WO2002067636A1

WO2002067636A1 - A digital lamp controller for low frequency operation

Info

Publication number: WO2002067636A1
Application number: PCT/IE2002/000021
Authority: WO
Inventors: Sean Noone; Michael Quinlan; Thomas Durkan
Original assignee: Noontek Limited
Priority date: 2001-02-20
Filing date: 2002-02-20
Publication date: 2002-08-29
Also published as: WO2002067636A8

Abstract

The invention provides a lamp ballast unit (1) having a control circuit (6) for controlling the power delivered to a low frequency operated lamp (2).

Description

"A Digital Lamp Controller for Low Frequency Operation"

lntrorl.ir.tion

The present invention relates to a lamp ballast unit having a control circuit for controlling the power delivered to a low frequency operated lamp . The invention is particularly directed towards providing a digital regulation controller for controlling the operation of a lamp ballast operating at low frequencies.

Generally, such lamp ballast units have up to now used mainly analogue methods to control the power to a lamp. This is carried out by a variety of techniques, including frequency shifting as with high frequency ballast devices, or by pulse width modulation techniques which are used on low frequency synthesised waveform ballasts.

A problem with analogue circuitry is that the circuitry suffers from instability over time. Further the circuitry can become unstable due to variations in temperature. This results in the inefficient operation of the lamp as incorrect current is delivered to the lamp due to the instability of the analogue circuitry leading to unnecessary losses in power.

A further problem with analogue circuitry is that there is usually a large number of components required to achieve effective ballast operation. Further, in a manufacturing process using several analogue components, the failure rate of these components is normally quite high resulting in a large amount of waste. Also a significant amount of testing is required on each component to ensure there is no fault in each component. This adds considerably to the expense and complexity of the manufacturing process.

Another problem is that when upgrades of analogue components are required, it is necessary to physically replace them. This is a time consuming and expensive task. Indeed, in some instances, it can be cheaper to replace the entire circuit with a new one. Maintenance of these analogue circuits is difficult, as each component must be individually tested from time to time to ensure that the correct power is delivered to the lamp. Again, this is a time consuming and expensive task.

For high frequency operation the lamp voltage and lamp current changes very rapidly. In order to maintain stability of the lamp these values have to be sampled at high speed. This is necessary to allow for effects such as phase differences between voltage and current to be accounted for power calculation. Because of the high frequency operation it is necessary to provide very fast sampling for analogue to digital conversion which is expensive and complex to implement. Similarly peak current and voltages occur very quickly in high frequency operation so fast sampling is required. Some digital solutions have been proposed to overcome the aforementioned problems specific to high frequency operation. An example of such a digital solution is disclosed in PCT Publication No. WO 01/45473 "Koninklijke Philips Electronic NV". This PCT publication discloses a digital ballast using a control integrated circuit. However, this digital ballast is directed at fluorescent lamps operating at high frequencies only. Further, this PCT publication does not teach or suggest to employ a digital control for lamp ballasts operating at low frequencies to overcome the problem inherent in low frequency operation. Indeed a person skilled in the art would consider the use of a digital ballast for low frequency use inappropriate.

In this specification it is to be regarded that low frequency operation is any frequency operation below 1 kHz while high frequency operation is anything above this value.

No solution or circuit has yet been developed to overcome the above problems. The present invention is directed towards providing a digital regulation circuit to overcome the above mentioned problems for a low frequency operated lamp.

Statements of Invention

According to the present invention there is provided a lamp ballast unit having a control circuit for controlling the power delivered to a low frequency operated lamp comprising :- a power supply;

an ignition circuit for the lamp;

a drive circuit having a number of switches connected between the power supply and the lamp;

characterised in that:

the control circuit has digital control means for providing digital reference values to control the operation of the drive circuit to ensure correct current is delivered to the lamp at low frequencies.

Ideally, the circuit comprises a micro-controller to operate the digital control means and is connected to each switch of the drive circuit. In one embodiment a digital-to- analogue converter, connected to the microcontroller, feeds a comparator with the digital reference value converted to an analogue value by the digital-to-analogue converter.

The advantage of carrying out the invention in this way is that it provides a simple effective, stable means of comparing the current across the lamp with a desired operational value representative of a set digital value which is converted by the digital to analogue converter for comparison with the measured current across the lamp thus ensuring correct power is delivered to the lamp operating at low frequencies. Because of the low frequency operation, the lamp need only be monitored or sampled every few milliseconds or so long as over current protection is also included. This provides a much simpler less complex circuit. It is the realisation of this that led to the present invention.

In another embodiment an analogue-to-digital converter converts the current value through the lamp to a digital value and then compared with the digital reference values in the microcontroller. Alternatively in this embodiment the lamp current value is converted to a digital value and the microcontroller compares the two values in digital form and in which the microcontroller responds to the comparison to ensure correct operating conditions are delivered to the lamp.

Ideally there is provided current sense means for sensing the current value during the positive or negative cycle of operation of the lamp. Preferably the current sense means is connected to the comparator. In one embodiment the current sense means is a resistor.

