Lamp ballast for accurate control of lamp intensity

An electronic ballast for an electron discharge lamp includes a resonant inverter driven by a high frequency switching signal supplied by a drive circuit which is substantially comprised in an integrated circuit, the lamp being connected in an output circuit of the inverter and powered thereby. The lamp intensity is controlled by changing the switching signal frequency, thereby changing the power supplied to the lamp. In order to prevent parasitic capacitance of remote wiring between the ballast and the lamp from causing inaccuracies in determination of the power being supplied to the lamp, the ballast takes into account the phase difference between lamp current and voltage in making such determination. For example, by deriving the product of rectified lamp voltage and rectified lamp current, and using the arithmetic summation of such products during each operating cycle as a measure of lamp power.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to an electronic ballast for a gas discharge lamp, 
and more particularly to such a ballast which enables accurate control of 
the lamp intensity by an externally supplied dimming signal for adjusting 
the power supplied to the lamp even at very low illumination levels (e.g. 
1 or 2% of maximum intensity) and even when the ballast is coupled to the 
lamp by remote wiring having significant stray capacitance. 
2. Description of the Related Art 
U.S. Pat. No. 5,742,134, issued Apr. 21, 1998 to the present Applicants and 
assigned to the present Assignee, Philips Electronics N.A., discloses an 
electronic ballast comprising a half-bridge inverter which includes a pair 
of serially connected MOSFET switches for generating a high frequency 
square wave in a resonant output circuit in which the lamp is connected. 
The inverter is driven by a drive control circuit principally consisting 
of an integrated circuit (IC) having pins corresponding to various 
operating parameters of the ballast, such as lamp current, voltage and 
power, as well as a pin for receiving an external dimming control signal. 
A feedback loop in the IC controls the lamp intensity by varying the 
switching frequency of the inverter, a change in frequency in the vicinity 
of resonance of the inverter output circuit causing a substantial change 
in lamp current and voltage and consequently in the power supplied to the 
lamp. A signal which is used as a measure of lamp power is obtained as the 
product of measured average lamp current and measured average lamp 
voltage, which power signal is used to derive an error signal for 
adjusting the lamp intensity to a level signified by an externally 
supplied dimming signal. Linear control of lamp intensity is thereby 
provided over a range down to as low as 1 or 2% of full intensity. Such 
patent is incorporated herein by reference, and constitutes a part hereof 
as fully as if set forth herein. 
A problem is encountered with such a ballast when it must be located at 
some distance from the lamp, so that remote wiring is necessary which 
introduces significant parasitic capacitance there-between. As a result of 
such capacitance the lamp current and voltage as detected at the ballast 
can have a significant phase difference. Consequently, the actual lamp 
power is no longer simply the product of average (i.e., DC) lamp current 
and voltage but rather is given by an integration of the product of actual 
lamp voltage and current over each periodic cycle thereof. At low 
intensity levels the lamp current may be of the same order of magnitude as 
the parasitic capacitive current, and so the current and power as measured 
at the ballast can become altogether erroneous as a measure of actual lamp 
current and power. That will result in erroneous control of lamp 
intensity, and can also cause difficulty in lamp start-up because the 
parasitic current will be interpreted by the ballast as lamp current and 
consequently as an indication that ignition has occurred and therefore 
that the lamp voltage can be reduced from the required start-up level. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a modification of the 
known ballast whereby instead of measuring lamp power as a product of 
average lamp current and lamp voltage it is measured as a product of 
actual lamp current and voltage, taking into account the phase 
relationship there-between. More particularly, the invention provides an 
auxiliary IC for use with the basic IC (known as the ".alpha." IC) of the 
known ballast, the auxiliary IC therefore being referred to as an 
".alpha.2" IC. It includes a first rectifier for rectifying a differential 
voltage representative of lamp current and producing a rectified ac 
current corresponding to said rectified voltage, a second rectifier for 
rectifying a current representative of the lamp voltage, and a 
current-mode single quadrant multiplier for multiplying the two rectified 
currents. The output current of the multiplier is averaged to derive a dc 
voltage proportional to the actual power being supplied to the lamp. Such 
multiplication takes into account whether the lamp voltage and current are 
of the same or different signs during each power cycle, the then existing 
dc voltage representative of lamp power being increased when the lamp 
voltage and current are of the same sign and being decreased when they are 
of opposite sign. Thus, the phase relationship there-between during each 
quadrant of each cycle is taken into account in deriving the value of the 
dc voltage representative of lamp power. Since the parasitic capacitive 
current is 90.degree. out of phase with the lamp voltage, the average 
value of the product of that current and lamp voltage is zero and so it 
does not affect the accuracy of the power signal derived as described.

