signals, and then the power is increased till the power level desired by the user is reached. The power is monitored by monitoring the peak value of the inductor current (610); this value is directly related to the output power.
The algorithm of the microprocessor (18) program is described with the aid of the flow diagram shown in Figure-6a and 6b. When the system is energized (100) the microprocessor (18) reads the user inputs as shown by the Routine 1: GET USER INPUTS (110), which is shown in Figure-7a.
In Routine 1 (110) the microprocessor (18) reads the variables MODE and LEVEL (111), defined by the user via the user panel not necessarily shown in the figure. MODE could be TEMP or POWER (112). If it is TEMP that means the induction cooktop will operate as a temperature controlled system (114). So it will operate at maximum power, defined as Max_Power, until the temperature reaches to the value desired by the user, defined as Final_Temp, which is equal to the variable LEVEL. If the MODE is POWER (113), that means the cooktop will operate at the power desired by the user, which is equal to LEVEL variable. Also the operation will stop as a safety precaution, if the temperature reaches the maximum permissible value, defined as Max_Temp. Max_Temp and Max_Power are the constant system parameters and cannot be modified by the user.
After getting the user inputs; the current power level, Power_Level, which the cooktop operates at, is set to minimum power level of the system, Min_Power (120). The turn-on durations are predetermined values that change with the current power level accordingly. Each power level has its own predefined turn-on durations (120). Min_Power is a constant system parameter and cannot be modified by the user.
Turn-off durations are dependent on the resonant frequency of the power stage (10), namely the load variations; so it should be updated dynamically. During startup to determine turn-off durations, first a single gate pulse (540) is produced (130). "Gate" is a built-in function of the microprocessor's PWM (Pulse Width Modulation) output. The first argument of "Gate" function is the turn-on duration of the gate signal (520), and the second argument is the turn-off duration. To produce a single pulse, predefined turn-on duration is sent as the first argument and a sufficiently long duration of 1 second (this value is not obligatory, just a preference) is entered as turn-off duration (130), so that only one pulse will be produced at the gate output (520) of the microprocessor(18).
After producing the single gate pulse Routine 2: DETERMINE TURN-OFF DURATION (140), shown in Figure-7b is called. In this routine at the turn-off instant (141) of the gate signal (520) a timer is initiated (142) and the duration till the falling edge of the Zero-Cross Sensor occurs is counted (143). This duration is saved as the turn-off duration (144).
After determining the turn-on and turn-off durations of the gate signals, gate signals could be started using function "Gate" (150).
In every one second the user inputs are checked if the user has made any updates and in every 100 milliseconds (this value is not obligatory, just a preference) the inductor current peak level (610) is checked; Power_Level is updated so that Final_Power_Level could be reached; and finally the temperature of the cookware (20) is checked. Two timers, timer1 and timer2, are used to count these durations. These timers are initiated just after the gate pulses (540) are started (160,170). When DC link voltage reaches its peak value, the turn-off duration is updated using Routine 2 (140), as described above. This action is repeated in a loop every 10 milliseconds (this value is not obligatory, just a preference).
So there are 3 nested loops (each having own timers: timer1, timer2, timer3) that repeat continuously unless the operation of the system is interrupted. The envelope of the inductor current value (610) is checked every 100 milliseconds, if the current peak value (610) is not in ±20% band (this value is not obligatory, just a preference) of the expected value then the operation is halted for 3 seconds (200) and the overall procedure restarts, otherwise the software continues its operation (190).
The current power level, Power_Level, is compared with Final_Power_Level, which is the power level the user desired (190). If Power_Level is greater than Final_Power_Level, then Power_Level is decremented (210). If they are equal, no update is made (220); otherwise Power_Level is incremented (230). The temperature of the cookware (20) is received from the analog peripheral circuit (21) and saved to the variable TEMP. TEMP is compared with the value, Final_Temp, which the user desired (240). If the temperature of the cookware (20) reaches the desired value, the operation of the system is halted for 10 seconds (this value is not obligatory, just a preference) and the overall procedure restarts from the beginning (250). Otherwise the software continues its operation (260).
