Synchronous Flyback Converter for Driving LED Loads

Abstract

The present invention relates to a synchronous flyback converter (100) having a terminal for supplying power to an LED load. The synchronous flyback converter (100) comprises: a sensing winding (L w ), the sensing winding is coupled to the primary winding (L) of the transformer T of the flyback converter (100) p ); a control unit (106) that uses the feedback signal to control the primary winding (L) of the flyback converter (100) p ) is connected in series with a primary side switch (S1) for closed-loop control of the secondary side voltage of the flyback converter (100) by controlling the switching frequency and / or duty cycle of the primary side switch (S1), wherein the feedback signal is a voltage across the sensing winding (L) sampled once during the on-time of the secondary side switch (S2) of the flyback converter (100). w )'s sense voltage is derived.

Description

Technical Field

[0001] The present invention relates to a synchronous flyback converter having terminals for powering an LED load. Background Art

[0002] In the switched DC-DC power supplies of different kinds of electrical devices, a power rectifier is used to output a rectified output voltage. Typically, a diode is used on the secondary side to obtain the rectified output voltage.

[0003] A flyback converter with a following rectification stage is a typical known example of a switched-mode DC / DC converter.

[0004] A conventional flyback converter may include a primary winding of a transformer and a switch on the primary side, and a secondary winding of the transformer connected to one or more rectifier diodes and an output capacitor on the secondary side, through which a load may be connected. Such a converter has a large voltage drop across the diode.

[0005] A so-called synchronous flyback converter is provided with a preferably actively controlled switch, such as a FET transistor, on its secondary side. Such an arrangement will reduce the losses on the secondary side.

[0006] The control unit for controlling the primary side switch of the flyback converter is usually arranged to be at the potential of the primary side, so that feedback signals from the secondary side cannot be easily fed back to the control unit, for example for closed-loop control of the LED current and / or the output voltage of the flyback converter. Therefore, it is known to detect secondary side entities indirectly, for example to detect the secondary side output voltage indirectly, i.e. based on primary side measurements. In this way, no feedback signal has to be transmitted through the galvanic isolation of the flyback converter.

[0007] It is therefore an object of the present invention to provide an improved synchronous flyback converter, in particular a synchronous flyback converter with regard to indirect detection of a secondary side voltage (the output voltage of the flyback converter) which may power an LED load. Summary of the invention

[0008] According to a first aspect, the present invention relates to a synchronous flyback converter having terminals for powering an LED load, comprising: a sensing winding coupled to a primary winding of a transformer of the flyback converter; and a control unit (at the potential of the primary side of the flyback), the control unit using a feedback signal to control a primary side switch connected in series with the primary winding of the flyback converter for closed-loop control of the secondary side voltage of the flyback converter by controlling the frequency and / or duty cycle of switching of the primary side switch, wherein the feedback signal is derived from a sensing voltage across the sensing winding sampled once during an on (i.e., conduction) period of the secondary side switch of the flyback converter or at every nth on period thereof, n being an integer.

[0009] The control unit may control the secondary side switch such that there are time periods with a negative current on the secondary side. Thus, the current on the secondary side will flow in two different directions (during different time periods).

[0010] In one embodiment of the first aspect, the sense voltage is sampled at a point in time when the secondary side current through the secondary side switch is at least close to zero.

[0011] In an embodiment of the first aspect, at least close to zero corresponds to 10%, preferably 5%, of the total period of time that the switch is on.

[0012] In an embodiment of the first aspect, the control unit is designed to compensate any contribution of the secondary side resistive element to the sense voltage by applying a calibration value if the sense voltage is not sampled at a point in time when the secondary side current is zero.

[0013] In an embodiment of the first aspect, the control unit controls the secondary side switch.

[0014] In an embodiment of the first aspect, the converter comprises a comparator configured to compare the primary side current ip with a reference value iref.

[0015] In an embodiment of the first aspect, the control unit is configured to measure the time (Tp) that the primary switch is closed and calculate the ratio:

[0016] Ratio = △ / T p

[0017] The time Δ corresponds to the time during which the primary-side current ip is greater than the reference value iref.

