Patent Description:
In currently available LED gears, with e.g. a half-bridge LLC resonant circuit as at least one stage of a LED driver, the following two approaches are used to detect secondary side errors, namely short-circuit and over voltage conditions, as described in the following.

The first approach makes use of two separate controller (microcontroller or ASIC) pins, wherein one pin is used to evaluate the sensed output (LED) voltage e.g. to detect an open load condition when the secondary side voltage exceeds a given threshold. Another pin is used to evaluate the sensed output (LED) current. The first pin utilizes a comparator to detect over voltage (open load) conditions, the second pin utilizes a comparator to detect over current (short circuit) conditions. In <FIG>, such a situation is shown, wherein the output voltage sensing module generates the voltage signal VSNS which is given as input in a pin of a controller, while the output current sensing module generates the current signal ISNS_PK which is given as input to another pin of the controller.

With this approach open-load and short-circuit can both be detected fast, because positive edges are used in both cases as detection criteria. As the sensing circuity uses peak hold circuits (namely, rectifier diode and filter capacitor) the sensed voltage can follow the real output voltage very fast in positive going direction. However, it follows slow in negative going direction as it takes some time for the filter capacitor to discharge.

The second approach makes use of only one controller pin, wherein only the output (LED) voltage is evaluated. Two comparators are used: one to detect over voltage (open-load) conditions and one to detect under voltage (short-circuit) conditions.

In this approach the reaction times (time until the error is detected) depend on the filtering of the sensed output voltage signal. If the sensed secondary side voltage of the LLC (output voltage) is fed to a peak hold circuit (namely, rectifier diode and filter capacitor), overvoltage conditions can be detected very fast (because peak hold capacitor is charged immediately). However, undervoltage (short-circuit) conditions are detected delayed, because it takes some time for the peak hold capacitor to discharge. This leads to the problem that high output currents flow for several milliseconds that could probably destroy parts of the circuit.

In the case where an LED load open and short-circuit situation in a micro-controller based control circuit is detected by an ADC measurement of LED voltage, a slow response (about <NUM> to <NUM> to detect) occurs. For precautionary measure under LED open condition, a Zener diode is connected across the LED load to prevent the further rise of LED voltage, before the micro-controller can act by shutting down the LED driver. For SELV products the voltage overshoot value requirement is quite tight and Zener diode as well as ADC tolerance becomes unacceptable. Further the short-circuit detection based on LED voltage detection needs to be fast enough before the current rises and causes a voltage drop across the short-circuit or low load voltage condition which can cause the sensed voltage above the detection threshold.

Furthermore, using a small size micro-controller poses a considerable challenge in wisely using the available pins (and associated analog and digital resources) by multi-tasking them to achieve all the advanced functionality or features of an, e.g., emergency LED driver, as it is the case when there is a quest to combine the sensing of LED open or short-circuit situation.

<CIT> discloses an isolated LED driver according to the preamble of independent claim <NUM> with combined open-circuit and short-circuit detection at one pin.

<CIT> discloses a lighting system which allows a defective LED to be disconnected, in particular automatically, from a current source of the lighting system.

Thus, it is an objective to provide an improved LED driver allowing to efficiently detect short current and over voltage conditions, all by reducing the complexity and especially the pin number requirements.

According to a first example not forming part of the invention, an isolated LED driver with open circuit and short-circuit at one pin is provided. The isolated LED driver comprises a control unit controlling at least one switch on the primary side of an isolation stage of the LED driver, means for detecting the voltage on the secondary side of the isolation stage and producing a voltage-representing signal, means for detecting the current flowing on the secondary side, and especially the current through an LED load when connected at supply terminals of the LED driver, and producing a current-representing signal, and means combining for a current-representing signal, optionally the decoupled AC component of the current representing signal, with the voltage representing signal and feeding it to an input pin of the control unit.

Advantageously, while only utilizing one controller pin, this aspect of the invention allows fast detections of both over voltage and over current conditions. In fact, in this approach, two different signals (one representative for the output voltage and one representative for the output current) are fed to a single controller pin.

According to a first aspect of the invention, an isolated LED driver with open-circuit and short-circuit detection at one pin is provided. The isolated LED driver comprises a control unit for controlling at least one switch on a primary side of an isolation stage of the LED driver, means for detecting a voltage on a secondary side of the isolation stage and producing a voltage-representing signal, a comparator supplied with the voltage-representing signal at its non-inverted input, wherein a reference level of the comparator and, in synchronization therewith, a polarity of a comparator output signal are switched in order to produce a signal indicating, in time-multiplex, an LED load open-circuit and short-circuit indicating signal.

In an example not forming part of the invention, the current representing signal is supplied to a further input of the control unit for a feedback control of the LED load current.

In a further preferred embodiment, the control unit is a microcontroller or an application-specific integrated circuit, ASIC.

