Control circuitry for increasing power output in quasi-resonant converters

Disclosed herein is a circuit including a transistor, with a resonant tank coupled between a DC supply node and a first conduction terminal of the transistor. A gate driver generates a gate drive signal for biasing a control terminal of the transistor to cause it to conduct current through the resonant tank. Control circuitry monitors a voltage across the transistor to determine that the transistor is an overvoltage condition if that voltage exceeds a threshold, and monitors a current through the transistor to determine that the transistor is an overcurrent condition if that current exceeds a threshold. If overvoltage is determined, the control circuitry causes the gate driver to pull up the gate drive signal. If overcurrent is determined, the control circuitry causes the gate driver to pull down the gate drive signal. If either overvoltage or overcurrent is present, a pulse width of the gate drive signal is reduced.

TECHNICAL FIELD

This disclosure is related to control circuitry for transistors in quasi-resonant converters, such as may be used in the design of induction geysers.

BACKGROUND

According to Faraday's law of induction, when an alternating current (AC) flows through a conductor, that alternating current creates a magnetic field around the conductor due its periodically changing direction. If another conductor, which can be referred to as a secondary conductor, is placed in the vicinity of this magnetic field, current is induced in the secondary conductor. This current is also alternating in nature. Due to the electrical resistance of the secondary conductor to the flow of current, some amount of energy is dissipated as heat. This heat can be harvested for a variety of uses, such as in the well-known induction stove top.

In an induction stove top, the induction stove top itself contains a resonant coil through which alternating current flows when the induction stove top is activated. Cookware made from a ferromagnetic material, such as stainless steel or iron, is placed on the induction stove top.

This system of the resonant coil and cookware can be considered as a transformer in which the cookware acts as a shorted secondary (load). As stated, alternating current flows through the resonant coil when the induction stove top is activated, which results in the generation of an oscillating magnetic field. This oscillating magnetic field induces electric currents inside the cookware, which results in heating of the cookware due to the electrical resistance of the cookware. The purpose of the cookware being constructed from ferromagnetic material is so that high eddy currents in the cookware are produced in the presence of the oscillating magnetic field, resulting in high energy dissipation, and thus, sufficient heating of the cookware.

Induction geysers (water heaters) represent a new application for the use of induction heating. An induction geyser includes a resonant tank through which rectified power from an AC power source flows, and a fluid tank containing water. The fluid tank is constructed from ferromagnetic material.

When alternating current flows through the resonant tank, when operated in a quasi-resonant mode, eddy currents are induced in the ferromagnetic material of the fluid tank, resulting in the fluid tank heating up, which in turn heats the water.

Using inductive heating as opposed to well-known resistive heating brings a variety of advantages. For example, with resistive heating, the power rating is necessarily dependent on the AC voltage it receives, which may be inconsistent. In addition, for a water heater, resistive heating typically uses a resistive heating element within a fluid tank, and hard water can form white scaling around the resistive heating element, causing degradation in ability to heat the water effectively. Since the heating element in an induction geyser is the ferromagnetic material of the fluid tank itself, this issue is not present. Moreover, since the heating element of the induction geyser is the ferromagnetic material in the fluid tank, the induction geyser is capable of more rapid heating than a water heater relying on resistive heating, as the heating element of the induction geyser has a greater surface area.

This makes induction geysers particularly useful for energy conscious applications, such as developing areas, and for applications where quick heating of water is desired, such as vacation homes in which a water heater will typically only be turned on once the vacationers have arrived.

A known technique for driving the resonant tank of a quasi-resonant induction converter in an induction geyser is to use a low-side drive transistor coupled to pull current from a rectifier6through the resonant tank8. An example of such a circuit1is shown inFIG. 1, in which a pulse width modulation (PWM) generator2generates a PWM control signal3that is applied to a gate driver4, which generates a gate drive signal5that is applied to the gate of an insulated gate bipolar transistor (IGBT) T0. A rectifier6rectifies AC power from an AC Mains line. The IGBT transistor T0has its collector coupled to the resonant tank and its emitter coupled to ground, and serves to pull current from the rectifier6through the resonant tank8.