Ideally, when the sensed current value equals the reference value, an interrupt pulse generated by the output of the comparator is sent to the microcontroller causing a digital pulse width modulator to change the drive signals of the switches.

In a further embodiment the microcontroller comprises a digital pulse width modulator for controlling the drive signals to the switches.

Ideally the microcontroller comprises a processor and a memory.

Preferably the processor has means for processing programmed instructions stored in memory for controlling the operation of the digital pulse width modulator.

Ideally the memory has means to receive new program instructions remotely. It will be appreciated that the new program instructions can be downloaded directly or remotely via a radio link for example.

Detailed Description of the Invention

The present invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only, with reference to the accompanying drawings, in which:-

Fig. 1 is a circuit diagram of the present invention, and

Fig. 2 is a waveform of the voltage across a sense resistor of the circuit and the current at the load.

Fig. 3 is an alternative embodiment of the present invention.

Referring to the drawings and initially to Fig. 1 , there is illustrated a lamp ballast unit indicated generally by the reference numeral 1 , for controlling the operation of a lamp 2. An ignition circuit 3 connected to an inductor 4 is used to control the start-up operation of the lamp 2. A number of switches 5(a) to 5(d) forming a drive circuit are used to ensure correct power is delivered to the lamp 2. The power is delivered from a mains supply 10. The switches 5(a) to 5(d) may be a number of transistors, for example, a MOSFET. Each of the switches 5(a) to 5(d) is connected to a control circuit 6 having a microcontroller. The microcontroller is connected to a digital to analogue converter (DAC) 7 whose output feeds a comparator 8 with a reference value set by the microcontroller. Signals representing lamp voltage are fed to the microcontroller after suitable attenuation. A current sense resistor 9 which measures the current value through the lamp 2 during operation feeds the comparator 8 with a voltage representing the current during the drive phase of the switching cycle. All ballast functions with the exception of power factor correction are carried out by the microcontroller.

Referring now to Fig. 1 in more detail, current to the lamp 2 flows when switches 5(a) and 5(d) switch "on". Pulses derived from a digital pulse width modulator located within the control circuit 6 drive switch 5(a). As the current in the lamp 2 builds up, the voltage across the current sense resistor 9 will increase. This voltage is fed to the comparator 8. A digitally derived reference input to the comparator 8 from the DAC 7 is generated by the control circuit 6. An interrupt pulse is generated when the load current reaches the reference input. This interrupt pulse causes the digital pulse width modulator to reset thereby removing the drive signal from the switch 5(a). This cycle is repeated continuously ensuring the correct load is delivered to the ballast lamp 2 by the reference values set by the control circuit 6. This is the basic current control and the switching between switches 5(a) and 5(d) is repeated for example every 20 microseconds. The lamp current is therefore limited. The microcontroller comprises internal analogue to digital converter blocks (not shown) which reads the lamp voltage and the peak version of lamp current. From these values the microcontroller generates a digital error signal which is used to either increase or decrease the comparator 8 threshold value. This is accomplished by software means stored on the microcontroller. Therefore the lamp current is adjusted according to what power is actually present in the lamp 2. It will be appreciated that the lamp voltage is obtained from the regulation indicator 4 voltage which can be filtered using standard resistors and capacitors. This gives an attenuated version of the lamp voltage.

Referring now to Fig. 2, there is illustrated a waveform of the voltage across the current sense resistor 9 and the current at the load, indicated generally by the reference numeral 20. Value l₂ is set by the control circuit 6 to ensure the correct power is delivered to the lamp 2. The value of l₂ is converted by the DAC 7 to an analogue value and is fed into the comparator 8 to provide a reference value for the positive and negative cycles of the ballast lamp 2 operation. The period between I, and 1₂ represents the positive cycle in which current is being delivered to the lamp.

The lamp cycle refers to the polarity of the current through the lamp 2. This remains fixed for the duration of the lamp half cycle which is approximately 7 mS. The rising current between I, and l₂ is the ramp up period of the switching cycle. The dead period between l₂ and I, is the free-wheeling period.

This can be seen by the increasing current of the waveform. When the current reaches the limit set by l₂ no more external power is applied to the inductor 4 and the lamp 2 then freewheels until the onset of the next cycle.

in operation, when switch 5(a) is switched on the current jumps to the I, value. Note I, is not set by the microcontroller, this value is whatever value is present from the freewheeling ramp-down when the next cycle begins. The interrupt pulse that occurs when l₂is reached does not alter the state of the switch 5(d). The switch 5(d) remains in the conducting state until the microcontroller 6 signals a load polarity reversal of the switches. There are two states of operation, switch 5(d) conducting with switch 5(a) pulsing and switch 5(c) conducting with switch 5(b) pulsing.

