EP2208399B1

EP2208399B1 - Operating lighting means

Info

Publication number: EP2208399B1
Application number: EP08847992.8A
Authority: EP
Inventors: Günter MARENT; Stefan Zudrell-Koch
Original assignee: Tridonic GmbH and Co KG
Current assignee: Tridonic GmbH and Co KG
Priority date: 2007-11-06
Filing date: 2008-11-06
Publication date: 2013-04-17
Anticipated expiration: 2028-11-06
Also published as: CN101849439A; WO2009059772A1; AT517937B1; CN101849439B; DE112008002754T5; EP2208399A1; AT517937A5

Description

The present invention generally relates to the supply of electrical power to lighting means, such as for example fluorescent lamps, high intensity discharge lamps, organic or inorganic light emitting diodes, etc.
The requirements as to the allowable harmonics for electronic ballasts for lamps supplied with mains power are lower at lower power ratings, e.g. below 25W in comparison to higher ratings. Therefore, at the lower power ratings, an active PFC (Power Factor Control Unit with controlled switch) is not necessary.
On the other hand, if no such active PFC is provided in the ballast, there can be the problem that a storage capacitor will always charged up to the maximum peak of the mains voltage connected to the electronic ballast, such that the bus voltage will vary synchronously to the mains voltage. To be more precise, the DC voltage across the storage capacitor will show an AC ripple with twice the frequency of the supplying mains voltage.
Now, if for cost reasons, a DC/AC converter supplied with the bus voltage and supplying AC voltage to the lighting means is used, the operational frequency of which can not varied, there is the problem that the power supply to the lighting means and thus the light Intensity will vary synchronously to the mains voltage
Document US 2005/0231133 A1 discloses providing and controlling power to loads including one or more LEDs. A power control apparatus receives a DC input voltage and draws an input current from a source of power. The input current passes through an energy transfer arrangement via the operation of a switch. The energy transfer arrangement is a buck-boost or inverting converter, including an inductor as well as other DC-DC converter circuitry that is configured to provide the DC output voltage to the load. The power to the load is adjusted by varying the duty cycle of the switching operation and holding the switching frequency constant.

Object of the present invention:

It is the object of the invention to propose a technique for reducing the impact of ripple of a DC voltage of a power modulator for lighting means on the characteristics of the lighting means operation. "Ripple" is top be understood of a variation of a preferably unipolar DC voltage, the variation being smaller than the DC level.

Summary of the invention:

This object is achieved by means of the features of the independent claims. The dependent claims develop further the central idea of the present invention.
Generally the invention proposes a feed-forward control approach. A parameter of the supply voltage for the power modulator is measured and evaluated in order to set a power-determining parameter of the power modulator. There is no feedback signal from the lamp circuit (supplied by the power modulator) in order to compensate for the impact of the ripple. This does not exclude the presence of feedback signals for other purposes.
To this regard the invention proposes a method for supplying these lighting devices with power, to a computer software program product executing such a method when running a computing device, to a power supply unit as well as to an integrated circuitry, such as for example an ASIC for implementing the power supply control method.
According to a first aspect of the invention a method for supplying lighting means with feed-forward controlled power is proposed according to claim 1.
Further aspects of the invention are disclosed in the dependent claims.
A further aspect of the invention relates to an integrated circuitry, such as e.g. a ASIC or a microcontroller, which is designed to perform a method according to any of the preceding claims.
Further features, advantages and objects of the present invention will become evident when reading the following description of non-limiting embodiments when taken in conjunction with the figures of the enclosed drawings.

Figure 1 shows schematically a circuitry according to the present invention using a half-bridge converter as power modulator, and
Figure 2 shows schematically a circuitry according to the present invention using a buck converter as power modulator.

