EP3967110A1

EP3967110A1 - Led converter

Info

Publication number: EP3967110A1
Application number: EP20734235.3A
Authority: EP
Inventors: Ludwig Erasmus DE CLERCQ
Original assignee: Tridonic GmbH and Co KG
Current assignee: Tridonic GmbH and Co KG
Priority date: 2019-06-27
Filing date: 2020-06-29
Publication date: 2022-03-16
Anticipated expiration: 2040-06-29
Also published as: WO2020260686A1; EP3967110B1

Abstract

The invention relates to a LED converter (100), comprising: - an isolated switched resonant stage (LLC) supplying terminals for driving an LED load (106), wherein an LED current iLED is feedback-controlled based on feedback- control signals preferably all sensed at a primary side stage (102) of an isolation stage of the isolated switched resonant stage; - a control unit (108) controlling at least one switch (S1, S2) of the switched resonant stage on the basis of the feedback-control signals supplied to the control unit (108), wherein said feedback-control signals comprise: a.) a signal representing the voltage across the LED load VLED; b) a signal indicating a DC supply voltage of the primary side VBUS; and c.) at least one signal representing the current sensed at the primary side and representing the current flowing on the primary side of the isolation stage of isolated switched resonant stage.

Description

LED converter

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a converter for the operation of at least one light source, in particular a converter circuit for the operation of a LED load having at least one LED.

BACKGROUND OF THE INVENTION

A switched resonant circuit, such as an isolated LLC converter can be included in driver circuits for operating LEDs which are basically known from the prior art. Such driver circuits are powered by an electric supply source and the (isolated) resonant, e.g. LLC, LCC etc., converter is responsible for transferring power over a galvanic barrier from a primary side to a secondary side of the isolated resonant converter.

"LLC" converter is to be understood as any resonant converter and to include at least a LCC converter.

The LED load is driven off terminals at the secondary side of the LLC.

It is known that such a resonant circuit or such a driver circuit is operated as a constant current converter. To this aim, a control circuit for controlling the LED current can be provided, wherein an actual value of the LED current can be measured on the secondary side of the galvanic isolation barrier. However, this actual value measurement can be fed- back, crossing the isolation barrier, to a primary-side control circuit to control one or more switches, especially the switching frequency and/or duty cycle of the LLC accordingly .

However, a disadvantage of this approach is the fact that the feedback of the actual value of the LED current back to the primary-side control circuit requires a potential separation and, thus e.g. an optocoupler. Therefore, attempts have been made in the prior art to abstain from a secondary-side detection of the LED current and to detect the value of the current indirectly via the current on the primary side of the transformer .

Thus, it is an objective to provide for a circuit or an LED converter for operating an LED and a corresponding operating method in which the primary-side control can be improved and in particular the abovementioned disadvantage can be overcome.

SUMMARY OF THE INVENTION

The object of the present invention is achieved by the solution provided in the enclosed independent claims. Advantageous implementations of the present invention are further defined in the dependent claims.

According to one aspect, the LED current is determined by putting into the relation the following physical entities:

- the LED voltage

- the electrical power transmitted by the LLC

- the power lost in parts of the circuitry, such as e.g. the rectifier stage on the secondary side According to a first aspect, the invention relates to a LED converter, comprising an isolated switched resonant stage supplying terminals for driving an LED load, wherein an LED current is feedback-controlled based on feedback-control signals. The signal indicating the LED current is sensed at a primary side stage. The control unit controls at least one switch, preferably to alternatively clicked switches connected in series, of the switched resonant stage on the basis of the feedback-control signals. Said feedback-control signals comprise a signal representing the voltage across the LED load V_LED, a signal indicating a DC supply voltage ("bus voltage") of the primary side, and a signal sensed at the primary side and representing the current flowing on the primary side of the isolation stage of isolated switched resonant stage.

The at least one signal representing the current on the primary side of the isolation stage of isolated switched resonant stage may comprise two differentially sensed signals.

