CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C. § 371 of International Application No. PCT/EP2019/081828, filed on Nov. 19, 2019, which claims the benefit of European Patent Application No. 18209522.4, filed on Nov. 30, 2018. These applications are hereby incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to switch mode power converters for example for use in LED drivers.

BACKGROUND OF THE INVENTION

Switch mode power converters are well known for use both in AC/DC and DC/DC conversion. Resonant converters for example have a resonant circuit, which can be a series or parallel resonant circuit. Resonant converters which comprise an LLC resonant circuit having two inductances and one capacitance, or having an LCC resonant circuit having two capacitances and one inductance, are well-known for use within LED drivers.

The switch mode power converter can be configured or operated as a constant current source or a constant voltage source. A constant current source can be used to drive an LED arrangement directly, thus enabling a single stage driver. Constant voltage sources can be used, for example, for LED modules which have further driver electronics in order to ensure a corresponding power supply to the LEDs. A predetermined current is then drawn from the output voltage provided by the constant voltage source.

Galvanic isolation is typically provided between the input and any outputs. A transformer is used to provide this isolation between the input power supply (typically a high voltage mains supply) and the load. The isolation requirements between the primary and secondary sides normally mean that the primary and secondary side windings of the transformer need to be physically separated or otherwise disposed such that the magnetic coupling between the primary and secondary side windings is not perfect. This imperfection in magnetic coupling manifests itself as leakage inductance of the output transformer.

In the application of LED drivers, current regulation is normally used, by which the switch mode power converter is controlled to deliver a desired LED current, at the secondary side. The regulation function also may be used to limit or control the output voltage being generated by the LED driver, for example in conditions when the LED load is (or becomes) disconnected from the driver.

It is then highly preferred that circuitry for measuring and limiting or controlling the driver output voltage should be disposed at the primary side of the driver rather than at the isolated secondary side of the driver. In the latter case, a signal needs to be brought from the isolated secondary side back to the primary side via a properly isolating path, such as via an opto-isolator.

Thus, primary side current and/or voltage regulation is more cost effective and robust over secondary side regulation. First, the high voltage or common mode insulation of the control circuit is not needed in the primary side control approach. Second, auxiliary circuitry for powering the control part is simplified. Moreover, being placed on the primary side, the regulation circuitry is capable of processing any information from the mains in a very simple and effective way.

Although the voltage at the primary side of the output transformer has a relation to the output voltage of the driver, that primary side signal is distorted because of the voltage drop across the leakage inductance of the output transformer.

In some cases a separate winding for output voltage sensing is disposed at the output transformer such that it has a magnetic coupling to the secondary side winding, and indeed a considerably better magnetic coupling than between normal primary and secondary side windings. It is then electrically connected to the primary side circuitry. This is feasible as long as no substantial power needs to be transferred via this sensing winding, hence no substantial current needs to flow in the sensing winding, and therefore no substantial voltage drop will occur across the sensing winding leakage inductance.

However, it is challenging to dispose such a sensing winding with sufficient magnetic coupling to the secondary side winding while maintaining creepage and clearances associated with the isolation requirements between the primary and secondary sides of the output transformer.

There is therefore a need for an improved primary side sensing approach which generates a signal representative of the output voltage of the driver, and in particular that compensates for the impact of the leakage inductance of the output transformer.

SUMMARY OF THE INVENTION

The invention is defined by the claims.

According to examples in accordance with an aspect of the invention, there is provided a switch mode power converter, comprising:

an inverter comprising a high side switch and a low side switch connected in series with a first node between them;

a transformer having a primary side winding and a secondary side winding; and

a first inductor in series with the primary side winding and connected to the primary side winding at a second node;

a second inductor magnetically coupled to the first inductor, the second inductor defining an output terminal at one end; and

a controller for controlling the inverter, wherein the controller has an input for receiving a signal derived from the voltage at the output terminal.

This power converter makes use of an additional (second) inductor coupled to a first inductor. The first inductor may for example be an already required part of the switch mode power converter such as used for energy storage or forming part of a resonant tank, or else the first inductor may be added for the purposes of enabling the output terminal to be provided. This first inductor is in series with the transformer primary side winding. The primary side circuit for example already includes the first inductor that carries essentially the same current as the current that flows through the transformer primary side winding. This may be the case in LCC and LLC converters, for example.