In figure 1 there is shown a mains voltage supply 1, i.e. a AC voltage having a peak value of e.g. 220V or 240V and a frequency of 50Hz or 60Hz. The mains voltage 1 is rectified e.g. by a bridge rectifier 2 and supplied to a storage capacitor 3. The storage capacitor 3 will supply a DC voltage ("bus voltage") which may be modulated synchronously to the mains voltage 1. The bus voltage is supplied to a power modulator 4 which supplies power to a light source 7.
As shown in figure 1 no actively switched PFC is provided between the rectifier 2 and the storage capacitor 3 such that the DC voltage 8 of the storage capacitor 3 will present an AC ripple with twice the frequency of the mains voltage 1.
The invention proposes to detect the mains voltage 1 or the bus voltage 8 supplied to the power modulator (the DC/AC converter being just one example thereof) 4, i.e. the voltage 8 across the storage capacitor 3 and to supply this information to a control unit 5.
The control unit 5 will then feed-forward control the power modulator 4, i.e. a power-setting parameter thereof pending on the sensed mains voltage 1 or bus voltage 8. The invention particularly proposes a digital and/or integrated implementation of such a control unit 5.
The shown half-bridge DC/AC converter 4 is one example of the power modulator. Other examples are e.g. a buck converter e.g. for supplying power to LEDs (organic or inorganic light emitting diodes) or a Push-Pull converter for use in ballasts for fluorescent lamps with low supply voltage of e.g. 12V. One alternative, being just one example of many, will be explained later on with reference to figure 2.
In the shown example the power modulator comprises a half bridge DC/AC converter 4 with two serial connected switching elements S1, S2 (such as for example MOSFETs) and a load comprising a resonance circuitry 6 and the light source 7.
As also shown in figure 1, an analogue signal 11 is sensed representing at least the time behaviour (not necessarily also the absolute level) of the bus voltage 8 and/or the mains voltage 1. This sensed analogue signal 11 can be digitized by a A/D converter 9 in a N-bit digital value (N being an integer greater than 1 and preferably greater than 2) and then be supplied to a transformation unit 10 of the control unit 5. It is also possible that the analogue signal 11 is sensed representing at least the absolute level of the bus voltage 8 and/or the mains voltage 1.
The transformation unit 10 transforms the supplied digital current sample value e(k) and optionally also one or more past values e(k-1) to a power modulating parameter x(k) supplied to the power modulator 4 e.g. via a driver circuitry 12.
The transformation can be performed e.g. by using a look-up table stored in a memory 13 or by using a preferably digitally implemented function.
The current sample rate e(k) or the transformed value x(k) can be stored in a register 14 for use in the transformation of subsequent values of x(k).
Thus the power modulator 4 is controlled depending on the sensed analogue voltage signal. Thus, in a digital manner the value x(k) for the control signal for the power modulator 4 is computed. Generally, the power modulator 4, i.e. a power-setting parameter thereof will be controlled by the transformation unit 10 depending on the sensed mains voltage 1 and/or bus voltage 8. In the present illustrative example the power modulator will set the operational frequency f of the half-bridge converter with connected load circuitry 6 depending on the sensed bus voltage 8 and/or mains voltage 1.
In the present example, e.g. the power supply to the light source is a direct function of the frequency control signal supplied to the power modulator, such that the currently supplied power for the light source can be derived from the frequency control value.
The data register thus can hold information about the currently delivered power sample value or one or more previous values thereof to the load 6.
The transformation unit preferably is not only supplied with the bus voltage or main voltage signal, but also with an output of the data register 14 and thus the current and/or previous power value(s).
Generally the transformation unit 10 will then compute a power setting value depending on the previous data register value and the value of the sensed bus or mains voltage.
In figure 2 there is shown a buck converter as an additional example for a power modulator driving a light source LED. This circuit can be supplied by a mains voltage supply, i.e. an AC voltage having a peak value of e.g. 220V or 240V and a frequency of 50Hz or 60Hz. The mains voltage can rectified e.g. by a bridge rectifier and supplied to a storage capacitor. The storage capacitor will supply a DC voltage ("bus voltage") which may be modulated synchronously to the mains voltage. The bus voltage is supplied to the buck converter (power modulator) which supplies power to a light source LED.
As an alternative it is also possible to supply the power converter (buck converter) with a low supply voltage, e.g. 12V from a battery source.
Similar to figure 1 no actively switched PFC needs to be provided between the rectifier and the storage capacitor such that the DC voltage of the storage capacitor will present an AC ripple with twice the frequency of the mains voltage.
The invention proposes to detect the mains voltage or the bus voltage supplied to the buck converter, i.e. the voltage across the storage capacitor at the midpoint of the resistive voltage divider R9 and R10 and to supply this information Vs to a control unit.
When the switch S1 is closed, a current will flow from the storage capacitor through the light source LED, the capacitor C1, the choke L1 and the switch S1. The choke L1 will be magnetised. When the switch S1 is opened, the current through the choke L1 will continue to flow in the freewheeling path through the diode D1 and the light source LED, the capacitor C1, until either the choke L1 is demagnetised or the switch S1 is closed again.
The buck converter may be controlled by the control unit depending on the information Vs supplied to the control unit. The power delivered to the light source LED is determined by the switch ratio of the switch S1. The power may be varied by modulation of the frequency or the pulse width of the control signal S1D which is controlling the switch S1.
It is not necessary to use a closed loop control in order to maintain constant power at the light source LED. For example, a current control by measuring the current through the LED needs a high effort for the measurement as the LED is not connected to the ground. When the current through the switch S1 is measured, there is no information about the current through the LED during the time where the switch S1 is open.
It may be sufficient, when the switch S1 is controlled at a given duty ratio (pulse width) and given frequency. As the light source LED and the values of the components contained in the power converter are known, the only variation may be in the supply voltage. By sensing the bus voltage or the supply voltage at the midpoint of the resistive voltage divider R9 and R10 this information Vs can be delivered to the control unit and the control of the switch S1 can be controlled depending on the sensed voltage or power.
In the present example, e.g. the power supplied to the light source is a direct function of the pulse width control signal supplied to the power modulator, such that the currently supplied power for the light source can be derived from the pulse width control value.
The function in the present example would be following: When the voltage Vs is decreasing, the pulse width will be increased in order to maintain the power delivered to the light source LED. When the voltage Vs is increasing, the pulse width will be decreased in order to maintain the power delivered to the light source LED.
The power modulator for the drive of a light source may be e.g. a half bridge converter, a full-bridge converter, a push-pull, a buck-boost or a boost converter.
As illustrated the invention encompasses the following aspects:

