Flyback converter and LED driver using the same

Abstract

A two-switch flyback converter in which the output current in the operating region where a constant output current is required can be regulated from the primary side. The output current is constantly regulated regardless of the input voltage and the output voltage by compensating the peak current in the primary winding with the average current in the clamping diode or the conduction duration of the clamping diode.

Description

Technical Field

[0001] The present invention relates to a flyback converter for use, for example, in an LED driver. Background Art

[0002] Flyback converters are used for AC / DC and DC / DC conversions, providing galvanic isolation between the input and any output. The flyback converter acts as a buck-boost converter, the inductor is split to form a transformer, so the voltage ratio is doubled and has the added benefit of isolation.

[0003] In the on-state of the converter, energy is transferred from the input voltage source to the transformer, during which time the output capacitor supplies energy to the output load. In the off-state, energy is transferred from the transformer to the output load (and the output capacitor). This is called the freewheeling phase.

[0004] One design of particular interest is the dual switch design. This utilizes two transistors between the input voltage and the transformer so that the total voltage stress is evenly divided across the two transistors. Instead of converting the leakage energy into losses, the leakage energy is returned to the input supply via two diodes. The diodes also clamp the drain-source voltage of both transistors to the input voltage, so the voltage rating of the transistors can be selected based on the input voltage without requiring a large margin.

[0005] In low to medium power applications such as switched mode power supplies used in LED drivers, dual switch flyback converters have become the preferred choice of DC-DC conversion topology compared to the resonant half-bridge converter family. The dual switch design provides high power density, low voltage stress, reliable structure and simple control scheme. A prior art document showing a dual switch flyback converter for driving LEDs is US 2012 / 0176060A1. Another prior art showing such a converter is US20100254167A1, which has a sensing element for sensing zero voltage and achieving zero voltage switching. US6175484B1 discloses a converter that can sense the peak current in the main inductor in the charging phase and can sense the current in the freewheeling phase.

[0006] In LED driver applications, current regulation is required by which the switching mode power converter is controlled to deliver the required LED current on the secondary side. However, primary side current regulation is preferred to control the LED current and voltage from the primary side. It is more cost-effective and robust compared to secondary side regulation because firstly, no high voltage or common mode isolation of the control circuit is required in the primary side control method and secondly, the auxiliary circuit arrangement for powering the control part is simplified. Furthermore, in case the regulation circuit arrangement is placed on the primary side, the regulation circuit arrangement is able to process any information from the mains in a very simple and efficient way.

[0007] Typically, primary-side regulation in a single-switch flyback converter uses the peak current in the primary winding and the sensed duty cycle of the output diode to derive a signal representing the average output current. The peak current in the primary winding of a single-switch flyback converter is not the same as the peak current in the secondary winding, but the difference between the two currents is typically a fixed value over the entire operating range. Therefore, a fixed correction can be used to compensate for the difference.

[0008] However, in a two-switch flyback converter, the demagnetization voltage of the leakage inductor has a strong dependence on the input voltage and the load voltage. If the input voltage is high, the inductor is intended to discharge more on the secondary side; otherwise it tends to leak power to the primary side. Therefore, the difference between the peak currents in the primary and secondary windings cannot be compensated with a fixed correction. This will be discussed in detail below.

[0009] Therefore, there is a need for a flyback converter circuit that can achieve efficient primary side regulation and can be implemented based on a two-switch flyback topology. Summary of the invention

[0010] The idea of ​​the invention is to provide a two-switch flyback converter in which the output current is regulated from the primary side. To make the current regulation more accurate, the peak current measured in the primary winding is compensated by taking into account the clamping diode current. It can be measured or it can be derived from the conduction duration of the clamping diode.

[0011] The invention is defined by the claims.

[0012] According to an example of one aspect of the present invention, there is provided a flyback converter, comprising:

[0013] a transformer including a primary side for connection to an input power source and a secondary side for connection to a load;

[0014] a first switching circuit on the primary side adapted to allow the primary side to accumulate power from the input power source during a charging phase and adapted to allow the secondary side to release a portion of the accumulated power to the load during a freewheeling phase;

[0015] A first sensing circuit, configured to sense a primary-side current during a charging phase; and

[0016] a second switching circuit on the primary side for allowing another portion of the power accumulated on the primary side to flow back to the input power supply as leakage current in the freewheeling phase;

[0017] a second sensing circuit for deriving a parameter dependent on the leakage current; and

[0018] a controller, configured to derive a control signal for controlling the first switching circuit according to outputs of the first and second sensing circuits;

[0019] The converter may further include a time detection circuit, the time detection circuit being suitable for detecting a time period related to the freewheeling phase, and the controller being suitable for calculating: a difference between a total charge released from a peak current of the primary winding for a time period related to the freewheeling phase and a charge released by a leakage current in the freewheeling phase, wherein the difference is associated with the secondary side current.

