EP2420108B1 - Driving circuit for leds - Google Patents

Description

The invention relates to a driver circuit for an LED and a method for driving an LED.

Technical field

Such driver circuits are used in lighting systems to achieve colored or flat lighting of rooms, paths or escape routes. The illuminants are usually controlled by operating devices and activated if necessary. Organic or inorganic light-emitting diodes (LED) are used as the light source for such lighting.

State of the art

Instead of gas discharge lamps and incandescent lamps, light-emitting diodes are increasingly being used as lighting sources for lighting. The efficiency and luminous efficacy of light emitting diodes is increasing more and more, so that they are already being used in various general lighting applications. However, light emitting diodes are point light sources and emit highly concentrated light.

However, today's LED lighting systems often have the disadvantage that the color or the brightness can change due to aging or by replacing individual LEDs or LED modules. Secondary optics also have an impact on thermal management, since heat radiation is impeded. In addition, the phosphor of the LED may change due to aging and the effects of heat.

A change in brightness is often only possible with an elaborate control circuit, there is no easy connection to commercially available dimmers, because in conjunction with most dimmers the light flickers or the dimmers do not work at all.

The document WO 2008/137460 A2 shows an operating device for LED-based lights. In particular, a switched supply is shown.

The Rand et al .: "Issues, Models and Solutions for Triac Modulated Phase Dimming of LED Lamps" shows a system for dimming LED lights .

Presentation of the invention

It is the object of the invention to provide a light source and a method which ensure trouble-free and energy-saving operation by means of a Illuminants with light-emitting diodes without the disadvantages mentioned above or with a significant reduction of these disadvantages.

This task is solved by the independent claims. Particularly advantageous embodiments of the invention are described in the subclaims. The explanations given below for exemplary embodiments which are not covered by the claims are to be understood as helpful examples for understanding the invention and not as exemplary embodiments according to the invention.

Description of the preferred embodiments

• Fig. 1 zeigt die erfindungsgemässe Vorrichtung
• Fig. 2 zeigt ein erstes hilfreiches Beispiel
• Fig. 3 zeigt ein weiteres hilfreiches Beispiel
• Fig. 4 zeigt ein weiteres hilfreiches Beispiel

The invention will be explained in more detail below with reference to the accompanying drawing. Show it:

• Fig. 1 shows the inventive device
• Fig. 2 shows a first helpful example
• Fig. 3 shows another helpful example
• Fig. 4 shows another helpful example

The invention is described below based on a first exemplary embodiment Fig. 1 explained with a driver circuit for an LED.

The driver circuit for an LED has a connection for a mains voltage, a filter circuit (L1) and a rectifier (GR1), an inductor (L2) and a switch (S1). The rectifier (GR1) is followed by a buffer element (C1), which is preferably only used to filter out high-frequency voltage changes and does not perform a strong smoothing of the voltage at the output of the rectifier (GR1). The intermediate storage element (C1) can be, for example, a capacitor, preferably a filter capacitor. The inductance (L2) preferably has a primary winding (L2p) and a secondary winding (L2s) coupled to it.

The inductor (L2) is magnetized when the switch is closed, and the inductor (L2) is demagnetized when the switch S1 is open, and at least during the demagnetization phase, the current through the inductor (L2) feeds the LED.

The switch S1 is only ever opened when the current through the switch S1 has reached a predetermined threshold value.

The current through the switch S1 can be detected by means of a current detection Ip (for example a current shunt). The current detection Ip can, however, also take place directly at the switch S1 (for example in the case of a so-called SENSE FET, which contains an integrated monitoring of the current). In particular, there is no time limit on the switch-on time, but an infinite switch-on time of switch S1 is also possible.

The switch-off duration of the switch S1 can depend on the detected amplitude of the current through the LED. The feedback of the detection of the amplitude of the current by the LED is preferably electrically isolated (i.e. the control loop for the dependence of the switch-off duration of the switch S1). However, the switch-off time can, for example, also be fixed (fixed setting).

