WO2001052602A9 - Apparatus and method for inductive heating - Google Patents

Apparatus and method for inductive heating Download PDF

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Publication number
WO2001052602A9
WO2001052602A9 PCT/US2001/001447 US0101447W WO0152602A9 WO 2001052602 A9 WO2001052602 A9 WO 2001052602A9 US 0101447 W US0101447 W US 0101447W WO 0152602 A9 WO0152602 A9 WO 0152602A9
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WO
WIPO (PCT)
Prior art keywords
response
control voltage
power
resonant
voltage
Prior art date
Application number
PCT/US2001/001447
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French (fr)
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WO2001052602A1 (en
Inventor
Nicholas Bassil
Jih-Sheng Lai
Original Assignee
Electric Power Res Inst
Nicholas Bassil
Lai Jih Sheng
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Electric Power Res Inst, Nicholas Bassil, Lai Jih Sheng filed Critical Electric Power Res Inst
Priority to AU2001230947A priority Critical patent/AU2001230947A1/en
Priority to US10/181,259 priority patent/US6727482B2/en
Publication of WO2001052602A1 publication Critical patent/WO2001052602A1/en
Publication of WO2001052602A9 publication Critical patent/WO2001052602A9/en

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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
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M5/00—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases
    • H02M5/40—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc
    • H02M5/42—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters
    • H02M5/44—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac
    • H02M5/443—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a thyratron or thyristor type requiring extinguishing means
    • H02M5/45—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only
    • H02M5/451—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases with intermediate conversion into dc by static converters using discharge tubes or semiconductor devices to convert the intermediate dc into ac using devices of a thyratron or thyristor type requiring extinguishing means using semiconductor devices only with automatic control of output voltage or frequency
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P80/00—Climate change mitigation technologies for sector-wide applications
    • Y02P80/10—Efficient use of energy, e.g. using compressed air or pressurized fluid as energy carrier

Definitions

  • the present invention relates generally to inductive heating. More particularly, the invention provides a technique for variable frequency, variable duty cycle inductive heating.
  • a resonant power converter converts the current or voltage available from an electrical power source into a predetermined current or voltage.
  • Applications of resonant power converters include inductive heating and cooking. Power converter output power is determined by the control voltage, v c , applied to the power converter. Power converter output power is maximum when the switching frequency of v c equals the resonant frequency of the power converter. Increasing the switching frequency above the resonant frequency enables zero voltage switching; however, it also lowers power converter output power. Conversely, decreasing the switching frequency limits power converter output power range. For applications such as inductive heaters and stoves, switching frequency must be limited to a certain range to achieve the desired heating depth.
  • Figure 1 illustrates, in block diagram form, a prior power converter controller, which generates a control voltage, or voltages, in response to a power setting.
  • a prior power converter controller which generates a control voltage, or voltages, in response to a power setting.
  • three power settings are available: high, medium, and low.
  • Figure 2A illustrates prior art complementary control signals v c , and v c2 generated in response to the high power setting;
  • Figure 2B illustrates prior art complementary control signals v cl and v c2 generated in response to the medium power setting;
  • Figure 2C illustrates prior art complementary control signals v cl and v c2 generated in response to the low power setting.
  • Figure 2A reveals that the control voltages associated with the high power setting have a maximum switching period, T H , and the lowest switching frequency.
  • Figure 2B shows that the control voltages associated with the medium power setting have a higher switching frequency.
  • Figure 2C shows that the control voltages associated with the lower power setting alternate between periods of medium setting switching and long periods of no switching; i.e., long periods in which both v,., and v c2 are held at the same voltage level. Consequently, the low power setting does not produce a continuous power level, but rather a pulsating power level that may annoy users and produce poor cooking quality.
  • a controller for a resonant power converter uses some type of modulation: frequency modulation, phase-shift modulation, pulse-width modulation or phase-angle modulation. Perhaps the most popular of these is pulse-width modulation. However, its application is limited because its reduced conduction period prevents balancing of the energy in the resonant inductive and capacitive components, thereby making it difficult to achieve zero voltage switching.