[0020] The flyback converter utilizes two primary side sensing methods - the primary side transformer current and the leakage current (the leakage current is the current generated during demagnetization induced by leakage). In this way, the primary side sensing is able to more accurately determine the current on the secondary side, thereby providing improved primary side regulation of the output current on the secondary side. The converter is of particular interest when the leakage current is not determined solely by the converter and its load (so that the difference between the primary side current and the secondary side current is not constant) but also depends on changes in the input voltage (such as is the case with a two-switch flyback topology). The second sensing circuit provides a variable compensation for the primary side current sensing to enable more effective primary side current regulation.

[0021] The second sensing circuit may be based on a measurement of the amplitude of the leakage current, or determine the leakage current based on a time period over which the leakage current flows. These options will be apparent from the following discussion.

[0022] The controller is suitable, for example, for:

[0023] determining a secondary side current at the secondary side based on the sensed primary side current and a parameter dependent on the leakage current; and

[0024] A control signal for controlling the first switching circuit is derived as a function of the determined secondary-side current and the reference current.

[0025] Thus, the two sensing circuits can provide a determination of the secondary side current, which can then be used as a feedback control parameter for regulating the secondary side current (ie, the load current) without the need for load sensing on the secondary side.

[0026] The first switching circuit may include synchronized first and second transistors between the positive input and the negative input, the synchronized first and second transistors are connected in series on each side of the primary side winding of the primary side, and the second switching circuit includes first and second diodes, the first and second diodes are connected in series between the negative input and the positive input, on each side of the primary side winding and in series with the input power supply, thereby forming an H bridge, and the control signal is a gate or base drive signal for the transistor, and the flyback converter further includes:

[0027] The flyback converter also includes an input power supply, which includes:

[0028] The input capacitor between the positive input and the negative input, and the second switching circuit is adapted to allow the leakage current to flow back to the input capacitor, and the input capacitor (C in ) is suitable for connection to a time-varying voltage supply.

[0029] This architecture defines a more specific circuit for a two-switch flyback converter, which is a high efficiency, high power density, and low operating voltage converter design. This converter design typically requires secondary-side regulation, but the sensing method enables effective primary-side regulation.

[0030] The flyback converter may further include an output diode between the secondary side and the output load connection, the output diode being adapted to block the inductive power from flowing to the load during the charging phase and to allow the inductive power to flow to the load during the freewheeling phase.

[0031] The output diode is conducting during the freewheeling phase. Thus, the freewheeling phase may be defined as the time period during which the output diode is conducting.

[0032] The first sensing circuit may include:

[0033] a primary winding current sensing resistor connected in series with the first switching circuit and the primary side winding; and

[0034] A sample and hold circuit is used to sample a peak voltage across the primary winding current sensing resistor, the peak voltage representing a peak current through the primary side winding.

[0035] The first sensing circuit samples the peak primary side current and provides it as a first feedback parameter, but scaled according to the time of the freewheeling phase or a portion of the freewheeling phase.

[0036] Preferably, the controller is adapted to derive a control signal (V C ).

[0037] The second sensing circuit may include:

[0038] a freewheeling current sensing resistor, connected in series with the second switching circuit and the primary side winding, adapted to sense the amplitude of the leakage current in the freewheeling phase as a parameter;

[0039] Filters; and

[0040] A comparator, wherein an output of the comparator is provided as an output of the second sensing circuit.

[0041] The comparator enables scaling of the sensed freewheeling current and adding an offset. It thus acts as an adder / amplifier / comparator. The scaling and offset are selected so that an accurate representation of the secondary side current is obtained when used to modify the sensed primary side current.

[0042] The filter output is provided, for example, to a comparator, which compares the filter output with a reference and comprises a feedback loop having a certain gain to the filter output.

[0043] In this way, the output of the second sensing circuit is used to provide an additional compensation input to the primary side current sensing to make the estimate of the secondary side current more accurate.

[0044] The time detection circuit is suitable, for example, for detecting:

[0045] The first part of the freewheeling phase is taken as a parameter, during which the leakage current on the primary side decreases from a peak value to zero; and

[0046] The second part of the freewheeling phase, the second part is from the leakage current on the primary side falling to zero to the secondary side current to the load falling to zero.