[0018] In one embodiment of the first aspect, the control unit is configured to, at time point t s Measure the winding voltage:

[0019] t s =(TsΔ) / T p

[0020] Where T s corresponds to the time when the secondary switch is closed.

[0021] The control unit may be configured to send a s Adding a constant correction factor, which can optionally result in the time point t s decrease or increase.

[0022] The correction factor may be dependent on a sense signal indicative of one or more of, for example, a value of the load, for example a temperature of the LED load, and the like.

[0023] The time point for the sampling is preset to be within a preset minimum value, preferably between 1ns and 40ns, preferably adjustable, preferably the clock cycle of the control unit, and a maximum value of (TS-settlingTimeSH), where settlingTimeSH is the stabilization time of the sample-and-hold module in the signal path of the sampled signal.

[0024] According to a second aspect, the present invention relates to a method for a synchronous flyback converter having terminals for powering an LED load, comprising the following steps: using a feedback signal to control a primary side switch connected in series with the primary winding of the flyback converter for closed-loop control of the secondary side voltage of the flyback converter by controlling the frequency and / or duty cycle of switching of the primary side switch, wherein the feedback signal is derived from a sense voltage across the sense winding sampled once during the on-time of the secondary side switch of the flyback converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The present invention will be described below with reference to the accompanying drawings.

[0026] Figure 1 A synchronous flyback converter according to one embodiment is shown;

[0027] Figure 2 shows current in a synchronous flyback converter as a function of time according to one embodiment;

[0028] Figure 3 shows the voltage of a sense winding of a synchronous flyback converter as a function of time according to one embodiment;

[0029] Figure 4 shows current (a) and a comparator (b) in a synchronous flyback converter according to one embodiment;

[0030] Figure 5 shows the current and the output of a comparator in a synchronous flyback converter as a function of time according to one embodiment;

[0031] Figure 6 A schematic diagram showing a method of synchronizing a flyback converter according to an embodiment is shown, and

[0032] Figure 7 In particular, signals are shown which are supplied to and emitted by a control unit, which may be an ASIC. DETAILED DESCRIPTION

[0033] Aspects of the invention are described herein in the context of a synchronous flyback converter.

[0034] The present invention is described more fully below with reference to the accompanying drawings, in which various aspects of the invention are shown. However, the present invention can be embodied in many different forms and should not be construed as limited to the various aspects of the invention presented by the present disclosure. On the contrary, these aspects are provided so that the present disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The various aspects of the invention shown in the accompanying drawings may not be drawn to scale. On the contrary, the sizes of various feature structures may be enlarged or reduced for clarity. In addition, some drawings may be simplified for clarity. Therefore, the drawings may not show all the components of a given device.

[0035] Various aspects of a synchronous flyback converter will be presented. However, as will be readily appreciated by those skilled in the art, these aspects may be extended to various aspects of a synchronous flyback converter without departing from the invention.

[0036] It should also be understood that aspects of the present invention may include integrated circuits that are readily fabricated using conventional semiconductor technology (such as complementary metal oxide semiconductor technology, referred to as "CMOS" for short). In addition, aspects of the present invention may be implemented using other manufacturing processes used to manufacture optical devices as well as electrical devices. Reference will now be made in detail to specific implementations of exemplary aspects as illustrated in the accompanying drawings. The same reference numerals will be used throughout the drawings and the following detailed description to refer to the same or similar components.

[0037] Figure 1 A synchronous flyback converter 100 is shown according to one embodiment.

[0038] The synchronous flyback converter 100 includes a primary side 102 and a secondary side 104 of a transformer and has terminals for powering an LED load (LED).

[0039] Furthermore, the synchronous flyback converter 100 comprises:

[0040] - Sensing winding L w, the sensing winding is coupled to the primary winding L of the transformer T of the flyback converter 100 p ;and

[0041] a control unit 106 which is potentially located on the primary side of the transformer and controls the primary winding L of the flyback converter 100 p The primary-side switch S1 is connected in series, and the switching characteristic is based on the feedback signal for closed-loop control of the secondary-side voltage of the flyback converter 100 by controlling the switching frequency and / or duty cycle of the primary-side switch S1 .

