AU675177B2 - Class E fluorescent lamp controller with locked loop - Google Patents

Description

P/00/011 213MI Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: I UL~2 ~.o.kw :.The following statement Is a full description of this invention, including the best ,.*.ijethod of performing it known to me:- I o1 S 1* JvS I CLASS E FLUORESCENT LAMP CONTROLLER with Phase Locked Loop by Dale E. Siver B.A.Sc(E.E),P. Eng, Dip. Tech.

This invention relates in general to control of power or frequency in resonant circuits. More specifically it relates to a class of switching amplifier known as Class (after the Sokals) which is defined by a switching waveform that has minimum switching losses. The Class amplifier has many applications in power control in which high efficiency and minimum losses in the power transistor (or main switch) are desirable. Such applications widely commercialized are in DC-to-DC converters or power supplies in general and RF amplifiers. The prior art control of fluorescent lamps known to the applicant can be found in "HIGH VOLTAGE HIGH FREQUENCY CLASS-E CONVERTER SUITABLE FOR MINIATURIZATION" by Georg Lutteke and Hubert Raets from IEEE Transactions G, Power Electronics VOL PE-1, NO. 4 October 19-S6.

The converter described in the prior art does not address the control of frequency or power. The prior art does not suggest using a phase locked loop to achieve automatic control.

The present invention teaches a technique for self-tuning the Class amplifier in the presence of a non-linear circuit load, such as a fluorescent lamp.

S The controller can be used for any such load and is not limited to lamp control.

Other loads could be transmit antennas for RF amplifiers or ultrasonic transducers, lasers or even generally resistive or inductive loads heating elements or motors).

The class amplifier topology (in FIG. 1) in it's simplest form "takes a DC input voltage and passes the current through an inductor (L1) for the purpose of producing a constant current. The constant current is switched by a single switch (Q1) which needs to have some minimum parallel capacitance The resonant circuit is usually a series resonant circuit comprised of a second inductor (L2) and capacitor The resonant circuit is connected from the junction of the switch and the first inductor (LI) to the circuit ground. A load may be placed across either the second inductor (L2) or the capacitor (CX).

FIG. 1 shows an equivalent series load representing all such circuit losses.

The current through the resonant circuit is more or less a sine wave and reaches a peak in amplitude at or near the resonant frequency. The resonant frequency is selected with the application load in mind and comprised of preferably low loss (high Q) components. The resonant frequency is set by the value of L2 and CX. The phase relationship between the current and the fundamental component of the voltage of such a second order system with respect to changing frequency is well known to those skilled in the art of engineering.

Of particular importance to this invention is that the voltage and current are very nearly in phase at the resonant frequency. If theVCO frequency of a phase locked loop controls the class switch and the current phase is used as the second PLL input (or phase feedback) the PLL VCO frequency will converge to the resonant frequency where the phase error is nil.

1 i1 R *7 I For a large number of class topologies the optimum frequency for achieving minimum switching losses is not the resonant frequency but a marginally higher frequency where the current lags the voltage switch waveform by an angle less than 90 degrees. Preferably in sonmc topologies the optimum phase angle is (for example) 34 degrees. By introducing a phase adjustment or shift in the feedback with a closed phase locked loop, tile optimum class "E" angle can be achieved continuously, despite changes in load resistance.

SUMMARY OF THE INVENTION The essence of the present invention is to use the widely available PLL circuit building block to achieve optimum switching frequency for the class E" amplifiter. Since the optimum frequency changes when load resistance changes, the presenlt invention adjusts the VCO frequency whenever load resistance changes to re-establish minimum switching losses. Over a wide range of load resistances the class "E'l frequency is characterised by a more or less constant phase relation between current and voltage. Another way of looking at this principle is to observe that for a 50% duty cycle switching waveform the class frequency coincides with constant which is tile ratio of reactive energy to real energy. In a series resonant circuit this constant "Q"I also corresponds to a constant angle between voltage and current The advantages of this circuit in various applications caln best be understood by examining the preferred embodiment of FIG. 2. Tilis circuit is useful for powering fluorescent lamps from a 12-35 Volt battery (or DC voltage source) with high efficiency, thus conserving power and extending battery life. Roughly similar controllers are required for emergency lamps, torches, remote area power systems, recreationlal vehicles and the like.

A prior art conventional fixed frequency controller comprised of a centre-tapped primary transformer with dual pull down transistors, found for example in SILICON CHIP February 1991 pages 46-51 entitled "Three inverters for fluorescent lights", by OTTO PRI7BOJ.

Such inverters (or DC to AC converters) are widely used commercially.

