WO1999014990A1

WO1999014990A1 - Floating driver having common emitter drive stage

Info

Publication number: WO1999014990A1
Application number: PCT/US1998/019445
Authority: WO
Inventors: David Pacholok
Original assignee: Everbrite, Inc.
Priority date: 1997-09-18
Filing date: 1998-09-17
Publication date: 1999-03-25
Also published as: AU2246999A

Abstract

A driver for a gas discharge lamp is claimed. The driver comprises a timer having an output that varies from a high state to a low state and has a pin connected to a common line. A first and a second transistor is electrically connected to the timer output in a half-bridge complimentary common-emitter configuration. A common node is disposed between the first and second transistors, connecting in circuit to an output transformer that drives a lamp. An electrical conduit is connected between the common node and the common line such that the potential of the common line is substantially the same as the potential at the common node. The present invention also includes a source of DC power whereby the output of the timer going to a high state turns on the first transistor to apply a positive DC voltage to the transformer and to the common line, and whereby the output going to a low state line, and whereby the timer output going to a low state turns off the first transistor and turns on the second transistor to apply a low state signal to the transformer and to the common line connected to the timer.

Description

FLOATING DRIVER HAVING COMMON EMITTER DRIVE STAGE

This application claims priority of United States provisional patent application number 60/059,240, filed on September 18, 1997.

BACKGROUND OF THE INVENTION

The present invention comprises a power supply that is particularly suitable for gas discharge lamps. More particularly, the present invention comprises a floating high side driver with a common emitter drive stage for providing power to a gas discharge lamp. The power supply according to the present invention is self-oscillating, and is capable of switching high voltages without using high power devices.

Gas discharge lamps are highly efficient sources of light energy. Gas discharge lamps are typically formed of gas filled vessels having electrodes. In order to power a gas discharge lamp, a high voltage is applied across electrodes at a high frequency. Gases within the tube become ionized as they are excited by high energy radiation. Light output of the gas output lamp is thus directly related to the input power signal characteristics.

In the past, numerous methods have been devised to provide floating gate or base drive for the top transistor in the familiar half bridge power converter topology; these include base or gate drive transformers and discrete and integrated semiconductor circuits that use high voltage current sources or optoelectronic means to achieve the necessary isolation as the emitter or the source of the top switching device "bounces" from ground to the full supply voltage at the switching frequency. While these techniques work well, they are both rather expensive for a low cost low power device such as an electronic ballast. In production quantities, the lowest cost device to date is the IR2151 by International Rectifier Inc. and sells for about $.50 in large volumes. Transformers on the other hand tend to cost at least this much each and have had difficulties with interwinding insulation failures and production consistency issues.

SUMMARY OF THE INVENTION

In a preferred embodiment of the present invention, the driver for a gas discharge lamp includes a timer which has an output that varies from a high state to a low state, and a pin electrically connected to a common line. The driver also includes a pair of transistors electrically connected to the timer output in a half-bridge complimentary common-emitter configuration. The common node is electrically connected between the two transistors and electrically connected in circuit to an output transformer that drives a load, such as a gas discharge lamp. An electrical conduit connects the common node with the common line such that the potential on the common line is substantially the same as the potential on the common node. The present invention also includes a source of DC power, whereby the output of the timer going to a high state turns on the first transistor to apply a positive DC voltage to the transformer and to the common line. When the timer output goes to a low state, the timer output turns off the first transistor and turns on the second transistor to apply a low state signal to the transformer and to the common line, electrically connected to the timer. The present invention may also include a blocking diode and a bootstrap capacitor electrically connected in circuit between the blocking diode and the common line.

The circuit is comprised of an integrated timer device, such as a 555 timer, two discrete bipolar power transistors, a blocking diode and additional resistors and capacitors as described herein. Although the present invention is preferably used to provide high side and low side drive, it may also be used with a separate low side driver.