The second inductor provides a simple method to measure the secondary side voltage at the primary side. The second inductor may be implemented by providing an extra winding to the existing first inductor, which then has a tap so that two inductors are defined by the single arrangement. The voltage across the extra winding is identical in shape and phase to the voltage across the leakage inductance and is used to compensate the error in voltage signal at the primary side winding of the output transformer, as caused by the leakage inductance. Thus, voltage sensing of the secondary side voltage is made more accurate based on voltage sensing at the primary side.

The sensed voltage may then be used as part of the control scheme of the converter. For example, it may be used to trigger protection when an excess voltage is detected at the output.

The converter may further comprise a series capacitor in series with the first inductor and a capacitor in parallel with the secondary side winding. This defines an LCC structure. Multiple secondary side windings are possible.

The second inductor may have an inductance approximately equal to a combined primary side series leakage inductance of the transformer.

By “combined primary side series leakage inductance” in this context is meant the sum of the primary side leakage inductance and the reflected secondary side leakage inductance. In particular, this combination represents (in a model of the transformer electrical characteristics) the effect of the imperfect coupling between the primary side and secondary side of the transformer. In the case of multiple secondary side windings, there might be multiple secondary side leakage inductances, that can still be translated as a secondary side leakage inductance reflected to the primary side.

In this way, the inductance of the second inductor is made equal to the leakage inductance it is intended to compensate. The leakage inductance may vary with temperature for example and it may not be known to a high degree of accuracy. Thus, the second inductor is selected to have a value which approximates the leakage inductance.

The second inductor may instead have a winding ratio with respect to the first inductor such that the second inductor has an inductance greater than a combined primary side series leakage inductance of the transformer.

In this way, the inductance of the second inductor is made larger than the leakage inductance it is intended to compensate. The measured voltage may then be scaled down.

The controller may have an input directly connected to the output terminal.

The voltage at the end of the second inductor then functions directly as a feedback signal for the controller.

Alternatively, the controller may have a combining circuit for combining signals at the second node and at the output terminal to derive a sensing signal which is provided to the controller.

In this case, the voltages at each end of the second inductor are processed, for example scaled, before being used as a feedback signal for the controller. This may for example be needed because only a ratio between integer numbers of turns of the second inductor and the first inductor is possible. The combining function is then used to improve the accuracy with which the leakage inductance is simulated.

The combining circuit may comprise a resistor network for combining voltages at the second node and the output terminal to provide a sensing signal voltage.

This resistor network may be used for providing a weighted combination of voltages or deriving any other function between the voltages at each end of the second inductor.

The combining circuit may instead comprise a circuit for generating a sensing signal current. The feedback signal provided to the controller may thus be a voltage or a current. However, even when a current is used as the feedback signal, it is still dependent on the voltage at the output node and hence the voltage across the second inductor.

The second inductor may have a first end connected to the second node and the output terminal at a second end. In this way, the second inductor is connected such that the compensation (e.g. signal subtraction) for the combined leakage inductance (represented at the primary side) is achieved by the component placement.

Alternatively, the second inductor may have a first end connected to ground and the output terminal at a second end. The compensation (e.g. signal subtraction) may then be carried out by the controller.

The converter may comprise a resonant converter with a resonant tank connected to the first node, the resonant tank comprising at least the first inductor.

The converter may further comprise a rectifier connected to the secondary side winding, and a storage capacitor across the rectifier output.

The converter thus delivers a DC output for a DC load.

In one set of examples, the rectifier comprises a four-diode bridge connected across the secondary side winding. In another set of examples, the transformer has first and second secondary side windings in series, with a node defined between them, wherein the rectifier (D1-D4) comprises a two diode arrangement.

By “two diode arrangement” is meant that the rectifier function is implemented with only two unidirectional conduction paths. Each path may have a single diode, but of course the same functionality would be achieved with each path having multiple diodes in series.

The series connection of the first and second secondary side windings may include a component between them, for example a diode of the rectifier.

Thus, different rectifier designs are possible according to the design of the secondary side of the transformer.

The invention also provides a lighting circuit comprising:

a converter as defined above; and

a lighting load in parallel with the storage capacitor.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

