Patent Description:
The invention relates to a particularly kind of buck converter in which the buck switch is ground-referenced.

Such a buck converter may find application as a charging circuitry for being supplied with a DC voltage derived from an AC mains voltage for example, for charging at its output terminals, a battery. This battery may be used in order to drive lighting means, such as for example a LED load, in the framework of emergency lighting means. To that regard, one or more additional converter stages may be arranged between the battery supply voltage and the lighting means.

Typically, a control circuitry, such as for example an ASIC or a microcontroller is provided which controls at least the buck switch between the conducting and the non-conducting state. In addition, typically, the control circuitry is supplied with a plurality of feedback signals on the basis of which the control circuitry determines the appropriate switching-over operations of the buck switch.

A typical control approach is that during the conducting state of the buck switch, the current through the switch is monitored and the switch is controlled into the non-conducting state as soon as the increasing buck switch current reaches a given switching-off threshold.

Once the buck switch is switched off, the LED current continues to be driven via an inductor of the buck converter in a freewheeling path comprising a diode.

Typically, it has to be determined when this decaying current, driven by the inductor, reaches the zero level. In the so-called critical conduction mode, the conducting state of the switch is immediately controlled as soon as the current reaches the zero level. In the discontinuous mode a given time period is allowed to lapse between the inductor current reaching the zero level and the switching-on of the buck switch.

Furthermore, the control parameters, such as for example the switching-off threshold for the buck switch current, have to be adapted to feedback-control the required battery voltage. To that regard, the control circuitry needs a feedback information as to the current level of the battery voltage. Thus, typically a plurality of feedback paths with matching input pins of the control circuitry have to be provided for the buck switch current, the timing of the zero crossing of the freewheeling current (inductor current), the battery voltage, and typically also the supplying DC bus voltage.

Relevant prior art can be found, for example, in the patent literature <CIT>, <CIT>, <CIT> and <CIT>.

The invention now targets of a simplification of this feedback path in order to reduce the required number of input pins at the control circuitry.

This object is achieved via the means of the independent claims. The depending claim develops further the central idea of the invention.

According to a first aspect of the invention, a buck converter for supplying a battery is provided. The buck converter comprises a serious connection of a diode and a buck switch, the serious connection being connected between a DC bus voltage (which may have a ripple) and a ground reference. An inductor is connected at its first terminal to the common node of the diode and the buck switch. A series connection of two resistors is provided in parallel to the buck switch. A control circuitry is provided for controlling the buck switch. A feedback path is connected from the common node of the resistors to the control circuitry. Output terminals are provided, which are connected to a battery, wherein one output terminal is connected to the DC bus voltage, and the other terminal is connected to the second terminal of the inductor.

The inductor, the diode and the battery form a freewheeling path for a current driven by the inductor in the non-conducting state of the buck switch.

The control circuitry is designed to evaluate the signal on the feedback path as a signal indicating the current through the buck switch when the control circuitry controls the buck switch to be conducting, by comparing this signal in this time period with a switching-off threshold value. As soon as the inductor current reaches the threshold value, the control circuitry may control the buck switch into the non-conducting state.

The control circuitry is designed to evaluate and sample a signal on the feedback path as a signal indicating the DC bus voltage in the period following the control of the buck switch into the non-conducting state and prior to the freewheeling current in this period reaching the zero level.

The control circuitry is designed to evaluate and sample the signal on the feedback path as a signal indicating the DC bus voltage minus the battery voltage when the current through the battery has decayed to zero. As previously the DC bus voltage has been sampled, thus the value of the battery voltage can be computed.

The battery may be charged by the buck converter when an AC voltage is present on the input terminals of the LED converter. In case that the AC voltage on the input terminals of the LED converter falls below a certain level or even is interrupted than the buck converter will stop to charge the battery. Such a condition may be an emergency lighting event where emergency lighting is necessary.

A further aspect of the invention relates to an LED converter, especially an emergency light LED converter, comprising a buck converter as explained above, and at least one LED, which can be supplied starting from the battery voltage.

The invention further relates to a method for operating a buck converter for supplying a battery.

Further aspects, features and advantages of the invention will become evident to the skilled person when reading the following explanations of a preferred embodiment of the invention, when taken in conjunction with the figure of the enclosed drawings.

<FIG> shows a buck converter generally referenced with the numeral <NUM>.