[0047] The peak primary current is therefore used as the feedback control input. The peak value is used only during the freewheeling phase. By taking into account the time period of the freewheeling phase (when the primary current drops to zero, and when the output diode current drops to zero), the compensation can be made accurate. By providing an accurate estimate of the secondary current, a feedback control loop can be established.

[0048] The controller may include a filter that is charged and discharged according to the detected time portion.

[0049] The controller may also include:

[0050] The charging circuit is used to provide the sampled peak voltage to the filter during the second part of the freewheeling phase from when the leakage current on the primary side drops to zero to when the secondary side current to the load drops to zero.

[0051] This is part of the freewheeling phase during which the current to the load drops to zero.

[0052] The chopper circuit may also be used to provide a ground signal to the filter outside of this portion of the freewheeling phase.

[0053] In this way, the freewheeling current itself does not need to be sensed, since the modification of the primary side sensed current can be obtained based on the on-time period during which the freewheeling current flows.

[0054] The chopper circuit provides the sensed peak primary current to the filter for a certain period of time, which can be considered as a separate output diode stage, during which the output diode conducts current from the secondary side to the load, and the diode of the second switching circuit is no longer turned on, that is, the primary side freewheeling has been completed.

[0055] It can be shown that this manipulation of the peak primary current, combined with the time period of the individual output diode phases, also enables an estimate of the secondary current (without actually measuring it) so that a feedback control loop can be established.

[0056] The time detection circuit may include:

[0057] A zero-crossing detection circuit, and a time detection circuit for deriving a time period according to a zero-crossing signal; or comprising

[0058] A zero crossing detection circuit for detecting a zero crossing signal and a latch circuit for deriving a time period, the latch circuit having as inputs a base or gate drive signal for the first switching circuit and the detected zero crossing detection signal.

[0059] The use of a latch resists resonances that may be caused in discontinuous operation mode and prevents false triggering.

[0060] The flyback converter may include a regulated current driver, wherein a control signal for controlling the first switching circuit is used to regulate an output current delivered by the secondary side.

[0061] The present invention also provides an LED driver, comprising the flyback converter as described above, the LED driver comprising an input for connecting to an external power source and an output for connecting to an LED load.

[0062] For example, the driver is of particular interest for power ratings in the range of 30W to 300W.

[0063] The present invention also provides a lighting device, which includes the LED driver defined above and an LED load connected to the LED driver.

[0064] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] For a better understanding of the invention, and in order to show more clearly how the invention may be implemented, reference will now be made, by way of example only, to the accompanying drawings, in which:

[0066] Figure 1 A known dual-switch flyback converter is shown;

[0067] Figure 2A known single-switch flyback converter with primary-side regulation is shown;

[0068] Figure 3 A known single-switch flyback converter with secondary-side regulation is shown;

[0069] Figure 4 A first example of a primary side sensing circuit for application to a flyback converter is shown;

[0070] Figure 5 is a timing diagram used to illustrate parameters that allow the operation of a circuit to be understood;

[0071] Figure 6 A second example of a primary side sensing circuit for application to a flyback converter is shown;

[0072] Figure 7 A third example of a primary side sensing circuit for application to a flyback converter is shown; and

[0073] Figure 8 A latch circuit is shown that may be used as part of a circuit implementation. DETAILED DESCRIPTION

[0074] The present invention will be described with reference to the accompanying drawings.

[0075] It should be understood that the detailed description and specific examples, although indicating exemplary embodiments of the apparatus, system and method, are intended for illustrative purposes only and are not intended to limit the scope of the invention. These and other features, aspects and advantages of the apparatus, system and method of the present invention will be better understood from the following description, the appended claims and the accompanying drawings. It should be understood that the drawings are only schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the drawings to indicate the same or similar parts.

[0076] The present invention provides a two-switch flyback converter, in which the output current in an operation region requiring a constant output current can be regulated from the primary side. By compensating the peak current in the primary winding with the average current in the clamping diode or the conduction duration of the clamping diode, the output current can be constantly regulated regardless of the input voltage and the output voltage.

[0077] Figure 1A known two-switch flyback topology is shown. The circuit comprises a first branch of transistor 100 and diode 101, and a second branch of transistor 102 and diode 103. The branch is connected to the primary side of a transformer 104. The secondary side of the transformer is connected to a load 106 represented as a capacitor through an output diode 105. The branch is connected in parallel between input power rails 107, 108. An optional additional inductor 109 is shown in series with the transformer primary winding. Input capacitor C in The charge flowing through the diode is received during the freewheeling phase.