The at least one signal representing the current on the primary side of the isolation stage of isolated switched resonant stage may be sensed by a differential current sensing block in series to a capacitor stabilizing a DC supply voltage of the converter, or in series to said at least one switch.

The differential current sensing block is also designated as a "tapping unit".

The control unit may control the at least one switch by frequency modulation and / or duty cycle control.

In an implementation form of the first aspect, the LED converter is an isolated half-bridge LLC converter.

In a further implementation form of the first aspect, the LLC converter comprises on the primary side a tapping unit, wherein the tapping unit comprises a sensing resistor R_s , a capacitor C_f in parallel to R_s , two resistors R_{f i} , R_f2 , wherein R_{f i} is in parallel to R_f2 and wherein the value of R_{f i} is equal to the value of R_f2, and wherein the capacitor C_f is connected between R_{f i} and R_f2 , and wherein the resistor R_s is connected between R_{f l} and R_f2.

In a further implementation form of the first aspect, a first current value i_snsp is tapped off between the resistor R_fi and the capacitor C_f , and wherein a second current value i_snsi is tapped off between the resistor R_f2 and the capacitor C_f .

In a further implementation form of the first aspect, the controller is configured to determine the following value based on a differential sensing of a current flowing into the converter :

<sensing> = i_snsp-isnsi.

In a further implementation form of the first aspect, the LED converter further comprises a rectifier and the controller is further configured to determine the current flowing through the LED i_LED on the basis of the following equation:

wherein V_D is a voltage drop of a diode of the rectifier. Losses are considered depending on the selected implementation/topologies .

In a further implementation form of the first aspect, the controller is further configured to estimate a power (loss) of the rectifier P_rect on the basis of the following equation:

Prect — LED^VLED

In a further implementation form of the first aspect, the LLC converter further comprises a capacitor on the secondary side.

In a further implementation form of the first aspect, the LLC converter further comprises an auxiliary winding coupled to a primary winding of a transformer of the LLC converter.

According to a second aspect, the invention relates to a LED luminaire comprising an LED converter according to the first aspect or any one of the implementation forms thereof.

According to a third aspect, the invention relates to a method for an LED converter, comprising the following steps: supplying terminals for driving an LED load, feedback controlling an LED current i_LED based only on feedback-control signals and controlling at least one switch of a switched resonant stage on the basis of the feedback-control signals. Said feedback-control signals comprise a signal representing the voltage across the LED load V_LED, a signal indicating a DC supply voltage of the primary side V_Bu_s/ and a signal representing the current flowing into the LED converter i_PFC. BRIEF DESCRIPTION OF THE DRAWINGS

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

Fig. 1 shows a schematic diagram of an LED converter according to an embodiment of the invention;

Fig . 2 shows a further schematic diagram of an LED converter according to an embodiment of the invention;

Fig. 2a shows an alternative embodiment to Fig.2,

Fig. 3 shows a schematic diagram of a method for an LED converter according to an embodiment of the invention, and

Fig. 4 shows a schematic diagram of an LED converter including a PFC according to an embodiment of the invention .

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Aspects of the present invention are described herein in the context of a LED converter or LED luminaire having such converter and a LED load.

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.

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

The term "LED luminaire" shall mean a luminaire with a light source comprising one or more LEDs. LEDs are well-known in the art, and therefore, will only briefly be discussed to provide a complete description of the invention.

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. Now referring to Fig. 1, a LED convert 100 is shown according to an embodiment .

The LED converter 100 comprises an isolated switched resonant stage supplying terminals for driving an LED load 106, wherein an LED current i_LED is feedback-controlled based only on feedback-control signals from a primary side stage 102.

Furthermore, the LED converter 100 comprises a control unit 108 controlling at least one switch Si, S2 of the switched resonant stage on the basis of the feedback-control signals.

The feedback-control signals comprise a signal representing the voltage across the LED load V_LED, a signal indicating a DC supply voltage of the primary side V_BUS, and a signal representing the current flowing into the LED converter i_PFC. "PFC" indicates that according to an example the current flowing into the LED converter e.g. LLC may be supplied by an actively switched power factor correction circuit (PFC) whereby this PFC provides a stabilized DC voltage (V_Bu_s) · The PFC is shown in Figure 4 explained later on.

