Method of operating a resonant inverter using zero current switching and arbitrary frequency pulse width modulation

A method of controlling a series-resonant, half-bridge inverter includes turning off the bottom switch and turning on the top switch the inverter when the current through the resonant inductor crosses the zero axis while the current is increasing, thereby insuring zero voltage switching of the inverter switches and increases the overall switching period so that the actual inverter frequency is closer to the resonant frequency of the series-resonant circuit. Using an on-time control circuit, the method further includes controlling the current delivered to the load (such as a gas discharge lamp) by varying the on-time of the top inverter switch.

CROSS-REFERENCES TO RELATED APPLICATIONS

BACKGROUND OF THE INVENTION

The present invention relates generally to resonant inverters, such as those used in an electronic ballast to provide power to a gas discharge lamp. More particularly, the present invention pertains to methods of operating a resonant inverter-type electronic ballast to provide stable operation of, and to control power to, a gas discharge lamp.

Electronic ballasts are commonly used to power gas discharge lamps, such as a fluorescent lamp. A typical electronic ballast will include a half-bridge resonant inverter driven by an inverter drive circuit. The inverter drive circuit controls the switching of the top and bottom inverter switches so that the inverter operates at or near the self-resonant frequency of the inverter. Using a driven resonant inverter allows for the lamp power to be controlled (for dimming and/or stable power regulation) by varying the inverter frequency and/or by varying the pulse width of the inverter output.

Stable operation of the lamp is important in the design and operation of an electronic ballast. For many applications, an ideal resonant inverter would act as an ideal current source in which the open loop output impedance is infinite so that all of the current generated by the inverter flows to the lamp. Also, to achieve optimal efficiency while lowering component stresses, a preferred resonant inverter design would insure zero-voltage switching of the inverter transistors.

Unfortunately, prior art resonant inverters that operate as current sources suffer from several problems caused by deficiencies in the inverter switching methods. For example, use of frequency modulation to control the inverter output does not guarantee zero voltage switching. Moreover, with frequency modulation, the higher frequencies and higher lamp voltages associated with lamp dimming create undesirable circulating currents through the component and parasitic capacitances in the circuit. Resonant inverters that use symmetric pulse width modulation (also known as dead time modulation) also have a high likelihood of non-zero voltage switching.

What is needed, then, is a method of operating a resonant inverter that maximizes the open loop output impedance by operating the inverter close to self-resonance and that guarantees zero voltage switching of the inverter transistors.

BRIEF SUMMARY OF THE INVENTION

The method of this invention is implemented in an inverter that has top and bottom inverter switches arranged in a half-bridge configuration and a series-resonant circuit connected across the output of the inverter. The inverter is also connected to a drive circuit that controls commutation of the top and bottom inverter switches. The series-resonant circuit includes a resonant capacitor in series with a resonant inductor. A load, such as a gas discharge lamp, is connected across the resonant capacitor.

The method of the present invention includes turning off the bottom switch and turning on the top switch of a series-resonant half-bridge inverter when the current through the resonant inductor crosses the zero axis while the current is increasing. This insures zero voltage switching of the inverter switches. This operation also increases the overall switching period so that the actual inverter frequency is closer to the resonant frequency of the series-resonant circuit. By operating the inverter close to resonance, the open loop output impedance of the inverter is increased, providing a more stable operation of a load such as gas discharge lamp.

Using an on-time control circuit, the method of this invention further includes controlling the current delivered to the load (such as a gas discharge lamp) by varying the on-time of the top inverter switch. This is accomplished in one embodiment by feeding back—cycle-by-cycle—a measurement of the current through the resonant inductor so that the top switch is commutated when the resonant inductor current reaches a reference value. In a second embodiment, the on-time of the top inverter switch is controlled by commutating the top switch based on a timer that is triggered when the top switch is turned on.

In one embodiment of the invention, the inverter drive circuit includes (a) a switching circuit coupled to the top and bottom inverter switches and (b) a zero-crossing detector circuit coupled to the inverter to monitor the AC load current through the resonant inductor. The zero-crossing detector circuit compares the measured AC load current with a first reference current value. The zero-crossing detector circuit also compares a derivative of the measured AC load current with a rate of change reference value. When the measured AC load current is at or near the first reference current value, and when the rate of change of the measured AC load current is one of either above or below the rate of change reference value, zero-crossing detector circuit generates an indicator signal. Preferably, the first reference current value is zero. In addition, the rate of change reference value is preferably zero so that the indicator signal is generated when the current is positive.