In a further preferred embodiment, the means for detecting the voltage on the secondary side of the isolation stage comprise an analog to digital converter, ADC.

In a further preferred embodiment, the LED driver comprises a LLC converter.

In a further example not forming part of the invention, the control unit is further configured to compare the current representing signal with a threshold value in order to derive regulation values for the LLC current transformer, for example, a frequency, a duty cycle value or an actuating variable.

In a further example not forming part of the invention, the means for combining the current representing signal further comprises a diode for combining the current representing signal with the voltage representing signal to obtain a total signal.

In a further example not forming part of the invention, the control unit further comprises a comparator configured to compare the total signal to a threshold value and, if the total signal is higher than the threshold value, to detect a short-circuit condition or an over voltage condition.

This has the advantage that, LED over-voltage detection and over-current detection are performed, while using a single micro-controller pin by using its onboard high-speed comparator.

In a further preferred embodiment, the comparator is a comparator with variable reference and polarity.

In a further example not forming part of the invention, the control unit is configured to switch off the means for combing the current representing signal in the short-circuit condition or over voltage condition.

According to a second example not forming part of the invention, a method for an isolated LED driver is provided. The method comprises the steps of: controlling at least one switch on the primary side of an isolation stage of the LED driver, detecting the voltage on the secondary side of the isolation stage and producing a voltage-representing signal, detecting the current flowing on the secondary side, and especially the current through an LED load when connected at supply terminals of the LED driver; producing a current-representing signal; combining for a current-representing signal, optionally a decoupled AC component of the current representing signal, with the voltage representing signal; and feeding it to an input pin of the control unit.

According to a second aspect of the invention, a method for an isolated LED driver is provided. The method comprises the steps of controlling at least one switch on a primary side of an isolation stage of the LED driver; detecting a voltage on a secondary side of the isolation stage and producing a voltage-representing signal; and switching a reference level of a comparator and, in synchronization therewith, a polarity of a comparator output signal in order to produce a signal indicating, in time-multiplex, an LED load open circuit and short-circuit indicating signal, wherein the comparator is supplied with the voltage-representing signal at its non-inverted input.

Aspects of the present invention are described herein in the context of an isolated LED driver.

Various aspects of an isolated LED driver will be presented. However, as those skilled in the art will readily appreciate, these aspects may be extended to aspects of LED drivers without departing from the invention.

Now referring to <FIG>, an exemplary embodiment of a circuit of an isolated LED driver <NUM> with open-circuit and short-circuit detection at one input terminal ("pin") of a preferably integrated control circuitry according to an example not forming part of the invention is shown.

The isolated LED driver <NUM> comprises a control unit (not shown in <FIG>) for controlling at least one switch on a primary side of an isolation stage of the LED driver <NUM>. The switching of the switch determines the power supplied to an LED load and especially the LED current. Thus, the LED current may be feedback-controlled back to the control circuitry by a current signal representing the LED current. The control circuitry compares the LED current signal with a nominal value (which may be varied for a dimming control) and controls the switching of the switch.

The LLC comprises a half-bridge DC/AC converter with two serially connected switches (FETs), M40, M41. The half-bridge arrangement M40, M41 is fed with a DC voltage. The mid-point of the switches M40, M41 is connected to a resonance capacitor C51 and the primary side winding L51a of the transformer of the LLC. The primary side winding L51a of the transformer of the LLC is coupled with a secondary side winding L51b connected to a rectifying Zener diode arrangement D52a, D52b, D52c and D52d. The output of the diode arrangement is fed to a capacitor C52, the DC voltage of which is applied to output terminals LED+ and LED- for supplying a LED load.

Furthermore, on the secondary side two primary side windings L52b, L52c of a sensing transformer for the secondary side current to the secondary side voltage are provided.

The primary side windings of the sensing transformers L52b, L52c are coupled with a secondary side of the sensing transformer L52a.

On the basis of the voltage across the secondary side winding L52a of the sensing transformer, a signal ISNS_PK indicating the peak current of the LED current is generated as well as a further signal ISNS_AVG indicating the average value of the LED current. Both signals ISNS_PK and ISNS_AVG can be used by the control circuitry <NUM> as feedback signals in order to set the clocking of the half-bridge switches M40, M41.

According to the invention the AC component of the LED current is decoupled via a transformer L53a, L53b in order to generate a signal which may be sent to a sensing terminal OVP_SCP_PIN of the control circuitry.

As furthermore, shown in <FIG>, this signal representing the decoupled AC component of this sensed LED current is thus combined with a signal produced by the circuitry <NUM> sensing, using a further transformer L51c, L51b, the voltage on the secondary side of the power transformer of the transformer L51a, L51b of the LLC.