The shortest loop is terminated at the instant of the peak of the DC link signal (500); this instant occurs 5 milliseconds after the 50 Hz zero-cross detector (23) output (580) becomes HIGH (if the line frequency is assumed to be 50 Hz). If this output (580) is not HIGH, the outputs (570A, 570B) of Protection Circuits (15) and the output (590) of Pan Detection Circuit (17) is checked (280). If any of them (570A, 570B, 590) is HIGH, that means an improper cookware has been placed on the cooktop or a hazardous condition has occurred, hence the gating signals (540) are interrupted for 3 seconds (200). After the completion of 3 seconds period, the overall procedure is restarted (100). If they (570A, 570B, 590) are not HIGH, then 50 Hz zero cross detector (23) output (580) is checked again in a loop manner (260).
When 50 Hz zero-cross detector (23) output (580) becomes HIGH, the microprocessor (18) resets "timer3" and starts to count 5 milliseconds (270). Counting this duration, it (18) also checks the outputs (570A, 570B) of Protection Circuits (15) and the output (590) of Pan Detection Circuit (17). If any of them (570A, 570B, 590) is HIGH (320), the gating signals (540) are interrupted for 3 seconds (200). Otherwise timer3 is checked if the duration of 5 milliseconds is completed (330). If it is not completed hazard warning signals (570A, 570B, 590) are checked in a loop manner (320). Else the turn-off duration of gate pulses (540) is updated by using Routine 2 (140), and the turn-off durations of the gate signals are updated accordingly (290).
Then, the timers are checked if the durations of the loops are over (300, 310). If the duration of 100 milliseconds is over (310) then timer2 is reset (170), and the procedure that are described above is repeated. If 1-second duration is over (300), using Routine I (110) user inputs are checked for the updates and then timer1 is reset (160) and the loop is repeated.
Current transformer block (16) transforms the inductor (19) current to a voltage value, which is defined as inductor current level (620). Figure-8 shows the internal diagram of the current transformer block (16). The primary side of the transformer is the heating coil (19), and secondary side is where the transformed voltage occurs on resistor, R10. The turn ratio, 1:N, defines the output voltage of the current transformer, V_OUT, as $V_{OUT} = R 10 * I_{L} / N$
Where I_L is the current flowing through heating coil (19).
Also by using D10 and C10, the peak value of V_OUT, hence the inductor current peak value (610) is stored and sent to the microprocessor (18).
Protection circuits (15) and pan detection circuit (17) are designed to prevent the power circuit (10) against unexpected hazardous conditions, such as malfunctioning of the microprocessor (18).
The analog protection circuit (15) monitors the inductor current level (620) and the collector voltage (510) of the semiconductor switch. When the inductor current level (620) exceeds the maximum permissible value (V_ref,current), the output of COMP1 becomes HIGH. This output turns on the bipolar transistor, T3, (530); hence latches the output of the gate signal (520) to low state so that the power inverter (10) is disabled. Therefore power circuit (10) and IGBT are protected against overcurrents. It also sends an alarm signal (570A) to the microprocessor (18) to make it disable the gate outputs (540) of the digital circuitry (18). Similarly when the collector voltage (510) of the transistor is above a predetermined value (V_ref,voltage) the output of gate signals (520) is latched to low state through T3 (530), so that the false gate signals that may be caused by malfunctioning of the microprocessor are eliminated. If a gate pulse (540) arrives at the instant the output of COMP2 is HIGH, this gate signal cannot turn on the IGBT. Because collector voltage of T3 (530) has already set gate signals (520) to LOW state. So that power stage (10) and IGBT could be protected. But the output (570B) of AND1 gate will go to HIGH state, indicating a hazardous condition. Hence, the microprocessor (18) stops producing gate pulses (540).
Since turn-off and conduction losses of the IGBT are unavoidable, only turn-on losses could be minimized. To achieve this goal one should investigate V_CE (510), since it is the most important parameter that affects the level of turn of losses. As it can be seen in Figure-10a, after the turn-off instant of the transistor, V_CE (510) oscillates at the resonance frequency of heating coil (19) and resonant capacitor, C_RES. So the minimum of V_CE (510) always occurs at the same instant after turn-off for the same load, heating coil (19), and C_RES.
The turn-off duration is constant during an AC half-cycle, and directly related to the resonance frequency of the power inverter (10). But due to load variations the resonance frequency may vary. To compensate these variations the inductor (19) current is monitored by the current transformer (16), which is used also for over-current protection. Theoretically, the minimum V_CE voltage (510) occurs at the zero cross of the inductor (19) current. Figure-11a and 11b show the typical waveforms of inductor (19) current and V_CE (510), respectively. So by observing the inductor current level (620) the turn-off durations with yielding minimum power loss could be achieved. The monitored inductor current level (620) is inverted by an inverting amplifier (22) and fed to the microprocessor as the output (560) of Zero Cross Detector. The microprocessor (18) updates the duration of turn-off signals every 10 milliseconds (290) by monitoring this signal. It (18) calculates the time elapsed between the turn-off instant of the gate signals (140) and the falling edge of the Zero-Cross Detector (22) output (560). Figure-11d shows the Zero-Cross Detector (22) output (560).