This has the advantage that the current sensing for the LED converter is done in an efficient, easy and cheap way.

Moreover, this has the advantage that the current sensing for the LED converter abstains from a secondary-side 104 detection of the LED current and the value of the current is indirectly detected via the current on the primary side of the transformer . Fig. 2 shows a further embodiment of an LED converter 200 according to the invention.

In this embodiment, the LED converter is an LLC converter 200. The LLC converter 200 comprises a primary side 102 and a secondary side 104.

The primary side 102 can be supplied by a DC voltage V_Bu_s , for instance by a PFC as shown in Fig. 4. The sensing of the DC voltage V_Bu_s may be used for PFC control, i.e. the control of a switch of the PFC, as well as for the control (frequency, duty cycle, deadtime between the on time of two switches connected in series) of at least one switch (SI, S2), preferably two switches connected in series, of the switched resonant stage by the control unit 108.

The DC voltage VBus isupplied to a capacitor CPFC, the voltage of which is the DC supply voltage of an isolated switches resonant converter stage, which in the present example comprises a half-bridge converter with two switches SI, S2 connected in series and controlled by the control unit. The midpoint voltage of the half-bridge converter SI, S2 is fed to a series resonance circuitry LI, C2. The inductor LI is the primary winding of a transformer T which comprises a secondary winding L2. The voltage across the secondary winding L2 is supplied to a rectifier 204 feeding a capacitor C3. The DC voltage across the capacitor C3 is the supply voltage of the LED load 106.

The switches SI, S2 are controlled by corresponding control signals from the control unit 108. The primary side 102 can comprise a tapping unit 200a, also called "differential sensing block", wherein the tapping unit 200a can comprise a sensing resistor R_s, a capacitor C_f in parallel to R_s, two resistors R_fi, R_f2.

In an embodiment the resistor R_fi is in parallel to the resistor R_f2 and the resistance value of R_fi is equal to the resistance value of R_f2. Moreover, the capacitor C_f can be connected between R_fi and R_f2, and the resistor R_s can be connected between R_fi and R_f2.

The tapping unit 200a can be configured to tap off a first voltage value used to detect the current i_Sns_P between the resistor R_fi and the capacitor C_f, and a second voltage value used to detect the current i_snsi between the resistor R_f2 and the capacitor C_f. Thus, this is an example of a differential sensing of the current flow through the resistor Rs .

In the present example, the resistor Rs senses the current flowing through the capacitor Cpfc. This is one example of sensing a current flow on the primary side of the isolation stage of isolated switched resonant stage, and which is used to calculate (see further below) a secondary side current, especially the LED current to be feed-back controlled by the control unit.

Moreover, once i_Sns_P and i_snsi are obtained as differentially sensed values, the controller 108 can be configured to determine the following value:

<sensing> = i_snsp - isns 1 wherein this value <sensing> corresponds to a voltage difference indicating the current i_PfC flowing through R_s .

Once the <sensing> value is obtained, in the power analysis picture, the controller 108 can be configured to perform the following steps:

1^st step: average the current i_PfC , sensed by a differential sensing, which in the present embodiment is flowing from a PFC capacitor C_PfC and through the shunt Rs : ipfc = <sensing>/R_s

2^nd step: multiply i_PfC with the voltage of V_Bus in order to obtain the power flowing into the LLC resonator:

Pin ^— l^BUS * ipfc·

In order to account for losses on the secondary side 104 due to rectification performed by the rectifier 204, and to include this losses in the equation giving i_LED, the controller 108 can be configured to perform the following steps:

Moreover, the power P_rect of the rectifier 204 can be expressed as :

Prect ^— ILED 4V_b, wherein V_D is a diode voltage drop and the scaling factor 4 reflect how many diodes are used in the rectifier 204, namely, in this embodiment, 4. Furthermore, the load power can be expressed as:

Lload ⁼ ILED VLED wherein, according to the invention, the bus voltage supplying the half bridge switches Si and S₂ of the LLC converter 200 is measured, as well as the current flowing into the LLC half bridge .

