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, xcexdc, applied to the power converter.
Power converter output power is maximum when the switching frequency of xcexdc 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.
FIG. 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. FIG. 2A illustrates prior art complementary control signals xcexdc1 and xcexdc2 generated in response to the high power setting; FIG. 2B illustrates prior art complementary control signals xcexdc1 and xcexdc2 generated in response to the medium power setting; and FIG. 2C illustrates prior art complementary control signals xcexdc1 and xcexdc2 generated in response to the low power setting. FIG. 2A reveals that the control voltages associated with the high power setting have a maximum switching period, TH, and the lowest switching frequency. FIG. 2B shows that the control voltages associated with the medium power setting have a higher switching frequency. FIG. 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 xcexdc1 and xcexdc2 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.
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.