BASIC FEATURES OF THE PRIOR ART BALLAST 
In order to describe how the present invention improves upon the prior art 
ballast of the above-identified patent, a description will first be given 
of the basic features of such ballast which are also applicable to the 
ballast of the present invention. Referring to FIG. 1, it shows a 
simplified block diagram of a ballast as in the aforesaid patent. A 
substantially constant dc voltage which is selectable over a range of 240 
to 500 volts is supplied to an inverter 60 which comprises a switch mode 
power supply driven by a high frequency switching signal produced by a 
drive control circuit 65 which includes the .alpha.IC. The switching 
frequency may be about 45 kHz, and results in a square wave of that 
frequency at the output of inverter 60. Such output is applied to a load 
70 which includes a series resonant inductor 75 and capacitor 80. The 
resonant frequency is somewhat below the switching signal frequency, 
whereby the lamp intensity can be increased or decreased by lowering or 
raising the switching signal frequency. 
The ballast circuit arrangement in FIG. 1 is shown in more detail in FIG. 
2, which is identical to FIG. 2 of the above-identified patent except for 
omission of the dimming interface circuitry connected to the DIM pin of 
.alpha.IC 109 since such interface is only one of many possible interfaces 
for supplying a dim control signal to the DIM pin. The dimming signal 
interface circuit 110 is therefore only shown in block form. Also omitted 
is the optional external offset circuit shown in dotted block 198 of FIG. 
2 of said patent for use at deep dimming levels down to 1% of full light 
intensity. It could, however, optionally be included. 
Since the ballast in FIG. 2 of said patent is described in detail therein 
further detailed description of the structure thereof is unnecessary. 
However, in order to fully explain the improvement achieved by the present 
invention, a description of certain of the operational characteristics of 
the ballast in FIG. 2 will now be given. 
The difference between the current sensed at pins LI1 and LI2 of the 
.alpha.IC 106 is representative of the current flowing through lamp 85. 
The voltage across the lamp, scaled by the voltage divider formed by 
resistors 174 and 177, is detected by diode 180 and capacitor 183, 
resulting in a dc voltage at junction 181 which is proportional to the 
peak lamp voltage. That voltage is converted into a current into pin VL of 
the .alpha.IC by resistor 189. Within the .alpha.IC 109, the current at 
pin VL (representative of peak lamp voltage) and the difference of the 
currents at pins LI1 and LI2 (representative of average lamp current) are 
multiplied together to obtain a rectified ac current which is fed out of 
pin CRECT into the parallel combination of capacitor 192 and resistor 195. 
Such rectified ac current is thereby converted into a dc voltage which is 
proportional to the average power of lamp 85. A feedback circuit contained 
in the .alpha.IC operates to change the switching frequency of inverter 60 
until the voltage produced by the current at the CRECT pin becomes equal 
to the voltage supplied to the DIM pin from an external dimming interface. 
To be noted is that the current produced at the CRECT pin, flowing to 
ground through the parallel combination of resistor 195 and capacitor 192, 
is indicative of the average power of lamp 85 (the product of average lamp 
current and voltage). 