[0078] The junction between transistor 100 and diode 101 is connected to one end of the primary winding of transformer 104, and the junction between transistor 102 and diode 103 is connected to the other end of the primary winding of transformer 104. They form an H-bridge circuit with diagonal transistors and diodes.

[0079] Thus, the primary side of the transformer is used to connect to the input power supply, and the secondary side is used to connect to the load. The transistors 100, 102 implement a first switching circuit on the primary side, which is suitable for allowing the primary side to accumulate power from the input power supply in the charging phase, and for allowing the secondary side to release a portion of the accumulated power to the load in the freewheeling phase.

[0080] Diodes 101, 103 implement a second switching circuit on the primary side to allow another portion of the accumulated power at the primary side to flow back to the input power supply to be stored as leakage current on the input capacitor C during the freewheeling phase. in superior.

[0081] For comparison and illustration, Figure 2 A single switch flyback converter with primary side regulation is shown. There is only a single transistor 110 and a diode 111. The diode 111 clamps the voltage of the transistor 110 to a level above the supply via a level shifting circuit 115.

[0082] Usually, if Figure 2 As shown in , the primary side regulation in the single switch flyback converter utilizes the peak current in the primary winding (or the peak current in the active device 110) sensed by the resistor 117 and the detected duty cycle of the output diode 105 (sensed from the induced voltage on the auxiliary winding 116 at the zero crossing detection (ZCD) terminal through a voltage divider) to derive a signal representing the average output current.

[0083] The controller 118 controls the single transistor 110 based on the detected duty cycle (based on zero-crossing detection of the auxiliary winding voltage) and the sensed primary-side current.

[0084] Due to the presence of leakage inductance in the power transformer, the peak current in the primary winding of the single-switch flyback converter is obviously different from the reflected peak current in the secondary winding. This means that there is a gap between the derived output current and the actual output current. From the control system point of view, there is an offset in the sensing of the feedback signal, so there is an error between the reference and the controlled output. In order to eliminate this error, the controller must provide some compensation means. However, the primary side current sensed in the single-switch flyback converter has a fixed relationship with the secondary side current, so compensation is easy to achieve. In particular, because of the principle of operation of the transformer, the voltage on the primary winding is proportional to the voltage on the secondary winding during the freewheeling phase, so the demagnetization voltage of the leakage inductor is independent of the load voltage.

[0085] In existing implementations of primary-side regulated single-switch flyback converters, fixed compensation is usually applied.

[0086] For the sake of completeness, Figure 3 A secondary side regulation method is shown. The converter is simply represented as a primary side block 120 and a secondary side block 122. High voltage common mode isolation 128 is required, which is achieved by an optical transmitter or a magnetic transmitter. This provides isolated feedback of the secondary side current sensed across a sense resistor 123. In the example shown, a drive circuit 124 drives a photodiode 128 from a low voltage supply 129. The photodiode provides an optical signal to a phototransistor at the primary side.

[0087] The advantage of primary side regulation is that the driver circuit 124 is not necessary and the low voltage supply 129 for the driver circuit is simplified. Furthermore, with the regulation circuitry placed on the primary side, the regulation circuitry can process any information from the mains (such as timing information) in a very simple manner.

[0088] When controlled in critical conduction or discontinuous mode, if the input voltage and output voltage are constant, the flyback converter (whether in single-switch or dual-switch configuration) is essentially a current source with constant linear control, which makes the control of the circuit easier to implement, especially for LED driver applications that require constant output current.

[0089] Compared with conventional single-switch flyback converters (such as Figure 2 Compared to the dual-switch flyback converter shown in FIG. 1 , the voltage stress on the active devices of the dual-switch flyback converter is limited to the input power rails ( 107 and 108 ), so that low-cost devices with normal voltage ratings can be used. The present invention implements a variable correction that compensates for the difference between the sensed current in the primary side and the controlled current in the secondary side, so that cost-effective and robust primary-side regulation can be applied to a compact, efficient and reliable dual-switch flyback converter.