  • Phase-shift modulation can be used only with full-bridge resonant power converters. The zero voltage switching range available using pulse-width modulation is slightly larger than that available with pulse-width modulation; however, the conduction losses associated with phase-shift modulation are greater than those of pulse-width modulation. This is due to the additional circulating energy during phase shifting.
  • Frequency modulation is widely used because it permits zero voltage switching over a wide frequency range.
  • frequency modulated control limits power converter output power.
  • Phase angle modulation ensures zero voltage switching by maintaining a fixed phase angle between the output voltage and current.
  • Phase angle modulated control also limits power converter output power.
  • a power converter controller would provide both the heating depth necessary for inductive heating and cooking.
  • such a power converter controller would provide zero voltage switching.
  • the inductive heat source of the present invention possesses a wide-range output power and a limited switching frequency range.
  • the inductive heat source of the present invention is efficient because of zero- voltage switching and has the heating depth necessary for inductive cooking.
  • the inductive heat source includes a variable frequency, variable duty cycle controller, a resonant power converter and an inductive coil.
  • the controller generates a variable frequency, variable duty cycle control voltage in response to a power setting.
  • the variable duty cycle of the control voltage decreases in response to an increase in the variable frequency of the control voltage.
  • the resonant power converter In response to the control voltage, the resonant power converter generates an output power between a first node and a second node. Coupled between the first and second nodes, the induction coil varies the amount of heat it produces in response to the output power.
  • the method of inductive heating of the present invention includes three steps. First, in response to a power setting a control voltage is generated that has a variable frequency and a variable duty cycle, which decreases in response to an increase in the variable frequency. Second, output power is generated in response to the control voltage. Third, an amount of heat is produced that depends upon a value of the output power.
  • Figure 1 illustrates a prior art power converter controller for generating a control voltage in response to a power setting.
  • Figure 2A illustrates prior art control signals v c] and v c2 generated in response to a high power setting.
  • Figure 2B illustrates prior art control signals v cl and v c2 generated in response to a medium power setting.
  • Figure 2C illustrates prior art control signals v cl and v c2 generated in response to a low power setting.
  • Figure 3 illustrates the Inductive Heat Source of the present invention.
  • Figure 4 illustrates complementary control voltages v c/ and v c2 produced by the Controller of Figure 3 in response to high, medium and low power settings.
  • Figure 5 illustrates a Full-Bridge Resonant Power Converter suitable for use with the Inductive Heat Source of Figure 3.
  • Figure 6 illustrates a Half-Bridge Resonant Power Converter suitable for use with the Inductive Heat Source of Figure 3.
  • Figure 7 A illustrates control voltage v c generated by the Controller of Figure 3 in response to the high power setting.
  • Figure 7B illustrates the current through the Induction Coil 80 of Figure 6 in the high power setting.
  • Figure 7C illustrates the voltage across the Induction Coil 80 of Figure 6 in the high power setting.
  • Figure 7D illustrates the voltage at Node 126 of Figure 6 in the high power setting.
  • Figure 8 A illustrates the control voltage v c2 generated by the Controller of Figure 3 in response to the low power setting.
  • Figure 8B illustrates the current through the Induction Coil 80 of Figure 6 in the low power setting.
  • Figure 8C illustrates the voltage across the Induction Coil 80 of Figure 6 in the low power setting.
  • Figure 8D illustrates the voltage at Node 126 of Figure 6 in the low power setting.
  • Figure 9 A illustrates the voltage across Induction Coil 80 of Figure 6 given an AC input voltage of 208V, 60 Hz and a high power setting.
  • Figure 9B illustrates the current through Induction Coil 80 of Figure 6 given an AC input voltage of 208 V, 60 Hz and a high power setting.
  • Figure 10A illustrates the voltage across Induction Coil 80 of Figure 6 given an AC input voltage of 208V, 60 Hz and a low power setting.