The switch 5(a) operates in conjunction with the switch 5(d) which is held on continuously for the positive cycle to drive current of positive polarity to the ballast lamp 2. The switch 5(d) acts as a steering switch. For the negative cycle or opposite polarity, the switch 5(b) is pulsed with the switch 5(c) and is held "on". Thus, by alternating these two combinations, the correct AC current is delivered to the ballast lamp 2.

It will be appreciated that the invention allows for relatively simple calculation of the lamp power of the lamp 2. This is achieved by feeding an average of the voltage at the junction of switches 5(a) and 5(b) to an analogue to digital converter (not shown) on the control circuit 6. The average voltage at that point reflects the actual lamp voltage. The averaging is done by simple RC filtering.

Referring now to Fig. 3 there is illustrated the ballast lamp unit 1 of Fig. 1 except the digital to analogue converter 7 and the comparator 8 is replaced by an analogue to digital converter 30. A filter 31 comprising standard resistors and capacitors is used to filter the regulation indicator (4) voltage in order to obtain the lamp voltage. This gives an attenuated version of the lamp voltage. The measured lamp current value is converted to a digital value by the analogue to digital converter 30 which is fed to the control circuit 6. The microcontroller compares the set reference digital value with the converted digital value representative of the lamp current. The microcontroller can then send the appropriate drive signals to the switches 5(a) and 5(d) to ensure correct current is delivered to the lamp in response to the comparison of the two digital values.

The use of a purely digital microcontroller allows direct software control of the lamp 2 without the temperature or time drift problems, complexity and inflexibility associated with analogue electronics.

It will be appreciated that MOSFETS can be used as switches in this embodiment but other switching technologies may be used.

It will also be appreciated that while the use of a digital pulse width modulator in the microcontroller is advantageous, other pulse width modulation techniques may be employed. Further, in this embodiment, the current sense means is in the form of a resistor but other electronic components may be used to carry out the same operation.

Another aspect of the present invention is the microcontroller further comprises a processor and a memory. It will be appreciated that the processor has means for processing programmed instructions set by an operator. The operator can store these instructions in the memory of the microcontroller. Thus, total control of the reference values may be achieved by software means. This has the advantage that the control circuit 6 can be easily updated without the need for adding or replacing hardware within the circuit. It will be further appreciated that the updating can be carried out remotely, for example, over a radio link.

In the specification the terms "comprise, comprises, comprised and comprising" or any variation thereof and the terms "include, includes, included and including" or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation.

The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail within the scope of the claims.

Claims

1. A lamp ballast unit (1) having a control circuit (6) for controlling the power delivered to a low frequency operated lamp (2) comprising:-

a power supply (10);

an ignition circuit (3) for the ballast lamp (2);

a drive circuit having a number of switches (5(a-d)) connected between the power supply (10) and the lamp (2); and

characterised in that:

the control circuit (6) has digital control means (7, 8) for providing digital reference values to control the operation of the drive circuit to ensure correct current is delivered to the lamp (2) at low frequencies.

2. A lamp ballast unit (1) as claimed in claim 1, in which the control circuit (6) comprises a microcontroller to operate the digital control means (7, 8) and is connected to each switch (5(a-d)) of the drive circuit.

3. A lamp ballast unit (1) as claimed in any preceding claim, in which the digital control means (7, 8) comprises a digital-to-analogue converter (7), connected to the microcontroller, feeds a comparator (8) with the digital reference value set by the microcontroller converted to an analogue value by the digital-to- analogue converter (7).

4. A lamp ballast unit (1) as claimed in claims 1 or 2 in which an analogue-to- digital converter converts the current value across the lamp ballast (2) to a digital value and then compared with the digital reference values by the microcontroller.

5. A lamp ballast unit (1) as claimed in any preceding claim, in which there is provided current sense means (9) for sensing the current value during the positive or negative cycle of operation of the lamp (2).

6. A lamp ballast unit (1) as claimed in claim 5, wherein the current sense means (9) is connected to the comparator (8) for comparing the sensed current value with the converted analogue reference value.

7. A lamp ballast unit (1 ) as claimed in any preceding claim, wherein when the sensed current value equals the converted analogue reference value, an interrupt pulse generated by the output of the comparator (8) is sent to the microcontroller causing the digital pulse width modulator to change the drive signals of the switches (5(a-d)).

8. A lamp ballast unit (1) as claimed in any of claims 5, 6 or 7, in which the current sense means is a resistor (9).

9. A lamp ballast unit (1) as claimed in any of claims 2 to 8, wherein the microcontroller comprises a digital pulse width modulator for controlling the drive signals to the switches (5(a-d)).

10. A lamp ballast unit (1) as claimed in any preceding claim in which the frequency operation of the lamp operates at a frequency less than 1kHz.

11. A lamp ballast unit (1) as claimed in any preceding claims, in which the microcontroller further comprises a processor and a memory.

12. A lamp ballast unit (1) as claimed in claim 11, wherein the processor has means for processing programmed instructions stored in memory for controlling the operation of the digital pulse width modulator.

13. A lamp ballast unit (1) as claimed in claim 12, wherein the memory has means to be downloaded with new program instructions.