[0090] In the case of a two-switch flyback, the demagnetization voltage of the leakage inductor is the difference between the input voltage (between power rails 107 and 108) and the reflected load voltage. The load voltage can still be predicted, especially if the load is an LED with a stable forward voltage; however, the rails and capacitor C in The input voltage on the flyback converter is more determined by the supply to the driver, which varies with time. Therefore, the demagnetization time of the leakage inductor (which ultimately translates into a difference in peak currents in the primary and secondary windings) depends heavily on the input voltage, which is more converter and load independent. In the case of fixed compensation, the output current in a single-switch flyback converter is load dependent, the same principle will not apply to a dual-switch flyback converter and the load regulation of the converter needs to be improved.

[0091] Figure 4 A first example of a circuit for implementing a primary side regulation scheme is shown for use with Figure 1 A dual-switch flyback converter is used in conjunction with

[0092] The circuit includes a first sensing circuit 400 to sense the primary side current during the charging phase. The circuit senses the current flowing through transistor 100 and therefore the current flowing through the primary side winding. Of course, the same current flows through transistor 102, so sensing can be performed anywhere along the current path. It includes a current sensing resistor 200 (R csp ).

[0093] The second sensing circuit 420 is used to derive a parameter that depends on the leakage current. This example of a second sensing circuit senses the current flowing through the diode 103, and therefore senses the demagnetization leakage current. Of course, the same current flows through the diode 101, so sensing can be performed anywhere along the current path. It includes a current sensing resistor 201 (R csn ) and a circuit for processing the sensed current. In this example, the processing circuit includes filters 202, 203 and an operational amplifier 206, which has resistors 204 and 205 to provide gain tuning / feedback compensation functions for the operational amplifier 206.

[0094] The controller 210 is used to derive a control signal V for controlling the transistors 100 and 102 (ie, the first switching circuit) according to the outputs of the first and second sensing circuits 400 and 420. C The controller 210 is implemented as two operational transconductance amplifiers (OTA) 211 , 212 , which condition the output signal.

[0095] Note that the boundary between the controller 210 and the sensing circuitry is somewhat arbitrary and is used for illustration purposes only. In practice, there is one integral circuit that provides the combined sensing and processing functionality.

[0096] The difference in the output currents of the two OTAs is integrated over a phase compensation network (in this embodiment, phase compensation is performed by a single capacitor 220). Note that the output of OTA 211 is a positive charging current and the output of OTA 212 is a negative discharging current relative to capacitor 220. A control voltage V is then generated. c , which controls the on-time of active devices 100 and 102 and compensates for leakage current errors. In this way, the average current in diode 103 is used to compensate for leakage current errors. Controller 210 is a typical structure of a primary side current regulation control IC for a flyback converter. Therefore, the present invention can be used under any conditions where such an IC can be applied. Therefore, the flyback converter using this circuit utilizes two primary side sensing methods-the primary side transformer current and the leakage current (which is the current generated during the demagnetization of the leakage inductance). In this way, the primary side sensing can more accurately determine the current on the secondary side, thereby providing improved primary side regulation of the output current on the secondary side. The second sensing circuit 420 provides variable compensation to the primary side current sensing to enable more effective primary side current regulation. When the control voltage V c When it increases (meaning that the leakage current increases and the secondary current is insufficient), the current regulation circuit can increase the duty cycle (duration of the charging phase) of the flyback converter so that more energy can be delivered to the secondary side; otherwise the duty cycle can be reduced.

[0097] Figure 4 An example of is based on compensating leakage current, where compensation is performed based on the average current of the clamping diode 103 being sensed.

[0098] The first sensing circuit 400 includes a sample and hold structure of a switch 216 and a capacitor 215. Optionally, gain can be applied by an amplifier and a buffer can be provided. The held signal is applied to a chopper switch 214, which applies the output to an RC filter 213 during a specific time interval, as described below. The output is provided to an OTA 212 of a controller 210.

[0099] The second sensing circuit 420 comprises an RC filter 202, 203 which receives the sense current signal of the clamping diode 103. The filtered signal is applied to an amplifier 206 which performs an addition and scaling function. It adds the signal to a reference signal "reference". This provides a corrected reference signal which then sets a compensation signal at the output of the amplifier 206 which is then applied to the first OTA 211 of the controller 210. Thus, the sense current from the first sensing circuit 400 is compared by the controller with a reference which depends on the leakage current.

[0100] The controller 210 generates a control voltage V for the power converter. C , control voltage V C The leakage current is thus compensated. The control voltage is held on capacitor 220.

[0101] Now I will explain in more detail Figure 4 The definitions shown in Table 1 below will be used to define the circuit operation. Figure 5 Some of the definitions are shown in the definition. Figure 5 In FIG. 5 , the primary side current is shown as curve 500 . The leakage current (through the clamping diodes 101 , 103 ) is shown as curve 502 . The secondary side current is shown as curve 504 .

