Circuit for quickly energizing electronic ballast

An electronic ballast having a boot strap capacitor 22 that becomes initially charged at a first rate and a high voltage storage capacitor 23 that becomes charged at a second, faster rate, wherein the boot strap capacitor 22, becoming initially fully charged initiates operation of a PWM driver 18 that in turn causes a power factor corrector and inverter 16 to energize corresponding gas discharge lamps 11. Upon activation of the PWM driver 18 and the corresponding activation of the power factor corrector and inverter 16, a voltage clamp 19 responds to these events by establishing a conductive path 20 between the high voltage storage capacitor 23 and the boot strap capacitor 22, such that continued operation of the PWM driver 18 is ensured. So configured, a relatively small valued capacitor can be utilized for the boot strap capacitor 22, thereby ensuring rapid activation of the lamps 11 without risking subsequent sporadic energization or other operational difficulties.

The technical field of this invention relates generally to electronic 
ballasts used to energize gas discharge lamps. 
BACKGROUND OF THE INVENTION 
Gas discharge lamps are well known in the art. Typically, such lamps are 
energized by a ballast. Unlike incandescent lights, gas discharge lamps 
and their accompanying ballasts as found in the prior art do not switch on 
instantly. When turn on time becomes too long, users of the product may 
become confused when trying to switch the light on, and may conclude that 
the light or the ballast is no longer functioning properly. 
An electronic ballast has a boost coupled to an inverter. The output of the 
inverter energizes the lamps. Before the lamps are fully energized, the 
boost and the inverter must begin to operate. This creates a delay which, 
if not controlled, is perceptible to the user. 
Some electronic ballasts have a boost circuit. The boost circuit provides 
power factor correction, as is well known in the prior art. The boost is 
composed of a bridge rectifier coupled to an AC (alternating current) 
power source. The bridge rectifier supplies pulsating DC (direct current) 
power to a boost inductor. A pulse width modulator (PWM) driver drives a 
semiconductor switch, supplying energy to an electrolytic capacitor 
through a diode. The output of the boost is coupled to a load. A switch, 
when closed, connects the boost to the AC power source. 
One problem that arises is with powering the pulse width modulator driver. 
The PWM driver is an integrated circuit, and thus will not begin operating 
until it is supplied with 10 volts DC (direct current). Since the circuit 
is coupled to a 60 Hz AC (alternating current) voltage source, there will 
be some amount of time elapsed before the 10 volt DC is supplied to the 
PWM driver. Until the PWM driver begins operating, reduced power is 
supplied to the load. 
It is highly desirable to have the PWM driver begin operating as soon as 
possible after the switch is closed. At the same time, of course, the 
circuit powering the PWM driver must be low cost. 
One known method for powering the PWM driver at start up uses current 
flowing through a resistor to charge a capacitor. The voltage on the 
capacitor increases until it reaches the turn-on threshold of PWM driver. 
After startup, the PWM driver must have a source of higher power. The 
operation of the PWM driver causes the semiconductor switch to begin 
operating, causing high frequency current to flow through a boost inductor 
. The high frequency current is coupled to a secondary winding, rectified 
by a diode and supplied to a capacitor, thus sustaining the energy in the 
capacitor at a sufficient level to power the PWM driver. If the switch is 
a field effect transistor (FET), the total current drawn by the PWM driver 
and the FET semiconductor switch is approximately 20 milliamps. With a 
capacitor having a capacitance of 47 mF (microfarads), a startup time of 
about 0.5 seconds is achieved. 
However, if a high voltage, on the order of 800 volts or more, is across 
the semiconductor switch, then an expensive, high voltage FET must be 
used. A bipolar junction transistor (BJT) would be more cost effective. 
Using a BJT for the semiconductor switch presents an additional problem. 
Because a BJT requires much more drive current, the amount of current 
drawn by the PWM driver is much more (on the order of 200 milliamps, as 
compared to 20 milliamps for an FET). 
To supply such a large current, the capacitor must also be larger 
(approximately ten times larger with a BJT as opposed to an FET). But, if 
the capacitor is ten times larger, in order to preserve the charging time 
of capacitor, the resistor must be 10 times smaller. But, if the resistor 
is ten times smaller, then the power dissipation by the resistor is ten 
times greater. Such a high power dissipation causes the ballast to become 
less efficient, since power is being wasted. Additionally, the heat 
generated by the dissipation in power may adversely effect the operation 
of the entire ballast. 
Thus, a more efficient circuit for quickly energizing the PWM driver is 
highly desirable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now of FIG. 1, the electronic ballast described herein couples to 
a pair of series connected gas discharge lamps 11. (Although a pair is 
shown, one or more lamps may be connected in their stead.) The electronic 
ballast couples to a source of alternating current 12 through a user 
operable switch 13, as is well understood in the art. A rectifier 14 
receives the alternating current and provides a full wave rectified 
output. This output couples to both a power factor corrector and inverter 
16 and to a PWM driver 18 via a resistor 21 and a boot strap capacitor 22 
(the boot strap capacitor 22 serves, amongst other things, to filter the 
rectified alternating current signal provided by the rectifier 14). The 
PWM driver 18 is coupled to and controls operability of the power factor 
corrector and inverter 16. A voltage clamp 19 couples to the power factor 
corrector and inverter 16 and also couples, via a conductive path 20, to 
the boot strap capacitor 22. Lastly, the power factor corrector and 
inverter 16 also couples to an output 17 which in turn couples to the gas 
discharge lamps 11. 