  • Figure 10B illustrates the current through Induction Coil 80 of Figure 6 given an AC input voltage of 208V, 60 Hz and a low power setting.
  • Figure 11 illustrates a first Single-Ended Resonant Power Converter suitable for use in the Inductive Heat Source of Figure 3.
  • Figure 12 illustrates a second Single-Ended Resonant Power Converter suitable for use in the Inductive Heat Source of Figure 3.
  • FIG. 3 illustrates, in block diagram form, the Inductive Heat Source 120 of the present invention. Unlike prior inductive heat sources, Inductive Heat Source 120 possesses a smooth, wide-range output.
  • Inductive Heat Source 120 includes Resonant Converter 125, Controller 130 and Induction Coil 80.
  • Resonant Converter 125 converts the AC input into a variable output power available between Nodes 126 and 128. Coupled between Nodes 126 and 128, Induction Coil 80 converts the output power into heat.
  • the amount of output power produced by Resonant Power Converter 125 depends upon a control voltage or voltages. Controller 130 generates its control voltage(s), v cn , in response to one of three power settings, high, medium or low.
  • Controller 130 varies both the frequency and duty cycle of its control voltage(s) for each power setting, producing a smooth wide-range output.
  • the duty cycle of the control voltage(s) automatically decreases as the frequency increases.
  • Figure 4 illustrates complementary control voltages v c/ and v c produced by Controller
  • the high setting produces a maximum duty cycle, D H , o ⁇ v cl and v c2 and a maximum switching period, T H .
  • the medium power setting reduces the duty cycle of v c/ and v c2 to D M and the switching period to T M .
  • the low power setting further reduces the duty cycle of v c/ and v c2 to D L and the switching period to T L .
  • the control voltages generated in response to the lower power setting differ from those generated by prior art controllers in three ways. First, the control voltages generated in response to the low power switch every Yi low switching period, rather than including extended periods without switching.
  • Controller 130 produces a smooth wide-range output between Nodes 126 and 128 because D H >D U > D L and T H > T M > T L .
  • Figure 5 illustrates schematically a Full-Bridge Resonant Power Converter 125a, which is one of several possible embodiments of Resonant Power Converter 125.
  • Full-Bridge Resonant Power Converter 125a is one of several possible embodiments of Resonant Power Converter 125.
  • Resonant Power Converter 125a includes Filter Inductor 65, Diode Bridge 60, Filter Capacitor 50,and Switches 10, 20, 30 and 40 and their associated Diode-Snubber Capacitor pairs.
  • Capacitor 70 and Induction Coil 80 are the resonant elements.
  • Induction Coil 80 heats cooking pan 82 in response to the power output across Nodes 126 and 128.
  • Control voltage v cl controls Switches 10 and 40, while control voltage v c2 controls
  • Switches 20 and 30 Across each Switch 10, 20, 30 and 40 is coupled a Diode-Snubber Capacitor pair 11 & 12, 21 & 22, 31 & 32, and 41 & 42. Diodes 11, 21, 31 and 41 allow negative directional current to flow while their associated Switches 10, 20, 30 and 40 are turned off. Snubber Capacitors 12, 22, 32 and 42 reduce the turn-off loss associated with their respective Switches 10, 20, 30 and 40. Snubber Capacitors 12, 22, 32 and 42 make zero- voltage switching desirable to improve power efficiency. Zero-voltage switching of Full- Bridge Resonant Power Converter 125 a can be obtained using a switching frequency greater than the resonant frequency of the resonant power converter. To ensure a pure AC output across Nodes 126 and 128, the duty cycle of control voltages v cl and v c2 must be less than 50%.
  • FIG. 6 illustrates schematically a second embodiment of Resonant Power Converter 125, Half-Bridge Resonant Power Converter 125b.
  • Half-Bridge Resonant Converter 125b includes Filter Inductor 65, Diode Bridge 60, Filter Capacitor 50,and a single pair of switches, Switches 10 and 20, and their associated Diode-Snubber Capacitor pairs, 11 & 12 and 21 & 22.
