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/EP2019/084800, filed on Dec. 12, 2019, which claims the benefit of European Patent Application No. 18214334.7, filed on Dec. 20, 2018. These applications are hereby incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to the generation of a DC voltage supply, in particular from a high voltage AC signal. For example it relates to resonant power converters, for example used to provide AC/DC conversion, or to provide DC/DC conversion, or for the generation of a DC supply voltage or current from the power converter.

BACKGROUND OF THE INVENTION

Converters which are in the form of so-called resonant converters 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 converters 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.

Resonant power convertors often use a low voltage supply which has been derived from a high voltage AC, e.g. semi-square-wave, node present in the converter via a charge pump. This low voltage supply is essentially a current source with a current output which is proportional to the converter frequency and close to proportional to the bus voltage. The low voltage supply, for example 13V, is for supplying the control or driver integrated circuit and/or other circuitry. Most commonly, a so-called dV/dt supply is used in LCC type resonant convertors, which are used in the output stages of driver circuits for fluorescent lamps and LEDs.

The low voltage supply current not consumed by the load needs to be dissipated to limit or control the low voltage. This dissipation reduces driver efficacy and may also pose thermal challenges.

It is known to provide a dissipation path for excess current so that it does not pass to the load. However, implementing this dissipation path is difficult in practice, because the amount of current that needs to be dissipated varies over time.

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 circuit for generating a DC voltage supply from a higher voltage AC signal, comprising:

an input for receiving the higher voltage AC signal;

a first capacitor in series between the input and a first node;

a first diode in forward direction between the first node and a DC voltage supply node;

a second capacitor between the DC voltage supply node and a reference terminal;

a third capacitor between the first node and a second node;

a second diode in forward direction between the reference terminal and the first node;

a controllable voltage supply; and

a diode arrangement comprising third and fourth diodes in series across the controllable voltage supply, with the junction between the third and fourth diodes defining the second node.

This circuit connects a capacitor in a charge pump type circuit to a controllable voltage supply. This provides a way to control when that capacitor provides a discharge path. In this way, the discharge path is controlled in dependence on the charge that needs to be diverted in order to generate a stable low voltage DC supply from a high voltage AC input with varying amplitude. By setting the voltage at the second node, the division of charge between the first diode (which flows to the load) and the third capacitor (which flows away from the load) is controlled.

The higher voltage AC signal is for example a rectified and chopped mains signal. Thus, it has a frequency which depends on the chopping rate, and an amplitude which varies at the slower mains rate.

The circuit may further comprise a voltage clamping device between one terminal of the first diode and the reference terminal, wherein the voltage clamping device for example is a Zener diode.

The magnitude of the voltage provided by the controllable voltage supply for example sets a voltage level for the input, which functions as a threshold which determines when charge will flow through the first diode. Thus, it controls, during each pulse of the AC input, whether charge is routed to the output via the first diode or is sunk into the controllable voltage supply.

When the magnitude of the voltage provided by the controllable voltage supply reaches an implementation limited minimum level (which may approximate zero), this corresponds to a minimum charge being delivered to the output node and a maximum charge being diverted into the controllable voltage supply.

This may be considered to be a saturation condition, in the sense that no more charge (per pulse of the AC signal) can be extracted/diverted from the first node. If the first capacitor (which functions as a charge pump) delivers even more charge into the first node, this excess charge will then still flow via the first diode into the DC voltage supply node.

A maximum magnitude of the voltage provided by the controllable voltage supply is smaller than the DC voltage supply voltage to enable the diversion of charge from the first node.

The magnitude of the voltage provided by the controllable voltage supply is for example smaller when more of a current received from the input needs to be diverted into the controllable voltage supply.

In one example, the magnitude of the voltage provided by the controllable voltage supply is controlled based on a current sensor which senses a current flowing to the DC output. This provides a current feedback mechanism for regulating the output current.

In another example, the magnitude of the voltage provided by the controllable voltage supply is controlled based on a voltage sensor which senses a DC output voltage. This provides a voltage feedback mechanism for regulating the output voltage.

The controllable voltage supply may comprise a transistor circuit having a control input at a voltage which depends on a current sensor output.

The controllable voltage supply for example comprises a bipolar transistor and a resistor connected in series, for example in parallel with the third and fourth diodes, and wherein the current sensor comprises a current sense resistor in series with the DC output, wherein a base voltage of the bipolar transistor depends on the current flowing through the current sense resistor.

The reference terminal may be ground.

The controllable voltage supply may have a terminal connected to a DC node or a low frequency AC node, directly or via a capacitor. This connection may for example be to ground, or to ground through a series capacitor or to the DC voltage supply node.

The invention also provides a driver circuit comprising:

a resonant switch mode power converter providing a first output for driving a load; and

a DC voltage supply circuit as defined above, wherein the high voltage AC signal is a second output from the resonant switch mode power converter.

The resonant switch mode power converter for example comprises an LCC converter.

The invention also provides a lighting circuit comprising:

a driver circuit as defined above;

a lighting load driven by the first output; and

auxiliary circuitry driven by the second output.

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