[0102] Table 1

[0103]

[0104] At time interval t 2 to 3 During this period, the circuit can be defined as being in the "output diode stage alone".

[0105] Any real transformer can be modeled according to the cantilever model, so the turns ratio can be considered as the equivalent turns ratio in the cantilever model rather than the ratio of the physical turns.

[0106] from Figure 5 The basic geometric shapes that can be exported are:

[0107]

[0108] This can be rewritten as:

[0109]

[0110] Where T sw =t 3 -t 1 +T res , and T res is the quasi-resonance time.

[0111] At time t 3 After that, the converter enters the so-called "quasi-resonant" time, where both the active devices (100, 102) and the output diode 105 are turned off. In critical conduction mode (CRM), T res The state ends at t 3 +T′ res , T′ res is the true quasi-resonant time, when the voltage on the lower active device cycles (resonates) to the valley of the sinusoid. In discontinuous mode (DCM), there is another controlled delay (T) after the voltage on the lower device reaches its valley before the next cycle begins. delay ). Therefore, under our conditions, we can use T res =T′ res +T delay Define T for both CRM and DCM res :

[0112] The left side of this equation is the reflected output current I out .

[0113] Therefore, divided by 2T SW The purpose is to make the left side of the equation equal to the output current I out , and this is the parameter to be controlled in a current regulating driver. The second term on the right hand side is the average current in the clamping diode, I dch .

[0114] Let the reference signal V ref =I out ·R scalo , where R scale is an arbitrary transimpedance depending on the application. This means that the previous equation can be replaced by:

[0115]

[0116] This can be rearranged as:

[0117]

[0118] Therefore, choose V ref level so that it is equal to the desired output current multiplied by the scaling resistor R scale Therefore, V ref It is used to set the output current I out control input.

[0119] In particular, the reference signal V ref So the desired output current I out Primary-side regulation is used by applying the appropriate Vref value to set the output current.

[0120] This last equation is given by Figure 4 Circuit implementation.

[0121] The first sensing circuit 400 provides I ppk x R csp The current sensing voltage is based on the current flowing through the resistor R csp The voltage of the peak current is sampled and stored on capacitor 215. The chopper operates by only 3 The output is applied to the filter to apply the factor t 3 / T sw .

[0122] I dch R csn is a voltage representing the average current in diode 103, so before reaching the operational amplifier ( Figure 4 The reference signal should be pre-scaled to R through a resistive voltage divider before the non-inverting input port of 206). csn / R scale , and the gain of amplifier 206 should be 2R csp / R csn .

[0123] The averaging is accomplished by filters 202, 203. Amplifier 206 implements R csn / R scale The gain and the V ref The addition of items.

[0124] The controller 210 adjusts the control voltage V in this way. C to make the two inputs equal. Therefore, the effect of feedback control is to provide a control voltage V C , so that the two inputs are equal, that is, satisfying the above equation.

[0125] Figure 6 Shows Figure 4 The second sensor circuit 420 is identical.

[0126] In the first sensor circuit 400, the sampling and holding function is implemented by a current mirror 600 including two NPN transistors. The peak current in the transistor 100 is held in the first filter circuit C 1 and R 1 superior.

[0127] The chopper is implemented by a comparator 602 having a reference input in the form of a zero crossing detection signal ZCD. The signal ZCD has the same phase as the voltage on the secondary winding of the transformer and is obtained, for example, from Figure 2 The auxiliary winding shown in FIG.

[0128] When the output diode 105 is turned on, the comparator 602 outputs the resistor R 5a The peak holding current is then amplified to the output of the operational amplifier 604. In other cases, the output of the comparator 602 is in an open drain configuration and the output of the amplifier 604 is low. The chopped signal is then pulled to ground by the second filter circuit R 2 and C 2 are averaged and the output V C Generated by operational amplifier 606.

[0129] Figure 4 and Figure 6 The example of is based on monitoring the average current in the clamping diode. However, the average current in the clamping diode is not a necessary input to achieve primary side regulation.

[0130] Instead, the conduction duration of the clamping diode can be used.

[0131] because:

[0132]

[0133] The reflected output current can be found by the following formula:

[0134]

[0135] Figure 7 An alternative embodiment is shown which avoids the need for current monitoring of the clamping diode current and is instead based on the above equation.