[0042] The feedback signal is obtained from a voltage across the sensing winding L sampled during the on-cycle (conduction cycle) of the secondary side switch (S2) of the flyback converter 100 or once every nth cycle. w The sense voltage is derived from n, where n is an integer. Preferably, the sampling is performed near the end of the on-period.

[0043] like Figure 1 As shown, the flyback transformer T has a primary winding L p , one end of the primary winding is connected to the input voltage V B And the other end thereof is connected to the drain terminal of the primary switch S1 , which may be implemented by a MOS transistor.

[0044] The control unit 106 may be connected to a gate terminal of the primary switch S1 , and configured to issue a pulse signal to control a switching operation of the primary switch S1 .

[0045] When the primary switch S1 is turned on, the primary winding L p Receive input current i from the input terminal p And thus the energy is stored in the primary winding L p At the same time, the second secondary winding L which can be realized by a MOS transistor and connected to the flyback transformer T s The synchronous switch or secondary switch S2 is preferably disconnected by the control unit 106. Since the flyback converter is designed as a synchronous flyback converter 100 according to the present invention, the secondary switch S2 is provided on the secondary side 104 instead of the rectifier (such as it would be for a single switch flyback).

[0046] In addition, when the primary switch S1 is turned off, the synchronous or secondary switch S2 is turned on by the control unit 106, so that the current stored in the primary winding L p The energy in the flyback transformer T can be transferred to the secondary side 104. This induces the second secondary winding L s The high frequency voltage on the secondary side causes the secondary current to flow through the synchronous switch S2.

[0047] By repeated switching operation of synchronous switch S2, the second secondary winding Ls The high frequency voltage variation induced on the output capacitor C can be rectified into a DC voltage. The rectified DC voltage may exhibit ripple and therefore will usually be f Filtering to generate a stable output DC voltage V that is provided to the LED load (LED) out .

[0048] The control unit 106 may be configured to (indirectly) detect an actual value of the output voltage of the synchronous flyback converter 100 , compare it with a nominal value, apply a control algorithm to any deviation between the actual value and the nominal value, and control switching of the primary side switch depending on the result of the control algorithm.

[0049] The control unit 106 may be formed by an integrated circuit such as an ASIC (Application Specific Integrated Circuit), an FPGA (Field Programmable Array), or a microcontroller.

[0050] Figure 2 shows the primary side current i in the synchronous flyback converter 100 as a function of time t according to one embodiment. p and the secondary side current i s .

[0051] The object of the present invention is to make the LED voltage measurement sampling (for example, once per on-cycle of the secondary side switch, or only per n-th on-cycle) as close as possible to the secondary side current i s The time point when the secondary side switch S2 intersects the neutral line (i.e., at Figure 2 Time T zxs ) is carried out, and the secondary side current flows from the positive current flow direction to the negative flow direction (the negative flow direction is a smaller part).

[0052] Furthermore, the fact that the ratio of positive current to negative current on the primary side 102 is the same as the corresponding ratio on the secondary side 104 may be exploited.

[0053] Therefore, by monitoring the time period of negative or positive current on the primary side 102, the total on-time T p , to understand the total on-time T of the secondary-side switch s , then the zero-crossing time point of the current on the secondary side is T zxs It can be determined indirectly by multiplying the total on-time of the secondary side switch S2 starting from zero by the corresponding ratio of the primary side current.

[0054] Therefore, the LED voltage measurement sampling will always be performed at least very close to the zero crossing, which has the advantage that since no current flows on the secondary side 104 , the resistive elements on the secondary side 104 cannot influence the LED voltage measurement through their own voltage drop.

[0055] Very close to the zero crossing corresponds to 10% of the total period of the switch on time, preferably 5%.

[0056] During calibration after production of the synchronous flyback converter 100 , any contribution to the voltage measurement from the resistive secondary side elements may be measured and then used as a calibration value in subsequent measurements during operation of the synchronous flyback converter.

[0057] Therefore, the present invention also relies on synchronous flyback, wherein the auxiliary winding L coupled to the primary side 102 of the synchronous flyback converter 100 w Used for LED voltage measurement.

[0058] At a given point in time, during a switching on-cycle of the secondary side switch S2, a value representing a measurement of the LED voltage may be sampled.

[0059] Furthermore, the measurement can then be corrected by calibration values ​​determined at the end of production and therefore at the factory.

