GB2068656A - A lamp drive circuit - Google Patents

Abstract

As a drive circuit for a discharge lamp it has been proposed to rectify a mains alternating supply, regulate the rectified supply with a switching regulator and switch the regulated direct current with an inverter across whose output the lamp is connected. This invention arranges switching transistors (22-24) of the regulator (18) to receive base drive controlled by a cyclic signal from the output transistor (T6) of a control circuit in cooperation with an inductor (L1) in such a way that if the output of the control circuit fails the energy stored in the inductor decays and the base drive is removed. <IMAGE>

Description

SPECIFICATION Lamp drive circuits This invention relates to lamp drive circuits, and especially to electronic lamp drive circuits for compact source metal halide discharge lamps.

Compact source metal halide discharge lamps are described in Section 15.1.4 of "Lamps and Lighting", Second Edition, 1972, pubiished by Edward Arnold.

Such lamps can be operated either with alternating current or with direct current. However, alternating current operation of a compact source metal halide discharge lamp can provide improved colour rendering, higher luminous efficiency and longer life, as compared with direct current operation. Nevertheless, direct current operation of such a lamp gives the characteristic of a steady light output whereas the light output of an alternating current operated lamp closely follows the current waveform, and with a simple choke in series with a mains supply, the light output is strongly moduiated at twice the mains frequency.This modulation causes a series problem when a compact source metal halide discharge lamp is used as the lamp in a cine-projector, since the modulation can beat with the modulations of the light caused by the cine-projector shutter, the result being an obtrusive fluctuation in the light on the screen receiving the light projected by the projector.

The beat fluctuation has a frequency typically-of a few cycles per second.

A similar problem of beats occurs when an alternating current driven lamp is used for illuminating a scene during the taking of a cine-film.

It has been proposed (see UK Patent Application No. 7930847 publication No. 2030388 - and US Patent No. 4,042,856) to operate a discharge lamp through a ballast which comprises a rectifier to rectify mains alternating current supply, a highspeed switch or chopper for chopping the rectified current at a rate in excess of 10 kHz to provide a square waveform, and an inverterorcommutatorfor continuously reversing the polarity of the chopped rectified voltage applied to the lamp. The circuit additionally may include an ignitor circuit to ensure proper lamp starting.

Problems have however arisen in two respects.

The first is to construct a lamp supply circuit which is capable of supplying high power (e.g. 1000 watt) discharge lamps such as OSI lamps, and which is light in weight and of reasonable cost. The second is to get over the problems of electrical noise. Particu larly when integrated circuits (ICs) are used faults can arise in noisy environments caused for example by (a) rectificaction of radiated noise, (b) rectification of conducted noise on external pin connections, and (c) latch-up and latch-down conditions on the inputs of an IC due to transients in the supply.

Furthermore lamp ignitors involve the use of voltages which can vary from a few kilovolts up to 40 KV or more. To achieve such high voltages with the minimum of gear, spark ignitors are employed.

These produce intense radio frequency energy on a broad spectrum. The peak power in such circuits can be in excess of 150 KW. To attenuate such large power levels adequately is very difficult.

It is an object of this invention to provide a lamp supply circuit suitable for discharge lamps including high power lamps and which allows light weight and economical construction.

According to the present invention there is provided a lamp drive circuit for a discharge lamp, the drive circuit including a switching regulator direct current power supply and an inverter powered by the said power supply and arranged to supply an alternating current between a pair of output terminals when a discharge lamp couples the said output terminals together, the direct current power supply including at least one switching transistor providing current pulses of controlled duration; means for smoothing said pulses to provide a direct current output; means for providing base drive for the at least one switching transistor; a control circuit for providing a cyclic control signal; and means for removing the base drive from the switching transistors in the event of failure of the control circuit.