The switch-off duration of the switch S1 can, for example, also be directly or indirectly dependent on the demagnetizing current.

The switch S1 can be switched on whenever a demagnetization of the inductance (L2) is detected. However, switching on can always only take place with demagnetized inductance (L2); there may also be a certain time between demagnetization and switching on again.

The driver circuit can be connected to a commercially available dimmer, and the switch S1 can be closed during the phases in which the dimmer cuts off a part of the phase in order to conduct a residual current through the inductance and the switch S1 and thus load the dimmer . However, this residual current through switch S1 is limited by the predetermined threshold value in order to avoid overloading switch S1.

The inductance (L2) can be a transformer (L2p, L2s), which serves as a potential-isolating element.

The driver circuit can thus be transmitted to the light source (LED) by high-frequency switching of the switch (S1) via the inductance (L2). The switch (S1) can be, for example, a field effect transistor, such as a MOSFET, or a bipolar transistor.

The predetermined threshold value can depend on the current amplitude of the supply voltage Vin. In a simple variant, for example, if the supply voltage Vin exceeds a certain value, the threshold value can be increased. However, the threshold value can also be increased in several stages.

In particular, the predefined threshold value can depend on the current amplitude value of the rectified sine half-wave of the AC AC voltage if an AC AC voltage with typically 50 Hz or 60 Hz frequency is present as the supply voltage. The current amplitude value is preferably monitored with a high-frequency sampling or constant monitoring, so the value of the supply voltage averaged over one or more periods is preferably not recorded.

This monitoring of the current amplitude of the supply voltage Vin can be carried out by a monitoring circuit U1. The monitoring circuit U1 can, for example, be an integrated circuit (for example an ASIC, microcontroller or DSP). Depending on the monitoring of the current amplitude of the supply voltage Vin, the monitoring circuit U1 can specify the threshold value for the opening of the switch S1.

The monitoring circuit U1 can detect, for example, via the buffer element C1 or at the (positive) output of the rectifier GR1 or, if present, before the decoupling element or the voltage difference across the decoupling element (preferably by measuring the voltage in front of and behind the decoupling element). In a simple variant, the voltage measurement is carried out by means of a voltage divider which taps the voltage across the buffer element C1 or at the (positive) output of the rectifier GR1 and reduces it to a potential which can be evaluated by the monitoring circuit U1.

However, the monitoring circuit U1 can also be designed (for example in high-voltage technology) in such a way that it can directly detect the voltage across the buffer element C1 or at the (positive) output of the rectifier GR1.

The monitoring circuit U1 can also control the switch S1. In this case, the monitoring circuit U1 can on the one hand monitor the current through the switch S1 by means of a current detection Ip (for example a current shunt) and additionally monitor the current amplitude of the supply voltage Vin.

The monitoring circuit U1 can always trigger the opening of the switch S1 when the predetermined threshold value for the current through the switch S1 is reached.

As already mentioned, the threshold value is preferably predetermined on the basis of monitoring the current amplitude of the supply voltage Vin. For example, only two values can be specified as the threshold value, the lower threshold value being specified when a supply voltage Vin is below a certain value and the upper threshold value is specified when a specific value for the supply voltage Vin is exceeded. However, it is also possible for several threshold values to be stored in a type of table and for these to be specified for different voltage ranges of the supply voltage Vin in accordance with the table.

The monitoring circuit U1 can also be designed in two parts (for example in the form of two integrated circuits which are linked to one another). On the one hand, there can be a first monitoring circuit U1a, which specifies a threshold value as a function of monitoring the current amplitude of the supply voltage Vin. The first monitoring circuit U1a can forward this threshold value to a second monitoring circuit U1b. The second monitoring circuit U1b can control the switch S1.

The monitoring circuit U1b can monitor the current through the switch S1 and, depending on this, actuate the switch S1. This activation can be dependent on the threshold value specified by the first monitoring circuit U1a.