Typically, the buck converter <NUM> is supplied starting from an AC voltage <NUM>, which may be rectified by an AC/DC block <NUM>, which may comprise also a PFC (power factor correction) functionality. If the PFC is an actively controlled PFC, a control circuitry <NUM> may issue a control command to one or more switches of the PFC unit. The AC/DC block <NUM> may be formed by a flyback converter acting as isolated PFC or may be formed by a non-isolated converter like a boost converter acting as PFC followed by a flyback converter which takes over the isolating functionality.

The output of block AC/DC <NUM> is typically stabilized by a capacitor <NUM>. The voltage at the output terminals of AC/DC block <NUM>, thus the voltage across the capacitor <NUM> will now be referred to as DC bus voltage (or rail voltage) with the reference numeral VDC. A ripple with twice the frequency of the AC voltage <NUM> may be present on this DC bus voltage VDC.

The DC bus voltage VDC is supplied to the buck converter <NUM>.

The buck converter has a switch <NUM>, which is ground referenced. A low ohmic resistor <NUM> (e.g. in the order of <NUM> to <NUM> ohms) may be connected between the ground potential and the switch <NUM>.

The switch is connected in series with a diode <NUM>. The diode <NUM> forms a freewheeling path together with an inductor <NUM> and a battery <NUM>.

The battery <NUM> is charged by the current produced by the buck converter <NUM> and thus represents the load of the buck converter <NUM>.

Preferably the battery <NUM> is charged by the buck converter <NUM> when an AC voltage <NUM> is present on the input terminals of the LED converter. In case that the AC voltage <NUM> on the input terminals of the LED converter falls below a certain level or even is interrupted then the buck converter <NUM> will stop to charge the battery <NUM>. Such a condition may be an emergency lighting event where emergency lighting is necessary.

The inductor <NUM> at its one end is connected to the common node of the switch <NUM> and diode <NUM>.

The other end of the inductor <NUM> is connected to the ground potential.

The battery <NUM> may be disconnected by a switch <NUM>, wherein the switch <NUM> may also be controlled by a control signal from the control circuitry <NUM>. The switch <NUM> may be formed by a MOSFET. Switch <NUM> is preferably closed in the emergency lighting mode to connect the negative pole of battery <NUM> to the ground, so that the battery <NUM> can in the emergency lighting mode supply the converter stage <NUM> from the battery efficiently by reducing the forward drop across the anti-parallel diode of switch <NUM> (as it is a MOSFET). During battery charging mode of the battery <NUM> the switch <NUM> remains off since the battery <NUM> remains floating in this battery charging mode.

The output terminals <NUM>, <NUM>' of the buck converter <NUM>, at which the voltage of the battery <NUM> occurs, can be used to drive directly an LED load or, as illustrated in the example, drive the LED load <NUM> through one or more converter stages <NUM>. The one or more converter stages <NUM> may comprise switch converter stages, e.g. a boost converter, a tapped boost converter or a buck-boost converter. The control signal for the switch converter stage(s) may be produced by the control circuitry <NUM> or an additional control circuitry <NUM>.

The additional control circuitry <NUM> may communicate with the first mentioned control circuitry <NUM> through a communication path <NUM>.

The LED load may be driven in an emergency lighting mode, e.g. when the AC voltage <NUM> on the input terminals of the LED converter falls below a certain level or even is interrupted.

Importantly, according to the invention a series connection of two resistors <NUM>, <NUM> is arranged in parallel to the buck switch <NUM>.

A feedback path is connected between the common node <NUM> of the resistors <NUM>, <NUM> and an input pin <NUM> of the control circuitry <NUM>.

As illustrated, further feedback path may be present for the control circuitry <NUM>, such for example a path <NUM> carrying a signal representing the AC voltage (especially in view of emergency light applications). However, preferably no further feedback path is required for the operation of the buck converter <NUM>, i.e. all feedback signals required for the operation of the buck converter <NUM> are sent to the control circuitry <NUM> at the sole input pin <NUM>.

Now the operation of the circuitry will be explained.

When the control circuitry <NUM> controls the buck converter switch <NUM> in the conducting state, the current through the buck switch <NUM> increases, charging the buck inductor. The increasing current produces a signal at the common node of the resistor <NUM> of the resistors <NUM>, <NUM> thus that the control circuitry <NUM> can monitor a signal representing the increasing current through the buck switch <NUM>.