The system further includes a circuit, called Pan Detection Circuit (17), for detecting if a suitable cookware (20) is placed on the cooktop. This circuit (17) monitors the DC link voltage signal (500). According to the monitored signal Pan Detection circuit (17) decides whether there is a suitable cookware (20) on the cooktop or not. If there is no suitable cookware present, this circuit (17) sends an alarm signal (590) to microprocessor (18) to make it disable the operation of the power inverter (10) for a predetermined duration.
If no suitable pan is present on the cooktop, there will not be any energy transfer from the power stage (10) to the cookware (20). Detection of the presence of a pan (20) is based upon the change in waveform of the DC link voltage (500). In the case where there is a suitable pan (20) this voltage (500) is plotted in Figure-13a. When there is no suitable pan the waveform (500) becomes the signal in Figure-14a. Since no energy transfer occurs when no suitable pan is present, the energy will be stored in the high-frequency bypass capacitor, C1. Therefore the presence of the pan (20) can be detected by the evaluation of this waveform (500). 50 Hz Zero Cross Detector (23) produces a HIGH signal (580) during the zero cross of the DC link signal (500) for duration of approximately 400 microseconds. This output (580) is also fed to the microprocessor (18) G to detect the peak instant of DC link voltage. DC link voltage (500) is monitored with a resistor divider by the inverting input of the comparator COMP3 in Figure-12. The level of the non- inverting input, namely the reference value, is a small fraction of the peak value of the DC link voltage (500). In this way when DC link voltage (500) falls below a certain value the output of the comparator (580) will go to HIGH state as seen in Figure-13b.
As seen in Figure-14b no zero-cross occurs when there is no suitable cookware. So 50 Hz Zero Cross Detector (23) circuit will produce no HIGH signal output when there is no suitable pan. The Pan Detection Circuit (17) observes these outputs and if no HIGH signal output (580) is produced by 50 Hz Zero Cross Detector (23) for a duration of 400 milliseconds, it sends a disable signal (590) to microprocessor (18) to stop the operation of the Power Stage (10). 3 seconds after the disabling the operation, it (17) restarts the system to detect if the user put a suitable pan (20) on the cooktop. Then it observes the output (580) of 50 Hz Zero Cross Detector (23) during 400 milliseconds and disables the system or remains idle according to level of this signal, that is the presence of a cookware (20).
The output (580) of 50 Hz Zero Cross Detector (23) feeds the input of NAND-A in Figure-13. The output of NAND-A charges the capacitor C23 and if its input is LOW, that means no zero crosses occur. The time elapsed to charge C23 to a value that sets the output of NAND-B to LOW state is approximately 400 milliseconds. If no zero cross occurs, then C23 is charged to the threshold value, output of NAND-B goes to LOW, and output (590) of NAND-C goes to HIGH, which is connected to the microprocessor (18).
Figure-15 shows the circuit diagram of the Temperature to Voltage Converter Block (21). A Negative Temperature Coefficient (NTC) thermistor is placed below the top plate of the cooktop and it senses the temperature of the cookware (20). The NTC thermistor and R11 forms a resistor divider, and the voltage of node (600) changes as the temperature changes. Thus, observing the voltage of node (600), the microprocessor could acquire the temperature of the cookware (20).
The invention contains a gate drive (11) with single totem-pole output designed for direct drive of IGBTs. This block is necessary because the gate pulses (540) of the microprocessor (18) are between 0 and 5V, but IGBT require between 0 and 15V for better performance. When a turn-on gate signal (540) arrives from the microprocessor (18), the lower transistor T1 will be turned off, and the upper transistor T2 will be turned on. Therefore 15 Volt signal will arrive to the gate of IGBT, through node (520) and upper transistor, T2. When a turn-off gate signal (540) arrives from the micro-processor (18), the lower transistor T1 will be turned on, and the upper transistor T2 will be turned off. Therefore, the gate of IGBT will be grounded through node (520) and lower transistor, T1. Through node (530), the output of the gate drive could be grounded by the protection circuits (15) regardless of the gate signal received from the microprocessor (18). This prevents false gate signals (540) caused by malfunctioning of the micro-processor destruct the power transistor.