In addition, the control can be dependent on further monitoring, for example on monitoring the demagnetization of the inductance L2, the detected voltage of the LED or the detected amplitude of the current through the LED. Preferably, all returns or monitors on the secondary side are electrically isolated, i.e. the feedback of the signals detected on the output side (secondary side) to the monitoring circuit U1 takes place via potential isolation (for example by means of an optocoupler or transformer). As already explained, the switch-off duration of the switch S1 is preferably dependent on the detected amplitude of the current through the LED.

The inductor L2 can be a transformer L2p, L2s, which serves as a potential-isolating element. The primary winding L2p of the transformer is connected in series with the switch S1. The secondary winding L2s magnetically coupled to the primary winding L2p is connected to a rectifier (D2) and a smoothing circuit (C2) to which the LEDs can be connected. The rectifier (D2) on the secondary winding L2s of the transformer can be formed by a diode D2 or by a full-wave rectifier.

When demagnetized, the inductor L2 can feed a smoothing circuit; this smoothing circuit can be, for example, a capacitor C2 or an LC (capacitor inductor C2-L3) or CLC (capacitor inductor capacitor C2-L3-C3) filter. The secondary side with the smoothing circuit (C2) is preferably designed so that a constant current supply of the LED is made possible.

A method for controlling an LED via a dimmer is made possible, the LED being controlled via the driver circuit, and energy being transmitted to the illuminant LED via the inductance L2 by high-frequency clocking of the switch S1. The switch S1 is also kept closed in phases if the dimmer cuts off the phase and is only ever opened when the current through the switch S1 has reached a predetermined threshold value. This means that even in the phases where the dimmer cuts off the phase (i.e. no mains voltage is let through), switch S1 is kept closed as long as the current through switch S1 has not reached a predetermined threshold value. Only then is the switch S1 kept open for a certain time (depending on the respective condition for determining the switch-off time as already mentioned) and switched on again. In the phases in which the dimmer cuts off the phase, compared to the phase with the mains voltage applied, switch S1 can be switched on longer because the current through inductor L2 and switch S1 rises more slowly due to the mains voltage not being applied .

The driver circuit with the monitoring circuit U1 can also be designed such that the switch (S1) is also kept closed when the illuminant (LED) is not in operation or is only supplied with a supply voltage Vin that is far below the nominal supply voltage Vin lies, and is only opened when the current through the switch (S1) has reached a predetermined threshold. The switch (S1) can be kept in the closed state, for example by a holding circuit, provided that it is not switched off by a corresponding active control. For example, the active control for switching off (opening) the switch (S1) can be carried out by bridging the holding circuit or by pulling down the control level for the control connection of the switch (S1).

The holding circuit can also be designed in such a way that, as soon as a low voltage is present at the input of the driver circuit, it already keeps the switch (S1) closed while the driver circuit itself is not yet starting.

Thus, a lamp for an LED can be formed, with a base for inserting the lamp into a commercially available lamp base, having a driver circuit according to the invention.

The invention is described below using a second exemplary embodiment Fig. 2 , Fig. 3 and Fig. 4 explained with a driver circuit for an LED.

The driver circuit has a connection for a mains voltage, followed by a rectifier GR1 and a filter circuit L1, as well as a buffer element. This is followed by an inductor L2 and a switch S1.

Inductor L2 is magnetized when switch S1 is closed, and inductor L2 is demagnetized when switch S1 is open, and at least during the demagnetization phase, the current through inductor L2 feeds the LED.

The driver circuit can be constructed as a step-up converter circuit or as a flyback converter circuit. The flyback converter circuit or the step-up converter circuit is advantageously designed to be electrically isolated, i.e. the clocked inductor L2 of the driver circuit has a secondary winding L2s which is magnetically coupled to the primary winding L2p of the inductor L2.