This invention will be described by way of example with reference to the accompanying drawings, in which: Figure lisa block diagram of a lamp drive circuit embodying the invention arranged to supply a CSl lamp, Figure 2 to 6 show the various components of Figure 1 in more detail, namely, Figure 2 is a circuit diagram of the main power supply circuits (16,32), Figure 3 is a circuit diagram of the switching regulator (18), Figure 4 is a circuit diagram of the control circuit (34) for the switching regulator, Figure 5 is a circuit diagram showing the inverter (20) and its associated power supply circuits (28), and also indicating the lamp (22) and the ignitor (30), and Figure 6 is a circuit diagram of the inverter control circuit (26).

The discharge lamp circuit shown in Figure 1 has a mains supply inlet 12 connected to a radio frequency interference (R.F.I) filter 14. The filtered mains supply is applied through a switch SW1 to a full wave bridge rectifier 16 with a smoothing capacitor, providing in this example a nominally 350 volt DC power supply.

This applied to a series switching regulator 18, which chops the current into DC pulses at a rate of about 20kHz, and regulates power to the lamp. The output of the switching regulator 18 is applied to an inverter 20 which operates at a rate of typically 100 Hz to 1000 Hz, and the output of the inverter 20 is applied to the lamp 22, which is in one example a 1000 watt CSI arc discharge lamp.

In Figure 1 there is shown a vertical chain-dotted line 24. This indicates the physical separation of the components; those to the right of line 24 are closely associated with the lamp 22 while those to the left of it can be remote from the lamp. The only connections crossing the line 24 are the switched DC power from circuit 18, and an AC mains supply.

The inverter 20 is preferably fitted to the lamp head for several reasons. The reduced lead length produces less loss of power and waveform, and causes less radiated interference.

The inverter 20 is operated by an inverter control circuit 26 powered from an auxiliary supply 28. This receives filtered AC mains from a switch SW1 through the b contacts of a lamp start switch SW2.

An ignitor 30 in series with the lamp 22 is also powered from these switch contacts, to produce ignition pulses to start the lamp.

Another auxilliary supply 32 is connected to the ON/OFF switch SW1 and supplies a control circuit 34 for the switching regulator 18. Associated with this control circuit 34 are switch contacts a of the lamp start switch SW2.

Referring to Figures 2 to 6, the detailed circuit construction will now be outlined. Insofar as the connections of the many constituent circuit elements are readily apparent from the drawings they are not repeated here.

Figure 2 shows the RFI filter 14 connected to the mains input 12. This filter 14 may be as described in the aforementioned UK Patent Application No.

7930847. The filter 14 brings the conducted mains frequency interference down to specified levels. The output of the filter 14 is applied to the double-pole mains ON/OFF switch SW1. Power from the switch is then applied to a rectifying bridge B1 and to the b contact of the lamp start switch SW2 where the line side is switched to provide an output comprising the switched line LS and neutral N. The auxiliary power is derived via an auxiliary inverter operating from the rectified side of B1. This allows the voltage tapping to be changed by a single switch SW3.

The various power output terminals are then as labelled at the right hand side of Figure 2. It should be noted that various terminals in the different figures which are similarly labelled are connected together in the completed circuit. In Figure 2 the main power output terminals are B1+ and B1 -.

Terminals B1 -, B3+ are strapped together (shown in Figure 3). Thus terminals B3- is negative with respecttoterminal B1-.

The circuit 18, shown in Figure 3, serves to provide a switched regulated current to the inverter 20. The switched regulator 18 is controlled by the control circuit 34 shown in Figure 4.

Referring to Figure 4, unijunction transistor UJ1 is the timing element for the 18 KHz switching frequency. A voltage ramp developed across capacitor C2 is buffered by means of an emitter follower transistor T3 to a voltage comparator transistor T4. A potential divider formed by Zener diode Z2 and resistor R14 with resistor R13 sets the emitter potential of transistor T4. When the voltage ramp across R8 exceeds the potential at the emitter of transistor T4, then T4 turns on. Transistor T5 also turns on, as does transistor T6 (Figure 3). When C2 discharges through the normal mechanism of the UJ1 operation, base drive is restored.