[0136] As in Figure 4 As in the circuit, a current sensing resistor of 200 (R csp ) to measure the current through transistor 100 (and therefore through 102). The peak current is again sampled and held by switch 216 and capacitor 215.

[0137] The second sensing circuit 420 again derives a parameter that depends on the leakage current. However, this does not involve measuring the current, but rather obtaining timing information. In particular, the timing instant t 2 and t 3 is used as a parameter depending on the leakage current.

[0138] Usually, there is a time detection circuit which is suitable for detecting the time difference between the freewheeling phase (0-t 3) and this also applies to Figure 4 and Figure 6 For the circuit. Figure 6 The time detection circuit (which can be considered as a part of the second sensing circuit 420) is suitable for detecting the freewheeling phase (0-t 3 ) first part (0-t 2 ), in the first part (0-t 2 ), the leakage current on the primary side decreases from the peak value to zero. This is used as a parameter that depends on the leakage current. 3 ) 2 -t 3 )The time from when the leakage current on the primary side drops to zero to when the secondary side current to the load drops to zero is also detected.

[0139] Figure 7 The circuit includes a first (high-side) switch 232 for coupling the output of the first sensing circuit 400 to a first (high-side) switch 232 having a voltage output V 214 The input of the output filter 214 is connected to ground. A second (low side) switch 233 is provided for coupling the input to the output filter 214 to ground.

[0140] When t 2 <t<t 3 When , switch 232 is turned on and switch 233 is turned off (condition 230), otherwise, switch 233 is turned on and switch 232 is turned off (condition 231).

[0141] In this way, a signal V representing the average output current is generated across the RC filter 214. 214 The amplifier 234 with feedback structure 235 generates a control voltage V C . Control voltage V C A higher R means a lower secondary current, and the current regulation circuit can increase the duty cycle (duration of the charging phase) of the flyback converter so that more energy can be delivered to the secondary side; otherwise the duty cycle can be reduced.

[0142] for Figure 7 circuit, the following equation for the average voltage across the RC filter 214 is true:

[0143]

[0144] Solving this equation leads to:

[0145]

[0146] therefore:

[0147]

[0148] In this way, a signal representing the average output current is established across the RC filter 214. It is based on determining the conduction time interval of the clamping diode.

[0149] In both embodiments above, there must be a signal indicating the diode phase (the time interval when either diode is conducting).For a two-switch flyback converter operating in critical conduction mode, it is reasonable to use a zero crossing detection (ZCD) signal.

[0150] In discontinuous mode including a resonant phase, an unwanted equivalent ZCD signal is usually generated during the resonant phase, which can erroneously trigger the chopper circuit. This in turn results in a higher feedback signal and a lower controlled output current.

[0151] Improvements can be obtained by introducing memory devices such as SR latches, as Figure 8 The “set” input is triggered by the falling edge of the low-side gate drive, while the “set” input is triggered by the falling edge of the zero-crossing ZCD signal.

[0152] The present invention may be implemented within an integrated circuit or discretely.

[0153] The present invention is of particular interest to dual-switch flyback platforms, for example for mid-range to high-end LED driver products with drive powers ranging from 30 W to 300 W, for both indoor and outdoor LED lighting applications.

[0154] By studying the drawings, the disclosure and the appended claims, those skilled in the art can understand and implement variations of the disclosed embodiments when practicing the claimed invention. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit can implement the functions of several items recited in the claims. The fact that certain measures are recited in mutually different dependent claims does not mean that a combination of these measures cannot be used to advantage. The computer program may be stored / distributed on a suitable medium, provided together with, for example, an optical storage medium or a solid-state medium or provided as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunications systems. Any reference numerals in the claims should not be interpreted as limiting the scope.

Claims

1. A flyback converter with primary side regulation of output current at the secondary side, the flyback converter comprising: A transformer (104) comprising a primary side for connection to an input power source and a secondary side for connection to a load; a first switching circuit (100, 102) at the primary side, adapted to allow the primary side to accumulate power from the input power source in a charging phase, and adapted to allow the secondary side to release a portion of the accumulated power to the load in a freewheeling phase (0-t3); A first sensing circuit (400) for sensing the primary side current during the charging phase; as well as a second switching circuit (101, 103) at the primary side for allowing another portion of the accumulated power at the primary side to flow back to the input power supply as leakage current during the freewheeling phase; a second sensing circuit (420) for deriving a parameter dependent on the leakage current; as well as A controller (210) is used to derive a control signal (V) for controlling the first switching circuit according to the outputs of the first sensing circuit and the second sensing circuit. C ); The flyback converter further comprises a time detection circuit, the time detection circuit is adapted to detect a time period of the freewheeling phase (0-t3), and the controller (210) is adapted to calculate: a difference between a total charge released from a peak current in the primary winding during the time period of the freewheeling phase (0-t3) and a charge released by the leakage current during the freewheeling phase, wherein the difference represents a secondary current, and the controller is adapted to derive the control signal (V C ).