The first indicator signal is delivered to an input to the switching circuit. The switching circuit is operable to commutate the top and bottom inverter switches. Using the first indicator signal, the switching circuit causes the top inverter switch to turn on, and the bottom inverter switch to turn off, when the current in the resonant inductor crosses the zero-axis, while the inductor current is increasing. This insures zero-current switching of the inverter switches and increases the overall switching period so that the inverter operates closer to resonance.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the inverter circuit10of the present invention is shown inFIG. 1. The inverter circuit10is particularly useful when the load12is a gas discharge lamp. The preferred embodiment stabilizes the operation of the lamp by maximizing the open loop output impedance of the inverter circuit.

The inverter circuit10includes a half-bridge inverter14coupled to a DC source16and a series-resonant circuit22. The inverter14and series resonant circuit22conventionally operate to convert a DC voltage from DC source16to a high frequency, high voltage AC current that is supplied to a load12, such as a gas discharge lamp. In this embodiment, the inverter14is a driven inverter in which commutation of the top and bottom inverter switches28,20is controlled by an inverter drive circuit.

A DC blocking capacitor19may be placed between the inverter14and the resonant circuit22. The inverter output18may have varying time, frequency, and envelope characteristics depending on the characteristics of resonant circuit22and the desired application.

In a preferred embodiment, the inverter14has a top switch28and bottom switch30arranged in a half-bridge inverter topology. Preferably, the series-resonant circuit22includes a resonant inductor24and resonant capacitor26. The load12is connected across the resonant capacitor26to receive an AC load current23. (Labeled ILOADonFIG. 3(a).)

The inverter drive circuit includes a zero-crossing detector circuit32coupled to the resonant circuit22. The zero-crossing detector circuit32monitors the current ILSerthrough the resonant inductor24so that commutation of the inverter switches28,30occurs (a) at a frequency that is close to the resonant frequency of the resonant circuit22and (b) in a manner that is synchronized to a particular rate of change of the current through resonant inductor24. By selecting certain current and current rate of change reference values, the zero-crossing detector circuit32can assure zero voltage switching of the top and bottom inverter transistors28,30.

The zero-crossing detector circuit32receives an AC current signal25corresponding to the current ILSerthrough resonant inductor24. The AC current signal25may also be any current signal associated with the AC load current23so long as the AC current signal25has a frequency and amplitude approximately proportionally related to the frequency and amplitude of the AC load current23. Preferably however, the AC current signal25corresponds to the same frequency as the AC load current23. Those of skill in the art will recognize that the AC current signal25can be generated in a variety of conventional ways, such as by inductively coupling to the resonant inductor or by measuring a voltage across a current sensing resistor (not shown).

The zero-crossing detector circuit32is operable to compare the AC current signal25with a first reference current value34. As shown inFIGS. 1-2, this may be accomplished using a first comparator35. The first comparator35produces a current value crossing signal37when the AC current signal25is at or near the first reference current value34.

The zero-crossing detector circuit32is also operable to compare a rate of change of the AC current signal25with a rate of change reference value36. This may be accomplished by utilizing a current derivative (rate of change) detector42and a second comparator44. The current derivative detector42generates a signal that corresponds to a rate of change of the AC current signal25. The current derivative detector42may be a combination of passive components or an active device. For example, the current derivative detector42may include an inductor that is coupled to other passive or active components. Alternatively, the current derivative detector42may be a processor device that calculates the derivative of the AC current signal25and outputs the appropriate rate of change current signal. This signal is then coupled to one input of second comparator44.

The second comparator44compares the rate of change of the AC current signal25with a rate of change reference value36. When the rate of change of the AC current signal25is one of either approximately above or below the rate of change reference value36, the zero-crossing detector circuit32generates a rate of change crossing signal39. A logic device33is coupled to the outputs of comparators35,44. The logic device33is functional to generate a first indicator signal49when the current value crossing signal37and the rate of change crossing signal39are generated by the first and second comparators35,44. Preferably, the logic device33is an AND gate.

The zero-crossing detector circuit32provides all of the measurements required for zero voltage switching. Zero voltage switching occurs when the inverter switches28,30are commutated near or at a zero current crossing and when the current rate of change is approximately positive. Consequently, the first reference current value34and the rate of change reference value36are at or near zero. In addition, the first indicator signal49is produced when the rate of change of the AC current signal25is approximately above the rate of change reference value36. In this manner, the indicator signal49is produced when the AC load current23is at or near a zero crossing and has a positive rate of change.