Although the circuit1ofFIG. 1allows for the realization of an induction geyser, such induction geysers are not tuned to deliver maximum power to their fluid tanks, so that they require a longer period of time to heat their water than would be possible with maximum power delivery. This lack of tuning to deliver maximum power to the fluid tank is done for a variety of reasons.

For example, in developing countries, the voltage of the AC power source (e.g. AC Mains) may be inconsistent. In addition, process and material variation in realizing the resonant tank and fluid tank may result in less than optimal eddy current generation. Moreover, magnetic coupling between the resonant tank and the fluid tank may be inconsistent. In addition, this existing solution lacks a comprehensive safety mechanism to protect the IGBT. Therefore, for all these reasons the IGBT is operated so as to maintain high safety margins, and the converter is thus operated at a low power, resulting in the longer period of time to heat water. Also, the protections to the IGBT provided by the existing solution are not foolproof.

It would be desirable to maximize, or come close to maximizing, power output of the converter while properly protecting the IGBT. Therefore, the development of further control circuitry for quasi-resonant converters used in induction geysers and in other applications is necessary.

SUMMARY

Disclosed herein is a circuit including a DC supply node, and a transistor having a first conduction terminal, a second conduction terminal, and a control terminal. A shunt resistor is coupled between the second conduction terminal of the transistor and ground. A resonant tank is coupled between the DC supply node and the first conduction terminal of the transistor. A gate driver generates a gate drive signal for biasing the control terminal of the transistor to cause the transistor to conduct current through the resonant tank, the gate drive signal being a series of pulses having a first pulse width. A microcontroller controls the gate driver and includes an analog to digital converter for receiving input.

Overvoltage determination circuitry monitors a voltage between the first and second conduction terminals of the transistor, generates an interrupt when the voltage between the first and second conduction terminals exceeds a threshold voltage, and causes the gate driver to pull up the gate drive signal when the voltage between the first and second conduction terminals exceeds the threshold voltage to thereby stop the transistor from being in an overvoltage condition.

Overcurrent determination circuitry monitors a current flowing between the first and second conduction terminals of the transistor and causes the gate driver to pull down the gate drive signal when the current flowing between the first and second conduction terminals exceeds a threshold current to thereby stop the transistor from being in an overcurrent condition.

The microcontroller, in response to receipt of the interrupt, causes modification of the gate drive signal by the gate driver such that the series of pulses subsequently have a second pulse width instead of the first pulse width. The analog to digital converter of the microcontroller is configured to read the current across the shunt resistor. The microcontroller, in response to the current across the shunt resistor as read by the analog to digital converter exceeding the threshold current, causes modification of the gate drive signal by the gate driver such that the series of pulses subsequently have the second pulse width instead of the first pulse width.

The microcontroller may, in a startup condition, gradually increase the first pulse width until either the interrupt is received or the current across the shunt resistor as read by the digital converter exceeds the threshold current, at which point the microcontroller causes modification of the gate drive signal by the gate driver such that the series of pulses subsequently have the second pulse width instead of the first pulse width.

The microcontroller, when causing modification of the gate drive signal, may cause modification of the gate drive signal such that the series of pulses subsequently have the second pulse width instead of the first pulse width for a given period of time and then revert to the first pulse width unless the interrupt is received or the current across the shunt resistor as read by the analog to digital converter exceeds the threshold current within the given period of time.

The overvoltage determination circuitry may include a first comparator that compares a first voltage proportional to the voltage between the first and second conduction terminals of the transistor to the threshold voltage, and asserts its output if the first voltage is more than the threshold voltage. A voltage divider may be coupled between the first conduction terminal of the transistor and ground, and the first voltage may be produced at a center tap of the voltage divider.

The overcurrent determination circuitry may include a second comparator that compares a second voltage proportional to the current flowing between the first and second conduction terminals of the transistor to a second threshold voltage and deasserts its output if the second voltage is higher than the second threshold voltage.

The second voltage may be produced across the shunt resistor.

A secondary conductor may be magnetically coupled to the resonant tank such that when the current is conducted through the resonant tank, eddy currents are induced in the secondary conductor.