The present invention enables a push-pull drive scheme to be used without crossover, while still allowing the transistors to be driven higher or lower than the power rails without using high voltage devices.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 is a schematic diagram of a circuit that incorporates the present invention. The inventive matter herein employs a low cost, low voltage oscillator Ul, such as the familiar 555 timer or a CMOS gate type oscillator and a complementary pair of

Bipolar transistors or metal-oxide-semiconductor fold effect transistors (MOSFETS) Ql and Q2. The emitter terminals of both the PNP and NPN devices Q2 and Ql, respectively (source terminals if P channel and N MOSFETS are used) are operationally connected together and further connected to the common or ground terminal GND of the low voltage oscillator. In this configuration, both the PNP and the NPN devices require alternate floating drive, the NPN requiring a (+) current and the PNP requiring a (-) current alternating on opposite half cycles of the inverter output frequency. Both of these currents are referenced to the common floating ground; that is, the junction 10 of transistors Ql and Q2 emitters which is the output of the half bridge and "bounces" from ground to the full supply voltage as described above.

When transistor Q2 is on, current flows through resistor R8, transistor Q3, and diode Dl into capacitor Cl and returns to ground by jumper J2 and transistor Q2. This action charges capacitor Cl every 1/2 cycle of inverter operation, maintaining a voltage substantially identical to the supply voltage across capacitor Cl, which provides power for oscillator Ul, the 555 oscillator. When transistor Ql is turned on during opposite 1/2 cycles, the voltage at the cathode of diode Dl rises to essentially twice the supply voltage and diode Dl is thereby turned off, while a voltage equal to the supply voltage still appears across capacitor Cl which provides uninterrupted power for oscillator Ul. Oscillator Ul provides 50 percent duty unipolar square waves at pin 3 whose frequency is determined by resistors Rl, R2 and R3 and capacitor C2 according to standard time constant calculations. This unipolar output has an average of 1/2 the capacitor Cl voltage at its 50 percent duty cycle.

Since the time constant of capacitor C3 and resistor R4 is long compared to the oscillator period, and since the parallel-connected base emitter junctions of transistor Ql and transistor Q2 allow current flow in both directions, a DC voltage of 1/2 the capacitor Cl voltage appears across capacitor C3, with the side connected to pin 3 being positive. When pin 3 goes high, a (+) current flows through capacitor C3, resistor R4 and the base-emitter junction of transistor Ql, turning it on. Similarly, when pin 3 drops low, energy stored in capacitor C3 produces a (-) current flow through resistor R4 and the base-emitter junction of transistor Q2 , turning it on. Resistor R4 also serves to limit current through the base-emitter junctions of both transistors. During the minority carrier storage time of each transistor, the base- emitter voltage of each is maintained near its respective on-state voltage until the carriers recombine and it turns off; this effectively prevents the opposite part from being turned on until its partner is completely off, thus preventing cross conduction or "shoot through" current in transistors Ql and Q2.

Other component functions are as follows: Capacitors C5, C6 and C7 are high frequency bypass capacitors. Inductors LI and L2 , and capacitors C8 and C9 form a Pi-L low pass network for electromagnetic interference (EMI) control, and capacitor C4 resonates the transformer inductance during start-up of the lamp load to provide maximum starting voltage and during lamp running to provide typical series resonant inverter ZIS to minimize switching losses in transistors Ql and Q2, and increase efficiency. Resistor R5 provides a leakage path to charge capacitor Cl to start oscillation of oscillator Ul when power is first applied; once the bootstrap charging action of capacitor Cl takes place, resistor R5 serves no additional purpose. Resistor R8 limits peak currents through diode Dl and transistor Q3 and isolates switching noise from transistors Ql and Q2 from the oscillator circuit Ul.