When the increasing current is determined to reach a given switching-off threshold, the control circuitry <NUM> controls this buck switch <NUM> into the non-conducting state. In this time period the inductor <NUM> continues to drive the battery current in the freewheeling path comprising at least the inductor <NUM>, the diode <NUM> and the battery <NUM>. This freewheeling current decreases until the freewheeling current (discharging current of the inductor <NUM>) reaches the zero crossing level.

During this discharging phase (this is the time period between switching off the buck switch <NUM> and the inductor current <NUM> reaching the zero level), the sole feedback pin <NUM> of the control circuitry <NUM> measures the level of the bus voltage VDC. Preferably, the control circuitry <NUM> is designed to sample a value representing the DC bus voltage towards the end of the discharging phase of the inductor current <NUM>, such that the effect of the discharging current is reduced.

Thus, during this discharging phase for the inductor current, the DC bus voltage is sampled and held within the control circuitry <NUM>.

As soon as the inductor current reaches the zero crossing level, the voltage at the input pin <NUM> rapidly changes to a value, which corresponds to the difference between the DC bus voltage reduced by the voltage across the battery <NUM>. The control circuitry is designed to sample this voltage and correspondingly compute the current value of the voltage of the battery <NUM> in view of the previously sampled value for the DC bus voltage.

In addition, the timing of the rapid change of the voltage at the feedback input pin <NUM> of the control circuitry <NUM> represents the timing of the zero crossing of the inductor current. This timing information can be used in order to determine the next switching-on cycle timing of the buck switch <NUM>, be it in the critical mode or in the non-continuous mode of the buck converter <NUM>.

As shown in <FIG> and <FIG>, thus a single feedback pin <NUM> is sufficient in order to convey to the control circuitry <NUM> the information as to the zero crossing timing, the value of the increasing switch current during the switching-on phase of the buck switch <NUM>, as well as the battery voltage and even the level of the DC bus voltage VDC.

In a sixth aspect of the present disclosure, a buck converter for supplying a battery comprises:.

wherein a feedback path is connected from the common node of the resistors to the control circuitry, and.

The control circuitry is designed to evaluate the signal on the feedback path as a signal indicating the current through the buck switch when the control circuitry controls the buck switch to be conducting, by comparing it with a switching-off threshold.

The control circuitry is designed to evaluate and sample the signal on the feedback path as a signal indicating the DC bus voltage in the period following the control of the buck switch into the non-conducting state and prior to a freewheeling current reaching a zero level.

The control circuitry is designed to evaluate and sample the signal on the feedback path as a signal indicating the DC bus voltage minus the battery voltage when the current through the battery has decayed to zero.

In an eighth aspect of the present disclosure, an LED converter, especially an emergency light LED converter, comprises a buck converter according to the sixth aspect of the present disclosure, and at least one LED which can be supplied starting from the battery voltage.

Claim 1:
Buck converter (<NUM>) for supplying a battery (<NUM>), comprising:
- a series connection of a diode (<NUM>) and a buck switch (<NUM>), the series connection being connected between a DC bus voltage (VDC) and a ground reference,
- an inductor (<NUM>) connected at its first terminal to a common node of the diode (<NUM>) and the buck switch (<NUM>),
- a series connection of two resistors (<NUM>, <NUM>) in parallel to the buck switch (<NUM>),
- a control circuitry (<NUM>) for controlling the buck switch (<NUM>), wherein a feedback path is connected from a common node (<NUM>) of the resistors (<NUM>, <NUM>) to the control circuitry (<NUM>), and
- output terminals (<NUM>, <NUM>') connectable to the battery (<NUM>), wherein one output terminal (<NUM>) is connected to the DC bus voltage (VDC), and the other terminal (<NUM>') is connected to the second terminal of the inductor (<NUM>);
wherein the control circuitry (<NUM>) is designed to evaluate and sample a signal on the feedback path as a signal indicating the DC bus voltage (VDC) in a period following a control of the buck switch (<NUM>) into a non-conducting state and prior to a freewheeling current reaching a zero level; and
wherein the control circuitry (<NUM>) is designed to evaluate and sample the signal on the feedback path as a signal indicating the DC bus voltage (VDC) minus a battery voltage when a current through the battery (<NUM>) has decayed to zero; and
wherein the control circuitry (<NUM>) is designed to evaluate the signal on the feedback path as a signal indicating the current through the buck switch (<NUM>) when the control circuitry (<NUM>) controls the buck switch (<NUM>) to be conducting, by comparing it with a switching-off threshold.