WO1987007996A1

WO1987007996A1 - A ballast for systems having multiple high-intensity discharge lamps

Info

Publication number: WO1987007996A1
Application number: PCT/US1987/000168
Authority: WO
Inventors: Donald S. Kuhnel; Sidney A. Ottenstein
Original assignee: Innovative Controls, Inc.
Priority date: 1986-06-19
Filing date: 1987-01-21
Publication date: 1987-12-30
Also published as: IL81400A0

Abstract

An electronic ballast is provided for systems having a plurality of high-intensity discharge lamps. The ballast operates on commercially available AC line current, and has overvoltage, undervoltage, and spike protection features. It also uses a current feedback loop to maintain precise control of the lamp current during start-up and otherwise.

Description

A BALLAST FOR SYSTEMS HAVING MULTIPLE HIGH-INTENSITY DISCHARGE LAMPS

Field of Invention:

This invention relates to the field of electronic solid state ballast systems for high-intensity discharge lamps. More particularly, this invention relates to the field of controlled systems for ballasting high-intensity discharge lamp systems having more than one lamp.

Background of the Invention;

In high-intensity discharge lamps, light is generated when an electric current is passed through a gaseous medium. The lamps have variable resistance charac¬ teristics that require operation in conjunction with a ballast to provide appropriate voltage and current limiting means. Control of the voltage, frequency and current supply to the lamps is necessary for proper operation and deter¬ mines the efficiency of the lamps. In particular, it determines the size and weight of the required ballast.

The appropriate voltage, frequency and current for efficient running of a lamp in its normal operating state is not appropriate for the lamp during its warm-up state. A high-intensity lamp typically takes several minutes to warm-up from the time it is struck or turned on to its normal operating state. Initially, the lamp is an open circuit. Short pulses of current are sufficient to strike the lamp, provided they are of adequate voltage. Subsequent to striking, the lamp's resistance drops radically. The resistance then slowly rises during warm-up to its normal operating level. Hence, subsequent to striking and during warm-up the current of lamp must be limited to prevent internal lamp damage.

Prior art ballasts for multiple lamp systems typically were of the wire iron type. However, such ballasts are both inefficient and heavy. Electronic ballasts are more efficient and lighter in weight.

In some applications, it is more crucial that the characteristics of the incoming current and voltage to the lamps be precisely regulated. An example of such an appli¬ cation is a multiple lamp system used in color sorting machines. Thus, it is desirable to provide an electronic ballast wherein the current and voltage characteristics are precisely regulated.

Summary of the Invention;

An electronic ballast for a multiple high-inten¬ sity discharge lamp system is taught. The ballast has a current integration feedback loop to control the lamp current during start-up or when other overcurrent condition are present.

Each of the lamps in the multiple lamp system has an inductor in series with it to limit current to that lamp This also allows the system to operate with one or more lamps removed.

The multiple lamp system also has overvoltage and low voltage protection circuits, as well as a voltage spike protector and a radio frequency interference (RFI) filter.

In one embodiment, a regulated, pulse width mod¬ ulated power supply precedes the ballast. This results in more regulated light output, making the ballast system suitable for use with fluorescent bulbs in color sorting machines.

It is an object of the present invention to desig an AC line powered electronic ballast to power multiple high-intensity discharge lamps in commercial watt ranges at a high frequency to obtain a higher efficiency than commer¬ cial wire iron ballasts.

It is another object of the present invention to provide an electronic ballast with precise regulation of the input signal by employing overvoltage and undervoltage protection, and a radio frequency interference filter.

It is yet another object of the present invention to use a regulated power supply preceding an electronic ballast to achieve a regulated light output suitable for use in color sorting machines.

It is yet another object of the present invention to provide a ballast which detects current imbalances in the lamp system, and controls the lamp system current during start-up and otherwise, by use of a current integration feedback loop.

These and other features of the present invention will be apparent to one skilled in the art from the drawings and the detailed description.

Brief Description of the Drawings;

Fig. 1 is block diagram illustrating the control sequences of a preferred embodiment of the present in¬ vention.

Figs. 2A, 2B and Fig. 3 are circuit diagrams of a preferred embodiment of the present invention.