Since the following equation holds: all that is left is to solve for the LED current i_LED by performing the lest step by the controller 108:

The LLC converter 200 can even comprise a smoothing circuit on the secondary side 104, the capacitor C₃ in the embodiment shown in Fig. 2, wherein the smoothing circuit is configured to smooth the ripple at the output of the LLC converter 200 in order to obtain a smoothed voltage.

The LLC converter 200 can further comprise an auxiliary winding coupled to a primary winding Li of a transformer T of the LLC converter 200.

Furthermore, according to an embodiment of the invention, the LED voltage V_LED is indirectly measured, indirectly meaning by an electric parameter tapped off the primary side 102 of the isolated LLC converter 200. Preferably, this is done using the auxiliary winding coupled to the primary winding LI of the transformer T.

Fig. 2a shows an alternative embodiment to Figure 2. The relevant difference to Figure 2 resides in the fact that the tapping unit 200a is connected as a shunt in series to the lower potential half-bridge switch S2. The tapping unit 200a acts as a current sensing block.

As the half-bridge comprising the switches SI, S2, of the resonant converter is clocked with approx. 50% duty cycle (with some dead time) , the current fed into the resonant converter flows during the switch on-times of the resp. switch through that switch. As it is a resonant converter the current flows during the first half through the first switch SI (upper) and second half back through the second (low side) switch S2 - thus sensing can be done at this sensing point in series to the switch S2.

The tapping unit (current sensing block) 200a of this embodiment is also designed for a differential current sensing. It comprises a shunt resistor Rs, two differential resistors Rfl, Rf2 connected by a capacitor Cf. The voltage across the capacitor Cf is lead to two output terminals Isnsl, Isns2 connected to corresponding input terminals of the control unit, such that the control unit can process these two differential sensing in order to obtain a value representing the current flowing into the converter.

Fig. 3 shows a schematic diagram of a method 300 for an LED converter 100. The method 300 comprises the steps of: supplying 302 terminals for driving an LED load 106, feedback-controlling 304 an LED current i_LED based only on feedback-control signals from a primary side stage 102; controlling 306 at least one switch Si, S2 of a switched resonant stage (preferably the switching frequency and/or the duty cycle) on the basis of the feedback-control signals .

The feedback-control signals comprise: a) a signal representing the voltage across the LED load VLED b) a signal indicating a DC supply voltage of the primary side VB_US; and c) a signal representing the current flowing into the LED converter i_PFC.

Fig. 4 shows schematic representation of an alternative embodiment of an LED converter 500 according to the invention.

The driver 500 can comprise a converter 200, for instance a half bridge LLC HB-LLC or LCC converter. The converter 200 can be any one of the converters 100 of the Figs. 1 to 2a. The converter 200 may comprise a primary side stage 102 and a secondary side 104. The primary side stage 102 comprises a primary winding LI of a transformer T and the secondary side 104 comprises a secondary winding L2 of the transformer T, whereby the secondary side 104 is magnetically coupled to the primary side stage 102 via the transformer T.

The driver 500 can further comprise an electromagnetic interference (EMI) filter 501 that forwards an input voltage to an activelely switched PFC 503, in particular a boost PFC circuit. The PFC 503 can in turn supply the primary side 102 of the converter 200 with a DC voltage V_Bu_s ·

The driver 500 can further comprise a control unit implemetee e.g. as an ASIC 508. The ASIC 508 can correspond to the control unit 108 or to a component of the control unit 108 from Figs. 1 to 2a. The ASIC 508 can be configured to perform a feedback control of the secondary side voltage of the converter 504 and/or the PFC circuitry 503 by means of a control of the switches of the half bridge of the half bridge LLC or LCC .