The transistor may be an insulated-gate bipolar transistor, the first conduction terminal may be a collector, the second conduction terminal may be an emitter, and the control terminal may be a gate.

DETAILED DESCRIPTION

The following disclosure enables a person skilled in the art to make and use the subject matter disclosed herein. The general principles described herein may be applied to embodiments and applications other than those detailed above without departing from the spirit and scope of this disclosure. This disclosure is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed or suggested herein.

With reference toFIG. 2, a quasi-resonant converter circuit10for controlling an insulated-gate bipolar transistor IGBT T1is now described. The quasi-resonant converter circuit10, broadly speaking, functions so as to control the IGBT T1in such a way that the output from the quasi-resonant converter circuit10is maximized, regardless of variations in input voltage, load material, or variations in coupling.

The quasi-resonant converter circuit10includes a bridge rectifier12that rectifies power from an AC mains line and outputs a rectified power signal11to an LC filter formed by inductor L1coupled in series between the bridge rectifier12and note N1, and a capacitor C1coupled between node N1and ground. A resonant tank14is coupled between nodes N1and N2, and is comprised of inductor L2coupled in parallel with capacitor C2. The IGBT T1, described in greater detail below, has its collector coupled to pull current I1through the resonant tank14.

A resistive divider comprised of resistors R1and R2coupled in series at center tap N3is coupled between node N2and ground. A comparator16capable of high speed operations has its non-inverting terminal coupled to node N3and its inverting terminal coupled to reference signal Vref, and provides its output to node N4. The comparator16serves to compare the voltage V1at node N3, which is proportional to the voltage Vce between the collector and emitter of the IGBT T1, to the reference voltage Vref, and its output17indicates whether the voltage V1is greater than the reference voltage Vref, and serves to turn on the IGBT T1to provide overvoltage protection.

A resistor R3is coupled between a DC voltage (here 5 V) and node N4, while a diode D1is coupled between node N4and node N5. A microcontroller (MCU)18has an input that is coupled to diode D1at node N5. The functions of the MCU18will be described in detail below.

The emitter of the IGBT T1is coupled to node N6, while its gate is coupled to receive a gate drive signal23output from an IGBT gate driver20. The MCU18generates a pulse width modulation (PWM) control signal21for the IGBT gate driver20, and passes it to the IGBT gate driver20through resistor R7, which is coupled to the IGBT gate driver20at node N5. The IGBT gate driver20generates a gate drive signal23that is based upon the PWM control signal21.

A resistor R6is coupled between the emitter of the IGBT T1at node N6and ground. A comparator22capable of high speed operation has its inverting terminal coupled to node N6through resistor R4, its non-inverting terminal coupled to the reference voltage Vref, and provides its output to node N5. The comparator22compares the voltage V2at node N6(which is proportional to current I1) to the reference voltage, and its output19indicates whether the voltage V2is greater than the reference voltage Vref, and serves to cause the IGBT gate driver20pull down the gate drive signal23to provide for overcurrent protection.

A resistor R5is coupled between nodes N6and N7. A capacitor C3is coupled between node N7and ground. An analog to digital converter (ADC)24receives input from node N7, and digitizes its input as output to the MCU18. Although the ADC24is shown as being external to the MCU18, it may also be an internal component of the MCU. A resistor R7is coupled between the output of the MCU18and node N5.

Operation of the quasi-resonant converter circuit10will now be described in detail with additional reference to the timing diagrams ofFIGS. 3-5.

In operation, the MCU18generates a PWM control signal21for the IGBT gate driver20, which in turn generates a gate drive signal23to the gate of the IGBT T1. This gate drive signal23is generated with a gradual increase in duration until it is asserted, providing for a “soft” start. Along with the gradual rise in duration of the gate drive signal23, the MCU18monitors the current I1flowing across the IGBT T1by monitoring the voltage V2as seen from the input of the ADC24. Regardless of the voltage of the AC mains (provided that it is within the range of 85 V to 300 V), the MCU18will generate the PWM control signal21so as to control the gate drive signal23to rise in duration until either an overvoltage or an overcurrent situation is reached by the IGBT T1.