Moreover, the LED driver <NUM> comprises means for detecting the voltage on the secondary side of the isolation stage <NUM> and producing a voltage-representing signal. Furthermore, the LED driver <NUM> comprises means for detecting the current flowing on the secondary side <NUM>, and especially the current through an LED load when connected at supply terminals of the LED driver, and producing a current-representing signal, means combining for a current-representing signal, optionally the decoupled AC component of the current representing signal, with the voltage representing signal and feeding it to an input pin of the control unit OVP_SCP_PIN in <FIG>.

This provides the advantage that the pins of the control unit, e.g. a microcontroller, can be used more efficiently and, thus, saving costs.

The control unit can further be configured to compare the current representing signal with a threshold value in order to derive regulation values for the LLC current transformer, for example, a frequency, a duty cycle value or an actuating variable.

The control unit can be a microcontroller or an application-specific integrated circuit (ASIC). Within the controller, e.g., an analog-to-digital converter, ADC, can be used to determine the LED voltage. Additionally, a comparator <NUM> can be used to detect over voltage conditions (when the voltage at that pin rises above a certain threshold). In this embodiment, the voltage signal is generated by the LED voltage source.

If a short-circuit of the LED takes place, then a current pulse is generated in the signal ISNS_PK. This current pulse can be transformed via current transformer to a voltage signal which can then be coupled to the voltage signal at a diode. In short circuit conditions, a peak occurs in this signal, which can be fed to a comparator within the controller. The comparator will detect the peak so that the controller can react to the short-circuit condition.

Advantageously, the current signal does not affect the voltage signal during normal operation (where ILED is a constant DC), but it increases the voltage signal during transients of ILED such as in the case of a short circuit of the output.

Moreover, in the short-circuit condition or over voltage condition the control unit can be configured to switch off the means for combining the current representing signal.

<FIG> shows exemplary behaviors of the current ILED and voltage VLED in the isolated LED driver <NUM> according to an example not forming part of the invention.

After being detected, VLED and ILED are combined into a single total signal and then fed to the comparator <NUM>, as shown in <FIG>.

The comparator <NUM> can be a comparator configured to compare the total signal to a threshold value and, if the total signal is higher than the threshold value, to detect a short-circuit condition or an over voltage condition. The comparator <NUM> can be a comparator with variable reference and polarity.

The circuit shown in <FIG>, further comprises two diodes D1 and D2, one resistance R1 and one capacitance C1. The signals VLED and ILED pass through the diodes D1 and D2, respectively. Afterwards, they are combined into a single signal PA3 which is fed to the comparator <NUM> and then compared to the reference voltage Vref,int in order to detect an over voltage or short current condition.

<FIG> shows an isolated LED driver <NUM> comprising a comparator <NUM> according to the invention.

This isolated LED driver <NUM> with open-circuit and short-circuit detection at one pin comprises a control unit for controlling at least one switch on a primary side of an isolation stage of the LED driver, means for detecting a voltage on a secondary side of the isolation stage and producing a voltage-representing signal, a comparator <NUM> supplied with the voltage-representing signal at its non-inverted input, wherein a reference level of the comparator <NUM> and, in synchronization therewith, a polarity of the comparator output signal are switched in order to produce a signal indicating, in time-multiplex, an LED load open-circuit and short-circuit indicating signal.

The LED driver <NUM> shown in <FIG> comprises an LED load, whose voltage VLED is compared to a reference voltage Vref by the comparator <NUM>. Moreover, the LED driver <NUM> comprises three resistors R1, R2, and R3 and two capacitors C1 and C2.

The comparator <NUM> can comprise an operational amplifier, wherein the polarity of the operational amplifier output circuit depends on the polarity of the difference between the two input voltages VLED and Vref.

<FIG> shows exemplary behaviors of the voltage VLED in an isolated LED driver as a function of time according to the invention.

In this embodiment, Vref represents the variable reference, namely voltage varying between VREFINT and ½VREFINT, wherein VREFINT=<NUM>,2V. The frequency of variation is <NUM>.

As it can be taken from <FIG>, in case of an over-voltage event or short-circuit event, a peak is produced in the output signal of the comparator <NUM> on a microcontroller.

<FIG> shows a method <NUM> for an isolated LED driver <NUM> according to an example not forming part of the invention.

The method <NUM> comprises the following steps:.

<FIG> shows a method <NUM> for an isolated LED driver <NUM> according to an embodiment.

All features of all embodiments described, shown and/or claimed herein can be combined with each other.

Claim 1:
Isolated LED driver (<NUM>) with combined open-circuit and short-circuit detection at one pin, comprising:
- a control unit for controlling at least one switch on a primary side of an isolation stage of the LED driver (<NUM>);
- means for detecting a voltage (<NUM>) on a secondary side of the isolation stage and producing a voltage-representing signal; and
- a comparator (<NUM>) supplied with the voltage-representing signal at its non-inverted input,
characterized by
a reference level of the comparator (<NUM>) and, in synchronization therewith, a polarity of a comparator output signal being switched in order to produce a signal indicating, in time-multiplex, an LED load open-circuit and short-circuit indicating signal.