SThis prior art conventional circuit suffers from the disadvantage of poor efficiency due to the lack of an optimum switch controller. Other drawbacks of this and other presently available .30 commercial systems are high cost, necessitated by the use of two large switches and a lossy transformer. The aforementioned lossy tr-ansformer is a second reason for low efficiency and highlights another novel feature of the present invention. The inductors Li and L2 in FIG. 2 are typically low loss (high Q) AIR CORE inductors. Using air cores saves the extra cost and weight of the ferrite core and the conlsequently larger batteries required in the prior art commercial systems.

A much more important limitation of the prior art commercial fluorescent lamp conltrollers is the high cost of controlling larger size lamps with higher currents to switch. Tllis results in higher transistor switching losses necessitating larger more expensive transistors and heat sinks.

The circuit of FIG. 2 works as follows and as previously described and with reference to the power electronics theory taught in the prior art documents.

The heart Gf the circuit is ICI (typically a 4046) CMOS Phase Locked Loop available from manufacturers suxch as RCA, Motorola etc. and available as a standard library cell in most VJLSI fabrication environments.

2 1A~ 1 The VCO (voltage controlled oscillator) output from pin 4 is a square wave (50 duty cycle) that is buffered and amplified by (open collector hex buffer) IC3 and drivers Q1 and Q2 before being applied to the gate of the main transistor switches Q3 and Q4. Switches Q3 Q4 are in parallel for lower resistance and hence lower conduction losses but one transistor works essentially just as well. C8 is the shunt capacitor.

The main DC voltage is fed (from a suitable power source) through TB1 and via LI to the switch and the series resonant circuit comprised of L2,C9,C10,C12,Cll.

Those skilled in the art of class design will recognise that C9 through ClI represent an impedance inverter. The total current in the resonant circuit and the lamp goes through the primary of TI which along with the secondary and R9 form a toroidal current sensor. The current sensor signal is passed through a phase lead network aimed at providing around 34 degrees of phase lead at 400 kHz.

The phase adjusted current sensor signal (current feedback) is input to the AIN signal input of the PLL via AC coupling capacitor The other input to the phase detector comes from the VCO output pin 4.

The internal PLL circuit arrangement is such that the phase comparator in the PLL outputs a signal proportional to the phase error between; the VCO voltage wave form and the phase adjusted current sensor signal feedback (eg. shifted 34 degrees).

The phase error signal output from pin 13 of the PLL is filtered by a standard error amplifier compensation network (a low pass filter comprised of R4, R5 and C7 before being fed into pin 9 of the PLL which controls the PLL Voltage Controlled Oscillator frequency. The operating limits of the VCO are set by frequency setting components C4,R1,R2.

The external manual or automatic control (for dimming) is fed to the VCO pin 9 via Rll and :"25 comes from either the pot P1 or the "External Frequency Control" input on TB1.

The fluorescent lamp connects across the capacitor C12 for the main voltage and is connected external to the printed circuit board via TB2.

When the lamp is not lit the phase versus frequency characteristic is very sharp (coinciding with a potentially higher Q) and the controller frequency goes quite close to the S. unloaded resonant frequency. Hf nce the current magnitude is very high and a large AC voltage is built up across the capacitors C9 to C11. The lamp is struck and the phase versus frequency characteristic becomes less steep(coinciding with a much lower potential Q).

Hence the controller frequency moves (higher) to keep the phase between current and voltage waveform around 30-40 degrees and the current naturally drops due to the loading of the resonant circuit.

It will be appreciated by those skilled in the art of class design that the controller automatically adjusts it's operating frequency so that the voltage across the switch and capacitor C8 is a minimum when the switch is turned on (conducting).

This adjustment protects the switch from dissipating unnecessary power and overheating.

FIG. 3 shows typical data for an open loop phase versus frequency of the actual circuit. One curve shown is with the lamp and the other curve is without the lamp.

When the controller is closed loop the phase is kept constant even with changes in the resistance of the lamp.

3 i i'4 1 Another use for the external frequency control input is to dim the lamp at a remote point using an external pot or voltage.

Yet another use is to provide intensity control via a photo-detector which would increase the VCO voltage until maximum light is obtained and then keep the lamp as bright as possible by closed loop control. Say for example the photo-detector input increased as light intensity increases and in turn raised the frequency until the lamp was brightest, then with any attempted further increase in frequency the VCO voltage would decrease and feedback to lower the frequency back to the bright spot.

These and other non-limiting variations of the present invention are considered to be within the scope of the following claims which we hope defines the essential features of the class PLL controller.

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