➢ Mains input with AC/DC converter (rectifier).
➢ Sensor Signal proportional to the DC voltage (= bus voltage).
➢ Analogue Sensor Signal is converted into a digital information with >= 1 Bit resolution.
➢ A power stage which is supplied by the DC bus voltage which has an input that controls the flow of power delivered to a load (light source, discharge lamp, LED). Normally the input is a control signal of a power switch.
➢ A modulator which has an input that takes information about the power that shall be delivered to the load as well as an output which is connected to the control input of the power stage. Normally the modulator is a PWM unit.
➢ A clock which is triggering an update cycle of the data register.
➢ A data register which holds information about the power delivered to the load at the current point of time.
➢ The input of the data register is connected to a transformation unit.
➢ The transformation unit has an input which is connected to the digital signal proportional to the bus voltage and an output which is connected to the input of the data register. The transformation unit can be connected also to a memory or external information (can be a data input or configuration information). The transformation can be a linear gain or a nonlinear function or a lookup table (LUT).

1. Embodiment: Linear function:

➢ The input of the data register X(k) is generated by a combining the previous data register value X(k-1) with the value of the bus voltage e(k). $X (k) = f (X (k - 1), e (k), I)$
➢ The combination is implemented as (synchronous) logic
➢ The combination is a linear gain $X (k) = K \cdot e (k)$
➢ The combination is a linear sum $X (k) = Σ K_{i} \cdot e (k - i) + Σ L_{j} \cdot X (k - j) + I$

2. Embodiment: Nonlinear function:

The combination is a nonlinear sum $X (k) = Σ K_{i} (e, X) \cdot e (k - i) + Σ L_{j} (e, X) \cdot X (k - j) + I (e, X)$

➢ The combination may be implemented as software.
➢ The load stage is a half bridge with a resonant circuit.
➢ The control signal into the modulator is a frequency.
➢ The transformation has a positive forward characteristic i.e. the frequency increases with increasing bus voltage.

Claims

A method for supplying lighting means (7) with feed-forward controlled power,
wherein
- a power modulator (4) is supplied with a DC voltage (8),

- the DC voltage (8) is generated on the basis of an AC voltage (1), and

- the power modulator (4) supplies power to the lighting means (7),
wherein a control unit (5):
- senses the DC voltage (8) supplied to the power modulator (4),

- digitizes the sensed DC voltage (11),

- transforms the digital DC voltage value (e(k)) to a power modulating parameter (x(k)) using a look-up table or a function and using at least one past value of the digital DC voltage or of the power modulating parameter, and

- controls the power modulator (4) in order to set a power-determining parameter of the power modulator (4) depending on the power modulating parameter (x(k)).
The method according to claim 1,
wherein the control unit (5) determines a value for power-determining parameter of the power modulator (4) on the basis of the sensed voltage.
The method according to claim 1,
wherein the power-determining parameter is the frequency and/or the duty cycle of a controlled switch of the power modulator (4).
The method according to any of the preceding claims, wherein the power-determining parameter is supplied to the power modulator via a driver circuitry (12).
The method according to any of the preceding claims, wherein the DC voltage supplied to the power modulator (4) presents a ripple with a frequency which is equal to the AC mains frequency or a multiple thereof.
The method according to any of the preceding claims, wherein the power modulator (4) is one of a half bridge converter, a full-bridge converter, a buck an/or a boost converter.
The method according to any of the preceding claims, wherein the lighting means (7) are one or more LEDs or discharge lamps.
An integrated circuitry, such as e.g. a ASIC or a microcontroller,
being designed to perform a method according to any of the preceding claims.
A ballast for lighting means,
comprising a circuitry according to claim 8.
A lamp,
comprising a ballast according to claim 9.