[0060] The closer the sampling time point is to the zero crossing of the secondary current, the less important the calibration value is.

[0061] Preferably, this calibration method is used for light loads, where the current decreases faster than at high loads, which results in a higher uncertainty about the exact zero-crossing time.

[0062] On the other hand, the method using an accurate zero-crossing sampling time is preferably used at higher loads.

[0063] Therefore, according to one embodiment, the above two specific implementation forms are used depending on the dimming level.

[0064] The LED voltage, which influences the steepness of the secondary-side current reduction, is essentially influenced by the LED load, which means that by changing the LED module this will change. Therefore, if a multi-load converter is foreseen, for the low load range the calibration method is preferably used, whereas if LED modules in the higher load range are used, the zero-crossing method is preferably used. In this embodiment, multi-load means various LEDs with different total forward voltages.

[0065] Dimming does not affect the steepness of the secondary-side current reduction, but rather the "duty cycle" of the positive to negative current.

[0066] The present invention advantageously uses this arrangement and improves the quality of the LED voltage measurement by having a dedicated sampling time point for each cycle in which the LED voltage measurement is taken.

[0067] In one embodiment, the synchronous flyback converter 100 is driven by a negative current i pIn this embodiment, then, when the primary current i p When it becomes positive for the first time, you can check i p .

[0068] In measuring p After the time is positive, the ratio of this time to the primary switch on time can be used to set the sample-and-hold current that should be drawn from the additional winding L w Capture the time of the voltage. By adjusting the time at the ratio, one can know roughly when the crossover will occur on the secondary side 104, and adjust the time accordingly.

[0069] This has the advantage that accurate measurements with information can be obtained.

[0070] Furthermore, this provides the advantage of achieving a more accurate current estimation and thus better light control performance.

[0071] Furthermore, this has the advantage of achieving a cheap way of determining the LED voltage.

[0072] In the following, for the sake of completeness, the steps leading to the determination of the LED voltage will be summarized.

[0073] Since the purpose is to measure only the LED voltage, voltage drops due to things like secondary FETs, resistive losses on the secondary side, etc. are not desirable. Furthermore, measuring the LED voltage from the primary side 102 has the advantage of being cheaper since no safe extra low voltage (SELVC) barrier needs to be crossed.

[0074] exist Figure 2 In, if i s = 0, no current flows on the secondary side 104, and therefore the LED voltage V LED No other voltage can be reflected.

[0075] Available voltage V w and a sample-and-hold chip with an analog-to-digital converter (ADC).

[0076] In order to obtain s , perform the following steps:

[0077] a) Using a shunt measurement of the current on the primary side 102, place the result Figure 4 (b) on the comparator 400 (see also Figure 5 );

[0078] b) Check when ZX (zero crossing) occurs;

[0079] c) Measure T p and T s ;

[0080] d) measuring the time interval Δ by means of, for example, a timer (see also Figure 5 to understand the definition of the time interval Δ); and

[0081] e) Calculate the ratio = Δ / Tp, and then from Figure 3 Set t s =T zxs =T s (Δ) / T p This corresponds to measuring the winding voltage V w The best time, the winding voltage V w Through the winding ratio and V LED Voltage related.

[0082] exist Figure 3 , shows the sense winding voltage V w , where V w The first part a corresponds to the input voltage V B * Factor, where the factor depends on the winding. V w The second part c corresponds to the reflected voltage on the secondary side, and the voltage V in b w Corresponding to V LED *factor.

[0083] Figure 6 A schematic diagram of a method 600 of a synchronous flyback converter 100 having terminals for powering an LED load is shown according to one embodiment.

[0084] The method 600 comprises the following steps:

[0085] - Using the feedback signal to control 602 the primary winding L of the flyback converter 100 p The primary side switch S1 connected in series is used to perform closed-loop control of the secondary side voltage of the flyback converter 100;

[0086] - Control 604 the frequency and / or duty cycle of switching of the primary side switch S1, wherein the feedback signal is obtained from a voltage across the sensing winding L sampled once during the on-cycle of the secondary side switch S2 of the flyback converter 100. w The sense voltage is derived.