  • the resonant elements are Capacitors 71 & 72 and Induction Coil 80.
  • Control voltage v c/ controls Switch 10
  • control voltage v c2 controls Switch 20.
  • Zero- voltage switching of Half-Bridge Resonant Power Converter 125b can also be obtained using a switching frequency greater than the resonant frequency. To ensure a pure AC output across Nodes 126 and 128, the duty cycle of control voltages v cJ and v c2 again must be less than 50%.
  • Figures 7 A, B, C and D illustrate the response of Half-Bridge Resonant Power Converter 125b to the high power setting.
  • Figure 7 A illustrates control voltage v c2 , which is coupled to the gate of Switch 20.
  • the duty cycle of v c2 is approximately 50% and the switching frequency is slightly greater than resonant frequency.
  • Switch 20 turns on, the current through Induction Coil 80 begins increasing, as illustrated in Figure 7B.
  • the increase in current through Induction Coil 80 produces a positive voltage across it, as illustrated in Figure 7C.
  • Figure 7D illustrates the voltage at Node 126, which voltage decreases as the current through Induction Coil 80 increases. This is the positive phase of operation.
  • FIG 8 A illustrates the control voltage v c2 generated in response to the low power setting.
  • the duty cycle, D L of control voltage v c2 is much less than 50%, approximately 10%, and the switching frequency is much higher than the resonant frequency of Half-Bridge Resonant Power Converter 125b, approximately three times that of the high power setting.
  • These changes in control voltage v c2 lead to reductions in the amplitude of the current through, and the voltage across, Induction Coil 80. (See Figures 8B and C)
  • the voltage at Node 126 remains nearly constant at approximately one-half of the DC bus voltage. Because the power output by Resonant Power Converter 125b is not interrupted even heating occurs at all three power settings.
  • Figures 9A &B illustrate the response of Half-Bridge Resonant Power Converter 125b given an AC input voltage of 208V, 60 Hz and a high power setting.
  • Figure 9 A illustrates the voltage across Induction Coil 80 under the input conditions
  • Figure 9B illustrates the current through Induction Coil 80.
  • Figures 10A & B illustrate the response of Half-Bridge Resonant Power Converter 125b given a low power setting and the same AC input voltage.
  • Figure 10A graphs the voltage across Induction Coil 80
  • Figure 10B graphs the current through Induction Coil 80.
  • Figure 11 illustrates schematically a third embodiment of Resonant Power Converter
  • FIG. 12 illustrates schematically a third embodiment of Resonant Power Converter 125, Single-Ended Resonant Power Converter 125d.
  • Both Single-Ended Resonant Power Converters 125c and 125d include a single switch, Switch 10, which is controlled by control voltage v cl .
  • Switch 10 which is controlled by control voltage v cl .
  • Single-Ended Resonant Power Converters 125c and 125d differ in the connection of their resonant capacitors.
  • Figure 11 depicts Resonant
  • Capacitor 70 connected across Induction Coil 80
  • Figure 12 show Resonant Capacitor 72 connected across Switch 10.
  • control voltage v cl causes Switch 10 to conduct
  • Induction Coil 80 charges.
  • control voltage v c/ causes Switch 10 to cease conduction
  • Induction Coil 80 and Resonant Capacitor 70 or 72 resonate. Zero-voltage switching is achieved in both Single-Ended Resonant Power Converts 125c and 125d using a switching frequency greater than the resonant frequency.
  • variable frequency, variable duty cycle controller may be used to control resonant power supplies.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Induction Heating Cooking Devices (AREA)

Abstract

An induction heating method and device comprise an inductive heat source (120) having a controller (130), a resonant converter (125) and an induction coil (80). The controller (130) generates a variable frequency variable duty cycle control voltage in response to a power setting. The variable duty cycle of the control voltage decreases in response to an increase in the variable frequency of the control voltage. In response to the control voltage, the resonant power converter (125) generates an output between a first node (126) and a second node (128). Coupled between the first and second nodes (126, 128), the induction coil (80) varies the amount of heat it produces in response to the output power.