Power control is derived by current feedback through transformer TX1 whose primary TX1 p is in the switching regulator 18 and which samples the current flowing in the series pass-switching elements T22, T23 and T24. The secondary of this transformer feeds into VR1 and R18. When a certain level of current is reached the voltage across these resistors exceeds the threshold voltage of zener diode Z3 which conducts, firing SCR1. This SCR1 over-rides T4 and transistor T5 is turned on. As the ramp across R8 continues to rise, T4 turns on, commutating SC1 by depleting it of anode current, and so the cycle repeats. During normal operation (on load) SCR1 always ends the cycle. In open circuit conditions SCR1 never fires.

Transistors T1 and T2 form an auxiliary rail voltage comparator to prevent the circuit from operating before proper conditions are established. If insufficient voltage is available the zener diode Z1 fails to conduct and T1 remains off, T2 in this case remains hard on and turns T5 on via R1 1. When sufficient voltage is available zener diode Z1 conducts and T1 turns T2 off, allowing normal circuit operation.

The function of the control circuit 34 of Figure 4 is to provide the main switching transistors (T22, T23, T24 in Figure 3) with the correct drive logic under all events, including for example short circuit and under-voltage.

The board of circuit 34 has only seven active components and is so arranged that common failure modes cause the main switching transistors to shut down. Shut-down will occur if T2, T3, T4, SCR1 or T5 go short circuit (the most common mode of failure of semiconductor devices).

Base drive to the main switching transistors occurs when T5 is in the off condition, which is not achieved when the voltage is insufficient as described.

R19 and C3 provide current limiting with a characteristic between constant current and constant power by making the current a function of output voltage.

This is achieved by recognising that the duration of each of the inverter output current pulses sampled by TX1 is a function of the output voltage of the convertor operating at a constant frequency. The amplitude of each pulse is a function of the load current. In this example rectifier D3 in the control path provides a peak current response and R19 and C4 modify this response as a function of the output voltage (represented by pulse duration).

Referring again to Figure 3, the switching regulator 18 provides smoothed DC between its output terminals X, Y. This is achieved by the co-operation of the main switching transistors T22, T23 and T24, which are in parallel to give a high switching current, with the inductor L3 and capacitors Cl 1, Cl 2. A base drive circuit controls the bases of the three parallel transistors T22, T23 and T24. Snubber protection is provided for turning on and turning off of by networks L2, R26, and D10, R25, C13 respectively. D9 is a free-wheel diode for L3 and is snubbed via R22 and C10.

The base drive circuit according to the invention is designed to provide power transistor protection both in the running mode and the off state. The base drive circuit is also such that if long term failure occurs in the control circuit 34 (that is when T5 is in the on or off states for more than two or three cycles) the switching transistors are turned off.

Transistor T6 may also fail in short circuit or open circuit condition with no risk being given to main power transistors. The base drive power rail (B3-) is, as has been described, negative with respect to the emitter connections of the power transistors (connected to B1-), since a positive voltage source is required for driving the power elements. Energy is stored on a cycle-to-cycle basis on choke Ll,so the operation of the circuit is dependent on repetitive switching of T6. If T6 fails one way or the other base drive is removed by the natural decay of current in the choke.

A further advantage of purely single negative drive is that only half power is required for the base drive compared to conventional symmetrical base drive circuits.

Another feature of the drive circuit is that the base current is self regulating depending on the load, reducing base storage charge problems associated with high power convertors. As the load is increased the main pass transistor'angle' is reduced, butthe energy storage time in choke L1 is increased, so it is possible to optimise the base current with the load and hence optimise switching losses.

When transistor T6 is on storage in L1 occurs and when T6 turns off the energy is diverted into the base circuit. The current in L1 is limited by R6.

Network D5, D7 and D8 clamps the collector saturation voltage to a desired value, thus reducing the storage time.

R23 enables a path for reverse base current to flow during turn off.

The invertor 20 is shown in Figure 5, and is driven by a transformer TX5. The inverter includes transistor T8, T9, T10 and T1 1. The transistors T8 and T11 form the switching elements of a full-wave bridge inverter; transistors T8 and T11 are always in phase, and transistors T9 and T10 are in phase. The phases of the two sets are opposite, i.e. when T8 and Tri 1 are on, then T9 and T10 are off.

