Power factor correction circuit

Abstract

The invention relates to a power factor correction (PFC) circuit (20), comprising an inductor (21) which is configured to provide a discharge current, a capacitor (23) which is connected to the inductor (21) via a switch (24) and which can be charged with said discharge current, a control unit (14) which is configured to alternately switch the switch (24) on and off based on a feedback control, wherein the control unit (14) has an input interface (42) for receiving a feedback signal (ZXCS) which represents a discharge voltage of the inductor (21), wherein the control unit (14), in a DCM mode, is further configured to calculate a switch on time (T.sub.on) of the switch (24) which is after a first local minimum of the discharge voltage, and wherein, after switching off the switch (24), the control unit is configured to: either switch on the switch (24) at a next or closest local minimum of the inductor voltage after Ton, in case Ton is less than a directly or indirectly set reference time (T.sub.ref), or close the switch (24) at T.sub.on, in case T.sub.on is equal to or exceeds T.sub.ref.

Claims

1. A power factor correction (PFC) circuit, comprising: an inductor which is configured to provide a discharge current; a boost diode which is connected to the inductor in series; a switch connected on one side between the inductor and the boost diode and connected to ground on the other side; a capacitor which is connected to the inductor and the boost diode and which can be charged with said discharge current from the inductor through the boost diode; a detection circuit that provides a feedback signal representing a discharge voltage of the inductor; a control unit which is configured to alternately switch the switch on and off; wherein the control unit has an input interface for receiving a feedback signal which represents the discharge voltage of the inductor, wherein the control unit, in a discontinuous conduction mode (DCM), is further configured to calculate a calculated switch on time (T.sub.ON_CALC) of the switch which is after a first local minimum of the discharge voltage of the inductor, and wherein, after switching off the switch, the control unit is configured to: a) either switch on the switch at a next or closest local minimum of the discharge voltage of the inductor after the calculated switch on time (T.sub.ON_CALC), in case the calculated switch on time (T.sub.ON_CALC) is less than a directly or indirectly set reference time (T.sub.ref), b) or close the switch at the calculated switch on time (T.sub.ON_CALC), in case the calculated switch on time (T.sub.ON_CALC) is equal to or exceeds said directly or indirectly set reference time (T.sub.ref).

2. The PFC circuit according to claim 1, wherein the control unit is configured to determine the local minimum of the discharge voltage based on the feedback signal.

3. The PFC circuit according to claim 2, wherein the control unit is configured to detect the local minimum of the discharge voltage at a positive zero crossing of the discharge current.

4. The PFC circuit according to claim 1, wherein the control unit is configured to calculate the calculated switch on time (T.sub.ON_CALC) during a closing phase of the switch.

5. The PFC circuit according to claim 1, wherein the control unit comprises a processing unit for analyzing the feedback signal and calculating the calculated switch on time (T.sub.ON_CALC).

6. The PFC circuit according to claim 1, wherein the control unit comprises an output interface for controlling the switch.

7. The PFC circuit according to claim 1, wherein the control unit comprises a restart timer which is configured to switch on the switch at a set time limit after switching off the switch, wherein the reference time (T.sub.ref) is less than the time limit of the restart timer.

8. The PFC circuit according to claim 1, wherein the reference time (T.sub.ref) is less than 300 μs.

9. The PFC circuit according to claim 1, wherein the switch is a transistor, in particular a power transistor, a field effect transistor (FET) or a metal oxide semiconductor field effect transistor (MOSFET).

10. The PFC circuit according to claim 1, wherein the PFC circuit is part of a driver circuit for driving light sources including a plurality of LEDs.