Description

APPARATUS AND METHOD FOR INDUCTIVE HEATING
The present invention relates generally to inductive heating. More particularly, the invention provides a technique for variable frequency, variable duty cycle inductive heating.
BACKGROUND
A resonant power converter converts the current or voltage available from an electrical power source into a predetermined current or voltage. Applications of resonant power converters include inductive heating and cooking. Power converter output power is determined by the control voltage, vc, applied to the power converter. Power converter output power is maximum when the switching frequency of vc equals the resonant frequency of the power converter. Increasing the switching frequency above the resonant frequency enables zero voltage switching; however, it also lowers power converter output power. Conversely, decreasing the switching frequency limits power converter output power range. For applications such as inductive heaters and stoves, switching frequency must be limited to a certain range to achieve the desired heating depth.
Figure 1 illustrates, in block diagram form, a prior power converter controller, which generates a control voltage, or voltages, in response to a power setting. Typically, three power settings are available: high, medium, and low. Figure 2A illustrates prior art complementary control signals vc, and vc2 generated in response to the high power setting; Figure 2B illustrates prior art complementary control signals vcl and vc2 generated in response to the medium power setting; and Figure 2C illustrates prior art complementary control signals vcl and vc2 generated in response to the low power setting. Figure 2A reveals that the control voltages associated with the high power setting have a maximum switching period, TH, and the lowest switching frequency. Figure 2B shows that the control voltages associated with the medium power setting have a higher switching frequency. Figure 2C shows that the control voltages associated with the lower power setting alternate between periods of medium setting switching and long periods of no switching; i.e., long periods in which both v,., and vc2 are held at the same voltage level. Consequently, the low power setting does not produce a continuous power level, but rather a pulsating power level that may annoy users and produce poor cooking quality.
Typically, a controller for a resonant power converter uses some type of modulation: frequency modulation, phase-shift modulation, pulse-width modulation or phase-angle modulation. Perhaps the most popular of these is pulse-width modulation. However, its application is limited because its reduced conduction period prevents balancing of the energy in the resonant inductive and capacitive components, thereby making it difficult to achieve zero voltage switching. Phase-shift modulation can be used only with full-bridge resonant power converters. The zero voltage switching range available using pulse-width modulation is slightly larger than that available with pulse-width modulation; however, the conduction losses associated with phase-shift modulation are greater than those of pulse-width modulation. This is due to the additional circulating energy during phase shifting. Frequency modulation is widely used because it permits zero voltage switching over a wide frequency range. Unfortunately, frequency modulated control limits power converter output power. Phase angle modulation ensures zero voltage switching by maintaining a fixed phase angle between the output voltage and current. Phase angle modulated control also limits power converter output power.
Thus, a need exists for a controller for a resonant power converter that supports both a wide-range output power and a limited switching frequency range. Such a power converter controller would provide both the heating depth necessary for inductive heating and cooking. In addition, such a power converter controller would provide zero voltage switching.
SUMMARY
The inductive heat source of the present invention possesses a wide-range output power and a limited switching frequency range. The inductive heat source of the present invention is efficient because of zero- voltage switching and has the heating depth necessary for inductive cooking. The inductive heat source includes a variable frequency, variable duty cycle controller, a resonant power converter and an inductive coil. The controller generates a variable frequency, variable duty cycle control voltage in response to a power setting. The variable duty cycle of the control voltage decreases in response to an increase in the variable frequency of the control voltage. In response to the control voltage, the resonant power converter generates an output power between a first node and a second node. Coupled between the first and second nodes, the induction coil varies the amount of heat it produces in response to the output power.
The method of inductive heating of the present invention includes three steps. First, in response to a power setting a control voltage is generated that has a variable frequency and a variable duty cycle, which decreases in response to an increase in the variable frequency. Second, output power is generated in response to the control voltage. Third, an amount of heat is produced that depends upon a value of the output power.