A current detector, preferably a unidirectional decoupling element, is included between the rectifier GR1 and the buffer element C1.

According to the examples of Fig. 2 and 3rd the decoupling element can be formed as a current detector by a diode D1. However, a full-wave rectifier DV1 can also serve as a decoupling element. The current flow can be monitored via the rectifier (GR1) into the inductance (L2) and the switch (S1) and / or the filter capacitor (C1) by means of the current detector.

At the output of the rectifier GR1 there is a bridging circuit (R40, Q4) which is deactivated when the current detector (for example the decoupling element) passes a current.

A bypass circuit (R40, Q4) is therefore always activated when a current flows into the driver circuit for an LED. A current flows into the driver circuit for an LED whenever a current flows through the rectifier GR1 via the inductor L2 and the switch S1 or into the intermediate storage element.

The decoupling element thus acts as a current detector. As soon as a current flows through the rectifier GR1 and a current flows through the inductance L2 and the switch S1 or into the intermediate storage element, a voltage drops across the decoupling element which is only slightly higher than the voltage across the intermediate storage element (i.e. the voltage behind the decoupling element). . This voltage across the decoupling element can be monitored. This monitoring can be carried out by a monitoring circuit U1. The monitoring circuit U1 can be an integrated circuit, for example.

Depending on the monitoring of the decoupling element as a current detector, the monitoring circuit U1 can activate or deactivate the bypass circuit (R40, Q4).

The monitoring circuit U1 can, for example, only detect the voltage in front of the decoupling element or the voltage difference across the decoupling element (preferably by measuring the voltage in front of and behind the decoupling element). The monitoring circuit U1 can also control the switch S1.

The decoupler as a current detector can be formed by a diode D1. However, a full-wave rectifier DV1 can also serve as a decoupling element.

The driver circuit can be connected to a commercially available dimmer, and the bypass circuit (R40, Q4) can be activated during the phases in which the dimmer cuts off part of the phase in order to generate a residual current via the bypass circuit (R40, Q4) and the inductor L2 and to guide the switch S1 and thus load the dimmer.

The buffer element can, for example, by a Valley Fill circuit ( Fig. 3 ) or by a capacitor as a buffer element C1 ( Fig. 2 ) are formed.

The switch S1 can be switched on whenever a demagnetization of the inductance L2 is determined.

However, switching on can always only take place with demagnetized inductance L2, and there can also be a certain period of time between demagnetization and switching on again.

The switch S1 can be activated, for example, by an integrated circuit for a power factor correction. The monitoring circuit U1 can contain a control circuit for a power factor correction.

The inductor L2 can be a transformer L2p, L2s, which serves as a potential-isolating element. The primary winding L2p of the transformer is connected in series with the switch S1.

The secondary winding L2s magnetically coupled to the primary winding L2p is connected to a rectifier (D2) and a smoothing circuit (C2) to which the LEDs can be connected. The rectifier (D2) on the secondary winding L2s of the transformer can be formed by a diode D2 or by a full-wave rectifier.

The on and / or off duration of the switch S1 can depend on the detected amplitude of the current through the LED. However, the on and / or off duration of the switch S1 preferably does not decrease to or close to zero. In a simple variant, for example, the current can be limited by the LED by limiting the on-time.

When demagnetized, the inductor L2 can feed a smoothing circuit (C2); this smoothing circuit (C2) can be, for example, a capacitor C2 or an LC or CLC filter.

The bypass circuit (R40, Q4) can be formed by a resistor R40 in series with a switch Q4.

However, the bypass circuit can also have a current source (constant current source) as a bypass circuit. An example of a current source (constant current source) is in Fig. 4 shown.

In Fig. 4 only a section of the driver circuit for a lamp is shown.

The current detector is formed here by current monitoring element R34. Depending on the current flow through the current monitoring element R34, the monitoring circuit U1 (formed by a transistor Q5 and a resistor R30, which is connected to an internal voltage supply Vcc) can activate or deactivate the bypass circuit.