Detailed Description;

Fig. 1 illustrates in a schematic block diagram fashion the elements of a preferred embodiment of the ballast system utilizing a push-pull DC to AC converter, ^' an autotransformer, a lamp circuit for multiple lamps, and a current integration feedback loop.

The scheme assumes an input of alternating cur¬ rent, preferably 115 VAC. In a preferred embodiment, an optional regulated power supply 135 may precede the ballast to achieve a more regulated light output. This makes the ballast system particularly suitable for use with fluores¬ cent bulbs in color sorting machine applications.

The AC to DC converter 10 then rectifies the alternating current in a traditional fashion into direct current waves. Converter 10 is comprised of a full wave bridge rectifier 11 and capacitor 128.

A spike protector 127 prevents intermittent high voltages or spikes from reaching the ballast. Also, the ballast contains a radio frequency interference (RFI) filte 129, comprising capacitors 130, 131, 132 and inductors 133 and 134 (see Fig. 2A) to prevent RFI signals generated by the ballast from being picked up on the power line.

Low voltage power supply 12, fed by input from converter 10, supplies low voltage direct current to an oscillator, a dead time controller, and a pulse width modu¬ lator. Low voltage power supply 12 comprises resistors 108 and 109, zener diode 125, and capacitor 126 (See Fig. 2A) . In a preferred embodiment, power supply 12 outputs 15 VDC, and is used to run the sensing electronics.

The oscillator, dead time controller and pulse width modulator, together with the switch control, form the means for driving the hexfet switches 28A and 28B.

The ballast also contains a 5 VDC reference power supply 126 internal to subcircuit 40, which provides 5 VDC on pin 14 of subcircuit 40.

Referring again to Fig. 1, oscillator 16 generate a high-frequency signal, high at least in relation to the line frequency. The frequency is controlled by resistor 11 and capacitor 115, .according to the following equation;

Frequency of operation = .55

RC

In a preferred embodiment, the frequency equals 66 kilocy¬ cles, although it is obvious that other frequencies may be used.

The frequency of oscillator 16 determines the frequency of the alternating current in the lamp circuit. The frequency of oscillator 16 and the voltage transformation performed by transformer 30 and taps 31A and 31B are chosen to permit the election of an efficient economic current limiting means, such as inductors 32A, 32B, 32C, and 32D, which are in series with lamps 34A, 34B, 34C and 34D respectively.

The high frequency wave formed by oscillator 16 is supplied to dead time controller 18 and pulse width modu¬ lator 20. Pulse width modulator 20 is also supplied with input from a current integration feedback loop 71.

Referring to Figs. 2A and 2B, pulse width modu¬ lator comparator 20 compares the input signal voltage from error amplifier 13 with the variable periodic signal voltage generated by oscillator 16. During that part of the oscil¬ lator signal cycle that the variable periodic signal voltage is greater than the signal voltage supplied by error ampli¬ fier 13, pulse width modulator comparator 20 is turned to an "on" state.

Pulse width control subcircuit 40 includes the following components;

1. Complete pulse width modulation control circuitry;

2. On chip oscillator 16;

3. Two user available operational amplifiers, error amplifiers 15 and 13;

4. Internal 5 VDC reference power supply 126;

5. Two output transistors 46 and 48 for driving the push-pull converter; and

6. Fixed or variable dead time controller 18. A primary purpose of control subcircuit 40 is to develop two alternate pulse trains at a fixed frequency. In a preferred embodiment, the^' pulse width of each pulse being controlled varies from 0 percent to 48 percent, with a minimum dead band (both pulses having a zero period) of two percent. Of course, dead bands of other lengths may be used. Subcircuit 40 uses two operational amplifiers for pulse width modulation control, error amplifier 15 and feed¬ back/pulse width modulator comparator 20.

Resistors 100 and 102 are connected to the 5 VDC regulated power supply 126, forming a voltage divider that is connected to the positive input of amplifier 15.

Referring again to Fig. 1, dead time controller 18 produces a modulated output signal that corresponds to a maximum duty cycle of slightly less than 100 percent. Such dead time controller provides a safety period to ensure that switch controller 24 cannot gate hexfets 28A and 28B "on" at the same time. As a result of dead time controller 18, switch controller 24 must gate both hexfets 28A and 28B "off" for a minimum dead time each oscillating signal cycle.