It is noted that an overvoltage or overcurrent condition can result from transients in the voltage received from the AC Mains, or simply due to the optimal duty cycle for the IGBT T1being reached. An overvoltage situation means that the voltage between the collector and emitter of the IGBT T1, Vce, has exceeded a set limit. As will be explained, in the presence of an overvoltage situation, the comparator16serves as a self-clamp and causes the IGBT gate driver20to generate the gate drive signal23at a high voltage so as to turn on the IGBT T1to bring the voltage Vce across the IGBT T1to a safe level. In parallel, the MCU18reads the output of comparator17as an interrupt, and will generate the PWM control signal21so that upcoming pulses of the gate drive signal23are reduced in pulse width.

An overcurrent situation means that the current I1flowing between the collector and emitter of the IGBT T1has exceeded a set limit. As will be explained, in the presence of an overcurrent situation, the comparator22causes the IGBT gate driver20to generate the gate drive signal23at a logic low so as to turn off the IGBT T1to bring the current I1through the IGBT T1to a safe level. In parallel, the MCU18reads the current I1flowing between the collector and emitter of the IGBT T1at the ADC24by measuring the voltage across the resistor R6, and generates the PWM control signal21so that upcoming pulses of the gate drive signal23are reduced in pulse width if the reading at the ADC24is consistently above the set limit.

Referring toFIG. 3, between time periods t1and t2, operation without reaching of an overvoltage condition is shown. Between time periods t1and t2, the current I1ramps up when the IGBT T1turns on, until at time t2, the gate drive signal23is deasserted, resulting in the IGBT T1turning off, the voltage Vge between the gate and emitter of the IGBT T1falling, and in the voltage Vce between the current and emitter of the IGBT T1rising to a nominal level Vnominal in accordance with the resonance characteristics of the resonant tank14. After time t2, the voltage Vce increases in accordance with the resonance characteristics of the resonant tank to time t3.

If Vce exceeds the set limit Vref, as it does between time t3and t4, then the self-clamping action performed by the comparator16turns the IGBT T1on for a short span of time between t4and t5to bring Vce to or below Vref. In parallel, an interrupt is generated to the MCU18, which works to alter the PWM control signal21so that the gate drive signal23decreases in pulse duration for a fixed interval of time (e.g. 45 seconds), and then returns to generating the PWM control signal21so that the pulse duration of the gate drive signal23returns to its original duration. If the overvoltage condition is repeatedly detected after multiple intervals (e.g. if the interrupt is repeatedly received), then the decreased pulse duration for the gate drive signal23is maintained until there is a further change in Vce.

This operation can be seen in detail starting at time t2inFIG. 3. Here, when current I1ramps down at time t2, the voltage V1begins to ramp up accordingly, resulting in the voltage V3produced by output from the comparator16ramping up to exceed the reference voltage Vref, indicating that the voltage Vce has exceeded a limit and that the IGBT T1is now in an overvoltage condition. When the voltage V3exceeds the reference voltage Vref at time t3, the diode D1permits current to flow, resulting in the voltage V4(which is based on the voltage V3) becoming sufficient for the MCU18to interpret as an interrupt and for the IGBT gate driver20to interpret as an asserted signal.

The IGBT gate driver20, upon receiving the asserted signal, turns on the IGBT T1at time t4, resulting in the voltage Vge rising and Vce quickly falling. This results in the voltage Vce falling to approximately the nominal level Vnominal at time t5, curing the overvoltage condition of the IGBT T1. Vge quickly begins to ramp down again at time t5, when the gate drive signal23deasserted. As explained, in response to the interrupt, the MCU18will alter the PWM control signal21such that the next pulse of the gate drive signal23produced by the IGBT gate driver20has a reduced pulse duration.

Due to the above operation, it should be appreciated that by changing Vref, the limit for Vce can be altered.

Detection of overcurrent is now described with reference toFIG. 4. Here, when the current I1through the IGBT T1exceeds a set limit, the output of the comparator22causes the IGBT gate driver20to immediately shut off the IGBT T1. In parallel, when the MCU18detects that the current I1through the IGBT T1has exceeded a set limit (performed by the ADC24reading the voltage across the resistor R6), the MCU18works to alter the PWM control signal21so that for the next pulse of the gate drive signal23, the gate drive signal23decreases in pulse duration for a fixed interval of time, and then returns to generating the PWM control signal21so that the pulse duration of the gate drive signal23returns to its original duration. If the overcurrent condition is repeatedly detected after multiple intervals then the decreased pulse duration for the gate drive signal23is maintained until there is a change in I1.