A resistor 156 connected between pin RREF and ground serves to set a 
reference current within the .alpha.IC, and a capacitor 159 connected 
between pin CF and ground sets the frequency of a current controlled 
oscillator (CCO) comprised in the .alpha.IC for generating the switching 
signals for gates G1 and G2 of switches 100 and 112 of inverter 60. A 
capacitor 165 connected between pin CP and ground is used for timing of a 
preheat cycle and also the timing of a nonoscillatory/standby mode. A pin 
FVDD connected to junction 110 by a capacitor 138 represents a floating 
supply voltage for the .alpha.IC. 
During an initial startup period capacitor 106 is charged in accordance 
with the RC constant of capacitor 106 and resistor 103. During that period 
switch 100 is nonconducting and switch 112 is in the conducting state, the 
input current into pin VDD of the .alpha.IC being maintained at a low 
level (less than 500 microamp). Capacitor 138, between pin FVDD and 
junction 110, charges to a relatively constant voltage of approximately 
VDD which serves as the supply voltage for the drive circuit of switch 
100. When the voltage across capacitor 106 reaches a threshold turnon 
value (e.g. 12 volts), the .alpha.IC enters its operating state 
(oscillatory/switching) with switches 100 and 112 switching back and forth 
between the conducting and the nonconducting states at a frequency which 
is well above the resonant frequency set by inductor 75 and capacitor 80. 
The .alpha.IC initially enters a preheat cycle when the inverter begins 
oscillating. During that cycle the lamp 85 is not yet in the ignited 
state. The initial operating frequency of the .alpha.IC, which is about 
100 kHz, is set by resistor 156 connected to pin RREF and capacitor 159 
connected to pin CF, and the reverse diode conducting times of switches 
100 and 112. That frequency is then reduced by the .alpha.IC at a rate 
determined thereby and the frequency is reduction continues until the peak 
voltage across resistor 162 as sensed at the RIND pin reaches a 
predetermined negative peak value such as -0.4 volts. The switching 
frequency of switches 100 and 112 is regulated by the .alpha.IC so as to 
maintain the sensed voltage at the RIND pin equal to -0.4 volts, which 
results in a substantially constant frequency of about 80-85 kHz at 
junction 110. A relatively constant rms current flows through inductor 75, 
which may be coupled to filaments 76 and 77 of lamp 75 to precondition 
them for subsequent lamp ignition. The duration of the preheat cycle is 
set by capacitor 165. If that capacitor is omitted, there will be no 
preheating and that will result in instant start operation. 
At the end of the preheat cycle, as determined by capacitor 165, the 
.alpha.IC starts sweeping the switching frequency down toward an unloaded 
resonant frequency (i.e. of inductor 75 and capacitor 80 before ignition 
of lamp 85, e.g. 60 kHz). As the switching frequency approaches such 
resonant frequency the voltage across the lamp rises rapidly (e.g. 600-800 
volts peak) and is generally sufficient to ignite the lamp. Once that 
occurs, the lamp current rises from a few milliamps to several hundred 
milliamps. The current through resistor 153, which is equal to the lamp 
current, is sensed at pins LI1 and LI2 of the .alpha.IC based on the 
difference between the currents thereat as proportioned by resistors 168 
and 171 respectively. The voltage of lamp 85, which is scaled by the 
voltage divider formed by resistors 174 and 177, is detected by diode 180 
and capacitor 183 resulting in a dc voltage proportional to the peak lamp 
voltage, at junction 181. The voltage at junction 181 is converted into a 
current by resistor 189 flowing into pin VL. 
The current flowing into pin VL is multiplied within the .alpha.IC 109 by a 
current corresponding to the differential current between pins LI1 and 
LI2, resulting in a rectified ac current fed out of pin CRECT into the 
parallel combination of capacitor 192 and resistor 195. That combination 
converts the ac rectified current into a dc voltage which is proportional 
to the average power of lamp 85. The voltage at the CRECT pin is forced 
equal to the voltage at the DIM pin by a feedback loop contained within 
the .alpha.IC 109. Thus, regulation of the power consumed by lamp 85 is 
obtained. 