Switch control 24 combines the outputs of dead time controller 18 and pulse width modulator 20, and sends the wave forms alternately to gate "on" hexfet 28A or hexfet 28B.

The duty cycle of each half wave of the induced current in the lamp circuit is a function of the on and off times of hexfets 28A and 28B, which is turn is a function of dead time controller 18 and pulse width modulator 20 of the switch driving means.

Figs. 2A and 2B represent a more specific diagram for the preferred embodiment of the ballast system illus¬ trated in Fig. 1. The subcircuit illustrated in Fig. 2A utilizes a pulse width modulating subcircuit 40 that is commercially available. One suitable IC chip is a Motorola TL 494. See Fig. 3. Use of such circuit is convenient but not necessary.

Referring now to Figs. 2B and 3, dead time con¬ troller 18 compares the variable periodic signal voltage from oscillator 16 each cycle with a minimum set control level voltage and is turned to an "on" state for all but a small percentage (2%) of each signal cycle of oscillator 16. The logic of the pulse modulator subcircuit 40 combines the output of dead time controller 18 with the output of pulse width modulator comparator 20, and permits NOR gates 42 and 44 to enable transistor switches 46 and 48 only when both controller 18 and comparator 20 are turned to the "on" state.

Dead time controller 18 generates the clock signal for flip-flop 19, corresponding to the frequency of oscil¬ lator 16, so that output switch transistors 46 and 48 may be driven alternately through control of the flip-flop by NOR gates 42 and 44. The output of the switch driver means is two pulse width modulated signals, at the frequency of oscillator 16, which open and close hexfets 28A and 28B.

Control subcircuit 40 has output transistors 46 and 48, the emitters of which drive the gates of hexfets 28A and 28B. These hexfets are the power switches of the push-pull DC to AC converter.

The operation of the power output stage of the ballast, including the push-pull converter and the lamp circuit, will now be described.

When a + 15 volt signal appears on the emitter of transistor 46 of subcircuit 40, the gate of hexfet 28B rises to + 14 volts through diode 85 and resistor 81. Diode 85 off biases transistor 23, causing hexfet 28B to conduct and bringing one side of transformer 30 to near 0 volts. The center tap of transformer 30 is then a"t 160 VDC. Transformer action in transformer 30 develops 320 volts at the drain of hexfet 28A. When the positive gate drive signal is removed from hexfet 28B, transistor 23 conducts and discharges an internal gate capacitor rapidly, i.e. in 100 nanoseconds. After a short dead time internal to subcircuit 40, the emitter of transistor 48 of subcircuit 40 goes to positive 15 volts, bringing the gate of hexfet 28A to positive 14 volts via diode 84 and resistor 80. Diode 84 keeps transistor 21 reversed biased. Hexfet 28B then conducts and brings one side of transformer 30 to near 0 volts. Transformer action in transformer 30 then develops 320 volts at the drain of hexfet 28B.

It can be seen from the above that the drains of hexfets 28A and 28B alternately switch from 0 to positive 320 volts.

Five filament windings, 120A, 120B, 120C, 120D an 120E, of 4 volts each provide filament power to the four lamps 3 A through 34D. Transformer 30 is tapped so that 25 volts is present between the tapped points. This is a preferred voltage to fire the lamps. Tap 31A is connected to filament winding 120E, tap 3IB is connected to the four inductors 32A through 32D. These four inductors are series connected to filament windings 120A through 120D respectively. The inductors limit the current to the lamps

The presence of one inductor in series with each lamp allows one lamp or more to be disconnected without affecting the current in the remaining lamps. The filament are connected -so that a 250 volt square wave is across each lamp while each lamp filament is heated with four volts.

As mentioned above, the present invention uses a current integration feedback loop 71 (Fig. 1) to control th lamp characteristics during lamp warm-up or when other overcurrent conditions are present. Current feedback loop 71 will now be described in greater detail.