BRIEF DESCRIPTION OF THE DRAWINGS Additional features of the invention will be more readily apparent from the following detailed description and appended claims when taken in conjunction with the drawings, in which:
Figure 1 illustrates a prior art power converter controller for generating a control voltage in response to a power setting. Figure 2A illustrates prior art control signals vc] and vc2 generated in response to a high power setting.
Figure 2B illustrates prior art control signals vcl and vc2 generated in response to a medium power setting.
Figure 2C illustrates prior art control signals vcl and vc2 generated in response to a low power setting.
Figure 3 illustrates the Inductive Heat Source of the present invention. Figure 4 illustrates complementary control voltages vc/ and vc2 produced by the Controller of Figure 3 in response to high, medium and low power settings.
Figure 5 illustrates a Full-Bridge Resonant Power Converter suitable for use with the Inductive Heat Source of Figure 3.
Figure 6 illustrates a Half-Bridge Resonant Power Converter suitable for use with the Inductive Heat Source of Figure 3.
Figure 7 A illustrates control voltage vc generated by the Controller of Figure 3 in response to the high power setting. Figure 7B illustrates the current through the Induction Coil 80 of Figure 6 in the high power setting. Figure 7C illustrates the voltage across the Induction Coil 80 of Figure 6 in the high power setting.
Figure 7D illustrates the voltage at Node 126 of Figure 6 in the high power setting. Figure 8 A illustrates the control voltage vc2 generated by the Controller of Figure 3 in response to the low power setting.
Figure 8B illustrates the current through the Induction Coil 80 of Figure 6 in the low power setting.
Figure 8C illustrates the voltage across the Induction Coil 80 of Figure 6 in the low power setting. Figure 8D illustrates the voltage at Node 126 of Figure 6 in the low power setting.
Figure 9 A illustrates the voltage across Induction Coil 80 of Figure 6 given an AC input voltage of 208V, 60 Hz and a high power setting.
Figure 9B illustrates the current through Induction Coil 80 of Figure 6 given an AC input voltage of 208 V, 60 Hz and a high power setting. Figure 10A illustrates the voltage across Induction Coil 80 of Figure 6 given an AC input voltage of 208V, 60 Hz and a low power setting.
Figure 10B illustrates the current through Induction Coil 80 of Figure 6 given an AC input voltage of 208V, 60 Hz and a low power setting.
Figure 11 illustrates a first Single-Ended Resonant Power Converter suitable for use in the Inductive Heat Source of Figure 3.
Figure 12 illustrates a second Single-Ended Resonant Power Converter suitable for use in the Inductive Heat Source of Figure 3.
DETAILED DESCRIPTION Figure 3 illustrates, in block diagram form, the Inductive Heat Source 120 of the present invention. Unlike prior inductive heat sources, Inductive Heat Source 120 possesses a smooth, wide-range output. Inductive Heat Source 120 includes Resonant Converter 125, Controller 130 and Induction Coil 80. Resonant Converter 125 converts the AC input into a variable output power available between Nodes 126 and 128. Coupled between Nodes 126 and 128, Induction Coil 80 converts the output power into heat. The amount of output power produced by Resonant Power Converter 125 depends upon a control voltage or voltages. Controller 130 generates its control voltage(s), vcn, in response to one of three power settings, high, medium or low. Unlike prior controllers, Controller 130 varies both the frequency and duty cycle of its control voltage(s) for each power setting, producing a smooth wide-range output. In particular, the duty cycle of the control voltage(s) automatically decreases as the frequency increases. Figure 4 illustrates complementary control voltages vc/ and vc produced by Controller
130 in response to high, medium and low power settings. The high setting produces a maximum duty cycle, DH, oϊvcl and vc2 and a maximum switching period, TH. The medium power setting reduces the duty cycle of vc/ and vc2 to DM and the switching period to TM. The low power setting further reduces the duty cycle of vc/ and vc2 to DL and the switching period to TL . The control voltages generated in response to the lower power setting differ from those generated by prior art controllers in three ways. First, the control voltages generated in response to the low power switch every Yi low switching period, rather than including extended periods without switching. Second, the low switching period, TL , is not equal to the medium switching period, T^; and, third, the low duty cycle, DL, is not equal to the medium duty cycle, DM. Controller 130 produces a smooth wide-range output between Nodes 126 and 128 because DH>DU > DL and TH> TM > TL.