The desired illumination intensity level of lamp 85 is set by the voltage 
applied to the DIM pin of the .alpha.IC 109. For that purpose, the 
.alpha.IC comprises the aforesaid feedback loop including a lamp voltage 
sensing circuit and a lamp current sensing circuit. The switching 
frequency of the inverter is adjusted by such feedback loop so that the 
CRECT pin voltage is made equal to the voltage applied to the DIM pin. The 
DIM voltage varies between 0.3 and 3.0 volts, which is a 1:10 ratio. When 
it rises above or falls below that range it is clamped internally by the 
.alpha.IC to 3.0 or 0.3 volts, respectively. 
The voltage at the CRECT pin is zero when lamp 85 ignites. As the lamp 
current increases the current at the CRECT pin, which is proportional to 
the product of average lamp voltage and current, charges capacitor 192 to 
a voltage proportional to said product. The switching frequency of the 
inverter circuit decreases or increases until the voltage at the CRECT pin 
becomes equal to the voltage at the DIM pin. When the dimming level is set 
to full (100%) light output, capacitor 192 is permitted to charge to 3.0 
volts, and therefore the CRECT pin voltage rises to 3.0 volts based on the 
feedback loop. During such rise in voltage the feedback loop remains open. 
Once the CRECT pin voltage reaches about 3.0 volts, the feedback loop 
closes. Similarly, when the dim level is set to minimum light output, 
capacitor 192 is permitted to charge to 0.3 volts and therefore the CRECT 
pin voltage rises to 0.3 volts based on the feedback loop. Generally, 0.3 
volts at the DIM pin corresponds to 10% of full light output. When the dim 
level is set to minimum light output, the CRECT capacitor 192 charges to 
0.3 volts before the feedback closes. 
MODIFIED BALLAST IN ACCORDANCE WITH THE INVENTION 
A modified ballast in accordance with the invention is as shown in FIG. 3, 
and is basically the same as the FIG. 2 ballast except for the addition of 
an auxiliary IC, denoted the .alpha.2 IC, which functions as a 
co-processor with the original .alpha.IC 109. In FIG. 3 the resistors 168 
and 171 which are connected to pins LI1 and LI2 of the .alpha.IC 109 are 
both connected to ground, thereby setting the differential input current 
at those terminals to zero. Consequently, the measured current and the 
voltage corresponding thereto at the CRECT pin will be zero. Instead, the 
CRECT voltage formerly used in the feedback loop of the .alpha.IC is now 
generated by the CPOW pin current of the .alpha.2 IC 301, which current is 
proportional to the instantaneous product of lamp current and voltage and 
consequently to the actual lamp power. The lamp current is now 
differentially sensed at the LI1' and LI2' pins of the .alpha.2 IC 301, 
which pins are connected across the resistor 153 between lamp 85 and 
ground. The lamp voltage is sensed at the IVL pin of the .alpha.2 IC 301, 
which pin is connected by a resistor 303 to the lamp terminal 170 
corrected to the junction between inductor 75 and capacitor 80. The CPOW 
pin of the .alpha.2 IC is connected to the CRECT pin of the .alpha.IC, and 
the ac current generated at the CPOW pin is converted by the parallel 
combination of capacitor 192 and resistor 195 into a dc voltage which is 
proportional to the actual lamp power. That sensed voltage is supplied as 
the CRECT voltage of the .alpha.IC, and so serves as the feedback voltage 
of the error amplifier in the .alpha.IC as described in said patent. The 
reference voltage of the feedback loop therein is controlled by the 
voltage supplied to the DIM pin, and so the supplied dimming voltage 
controls the actual lamp power level. As in the case of the original 
ballast, the differential lamp current sensed by the modified ballast in 
FIG. 3 will include parasitic capacitive current if there is remote wiring 
between the ballast and the lamp. However, since such capacitive current 
will be 90.degree. out of phase with the lamp voltage the average value of 
the product thereof will be zero and so it will not contribute to the 
CRECT voltage produced by the .alpha.2 IC. Thus, the detection of lamp 
power in the modified ballast including the .alpha.2 IC is no longer 
subject to error because of the parasitic capacitance of remote wiring. 