11. A method for performing a power factor correction (PFC) by means of a PFC circuit, wherein the PFC circuit comprises an inductor which is configured to provide a discharge current, a boost diode which is connected to the inductor in series; a switch connected on one side between the inductor and the boost diode and connected to ground on the other side, a capacitor which is connected to the inductor and the boost diode and which can be charged with said discharge current from the inductor through the boost diode, wherein the switch is alternately switched on and off, the method comprising the steps of: receiving a feedback signal, wherein the feedback signal represents a discharge voltage of the inductor, calculating a calculated switch on time (T.sub.ON_CALC) of the switch which is after a first local minimum of the discharge voltage of the inductor, and after switching off the switch: a) either switch on the switch at a next or closest local minimum of the discharge voltage of the inductor after the calculated switch on time (T.sub.ON_CALC), in case the calculated switch on time (T.sub.ON_CALC) is less than a directly or indirectly set reference time (T.sub.ref), b) or switch on the switch at the calculated switch on time the (T.sub.ON_CALC), in case the calculated switch on time (T.sub.ON_CALC) is equal to or exceeds (T.sub.ref).

12. The method according to claim 11, wherein the PFC circuit further comprises a control unit to alternately switch the switch on and off.

13. The method according to claim 12, wherein the control unit has an input interface for receiving the feedback signal and an output interface for controlling the switch.

14. The method according to claim 12, wherein the control unit is implemented as a micro controller, an application specific integrated circuit (ASIC) or a hybrid solution.

15. The PFC circuit according claim 1 wherein the reference time (T.sub.ref) is less than 100 μs.

16. The PFC circuit according claim 1 wherein the reference time (T.sub.ref) is less than 50 μs.

17. The PFC circuit according claim 1 wherein the reference time (T.sub.ref) is less than 25 μs.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in the followings together with the figures.

(2) FIG. 1 shows a schematic diagram of a PFC circuit according to an embodiment of the invention;

(3) FIG. 2 Shows a schematic plot of a voltage signal occurring in the circuit according to an embodiment of the invention;

(4) FIG. 3 shows a schematic diagram of a driver for light sources according to an embodiment of the invention; and

(5) FIG. 4 shows a schematic diagram of a method for power factor correction according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) Aspects of the present invention are described herein in the context of a PFC circuit.

(7) The present invention is described more fully hereinafter with reference to the accompanying drawings, in which various aspects of the present invention are shown. This invention however may be embodied in many different forms and should not be construed as limited to the various aspects of the present invention presented through this disclosure. Rather, these aspects are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. The various aspects of the present invention illustrated in the drawings may not be drawn to scale. Rather, the dimensions of the various features may be expanded or reduced for clarity. In addition, some of the drawings may be simplified for clarity. Thus, the drawings may not depict all of the components of a given apparatus.

(8) It is further understood that the aspect of the present invention might contain integrated circuits that are readily manufacturable using conventional semiconductor technologies, such as complementary metal-oxide semiconductor technology, short “CMOS”. In addition, the aspects of the present invention may be implemented with other manufacturing processes for making optical as well as electrical devices. Reference will now be made in detail to implementations of the exemplary aspects as illustrated in the accompanying drawings. The same references signs will be used throughout the drawings and the following detailed descriptions to refer to the same or like parts.

(9) FIG. 1 shows a power factor correction (PFC) circuit 20 according to an embodiment.

(10) The PFC circuit 20 comprises an inductor 21 which is configured to provide a discharge current, a capacitor 23 which is connected to the inductor 21 via a switch 24 and which can be charged with said discharge current, a control unit 14 which is configured to alternately switch the switch 24 on and off based on a feedback control, wherein the control unit 14 has an input interface 42 for receiving a feedback signal (ZXCS) which represents a discharge voltage of the inductor being applied to the switch 24.

(11) The control unit 14, in a DCM mode, can be configured to calculate a switch on time (T.sub.on) of the switch 24 which is after a first local minimum of the discharge voltage.

(12) After switching off the switch 24, the control unit can be configured to either switch on the switch 24 at a next or closest local minimum of the inductor voltage after T.sub.on, in case T.sub.on is less than a directly or indirectly set reference time (T.sub.ref), or close the switch 24 at T.sub.on, in case T.sub.on is equal to or exceeds T.sub.ref.