2. The flyback converter according to claim 1, wherein the controller (210) is adapted to: determining the secondary side current at the secondary side based on the sensed primary side current and the parameter depending on the leakage current; and The control signal (V C ).

3. A flyback converter according to claim 1 or 2, wherein the first switching circuit comprises a first transistor (100) and a second transistor (102) in synchronization between a positive input (107) and a negative input (108), the first transistor (100) and the second transistor (102) being connected in series on each side of a primary side winding at the primary side, and the second switching circuit comprises a first diode (101) and a second diode (103) between the negative input and the positive input, the first diode (101) and the second diode (103) being connected in series on each side of the primary side winding and in series with the input power supply to form an H-bridge, and the control signal is a gate or base drive signal for the transistors, The flyback converter further includes the input power supply, and the input power supply includes: The input capacitor (C in ), and the second switching circuit (101, 103) is adapted to allow the leakage current to flow back to the input capacitor, and the input capacitor (C in ) is suitable for connection to a time-varying voltage supply.

4. The flyback converter according to claim 1 or 2, further comprising an output diode (105) connected between the secondary side and an output load, the output diode being adapted to: block the inductive power from flowing to the load during the charging phase, and allow the inductive power to flow to the load during the freewheeling phase.

5. The flyback converter according to claim 1 or 2, wherein the first sensing circuit (400) comprises: Primary winding current sensing resistor (200, R scp ), connected in series with the first switching circuit and the primary side winding; as well as A sample and hold circuit (215, 216) is used to sample a peak voltage across the primary winding current sensing resistor, the peak voltage corresponding to the peak current through the primary side winding.

6. The flyback converter according to claim 5, wherein the controller is adapted to: derive the control signal (V C ).

7. The flyback converter according to claim 1, wherein the second sensing circuit (420) comprises: The freewheeling current sensing resistor (201, R csn ), connected in series with the second switching circuit (101, 103) and the primary side winding, the freewheeling current sensing resistor (201, R csn ) is adapted to sense the amplitude of the leakage current in the freewheeling phase as the parameter; Filter (202, 203); as well as A comparator (206), wherein an output of the comparator is provided as the output of the second sensing circuit (420).

8. A flyback converter according to claim 7, wherein the output of the filter is provided to the comparator (206), the comparator (206) compares the output of the filter with a reference, and the comparator includes a feedback loop having a certain gain for the output of the filter.

9. The flyback converter according to claim 1 , wherein the time detection circuit is adapted to detect: a first portion (0-t2) of the freewheeling phase (0-t3) as the parameter, during which the leakage current at the primary side decreases from a peak value to zero; and The second part (t2-t3) of the freewheeling phase (0-t3) is from when the leakage current at the primary side drops to zero to when the secondary side current to the load drops to zero.

10. The flyback converter of claim 9, wherein the controller comprises a filter (214) that is charged and discharged according to the detected time portion.

11. The flyback converter according to claim 10, wherein the controller further comprises: A charging circuit (232) for providing the sampled peak voltage to charge the filter (214) during a second portion (t2-t3) of the freewheeling phase (0-t3) of the freewheeling phase (0-t3), the second portion (t2-t3) being from when the leakage current at the primary side drops to zero to when the secondary side current to the load drops to zero.

12. The flyback converter according to claim 11, wherein the controller further comprises a chopper circuit, the chopper circuit being further configured to provide a ground signal outside the second portion (t2-t3) of the freewheeling phase to discharge the filter (214).

13. The flyback converter according to claim 7, wherein the time detection circuit comprises: A zero-crossing detection circuit for detecting a zero-crossing signal, and the time detection circuit is used to derive the time period according to the zero-crossing signal; or A zero crossing detection circuit for detecting a zero crossing signal and a latch circuit for deriving the time period, the latch circuit having as input a base or gate drive signal for the first switching circuit and the detected zero crossing signal.

14. An LED driver comprising a flyback converter according to any one of claims 1 to 13, the LED driver comprising an input for connection to an external power source and an output for connection to an LED load.

15. A lighting device comprising: The LED driver according to claim 14, and the LED load connected to the LED driver.

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