This operation can be seen in detail inFIG. 4. When the MCU18asserts the PWM control signal21at time t1, the IGBT gate driver20in turn asserts the gate drive signal23, turning on the IGBT T1, which pulls current from the bridge rectifier12, through the resonant tank14, and into its collector as current I1. Resultingly, at time t1, the current I1ramps up. As stated, the voltage V2, resulting from current I2flowing through resistor R6, is proportional to the current I1. Thus, the comparator22, when voltage V2exceeds the reference threshold to indicate that the current I1has exceeded a threshold current Ith and that the IGBT T1is in an overcurrent condition, deasserts its output19to cause the IGBT gate driver20pull down the pulse of the gate drive signal23at time t2to thereby shut off the IGBT T1, resulting in the current I1rampling down below Ith as shown, curing the overcurrent condition.

In addition, the MCU18also receives a digital representation of the voltage V2(representing the current I2, which is proportional to I1, across the resistor R6) from the ADC24. The MCU18measures the voltage V2multiple times and averages the voltage V2. If the averaged voltage indicates that the current I1flowing through the IGBT T1is above the set limit, the MCU18acts to reduce the pulse duration of the PWM control signal21such that the pulse duration of the gate drive signal23is subsequently reduced. It is noted that between time t2and t3, the PWM control signal21continues to be asserted, yet due to the deassertion received by the IGBT gate driver20at time t2, the gate drive signal23does not remain asserted.

The MCU18maintains this reduced pulse duration of the PWM control signal21(and accordingly that of the gate drive signal23) for a given period of time (e.g. 45 seconds) before reverting to the original pulse duration. If the digital representation of the voltage V2from the ADC24is regularly greater than the reference threshold, then the MCU18may maintain the reduced pulse duration of the PWM control signal21unless and until there is a change in the current through the IGBT T1such that it is below its set limit.

Through the overvoltage and overcurrent protection functionality described above, the quasi-resonant converter circuit10is free to attempt to have the MCU18to produce the PWM control signal21with a pulse width that will result in the IGBT T1reaching its maximum power output, without concern for the variation of conditions such as the voltage of the AC Mains, the load material, or variations in the magnetic coupling, as the IGBT T1will be protected against overvoltage and overcurrent conditions.

Operation in the absence of an overvoltage or overcurrent scenario (and thus when operation has been optimized as described above) will be described with specific reference toFIG. 5. As can be seen, in operation, the MCU18generates a PWM control signal21for the IGBT gate driver20, which in turn generates a gate drive signal23to the gate of the IGBT T1. When the gate drive signal23is asserted at time t1, the IGBT T1turns on, pulling current from the bridge rectifier12, through the resonant tank14, and into its collector as current I1, as shown. The current I1ramps up as it is pulled into the resonant tank14until the gate drive signal23is deasserted at time t2, resulting in the IGBT T1turning off. At that point, the voltage across the collector to emitter Vce of the IGBT T1rises and then falls to its nominal value, in accordance with the resonance characteristics of the resonant tank14.

An induction geyser40, such as a water heater system, incorporating a quasi-resonant converter circuit10and resonant tank14, as described above, is now described with reference toFIG. 6. The induction geyser40includes a water tank32, which acts as the secondary conductor. The quasi-resonant converter circuit10is coupled to pull current through the resonant tank14as described above, with the overvoltage and overcurrent protections as described above. The resonant tank14acts as a primary of a transformer, inducing eddy currents in the water tank32. Most of the energy of the eddy currents are dissipated as heat due to the resistance of the water tank32, heating the water in the water tank32.

This particular induction geyser design is particularly suited to use in developing countries, or locales with unreliable and/or inconsistent power, as it allows maximum efficiency without fear of damage to the IGBT within the quasi-resonant converter circuit due to overvoltage or overcurrent conditions.