FIG. 4 shows the basic circuit structure of the .alpha.2 IC 301, the pins 
of which correspond to the pins thereof shown in FIG. 3. It could, in 
addition, include voltage supply and voltage bias circuitry not relevant 
to control of lamp intensity. The lamp current rectifier 303 receives from 
pins LI1' and LI2' a differential voltage corresponding to the lamp 
current, and converts that voltage into a rectified ac current which is 
supplied to one input 305a of a current-mode single quadrant multiplier 
305. Such multipliers are well known in the art. The lamp voltage 
rectifier 307 receives from pin IVL a current representative of the lamp 
voltage and converts that into a rectified current which is supplied to a 
second input 305b of multiplier 305. The phase detector 309 is a logic 
circuit which outputs a high logic value if the lamp voltage and lamp 
current are both of the same sign, either positive or negative. If the 
lamp voltage and current are of opposite signs then the sign of the 
product thereof will be negative and phase detector 309 outputs a low 
logic value. The output thereof is supplied to a control input 305c of 
multiplier 305 and controls it to produce pin CPOW an output current which 
is outwardly directed and consequently adds to the then existing voltage 
level at that pin when the signal at control input 305c is high. When the 
signal at control input 305c is low, the output current produced at pin 
305 of multiplier 305 will be inwardly directed (sinked) and so will 
subtract from the then existing voltage level thereat. Thus, an arithmetic 
summation is effected in accordance with the phase relationship between 
actual lamp current and voltage, the resultant voltage at pin CPOW thereby 
being representative of the actual power being supplied to the lamp. 
The operation of the improved ballast can be analyzed as follows. The 
actual power (P.sub.real) consumed by a load such as lamp 85 can be 
expressed as 
##EQU1## 
where v(t) is the voltage across the load and i(t) is the load current. If 
both are sinusoidal, then 
##EQU2## 
where I.sub.peak and V.sub.peak are peak values thereof and .phi. is the 
phase difference there-between. 
In the .alpha.IC ballast power is calculated as 
##EQU3## 
Since i(t) is measured without regard to its sign. If i(t) is sinusoidal, 
then 
##EQU4## 
Equations (1) and (2) show that power as detected by the ac IC is 
representative of the actual or "real" power if there is a zero phase 
difference between load voltage and current. However, that assumption 
implies that there is no parasitic capacitance across the load. At low 
dimming levels of the lamp the lamp current will be small, and if the 
remote wiring is lengthy the resulting parasitic capacitance will be 
significant and will result in a phase difference approaching 90.degree.. 
For instance, if .phi.=85.degree. then cos .phi.=0.087. As a result, the 
value of P as detected by the .alpha.IC will be 11.5 times higher than the 
actual power P.sub.real. 
In contrast thereto, in the .alpha.2 IC ballast power is calculated as 
follows: 
##EQU5## 
where sign {i(t).multidot.v(t)}=1 if the product i(t).multidot.v(t) &gt;0, 
and said sign =-1if said product is &lt;0. Of particular significance is that 
the value of P as given by equation (3) is the actual power P.sub.real, 
for any type of waveform and any value of phase difference. Thus, 
nonlinearities in lamp voltage and/or current will not affect the accuracy 
of control of lamp intensity. 
In the .alpha.2 IC the value of i(t) is provided by the lamp current 
rectifier 303 and the value of v(t) is provided by the lamp voltage 
rectifier 307. The sign function is implemented by switching the direction 
of the current produced at the CPOW pin. The phase detector 309 detects 
whether the sign function is positive or negative. If positive, the CPOW 
pin current is directed outward ("sourced") from the pin, and if negative 
the CPOW pin current is directed inward ("sunk") into the pin. The 
averaging summation over each cycle of supplied power is implemented by 
the RC network of resistor 195 and capacitor 192 connected to the CPOW 
pin. 
While the invention has been described with reference to certain preferred 
embodiments and typical applications thereof, it will be apparent to those 
skilled in the art that various modifications and adaptations thereof may 
be made without departing from the essential teachings and scope of the 
invention as set forth in the ensuing claims.