[0087] Figure 7 Another more detailed embodiment of a synchronous flyback converter 100' is shown. The control unit 106' may be implemented as, for example, an ASIC (preferably) or a microcontroller.

[0088] The control unit has an output terminal OUTP which issues a command resulting in switching of the primary side switch S1 ′ of the synchronous flyback converter.

[0089] Furthermore, a shunt resistor R5 is connected in series with the primary side switch S1 '. Via the shunt resistor R5 and using an optional level adaptation circuit RISL6, R4, C1 , the primary side current signal is supplied to the control unit at the input terminal ISP.

[0090] The resistor R6 is supplied with a low DC voltage.

[0091] The output terminal OUT of the control unit issues a control signal for the secondary side switch S2 ′. A galvanically isolating coupler CP1 (eg an optocoupler) may be provided to transmit the control signal from the primary side to the secondary side switch across the isolation barrier ( ).

[0092] Furthermore, the control unit has an input terminal ISL for supplying it with a secondary side sense signal representing the LED current. A further galvanic isolation coupler CP2 (eg an optocoupler) is provided to transmit this feedback signal from the secondary side to the primary side across the isolation barrier ().

[0093] Finally, an auxiliary winding Lw' is shown coupled to the secondary winding Ls' and optionally the primary winding Lp' of the transformer T in order to sense a signal representative of the secondary side voltage on the primary side. w' For the LED voltage measurement according to the invention, the current states of the switches of the synchronous flyback are usually taken into account (these states are known to the control circuit 106 ′).

[0094] A voltage divider R1, R2, R3 is provided to feed this voltage signal across the auxiliary winding Lw' to the capacitor C R , the DC voltage of the capacitor is supplied to the input terminal VLS of the control unit 106'.

[0095] It is obvious from the above description that the LED voltage ( Figure 7 The exact timing of sampling the VLS in the primary side switch is important. Therefore, the timing of the primary side switch (e.g., the primary side on time TP, the secondary side on time TS, and the on time TP-T ZXP The accuracy of the measurement of the positive part of the LED is relevant. In the case of inaccurate measurements, the sensed voltage VLS may deviate from the true LED voltage.

[0096] Furthermore, there may be problems with extreme values ​​of the sampling time. For example, when there is no negative contribution of the primary side current (eg, at startup), and the secondary time is fixed, the sampling time point will be at the end of the secondary conduction phase.

[0097] In addition, it is necessary to consider the signal V SNSSettling time and control unit for the sample and hold process (within the control unit 106'). The sample and hold unit requires time to charge the sampling capacitor to the sensing voltage. If this time is not taken into account and the sampling trigger is close to the end of the secondary on-time (TS), inaccurate sampling can occur.

[0098] In this regard, it is suggested to add a correction factor to the determination of the sampling time. The correction factor is a constant time increment and can be set in steps, for example, between 10 Ns steps and 200 Ns steps, preferably in the timing steps of the control circuit, via a configuration unit CU internal to the control unit 106 or external to the control unit 106' (see Figure 7 ), for example in a microcontroller functionally connected to an input terminal INT of the control unit 106' (here an ASIC) via a serial interface SIF. It is foreseeable that the correction factor can have a positive or negative sign by changing the corresponding bit in the register of the configuration unit CU.

[0099] Figure 7 The control unit 106' in the embodiment may monitor the sampling time point to be higher than 10 ns to 200 ns, preferably constituting the duration of one clock cycle of the control unit (ASIC) as a minimum preset value.

[0100] In addition, a maximum preset value may be preset, which is the TS-settling time of the sample-hold unit of the control unit 106 ′.

[0101] The amount of the correction value and its sign may depend on an internal or external sense signal of the control unit 106' (eg, representing the temperature of the LED load) or the electrical load value in order to increase the voltage sensing accuracy over the entire load range.

[0102] All features of all embodiments described, illustrated and / or claimed herein may be combined with each other.

[0103] Although various embodiments of the present invention have been described above, it should be understood that these embodiments are given by way of example only and not by way of limitation. Without departing from the spirit of the scope of the present invention, various changes may be made to the embodiments disclosed in the present invention according to the disclosure herein. Therefore, the breadth and scope of the present invention should not be limited by any of the above-described embodiments. On the contrary, the scope of the present invention should be defined according to the following claims and their equivalents.