So configured, the power factor corrector and inverter 16 provides the high 
voltage/high frequency signal that is needed to energize the gas discharge 
lamps 11. The PWM driver 18 controls operation of the power factor 
corrector and inverter. 
The boot strap capacitor 22 has a corresponding charging rate (which 
charging rate is dependent upon a variety of factors, including the 
capacitance of the boot strap capacitor 22 itself). Similarly, the high 
voltage storage capacitor 23 has a corresponding charging rate in the 
context of the circuit depicted. Importantly, the charging rate for the 
boot strap capacitor 22 is slower than the charging rate for the high 
voltage storage capacitor 23. With this in mind, it will now be pointed 
out that, when the switch 13 is closed, a charging path exists between the 
rectifier 14 and the high voltage storage capacitor 23, as well as with 
the boot strap capacitor 22. So configured, once the switch 13 is closed, 
both capacitors 22 and 23 will begin to charge, with the high voltage 
storage capacitor 23 becoming completely charged first. In this 
embodiment, it is preferable that the high voltage storage capacitor have 
a charging rate that does not exceed 10 milliseconds, whereas the boot 
strap capacitor 22 should have a charging rate that does not exceed 500 
milliseconds. Although other time periods could be utilized, longer timing 
rates may give rise to delay start times that are, in turn, interpreted by 
a user as indicative of failure. 
The boot strap capacitor 22 must have a relatively low capacitance value in 
order to ensure that the charging rate for the boot strap capacitor 22 
will not exceed 500 milliseconds. Therefore, although the boot strap 
capacitor 22 will charge relatively quickly, it will not contain a large 
quantity of stored energy. Once the boot strap capacitor 22 becomes 
charged, an energizing signal is provided to the PWM driver 18, which in 
turn initially activates the power factor corrector and inverter 16. When 
the power factor corrector and inverter 16 becomes active, a drive signal 
is provided to the gas discharge lamps 11. 
At the same time, the voltage clamp 19 responds to operation of the power 
factor controller and inverter 16 by establishing a conductive path 20 
that selectively couples the high voltage storage capacitor 23 to the boot 
strap capacitor 22, thereby delivering energy from the high voltage 
storage capacitor 23 to the boot strap capacitor 22 and hence sustaining 
continued operation of the PWM driver 18. 
To summarize the above description, the boot strap capacitor 22 will charge 
relatively quickly (from the standpoint of an observer) and can provide 
sufficient energy to the PWM driver 18 to cause initial activation of the 
electronic ballast. .Its smaller size, ensures rapid initial activation. 
However, the boot strap capacitor 22 cannot long sustain operation of the 
PWM driver 18. Since, upon activation, a path 20 is formed between the two 
capacitors 22 and 23 through the voltage clamp 19, and since the high 
voltage storage capacitor 23 completed its full charge before the boot 
strap capacitor 22, energy from the high voltage storage capacitor 23 is 
thereafter made available to the boot strap capacitor 22 to sustain 
continued operation of the PWM driver 18 and hence continued energization 
of the gas discharge lamp 11. 
Referring now to FIG. 2, in more detailed description of an electronic 
ballast in accordance with the invention will be presented. 
The rectifier 14 can be comprised of a diode bridge 38. The power factor 
corrector and inverter 16 includes a circuit comprised of a 6 mH 
(microhenry) inductor 39 and a 0.1 mF capacitor 41. The circuit couples to 
a diode 40 and a MJE18004 bipolar transistor 42. (As an aside, the power 
factor corrector and inverter 16 contains this transistor 42 as the only 
active lo component in its design). The PWM driver 18 includes a drive 
element 43 and a pulse width modulation control element 44, provided 
through use of an MC3845 integrated circuit, as is well understood in the 
art. The boot strap capacitor 22 in this embodiment comprises a 47 mF 
capacitor. Resistor 21 that couples the boot strap capacitor 22 to the 
rectifier comprises a 220,000 ohm resistor. 
The voltage clamp comprises a transformer having a primary winding 46 and 
two secondary windings 47 and 52. A 0.1 mF capacitor 48 couples across the 
primary 46 and the first secondary 47. A ferrite bead 49 (for 
electromagnetic interference suppression) and a diode 51 are disposed as 
configured. The second secondary 52 couples to a diode 53 and to the path 
20 to the boot strap capacitor 22 as described above. 
In this embodiment, the high voltage storage capacitor 23 couples to the 
primary 46 and comprises a 22 mF capacitor. 
So configured, energy from the high storage capacitor 23 is inductively 
coupled through the primary 46 and second secondary 52 via the path 20 to 
the boot strap capacitor 22 when the voltage clamp circuit 19 is rendered 
fully operational via the transistor 42 of the power factor corrector and 
inverter 16. 
To conclude this more detailed description, the output 17 includes two 
inductors 33, 36 and two capacitors 34, 37 configured to form appropriate 
resonant circuits suited to properly maintained energization of the gas 
discharge lamp 11. The lamps 31 and 32 are themselves coupled into the 
electronic ballast circuitry via appropriate gas discharge lamp terminals 
30, as well understood in the art. 
So configured, a relatively simple and inexpensive circuit configuration 
provides for rapid activation of gas discharge lamps, with effective 
sustained operation of those lamps also being ensured.