(13) The discharge voltage refers to the voltage of the inductor at discharge. In particular, the discharge voltage refers to the discharge voltage of the inductor being applied to the switch 24.

(14) The PFC circuit 20 can be supplied with an input voltage V.sub.in e.g. in the form of an AC or DC voltage. The input voltage V.sub.in can be a rectified mains voltage from a rectifier (not shown).

(15) As output, the power factor correction circuit 20 can provide an output voltage which is a DC voltage. The output voltage V.sub.out can be used to supply a load to which the power factor correction circuit 20 is connected. The load can, for example, be a component of a control gear for a light source such as a fluorescent lamp, a halogen lamp, a light-emitting diode (LED) arrangement, etc.

(16) The input, respectively output, of the PFC circuit 20 can each be formed by an input terminal, respectively output terminal 27, and ground.

(17) The input voltage V.sub.in can be applied to a first terminal of the inductor 21 which can be a charging coil.

(18) The PFC circuit 20 can further comprises a boost diode 22 which is connected to a second terminal of the inductor 21. The capacitor 23 can be connected between the anode of the boost diode 22 and ground, and can thus be charged by the discharge current of the inductor 21 via the diode 22.

(19) The switch 24 can be a transistor, in particular a power transistor, a field effect transistor (FET) or a MOSFET.

(20) In an embodiment, when the switch 24 is switched on, the inductor 21 is connected to ground via the switch 24, whereby the diode 22 is blocking, so that the inductor 21 is charged. If the switch 24 is switched off the diode 22 is conductive, so that the inductor 21 can be discharged via the diode 22 into the charging capacitor 23.

(21) In FIG. 1, the inductor 21 is a charging coil that forms the primary winding of a transformer. A detection coil 31 forms the secondary winding of this transformer. The charging coil and the detection coil 31 are inductively coupled so that the current through the charging coil or the voltage at the charging coil can inductively be tapped by the control unit 14 at the input interface 42.

(22) Accordingly, a resistor 33 and a diode 32 are provided. One terminal of the detection coil 31 is connected to ground, the other terminal of the detection coil 31 to the anode of the diode 32. The resistor 33 is connected between the cathode of the diode 32 and the input interface 42 of the control unit 14.

(23) In an embodiment, while the switch 24 is switched off, the local minimum of the voltage via the inductor 21 or the (positive) zero crossing of the current (I.sub.L) flowing through the inductor 21 can be detected at the input interface 42. This detection of the voltage at the inductor 21 also indirectly detects the voltage at the switch 24.

(24) In an embodiment, while the switch 24 is switched on, the current through the switch can be measured at the input interface 42 via a measuring resistor 26. This preferably low-impedance measuring resistor 26 is connected between ground and the switch 24, so that, when the switch 24 is switched on, a current flows through the inductor 21, the switch 24 and the measuring resistor 26.

(25) In the exemplary embodiment of FIG. 1, another resistor 34 connects the input interface 42 and the connection point between the measuring resistor 26 and the switch 24.

(26) A further capacitor 25 can be connected between inductor 21 and ground, which is connected in parallel to a series circuit consisting of the switch 24 and the resistor 26. The capacitor 25 can be connected to the same terminal of the diode 22 as the inductor 21.

(27) The PFC circuit 20 further has an output interface 41 that is connected to the switch, in particular to a gate of the switch, for controlling the switch 24.

(28) The control unit 14 can have a further input interface 43 for recording further variables. For example, the control unit 14 can detect the output voltage in form of a bus voltage V.sub.bus via a voltage divider with resistors 36, 37.

(29) FIG. 2 shows a schematic plot 50 of a voltage signal occurring in the circuit according to an embodiment.

(30) The voltage in FIG. 2 is the fluctuating, or ringing, discharge voltage after switching off the switch 24 at time T.sub.0.

(31) FIG. 2 shows the switching with two different calculated switch on times T.sub.on_calc1 and T.sub.on_calc2, wherein T.sub.on_calc1<T.sub.ref and T.sub.on_calc2>T.sub.ref, and wherein T.sub.ref is the directly or indirectly set reference time.