The control unit (ASIC) 508 may be supplied by a secondary side feedback signal, preferably form a secondary side rectification stage, via an isolation stage (transformer) 505b. However, the control unit is not supplied with a secondary side LED current sensing signal.

The driver 500 can further comprise a low voltage power supply 507 which can be configured to supply integrated circuits of the driver 500, e.g. the ASIC 508, with a low DC supply voltage .

The driver 500 can further comprise a microcontroller 509, which can be configured to control the ASIC 508 and bidirectionally communicate with the ASIC 508. For instance, the microcontroller 509 can send signals to the ASIC 508 in order to control the ASIC 508, e.g. adjust a lamp brightness. Further, the microcontroller 509 can receive signals from the ASIC 508, e.g. lamp fault detection.

In an embodiment, the driver 500 comprises a rectification and sensing circuit 506, which is isolated from the other components of the driver 500 and coupled to the converter 504 and ASIC 508 via two transformers 505a, 505b.

The driver 500 can further comprise a dimming interface 513, e.g. DALI interface, which is isolated e.g. via an optocoupler 511a, 511b from the microcontroller 509. Signals can be exchanged between the dimming interface 513 and the microcontroller 509 via two optocouplers 511a, 511b.

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

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. 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.

Claims

1. An LED converter (100), comprising: an isolated switched resonant stage (LLC) supplying terminals for driving an LED load (106), wherein an LED current I_LED is feedback-controlled based on feedback- control signals which do not comprise a secondary side current sensing signal; a control unit (108) controlling at least one switch (Si, S2) , preferably two alternatively clocked switches connected in series, of the switched resonant stage on the basis of the feedback-control signals supplied to the control unit (108), wherein said feedback-control signals comprise : a) a signal representing the voltage across the LED load VLED ;

b) a signal indicating a DC supply voltage of the primary side VB_US; and c.) at least one signal representing the current sensed at the primary side and representing the current flowing on the primary side of the isolation stage of isolated switched resonant stage.

2. The LED converter (100) of claim 1, wherein the LED converter (100) is an isolated resonant converter, e.g. half-bridge LLC or LCC converter.

3. The LED converter (100) of claim 1 or claim 2, wherein the at least one signal representing the current on the primary side of the isolation stage of isolated switched resonant stage may comprise two differentially sensed signals.

4. The LED converter according to any of the preceding claims, wherein the at least one signal representing the current on the primary side of the isolation stage of isolated switched resonant stage may be sensed by a differential current sensing block in series to a capacitor stabilizing a DC supply voltage of the converter, or in series to said at least one switch.

5. The LED converter (100) of claim 3 or 4, wherein the LED converter (100) further comprises a rectifier (204) and wherein the controller (108) is further configured to determine the current flowing through the LED i_LED on the basis of the following equation:

wherein V_D is a voltage drop of a diode of the rectifier (204), and "sensing" is the difference between two current sensing values of the differential current values produced by the tapping unit (200a) acting as a differential current sensing block.

6. The LED converter (100) of claim 6, wherein the controller (108) is further configured to estimate a power of the rectifier P_rect on the basis of the following equation:

7. The LED converter (100) of any one of the preceding claims

2 to 5, wherein the LLC converter further comprises a capacitor on the secondary side.

8. The LED converter (100) of claims 2 to 7, wherein the LLC converter further comprises an auxiliary winding coupled to a primary winding (LI) of a transformer (T) of the LLC converter .

9. A LED luminaire comprising an LED converter (100) according to any one of the preceding claims.

10. A method (300) for an LED converter (100), comprising: supplying (302) terminals for driving an LED load (106), feedback-controlling (304) an LED current i_LED based only on feedback-control signals; controlling (306) at least one switch (SI, S2) of a switched resonant stage on the basis of the feedback- control signals, wherein said feedback-control signals comprise : a.) a signal representing the voltage across the LED load

VLED ;

b.) a signal indicating a DC supply voltage of the primary side VB_US; and c.) a differentially sensed current signal representing the current flowing on the primary side of the isolation stage of isolated switched resonant stage.