When the lamp is first struck or turned on, pulse width modulator 20 severely restricts current through the lamp circuit. Each hexfet is gated on only for a small fraction of each duty cycle. At the beginning of the warm-up cycle, the lamps' resistances are very low, and the lamps are susceptible to damage if an overcurrent condition exists. As the lamps begin to warm-up, their resistances increases. The current integration feedback loop compares the current sensed by the current sensors 116A and 116B wit a reference value, and communicates with the pulse width modulator. Assuming the sensed current is below the reference value, the pulse width modulator permits each hexfet to be gated on for a larger percentage of each duty cycle. The current is thereby gradually and precisely increased in correlation to a reference value yielding a precise control of the current during warm-up. If the current is higher than the reference value, the duty cycle is reduced. This increases the lives of both the lamp apparatus and the ballast.

When the lamp is completely warmed-up, the circuit will operate in what constitutes its normal operating mode. Each hexfet switch then remains gated on for its maximum design duty cycle, which in a preferred embodiment may be 48 percent of the time, if a 2% dead time is used.

Referring now to Fig. 2A, the current integration feedback loop operates as follows.

Current sensor 116A is comprised of diode 106 and resistor 73. Current sensor 116B is comprised of diode 107 and resistor 74. Diodes 106 and 107 sense the voltage drop, which is proportional to the current in the power output stage discussed, across resistors 73 and 74 respectively. The positive current is passed through the diodes and series dropping resistor 108, developing a peak voltage across capacitor 110 and resistor 109. The developed voltage, proportional to the peak current in the power output stage, is input to the positive input terminal of error amplifier 13.

Resistors 104 and 112 make a voltage divider from the output of the 5 volt regulated power supply 126. The voltage across resistor 112 is input to the negative input of error amplifier 13. Resistor 101 sets the gain of error amplifier 13 at a nominal gain of 27. The magnitude of the voltage across resistor 112 equals 1.56 volts. When the current in the power output stage develops a voltage in excess of 1.56 volts across resistor 73 or 74, a peak current of 4.72 amps is flowing in the power output stage. When the current in that stage exceeds 4.72 amps, the voltage developed across resistor 109 and capacitor 110 causes the positive output of error amplifier 13 to go positive with respect to its negative input. Thus, the output of error amplifier 13 goes high by a positive 2.5 volts, thereby reducing the pulse width of the driving pulses to keep the current below this specified level. The process is repeated whenever another overcurrent condition exists.

The present invention may also use an overvoltage control 136, which causes the lamps to extinguish whenever the line voltage exceeds a predetermined set point, such as 135 volts AC. The operation of overvoltage control 136 is as follows.

Referring now to Fig. 2A, resistors 102 and 100, connected to the 5 VDC regulated power supply 126 internal to subcircuit 40, develop a voltage of 2.5 volts at the negative input of amplifier 15, also internal to subcircuit 40. Resistors 100A and 100B, connected to the 160 VDC powe supply 10, develop a voltage across resistor 100B greater than the 2.5 volts at the positive input of amplifier 15. This causes the output of amplifier 15 to go high to a positive 5 VDC, thereby reducing the pulse width output to zero. This extinguishes the lamps. Resistor 113 provides schmitt trigger action in amplifier 15, making the set points for the drop out or pull-in to be several volts apart.

The ballast in the present invention may also contain an undervoltage protection circuit 137 to prevent damage to the power hexfets 28A and 28B. In the event that the AC line voltage drops below about 90 volts, the 15 volt regulated power supply 12 will drop out of regulation. Thi reduces the gate drive voltage to hexfets 28A and 28B when they are under a full current load, resulting in destructio of the hexfets unless undervoltage protection is achieved.

Undervoltage protection circuit 137 operates as follows. The collector of transistor 121 is connected to resistor 122 and pin 4 of subcircuit 40, corresponding to dead time controller 18. The emitter of transistor 121 is connected to the 5 volt regulated power supply 126, which remains in regulation even to a very low AC line voltage condition of 50 volts, or even lower. Resistors 124 and 123 are connected to the 15 volt regulated line output of power supply 126.

Under normal conditions, transistor 121 is turned off. When the 15 volt line drops to 13.5 volts, transistor 121 conducts. This raises the voltage at pin 4 of subcir¬ cuit 40 five volts, thereby reducing the pulse width modu¬ lation period to zero. This extinguishes the lamps and removes the load from hexfets 28A and 28B. The lamps cannot relight due to the presence of low voltage.