(32) In a first time period region of the ringing voltage, which is before the reference time T.sub.ref, the so-called valley switching can be performed. This means, that if a calculated switch on time T.sub.on_calc1 falls in this time period, then the switch 24 is not switched on at T.sub.on_calc1. Instead, it is switched on at T.sub.on_eff1, wherein T.sub.on_eff1 is the time of the next or closest valley to T.sub.on_calc1, and wherein often T.sub.on_calc1≠T.sub.on_eff1.

(33) In a second time period region of the ringing voltage, which is after T.sub.ref, the valley switching can be deactivated. A calculated switch on time T.sub.on_calc2, which falls in this time period, is directly applied to the actual switch on operation of the switch 24, i.e T.sub.on_calc2=T.sub.on_eff2. The switching at T.sub.on_calc2 is done if T.sub.on_calc2≥T.sub.ref, regardless of whether the discharge voltage at T.sub.on_calc2 is in a valley or not.

(34) This “adaptive valley switching” can prevent lost valley detection due to low signals. The PFC circuit 20 does no longer rely on a restart timer, that switches the PFC switch back on after a safety march in time period, which is much longer than the calculated switch on time T.sub.on. The use of such a restart timer can lead to visible low frequency flicker in the light output of a lighting means supplied by a converter stage, which supplied by the output or bus voltage. However, a restart timer can still be implemented in the PFC circuit 20 as a safety measure.

(35) The reference time can be less than 300 μs, 100 μs, 50 μs or 25 μs. In particular, the reference time, is less than the time limit of a restart timer.

(36) The control unit 14 can calculate the switch on time (Ton), in particular during a closing phase of the switch 24. The control unit can comprise a processing unit for calculating T.sub.on, e.g. based on the feedback signal (ZXCS).

(37) FIG. 3 shows a schematic diagram of a driver 60 for light sources 61, such as LEDs, according to an embodiment.

(38) The driver 60 comprises the PFC circuit 20, for instance the PFC circuit 20 as depicted in FIG. 1.

(39) The driver can be an electrical ballast for an LED converter or for a fluorescent lamp.

(40) FIG. 4 shows a schematic diagram of a method 70 for power factor correction according to an embodiment of the invention.

(41) The method 70 can be performed by a PFC circuit 20, in particular the PFC circuit of FIG. 1, wherein the PFC circuit 20 comprises an inductor 21 which is configured to provide a discharge current, a capacitor 23 which is connected to the inductor 21 via a switch 24 and which can be charged with said discharge current, wherein the switch 24 is alternately switched on and off.

(42) The method 70 comprises the steps of: receiving 71 a feedback signal (ZXCS), wherein the feedback signal (ZXCS) represents a discharge voltage of the inductor, calculating 72 a switch on time (T.sub.on) of the switch 24, which is after a first local minimum of the discharge voltage, and after switching off 73 the switch 24, either switch on 74 the switch 24 at a next or closest local minimum of the discharge voltage after T.sub.on, in case Ton is less than a directly or indirectly set reference time (T.sub.ref), or switch on 75 the switch 24 at T.sub.on, in case T.sub.on is equal to or exceeds T.sub.ref.

(43) The step of calculating 72 T.sub.on and/or the step of receiving 71 the feedback signal can be performed each before or after the switch off 73 of the switch 24.

(44) The control unit 14 as shown in FIG. 1 can perform the method 70 shown in FIG. 4.

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

(46) While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only and not limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit of scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalence.

(47) Although the invention has been illustrated and described with respect to one or more implementations, equivalent alternations and modifications will occur to those skilled in the art upon the reading of the understanding of the specification and the annexed drawings. In addition, while a particular feature of the invention may have been disclosed with respect to only of the several implementations, such features may be combined with one or more other features of the other implementations as may be desired and advantage for any given or particular application.