The AC current signal25does not need to have the same frequency as the AC load current23. The AC current signal25needs to have a frequency approximately proportional to that of the AC load current23. Thus, additional devices (not shown) may be included in the inverter circuit10so that the indicator signal49is produced according to a known proportion between the frequencies of the AC load current23and the AC current signal25. For example, the circuit may cause the indicator signal49to be produced at every other zero crossing if the frequency of the AC current signal25is approximately double the frequency of the AC load current23. However, in the preferred embodiment the AC current signal25has the same frequency as the AC load current23.

Referring again toFIGS. 1-2, the inverter drive circuit includes a switching circuit38coupled to the inverter14and to the zero crossing detector circuit32. The switching circuit38is responsive to the first indicator signal49from the output from the zero crossing detector circuit32to commutate the top and bottom inverter switches28,30. As described above, the first indicator signal49is preferably generated at a zero crossing of the AC current signal25and when the rate of change of the AC current signal25is positive.

As shown inFIGS. 1-2, the switching circuit38is coupled to the top and bottom inverter switches28,30such that when one switch28,30is open the other switch30,28is closed. Therefore, the switching circuit38may be a conventional S-R flip-flop device in which the set input S receives the first indicator signal49.

The power delivered to the load12can be controlled by varying the on-time of the top inverter switch28, using an on-time control circuit. In the embodiment ofFIG. 1, the on-time of top inverter switch28is controlled by a comparator circuit40coupled to the resonant circuit22. The comparator circuit40has a signal input coupled to the resonant inductor24and a reference input connected to a second reference current value41. The comparator circuit40compares the AC current signal25with second reference current value41. When the AC current signal25is at or near the second reference current value41, a second indicator signal50is sent to the reset input R of switching circuit38. The switching circuit38responds to the second indicator signal50to cause commutation of the inverter switches28,30. More particularly, the output Q of switching circuit38is coupled to the top inverter switch28. The complementary output Q′ is coupled to the bottom switch30. In one embodiment, the top and bottom switches28,30can be switching transistors in which the bases (or gates) of the transistors are coupled to the flip-flop outputs Q and Q′. When the set input S receives the first indicator signal49, the normal output Q is switched high while the complementary output Q′ is switched low. This signal is represent as STOPonFIG. 3A. This turns on the top inverter switch28and turns off the bottom inverter switch30. When the second indicator signal50is received by the reset input R, the normal output Q is switched low while the complementary output Q′ is switched high. This causes the top inverter switch28to turn off and the bottom inverter switch30to turn on.

In a second embodiment of the inverter circuit10shown inFIG. 2, the on-time of the top inverter switch28is controlled by a timer circuit41. The timer circuit41is preferably activated when the top inverter switch28is turned on. Thereafter, the second indicator signal50is generated after a time delay, causing the top inverter switch28to turn off. Preferably, and as shown inFIG. 3B, the time delay is synchronized so that the top inverter switch28is turned off at or near a peak current value for the AC current signal25. As described above, the zero-crossing detector circuit32insures commutation at or near zero current crossing when the load current is increasing.

A graphical illustration of the operation of the inverter circuit20is shown inFIGS. 3A-3B. Referring specifically toFIG. 3A, the graph depicts the output31from the normal output Q of switching circuit38. When the output31is low, top inverter switch28is turned off. When the zero crossing detector circuit32determines that the rate of change of the AC current signal25is positive, and that the AC current signal25is at or near zero, the first indicator signal49is generated at set input S and the output31is switched high. This turns top switch28on. When the second indicator signal50is received by the reset input R, the top inverter switch28is turned off. As shown inFIG. 3A, the output31is switched back to low at or near the peak value of the AC current signal25. The resultant AC load current23is illustrated in the graph.

FIG. 3Bshows a graphical illustration of the resultant voltage outputs from the inverter circuit10, including the voltage59across DC blocking capacitor19. The voltage60across the resonant inductor24is also shown. The voltage61across resonant capacitor26, which is the same as the AC voltage across the load12, is also shown.

Throughout this disclosure, the words “approximately” and “near” have been to describe when a various actions of the inverter circuit10are triggered or performed. These words recognize that no electronic device can perform a particular action precisely at a particular moment. While electronic devices are intended to be as accurate as possible, one of ordinary skill in the art recognizes that in practice no device performs at 100% accuracy. The words “approximately” and “near” are intended to recognize this inaccuracy in all electronic devices. Standard errors in electronic circuits and techniques utilized for correcting these errors should be considered when interpreting whether a particular action of the inverter circuit10is performed “approximately” and “near” a reference value.