Transistor Q3 and resistors R6 and R7 provide a means of remote control of inverter on/off and dimming functions. If pin CT is left open or pulled to the input supply voltage, transistor Q3 is off and no bootstrap power can be applied to capacitor Cl and oscillator Ul, and the inverter and lamp load are off. When pin CT is pulled low, however, transistor Q3 is turned on which allows normal inverter operation. As the inverter may be turned on and off in this way hundreds of times per second, a pulse width modulated square wave in this frequency range may be applied to this CT pin to effect dimming of the lamp load over a wide range. The human eye integrates the average light output of a source that is on, for example, 3 milliseconds (ms) and off 7 ms as being only 3/10 as bright as a source that is on all the time; no flicker is perceived.

In an installation comprising several ballasts and lamps, the CT pins of each ballast may be connected together to a common switch to ground, which will turn all lamps on and off simultaneously or to a common low frequency PWM source (see above) to provide substantially equal dimming of all lamps without the running of individual unsightly and expensive power wires for each ballast and dimmer or switch. Conversely, since all ballast/lamp assemblies are connected to the input power supply at all times, by connecting the CT terminal to ground for full light output and opening the CT terminal by the same switch to turn the lamp desired completely off, or even locally controlled in brightness with a local PWM source. When this local ballast/lamp switch is thrown to the "master dim" position, thereby connecting the CT pin to the master dim line, the master dimmer will control the brightness of that lamp.

Although several embodiments of the invention have been shown and described, alternate embodiments may be apparent to those skilled in the art. Therefore, the invention is to be limited only to the following claims.

Claims

CLAIMSWe claim:

1. A driver for a gas discharge lamp, comprising: a timer having an output that varies from a high state to a low state; first and second transistors connected to said timer output in a half bridge complementary common- emitter configuration; a common node disposed between said two transistors, connected in circuit to an output transformer that drives the lamp; an electrical conduit connected between said common node and said common line such that the potential on said common line is substantially the same as the potential at said common node; and a source of DC power; whereby the output of the timer going to a high state turns on the first transistor to apply a positive DC voltage to the transformer and to the common line; and whereby the timer output going to a low state turns off said first transistor and turns on said second transistor to apply a low state signal to the transformer and to the common line connected to the timer.

2. The driver of claim 1, further comprising: a blocking diode; and a bootstrap capacitor connected in circuit between the blocking diode and said common line.

3. The driver of claim 1, wherein said timer is a 555 timer. 4. The driver of claim 1, wherein said first and second transistors are bipolar transistors.

5. The driver of claim 1, wherein the first and second transistors are MOSFETS.

6. The driver of claim 1, wherein said first transistor is a P-channel device and said second transistor is a N-channel device.

AMENDED CLAIMS

[received by the International Bureau on 12 February 1999 (12.02.99); original claim 1 amended; new claim 7 added; remaining claims unchanged (2 pages)]

1. A driver for a gas discharge lamp, comprising: a timer having an output that varies from a high state to a low state and having a pin electrically connected to a common line; first and second transistors connected to said timer output in a half bridge complementary common- emitter configuration; a common node disposed between said two transistors, connected in circuit to an output transformer that drives the lamp; an electrical conduit connected between said common node and said common line such that the potential on said common line is substantially the same as the potential at said common node; and a source of DC power; whereby the output of the timer going to a high state turns ON the first transistor to apply a positive DC voltage to the transformer and to the common line; and whereby the timer output going to a low state turns OFF said first transistor and turns ON said second transistor to apply a low state signal to the transformer and to the common line connected to the timer.

3. The driver of claim 1, wherein said timer is a 555 timer.

4. The driver of claim 1, wherein said first and second transistors are bipolar transistors.

5. The driver of claim 1, wherein the first and second transistors are MOSFETS.

7. The driver of claim 1, further comprising: a third transistor that is connected in circuit to said timer; and a control line that delivers a control signal to said third transistor to switch said third transistor; whereby timer operation is interrupted in response to the switching of said third transistor to at least briefly shut off said lamp.

STATEMENT ACCOMPANYING ARTICLE 19

Claim 1 has been amended to recite that the timer has a pin connected to a common line. Claim 7 has been added to recite a feature of the invention that is discussed at page 5, lines 16 through 32.