Claims

WHAT IS CLAIMED IS;

1. An electronic ballast for high-intensity discharge lamps, comprising: a direct current source; a lamp circuit containing a plurality of lamps, each of said lamps having an inductor in series therewith; a current feedback means for sensing the current present in said lamp circuit, comparing said current with a reference value, and generat¬ ing an output signal; a high frequency oscillator; a pulse width modulator responsive to said output signal of said current feedback means; and a DC to AC converter that by control of at least one switch converts current from said direct current source to alternating current to power said lamps, said DC to AC converter including a switch driving means for driving said switch, said DC to AC converter being responsive to said high frequency oscillator and to said pulse width modulator.

2. An electronic ballast for high-intensity discharge lamps, comprising: a direct current source; a lamp circuit containing a plurality of lamps, each of said lamps having an inductor in series therewith; an overvoltage protection means for sensing the circuit voltage, comparing it with a refer¬ ence value, and extinguishing power to said lamps if the circuit voltage exceeds the reference value; a lamp circuit containing a plurality of lamps, each of said lamps having an inductor in series therewith; a high frequency oscillator; a current feedback means for sensing the current present in said lamp circuit, comparing said current with a reference value, and generat¬ ing an output signal; a pulse width modulator responsive to said output signal of said current feedback means; a DC to AC converter that by control of at least one switch converts current from said direct current source to alternating current to power said lamps, said DC to AC converter including a switch driving means for driving said switch, said DC to AC converter being responsive to said high frequency oscillator and to said pulse width modulator.

3. An electronic ballast for high-intensity discharge lamps, comprising: a direct current source; a lamp circuit containing a plurality of lamps, each of said lamps having an inductor in series therewith; an undervoltage protection means for sensing the circuit voltage, comparing it with a refer¬ ence value, and extinguishing power to said lamps if the circuit voltage is less than the reference value; a lamp circuit containing a plurality of lamps, each of said lamps having an inductor in series therewith; a high frequency oscillator; a current feedback means for sensing the current present in said lamp circuit, comparing said current with a reference value, and generat¬ ing an output signal; a pulse width modulator responsive to said output signal of said current feedback means; a DC to AC converter that by control of at least one switch converts current from said direct current source to alternating current to power said lamps, said DC to AC converter including a switch driving means for driving said switch, said DC to AC converter being responsive to said high frequency oscillator and to said pulse width modulator.

4. The ballast of claim 2 wherein said overvoltage protection means extinguishes power to said lamps by generating an output signal, in response to which said pulse width modulator outputs a signal corresponding to a zero duty cycle for said lamps.

5. The ballast to claim 3 wherein said undervoltage protection means extinguishes power to said lamps by generating an output signal, in response to which said pulse width modulator outputs a signal corresponding to a zero duty cycle for said lamps.

6. The ballast of claims 1, 2 or 3 further com- prising: a spike protection means for preventing damage to said lamp circuit caused by intermittent high voltage.

7. The ballast of claims 1, 2, or 3 further com- prising: a radio frequency interference (RFI) filter for preventing RFI signals generated by said ballast from affecting the ballast circuit.

8. The ballast of claims 1, 2 or 3 further co - prising: a dead time controller for controlling said switch so that said switch is gated off for a predetermined length of time.

9. An electronic ballast system for high-intensity discharge lamps, comprising: a regulated, alternating current source; an AC to DC converter for converting current from said alternating current source to direct current; a lamp circuit containing a plurality of lamps, each of said lamps having an inductor in series therewith; a current feedback means for sensing the current present in said lamp circuit, comparing said current with a reference value, and generating an output signal; a high frequency oscillator; a pulse width modulator responsive to said output signal of said current feedback means; and a DC to AC converter that by control of a least one switch converts current from said DC to AC converter to alternating current to power said lamps, said DC to AC converter including a switch driving means for driving said switch, said DC to AC converter being responsive to said high frequency oscillator and to said pulse width modulator.

10. The electronic ballast of .claims 1, 2 , 3 or 9 further comprising: a low voltage direct current source for providing power to said pulse width modulator.