Base driven is provided from four identical windings on the transformer TX5; the dot convention indicates the correct mode of connection. In this way the four switching transistors T8 to T1 1 are provided with base drive from a single drive circuit, thus reducing the bulk of and the number of components in the part of the power supply associated with the lamp.

As shown in Figure 6, an auxiliary rail is provided for the inverter 20 drive from auxiliary inverter B2.

Inverter 82 is a constant current generator providing constant current base drive to the full bridge inverter transistors T8-T11. This method of base drive limits the power dissipated in the full bridge circuit provided that the auxilary inverter is a switching regulator.

Transistor UJ2 is a unijunction oscillator providing negative edges to the flip-flop formed by transistors T13 and T12. UJ2 oscillates at twice the lamp frequency. TransistorsT12 and T13 provide a divideby-two circuit which gives a 1:1 mark space ratio.

Transistors T14 and T15 form a push-pull driver circuit which is emitter-coupled to the flip-flop. R50 and C29 form a snubber network for T14 and T15.

Thus it is seen that transformer TX5 is driven by a 'Jordon' flip-flop designed for maximum noise immunity. The unijunction oscillator is used for simplicity, the advantage of such a forced driven system over self-resonant types is that the frequency of operation can be varied easily by adjustment of R38, the lower frequency limit being imposed by TX5 and the upper frequency limit being fixed by losses in the bridge, due to storage times.

Operation of the inverter is stopped via switch SW2b by removing mains supply to the auxiliary inverter B2 and simultaneously shutting down the main inverter by SW2a.

The ignitor 30 itself can be of known type. It is connected in series with the lamp 22 across the inverter outputs. Protection of the bridge against ignitor transients is provided by filter C14, C15 and L4. Capacitors C20 to C23 provide additional protection.

The lamp drive circuit illustrated has the advantage that it can be relatively insensitive to noise while nevertheless requiring relatively few components. For this reason the lamp supply of the invention may be constructed to be of light weight and relatively low cost. The circuit is also believed to be particularly suitable for construction by thick film techniques allowing a further reduction in cost for volume production.

Claims (8)

1. A lamp drive circuit for a discharge lamp, the drive circuit including a switching regulator direct current power supply and an inverter powered by the said power supply and arranged to supply an alternating current between a pair of output terminals when a discharge lamp couples the said output terminals together, the direct current power supply including at least one switching transistor providing current pulses of controlled duration; means for smoothing said pulses to provide a direct current output; means for providing base drive for the at least one switching transistor; a control circuit for providing a cyclic control signal; and means for removing the base drive from the switching transistors in the event of failure of the control circuit.

2. A lamp drive circuit according to Claim 1 in which the means for providing base drive includes a single power rail and the means for removing the base drive includes an inductance in said power rail releasing energy to the bases of said switching transistors and means for maintaining energy stored in the inductor only in response to the cyclic signal from the control circuit.

3. A lamp drive circuit according to Claim 2 in which the base drive includes means for supplying energy to the inductor when the cyclic control signal is in one direction and means transferring the energy from the inductor to the base of the at least one switching transistor when the cyclic control signal is in the other direction.

4. A lamp drive circuit according to any preceding claim in which there are a plurality of said switching transistors driven by said one base drive.

5. A lamp drive circuit according to any preceeding claim in which the control circuit is responsive to the amplitude of current pulses supplied by the switching transistors to be representative of the current supplied by the switching regulator and to the duty cycle of the current pulses supplied by the switching transistors to be representative of the output voltage of the switching regulator and thereby to be representative at least in part of the power supplied to the inverter.

6. A lamp drive circuit according to any preceding claim in which the inverter includes four inverter switching transistors and a further base drive circuit supplying base drive thereto, wherein the further base drive circuit includes an auxiliary switching regulator providing a constant current generator to provide constant current base drive to all four of said inverter switching transistors.

7. A lamp drive circuit substantially as herein described with reference to Figures 1 and 3 of the accompanying drawings.

8. A lamp drive circuit substantially as herein described with reference to the accompanying draw ings. ~~~~~~~~~~~~~~