A. Resonant Power Converter Embodiments
Figure 5 illustrates schematically a Full-Bridge Resonant Power Converter 125a, which is one of several possible embodiments of Resonant Power Converter 125. Full-Bridge
Resonant Power Converter 125a includes Filter Inductor 65, Diode Bridge 60, Filter Capacitor 50,and Switches 10, 20, 30 and 40 and their associated Diode-Snubber Capacitor pairs. Capacitor 70 and Induction Coil 80 are the resonant elements. Induction Coil 80 heats cooking pan 82 in response to the power output across Nodes 126 and 128. Control voltage vcl controls Switches 10 and 40, while control voltage vc2 controls
Switches 20 and 30. Across each Switch 10, 20, 30 and 40 is coupled a Diode-Snubber Capacitor pair 11 & 12, 21 & 22, 31 & 32, and 41 & 42. Diodes 11, 21, 31 and 41 allow negative directional current to flow while their associated Switches 10, 20, 30 and 40 are turned off. Snubber Capacitors 12, 22, 32 and 42 reduce the turn-off loss associated with their respective Switches 10, 20, 30 and 40. Snubber Capacitors 12, 22, 32 and 42 make zero- voltage switching desirable to improve power efficiency. Zero-voltage switching of Full- Bridge Resonant Power Converter 125 a can be obtained using a switching frequency greater than the resonant frequency of the resonant power converter. To ensure a pure AC output across Nodes 126 and 128, the duty cycle of control voltages vcl and vc2 must be less than 50%.
Figure 6 illustrates schematically a second embodiment of Resonant Power Converter 125, Half-Bridge Resonant Power Converter 125b. Half-Bridge Resonant Converter 125b includes Filter Inductor 65, Diode Bridge 60, Filter Capacitor 50,and a single pair of switches, Switches 10 and 20, and their associated Diode-Snubber Capacitor pairs, 11 & 12 and 21 & 22. The resonant elements are Capacitors 71 & 72 and Induction Coil 80. Control voltage vc/ controls Switch 10, while control voltage vc2 controls Switch 20. Zero- voltage switching of Half-Bridge Resonant Power Converter 125b can also be obtained using a switching frequency greater than the resonant frequency. To ensure a pure AC output across Nodes 126 and 128, the duty cycle of control voltages vcJ and vc2 again must be less than 50%.
Figures 7 A, B, C and D illustrate the response of Half-Bridge Resonant Power Converter 125b to the high power setting. Figure 7 A illustrates control voltage vc2, which is coupled to the gate of Switch 20. The duty cycle of vc2 is approximately 50% and the switching frequency is slightly greater than resonant frequency. When Switch 20 turns on, the current through Induction Coil 80 begins increasing, as illustrated in Figure 7B. The increase in current through Induction Coil 80 produces a positive voltage across it, as illustrated in Figure 7C. Figure 7D illustrates the voltage at Node 126, which voltage decreases as the current through Induction Coil 80 increases. This is the positive phase of operation. When control voltage vc2 turns off Switch 20, control voltage vc7 switches on Switch 10, and the current through Induction Coil 80 begins decreasing, as does the voltage across it. (See Figure 7B and 7C) This is the negative phase of operation. The response of Half-Bridge Resonant Power Converter 125b during the positive phase of operation is symmetrical to its response during the negative phase of operation. Figures 8 A, B, C and D illustrate the response of Half-Bridge Resonant Power
Converter 125b to the low power setting. Figure 8 A illustrates the control voltage vc2 generated in response to the low power setting. The duty cycle, DL, of control voltage vc2 is much less than 50%, approximately 10%, and the switching frequency is much higher than the resonant frequency of Half-Bridge Resonant Power Converter 125b, approximately three times that of the high power setting. These changes in control voltage vc2 lead to reductions in the amplitude of the current through, and the voltage across, Induction Coil 80. (See Figures 8B and C) Further, as illustrated in Figure 8D, the voltage at Node 126 remains nearly constant at approximately one-half of the DC bus voltage. Because the power output by Resonant Power Converter 125b is not interrupted even heating occurs at all three power settings.
