Classifications

• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B6/00—Heating by electric, magnetic or electromagnetic fields
  • H05B6/02—Induction heating
  • H05B6/06—Control, e.g. of temperature, of power
  • H05B6/062—Control, e.g. of temperature, of power for cooking plates or the like

Definitions

• This invention relates to an induction heating system, more particularly induction cooking appliance for use as a home appliance.
• Induction cooking devices or cooktops are more reliable and efficient, flameless, and thus safer appliances when compared with other cooking appliances.
• 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.
• a removable or non-removable cookware pan, frying pan etc
• the cookware and the food contained therein is heated.
• the appliance 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.
• 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.
• 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.
• 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.
• 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.
• 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).
• IGBT Insulated Gate Bipolar Transistor
• the transistor is turned on and off continuously.
• 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.
• a removable or non-removable cookware which is placed on the cooktop and made of a magnetic material such as steel, iron, etc.
• 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.
• FIG-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.
• FIG.-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.
• FIG. 5a and 5c show the IGBT current waveform vs. time, Collector-Emitter voltage, V CE , of the
• FIG. 6a and 6b show the flow diagram of the microprocessor software.
• F ⁇ gure-7a and 7b show the "Routine 1 : Get User Inputs" and "Routine 2: Determine Turn-off
• Figure-8 shows the circuit diagram for the current transformer block.
• Figure-9 shows the circuit diagram for the protection circuits block.
• Figure-lOa 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 lla-lld show inductor current, I L , vs. time, V CE VS. time, gate signal, V GE , VS. time, and Zero
• Figure-12 shows the circuit diagram of the pan detection circuit.
• Figure-13a and 13b show DC link voltage, Vci, 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 D c, 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.
• FIG-1 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.
• IGBT Insulated Gate Bipolar Transistor
• “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 ol 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.
• 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.
• 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), Dl, 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, CI .
• the voltage across CI, Vci or V DC is also called DC link voltage (500).
• CI is not big enough to make V D c a smooth DC signal; so VQ C 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 ⁇ sec).
• the transistor 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).
• a short circuit current Ic r e s
• IGBT a very short duration (a few microseconds).
• 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 Ic res is dependent to the level of collector emitter voltage of IGBT (510), V CE , at the turn-on switching instant.
• V CE collector emitter voltage of IGBT
• 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.
• ⁇ t ⁇ is defined as the duration that V CE (510) falls to zero, namely turn- on switching time.
• ⁇ Vc re s 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 ( 10) at the instant of turn- on is zero volts, ⁇ Vc res is equal to zero, too. So using previous equation, Ic res 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.
• V CE Due to the nature of the resonant circuit V CE tends to go to negative values, but the anti-parallel diode, Dp, prevents these negative cycles. Therefore, V CE (510) is limited to zero volts when D is conducting as indicated in Figure-5b.
• 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.
• 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.
• 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.
• Routine 1 GET USER INPUTS (110), which is shown in Figure-7a.
• Routine 1 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 (1 12). If it is TEMP that means the induction cooktop will operate as a temperature controlled system (114).
• Max_Power maximum power
• Final_Temp the value desired by the user
• the operation will stop as a safety precaution, if the temperature reaches the maximum permissible value, defined as MaxJTemp.
• MaxJTemp and Max_Power are the constant system parameters and cannot be modified by the user.
• the current power level, PowerJLevel 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).
• MinJPower 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.
• 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.
• 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).
• gate signals could be started using function "Gate" (150).
• 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; PowerJLevel is updated so that Final_PowerJLevel could be reached; and finally the temperature of the cookware (20) is checked.
• Two timers, timerl and timer2 are used to count these durations. These timers are initiated just after the gate pulses (540) are started (160,170).
• 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).
• the current power level, PowerJLevel is compared with FinalJPowerJLevel, which is the power level the user desired (190). If PowerJLevel is greater than Final_Power_Level, then PowerJLevel 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, FinalJTemp, 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).
• the microprocessor (18) 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).
• 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 1 (110) user inputs are checked for the updates and then timerl is reset (160) and the loop is repeated.
• FIG. 6 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
• I L is the current flowing through heating coil (19).
• the peak value of V 0 u ⁇ 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 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 re f,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).
• V CE (510)
• C RES resonant capacitor
• 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.
• the inductor (19) current is monitored by the current transformer (16), which is used also for over-current protection.
• the minimum V CE voltage (510) occurs at the zero cross of the inductor (19) current.
• Figure-l la and l ib 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-1 Id 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.
• 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.
• 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).
• FIG-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 Rl 1 forms a resistor divider, and the voltage of node (600) changes as the temperature changes.
• 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.
• a turn-on gate signal (540) arrives from the microprocessor (18)
• the lower transistor Tl 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.
• a turn-off gate signal (540) arrives from the micro- processor (18)
• the lower transistor Tl will be turned on, and the upper transistor T2 will be turned off.
• the gate of IGBT will be grounded through node (520) and lower transistor, Tl.
• 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.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Cookers (AREA)
• Surgical Instruments (AREA)
• Dry Shavers And Clippers (AREA)
• Polishing Bodies And Polishing Tools (AREA)
• Induction Heating Cooking Devices (AREA)
• Control Of High-Frequency Heating Circuits (AREA)

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• 2003
  • 2003-05-28 WO PCT/TR2003/000047 patent/WO2004107819A1/en active Search and Examination
  • 2003-05-28 EP EP03728210A patent/EP1629698B1/de not_active Expired - Lifetime
  • 2003-05-28 AT AT03728210T patent/ATE349880T1/de not_active IP Right Cessation
  • 2003-05-28 DE DE60310774T patent/DE60310774T2/de not_active Expired - Lifetime
  • 2003-05-28 AU AU2003232875A patent/AU2003232875A1/en not_active Abandoned
  • 2003-05-28 ES ES03728210T patent/ES2279950T3/es not_active Expired - Lifetime

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