As soon as a sufficient current flow through the current detector (that is to say the current monitoring element R34) is ascertained, the bypass circuit is deactivated. The current flow through the current monitoring element R34 (current detector) is the current which flows via the rectifier (GR1) into the inductance (L2) and the switch (S1) or the intermediate storage element.

In the example according to Fig. 4 is the monitoring circuit U1 constructed discretely, but it can also as in the examples of Fig. 2 and 3rd be designed as an integrated circuit. If an integrated circuit is used as the monitoring circuit U1, further functions such as the activation of the switch S1 can also be integrated.

The bypass circuit is according to Fig. 4 formed by a current source (constant current source). The current source (constant current source) is formed in detail by transistors Q4 and Q6 and resistors R40, R27 and R29.

The bypass circuit can be as in Fig. 4 represented via a full-wave rectifier D3 via the filter circuit L2 with the connection for a mains voltage, parallel to the rectifier GR1.

The rectifier, via which the bypass circuit (R40, Q4) is connected to the connection for a mains voltage, can either be the same rectifier through which a current flows into the inductance and the switch or the buffer element (i.e. the rectifier GR1, see Fig. 2 and 3rd ), or a further rectifier D3 can be connected in parallel to this first rectifier GR1 (see Fig. 4 ) to be available.

This enables a method for controlling an LED, the LED being controlled via a driver circuit, the driver circuit being fed from a connection for a mains voltage via a filter circuit (L1) and a rectifier (GR1), and the driver circuit being an intermediate storage element, a Inductance (L2) and a switch (S1), and wherein a bypass circuit (R40, Q4) present at the output of the rectifier (GR1) is deactivated when a current flows through the rectifier (GR1) in the driver circuit.

Thus, a lamp for an LED can be constructed, with a base for using the lamp in a commercially available lamp base, having a driver circuit according to the invention.

It can also be the embodiment of the Fig. 1 with the 2 to 4 be combined. On the one hand, the switch S1 can always remain closed as long as the current through the switch S1 has not reached a predetermined threshold value, in addition there can be an activatable bypass circuit (R40, Q4), which is only activated when a sufficient current flow is detected by the current detector has been. In this way, two current paths are formed over which a current can flow, and thus the bypass circuit (R40, Q4) can be designed in such a way that it generates only slight additional losses when it is activated (compared to a solution without a second current path through the Switch (S1)).

It can also be a driver circuit for a lamp, preferably for an LED, having a connection for a mains voltage, a rectifier GR1 and a filter circuit, a buffer element (C1), an inductor L2 and a switch S1, formed by high-frequency clocking Switch S1 energy can be transmitted to the illuminant via the inductance, and a bypass circuit (R40, Q4) can be arranged at the output of the rectifier GR1 such that it is activated when the illuminant (LED) is not in operation.

This can be the case, for example, if there is no mains voltage or only a low voltage far below the mains voltage. The bypass circuit (R40, Q4) can thus be designed such that it is only deactivated when the illuminant (LED) is in operation. The bridging circuit (R40, Q4) can, for example, be connected in such a way that a current flows through it without actuation (activation) of this bridging circuit (R40, Q4) as soon as a voltage is present across the bridging circuit (R40, Q4). The bypass circuit (R40, Q4) can, for example, also be designed such that, as soon as a low voltage is present at the input of the driver circuit, it already keeps the switch (S1) closed while the driver circuit itself is not yet starting.

In this way, better compatibility with so-called mains isolators can also be achieved. Mains disconnectors recognize when no load is switched on (i.e. no significant current flows through a load) and disconnect the corresponding circuit from the mains in this case. To detect a load (i.e. a consumer) being switched on again, they usually switch on a voltage with a low level, for example 20V. The bridging circuit (R40, Q4), which would be activated (since the lamp was switched off), would represent a load when the lamp was switched on, which is sufficient to switch the mains isolator to a mains supply.