Claims

An induction cooking apparatus, comprising:
a power inverter circuit (10) drawing essentially a unity power factor current from mains, having an induction heating coil (19) that generates a high frequency magnetic field inducing a removable or non-removable magnetic cookware (20), a resonant capacitor parallel to said heating coil (19), an IGBT as a switching element, and a power diode (Dp) placed in an anti-parallel manner to said IGBT;

a microprocessor (18) taking the desired temperature of the cookware or desired power level of the cooking apparatus from a control panel, controlling the power transferred to said cookware (20) by adjusting the turn-on duration of gating pulses (520,540), controlling the temperature of said cookware (20) by interrupting the operation of the system when the temperature of said cookware (20) reaches desired value or by re-starting the system after waiting for a preset duration once it has been interrupted, adjusting the turn-off durations of gate pulses by observing zero-cross instants of current flowing through heating coil via zero-cross detector means (22), and interrupting the operation of the power stage (10) for a preset duration when a warning signal (570A, 570B, 590) from pan detection circuit (17) or protection circuits block (15) is received;

a pan detection circuit (17) detecting if a suitable cookware (20) is placed on the cooktop by observing DC link voltage (500) and comprising a 50 Hz zero-cross detector (23) means for detecting zero crosses of AC input voltage (V_ac) signal;

a block of analog protection circuits (15) protecting the power stage (10) against overcurrents by observing the current flowing through heating coil (19) and against inappropriate gating signals by observing the collector voltage of IGBT (510) and gating signals (540) at the same time.
An induction cooking apparatus of claim 1, wherein the said microprocessor (18):
in power mode, acquires the power input from user panel, assigns the desired temperature of the cookware (20) to the preset value of maximum permissible temperature of the cookware (20), and operates the power inverter (10) at chosen power input;

in temperature mode, acquires the temperature input from user panel, operates the power inverter (10) at the maximum power level, stops the operation of the system when the temperature of the cookware (20) reaches to desired value, and re-starts it after waiting for a preset duration.
An induction cooking apparatus of claim 1, wherein the said microprocessor (18):
starts to drive the power stage (10) with minimum power level of the cooking device, compares the present power level with the power level that user entered via control panel, increments or decrements the present power level to equalize both levels;

generates gate pulses (540) that have turn-on durations corresponding to the power level at that instant;

compares the inductor current peak level (610) with the expected value and interrupts the system for 3 seconds, if it is not in ±20 band of the expected value;
An induction cooking apparatus of claim 1, wherein the said microprocessor (18):
observes first 50 Hz zero-cross detector (23) output (580) and waits for 5 milliseconds, when the output (580) becomes HIGH;

calculates the time elapsed between the turn-off switching instant of the current gate pulse (540) and the instant zero-cross detector (22) output (560) changes its state from HIGH to LOW;

uses this calculated time as turn-off duration of gating signals (540), till the next calculation.
An induction cooking apparatus of claim 1, wherein the said microprocessor (18):
observes the output (590) of pan detection circuit (17) and output (570A, 570B) of protection circuits block (15);

stops gating signals (540) for 3 seconds and restarts the system after the completion of this duration, if any of these outputs (570A, 570B, 590) is HIGH.
An induction cooking apparatus of claim 1, wherein the said pan detection circuit (17):
comprises 50 Hz zero-cross detector (23) means comparing DC link voltage (500) with a reference value via COMP3 and producing HIGH output (580), if DC link voltage (500) is below this reference, LOW output (580) otherwise;

observes the inverse of this output (580) through NAND-A;

charges the capacitor (C23) with output of NAND-A through R56 (with a time constant of R56*C23), if output (580) of 50 Hz zero-cross detector (23) is LOW, discharges it through (R55//R56) otherwise (with a time constant of (R55//R56)*C23);

sends HIGH signal to microprocessor (18), if C23 is charged to a value above input threshold of NAND-B, LOW otherwise;

charges the capacitor (C22) with output of NAND-C through R59 and R52 (with a time constant of (R52+R59)*C22), if output (590) of pan detection circuit (17) is HIGH, discharges it through R59 otherwise (with a time constant of R59*C22);

discharges C23 through R58 by sets the output of COMP4 to LOW, if C22 is charged above a preset reference value in a duration determined by time constant of ((R52+R59)*C22), no discharges otherwise;

resets the output of NAND-C to LOW state, when C23 is discharged below the input threshold of NAND-B.
An induction cooking apparatus of claim 1, wherein the said protection circuits block (15):
comprises zero-cross detector (22) means inverting the inductor current level (620) received from current transformer block (16) and sending to the microprocessor (18) as the output (560); comprises a comparator (COMP1) comparing said inductor current level (620) with a reference value (V_ref,current) that corresponds to the maximum permissible current value, sends HIGH signal (570A) to the microprocessor (18), if said inductor current level (620) is above V_ref,current, LOW signal (570A) otherwise;

turns on BJT (T3) and sets gating signals (520) to LOW through node 530, if output of COMP1 is HIGH, no intervention to gate pulses otherwise;

comprises a comparator (COMP2) comparing collector voltage of IGBT (510) with a reference value that corresponds to maximum permissible collector voltage value (510) before turn-on of IGBT; producing HIGH output if collector voltage (510) is above the reference, LOW otherwise;

sends HIGH signal (570B) to the microprocessor (18), if output of COMP2 is HIGH and gate output (540) is HIGH at the same time, LOW otherwise;

turns on BJT (T3) and setting gating signals (520) to LOW through node 530, if output of COMP-2 is HIGH, no intervention to gate pulses otherwise.