[0104] Although the present invention has been shown and described with respect to one or more specific embodiments, equivalent alternatives and modifications will occur to those skilled in the art after reading this specification and understanding of the accompanying drawings. In addition, although specific features of the present invention may be disclosed only with respect to several specific implementations, such features may be combined with one or more other features of other specific implementations according to the expectations and advantages of any given or specific application.

Claims

1. A synchronous flyback converter (100) having a terminal for powering an LED load, the synchronous flyback converter comprising: - Sensing winding (L w ), the sensing winding is coupled to the primary winding (Lp) of the transformer T of the flyback converter (100), - a control unit, the control unit using a feedback signal to control a primary side switch (S1) connected in series with the primary winding (Lp) of the flyback converter (100) for closed-loop control of the secondary side voltage of the flyback converter (100) by controlling the frequency and / or duty cycle of switching of the primary side switch (S1), wherein the feedback signal is obtained from a voltage across the sensing winding (Lp) sampled by the control unit during an on-cycle of the secondary side switch (S2) of the flyback converter (100) or once every n-th on-cycle thereof. w ), n is an integer, and - a comparator (400) configured to convert the primary side current i p With reference value i ref For comparison, wherein the control unit controls the secondary side switch (S2) so that there is a time period with a negative current on the secondary side, The control unit is configured to measure the time T of the primary side switch being closed. p And calculate the ratio: Ratio = Δ / T p , The time Δ corresponds to the primary side current i p Greater than the reference value i ref The time interval The control unit is configured to s Measure winding voltage: t s =(T s Δ) / T p Where T s Corresponds to the time when the secondary side switch (S2) is closed.

2. The synchronous flyback converter (100) according to claim 1, wherein the sense voltage is sampled at a point in time when the secondary side current through the secondary side switch (S2) is at least close to zero.

3. The synchronous flyback converter (100) of claim 2, wherein at least close to zero corresponds to 10% of the total period of the switch on-time.

4. A synchronous flyback converter (100) according to any one of claims 1 to 3, wherein the control unit is designed to compensate for any contribution of the secondary side resistance element to the sensing voltage by applying a calibration value if the sensing voltage is not sampled at a time point when the secondary side current is zero.

5. The synchronous flyback converter (100) according to any one of claims 1 to 3, wherein the control unit controls the secondary side switch (S2).

6. The synchronous flyback converter (100) according to claim 1, wherein the control unit is configured to provide a synchronous flyback converter (100) to the time point t s A constant correction factor is added which can lead to a decrease or an increase in the time point ts.

7. The synchronous flyback converter (100) of claim 6, wherein the correction factor is dependent on a sense signal indicative of one or more of a value of the load and a temperature of the LED load.

8. The synchronous flyback converter (100) according to claim 1, wherein the time point for the sampling is preset to be within a preset minimum value and T s - a maximum value of settlingTimeSH, the preset minimum value corresponds to the duration of one clock cycle of the control unit and is between 1 ns and 40 ns, wherein settlingTimeSH is the stabilization time of a sample-and-hold module of the control unit that processes the sampled signal.

9. The synchronous flyback converter (100) of claim 3, wherein at least close to zero corresponds to 5% of the total period of time that the switch is on.

10. A method (600) for a synchronous flyback converter (100) having terminals for powering an LED load, the method comprising: - using a feedback signal to control (602) a primary side switch S1 connected in series with a primary winding (Lp) of the flyback converter (100) for closed-loop control of a secondary side voltage of the flyback converter (100) by controlling (604) a frequency and / or a duty cycle of switching of the primary side switch (S1), wherein the feedback signal is a voltage across a sensing winding (Lp) sampled once during the on-time of a secondary side switch (S2) of the flyback converter (100). w ), the secondary side switch (S2) is controlled so that there is a time period with a negative current on the secondary side, - The primary side current i p With reference value i ref For comparison, - Measure the time T during which the primary switch is closed p And calculate the ratio: Ratio = Δ / T p , The time Δ corresponds to the primary side current i p Greater than the reference value i ref The time interval - At time t s Measure winding voltage: t s =(T s Δ) / T p Where T s Corresponds to the time when the secondary side switch (S2) is closed.

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