Figures 9A &B illustrate the response of Half-Bridge Resonant Power Converter 125b given an AC input voltage of 208V, 60 Hz and a high power setting. In particular, Figure 9 A illustrates the voltage across Induction Coil 80 under the input conditions, while Figure 9B illustrates the current through Induction Coil 80.
Figures 10A & B illustrate the response of Half-Bridge Resonant Power Converter 125b given a low power setting and the same AC input voltage. Figure 10A graphs the voltage across Induction Coil 80, while Figure 10B graphs the current through Induction Coil 80. Figure 11 illustrates schematically a third embodiment of Resonant Power Converter
125, Single-Ended Resonant Power Converter 125c. Figure 12 illustrates schematically a third embodiment of Resonant Power Converter 125, Single-Ended Resonant Power Converter 125d. Both Single-Ended Resonant Power Converters 125c and 125d include a single switch, Switch 10, which is controlled by control voltage vcl. Single-Ended Resonant Power Converters 125c and 125d differ in the connection of their resonant capacitors. Figure 11 depicts Resonant
Capacitor 70 connected across Induction Coil 80, while Figure 12 show Resonant Capacitor 72 connected across Switch 10. Despite this difference, the operating principle of Single-Ended Resonant Power Converters 125c and 125d is the same. While control voltage vcl causes Switch 10 to conduct, Induction Coil 80 charges. When control voltage vc/ causes Switch 10 to cease conduction, Induction Coil 80 and Resonant Capacitor 70 or 72 resonate. Zero-voltage switching is achieved in both Single-Ended Resonant Power Converts 125c and 125d using a switching frequency greater than the resonant frequency.
Alternate Embodiments While the present invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims. For example, a variable frequency, variable duty cycle controller may be used to control resonant power supplies.

Claims

WHAT IS CLAIMED IS:
1. An inductive heat source, comprising: a controller generating a control voltage in response to a power setting, the control voltage having a variable frequency and a variable duty cycle, the variable duty cycle decreasing in response to an increase in the variable frequency; a resonant converter generating an output power between a first node and a second node in response to the control voltage; and an induction coil coupled between the first node and the second node, the induction coil producing an amount of heat depending upon a value of the output power.
2. A method of inductive heating, comprising: generating a control voltage in response to a power setting, the control voltage having a variable frequency and a variable duty cycle, the variable duty cycle decreasing in response to an increase in the variable frequency; generating an output power in response to the control voltage; and producing an amount of heat depending upon a value of the output power.
PCT/US2001/001447 2000-01-13 2001-01-12 Apparatus and method for inductive heating WO2001052602A1 (en)

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DE10304505A1 (en) * 2003-02-05 2004-08-26 Abb Patent Gmbh Process for feeding an induction furnace or inductor
CA2658615A1 (en) * 2006-08-07 2008-02-14 Messier-Bugatti Power control for densification of one or more porous articles
EP2774455A2 (en) * 2011-11-04 2014-09-10 BSH Bosch und Siemens Hausgeräte GmbH Induction heating device
US10690705B2 (en) * 2016-06-15 2020-06-23 Watlow Electric Manufacturing Company Power converter for a thermal system
US10908195B2 (en) 2016-06-15 2021-02-02 Watlow Electric Manufacturing Company System and method for controlling power to a heater
KR102670057B1 (en) * 2017-08-10 2024-05-29 와틀로 일렉트릭 매뉴팩츄어링 컴파니 System and method for controlling power to a heater

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