In addition, the bypass circuit (R40, Q4) can only be deactivated in the phases during operation of the illuminant if a current flow through the current detector is detected.

The bypass circuit (R40, Q4) preferably has a switchable element, such as a transistor (Q4), which can be driven and thus can deactivate the bypass circuit (R40, Q4). The bypass circuit (R40, Q4) can be deactivated by the monitoring circuit U1.

An operation of the illuminant (LED) is to be understood to mean that the driver circuit for controlling and energizing the LED is not in operation. In this state, however, it is possible for a low supply voltage Vin to be present, but which is not sufficient for the driver circuit to supply the LED, and in particular in this state there is no high-frequency switching of the switch (S1) by the driver circuit. (The switch (S1) can be switched on by the driver circuit, but there is no quick (high-frequency) change between switching the switch (S1) on and off.) However, the supply voltage Vin present can be sufficient for certain parts of the driver circuit how to activate the bypass circuit or the hold circuit.

The illuminant can also be a gas discharge lamp, for example.

According to the invention, a light source for an LED can therefore be formed, with a base for inserting the light source into a commercially available lamp base, comprising a driver circuit according to the invention.

Claims (10)

1. A driver circuit for an LED, having a connection for a mains voltage, a rectifier (GR1) and a filter circuit (L1), an inductor (L2) with a primary winding (L2p) and a secondary winding (L2s) coupled thereto and a switch (S1),
   wherein the inductor (L2) is magnetized, when the switch (S1) is closed, and the inductor (L2) is demagnetized, when the switch (S1) is opened, and at least during the phase of the demagnetization the current feeds the LED through the inductor (L2),
   wherein the switch (S1) is always opened only when the current through the switch (S1) has reached a predetermined threshold value,
   wherein a supply voltage is detected via the rectified voltage,
   characterized in
   that the predetermined threshold value depends on the current amplitude of the supply voltage (Vin), wherein the driver circuit can be connected to a commercially available dimmer not belonging to the driver circuit and
   wherein the switch (S1) is also kept closed, if the dimmer cuts off part of the phase, as long as the current through the switch (S1) has not reached a further predetermined threshold value.
2. The driver circuit for an LED according to Claim 1,
   characterized in that the switch-off time of the switch (S1) is dependent on the detected amplitude of the current through the LED.
3. The driver circuit for an LED according to Claim 1 or 2,
   characterized in that the switch-off time of the switch (S1) is dependent on the demagnetization current.
4. The driver circuit for an LED according to any one of the preceding claims,
   characterized in that a residual current is conducted via the inductor (L2) and the switch (S1), in order to load the dimmer.
5. The driver circuit for an LED according to any one of Claims 1 to 4,
   characterized in that the switch (S1) is always switched on when a demagnetization of the inductor (L2) detected.
6. The driver circuit for an LED according to any one of Claims 1 to 5,
   characterized in that the inductor (L2) serves as a potential-separating element and the inductor (L2) feeds a smoothing circuit (C2) during its demagnetization.
7. The driver circuit for an LED according to any one of Claims 1 to 6,
   characterized in that the switch-on and/or switch-off time of the switch (S1) is dependent on the detected amplitude of the current through the LED.
8. A lighting means for an LED, with a base for insertion of the lighting means into a commercially available lamp base, having a driver circuit according to any one the preceding claims.
9. The driver circuit according to any one of the preceding claims,
   wherein by means of a holding circuit the switch (S1) is held in the closed state.
10. A method for the control of an LED via a dimmer,
    wherein the LED is controlled via a driver circuit according to Claim 1, wherein a supply voltage (Vin) is detected via the rectified voltage, characterized in that the predetermined threshold value depends on the current amplitude of the supply voltage (Vin), and
    that the switch (S1) is kept closed, when the dimmer cuts off a part of the phase, as long as the current through the switch (S1) has not reached a further predetermined threshold value.

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