SWITCHED FLYBACK CONVERTER CIRCUIT

Description

[0001] The present invention relates to a clocked flyback converter circuit and a method for controlling a clocked flyback converter circuit. In particular, the invention relates to a clocked flyback converter for directly driving one or more light-emitting diodes.

[0002] A flyback converter, also called a boost-buck converter, is a DC-DC converter that transfers electrical energy between an input and output side using a transformer in a galvanically isolated manner. A flyback converter can convert a DC voltage supplied at the input into a DC voltage with a different voltage level with minimal circuitry.

[0003] WO 2012 / 167294 A1 discloses a clocked flyback converter circuit in which a control unit selectively switches on and off a switch, which is coupled to the transformer's primary coil and ground, at a specific frequency and duty cycle to clock the flyback converter. The current flowing through the controllable switch is monitored after switching off, and a restart occurs when the current crosses zero. WO 2012 / 167294 A1 proposes using such a clocked flyback converter circuit to directly power a light-emitting diode (LED).

[0004] However, WO 2012 / 167294 A1 does not disclose how dimming or varying the output power can be achieved when one or more light-emitting diodes are directly operated on the flyback converter circuit, in particular without a further actively clocked converter stage.

[0005] The light emission of an LED depends on the current flowing through it. For brightness control, LEDs are therefore typically operated in a mode in which the current flowing through the LED is modified by an operating device.

[0006] WO 2013 / 092734 A1 discloses a converter for controlling the brightness (dimming) of light-emitting diodes using pulse-width modulation (PWM). The duty cycle is varied in a first and a second dimming range with a first and second constant amplitude of the LED current, respectively, to cover a large overall dimming range. Brightness control is performed solely according to a dimming specification.

[0007] For precise control or regulation, especially with different loads, the control unit must record measured values in order to be able to counteract any deviations from specified operating parameters, such as light color and brightness, or shifts in the light spectrum at different dimming levels.

[0008] Additional components / circuits are required to record the measured variables and generate corresponding signals that can be fed to or processed by the control circuit, which makes the overall design complex and expensive.

[0009] WO 2010 / 135454 A discloses a clocked flyback converter circuit operating in discontinuous mode.

[0010] A clocked flyback converter circuit according to the preamble of the appended claim 1 is described in DE 11 2011 004983 T5.

[0011] The invention is based on the object of providing devices and methods that mitigate the described problems. In particular, the object is to provide a clocked flyback converter circuit for the direct operation of one or more lighting devices and a method for controlling a flyback converter circuit that allow precise control and regulation over a wide load range with a simple and cost-effective design.

[0012] This object is achieved according to the features of the independent claims. The invention is further developed by the features of the dependent claims.

[0013] According to the present invention, a clocked flyback converter circuit for operating one or more lighting devices includes a controllable switch, a transformer with a primary winding coupled to the controllable switch and a secondary winding to which the lighting devices can be coupled, a control unit for controlling the switch, and means for directly or indirectly detecting the current through the switch in the on state and for supplying a signal representing this current to the control unit. In order to cover a wide load / dimming range or to enable precise control or regulation even with low load / dimming ranges, the flyback converter circuit operates in marginal mode and in discontinuous mode with low load / dimming ranges.

[0014] As soon as the signal representing the current has reached a switch-off threshold after the switch has been switched on, which can be adjusted to change / adjust the power to be transmitted by the flyback converter circuit, the control unit switches the switch off again. However, the control unit only reduces the switch-off threshold to a predetermined minimum value, and to achieve a further reduction in the power to be transmitted by the flyback converter circuit, the control unit switches from limit mode operation to discontinuous operation with a switch-off threshold fixed at the minimum value. The control unit is configured to switch the switch off in both limit mode and discontinuous operation as soon as the signal representing the current reaches the switch-off threshold or the predetermined minimum value.The transformer has a primary-side auxiliary winding to which the control unit is coupled for determining a curve of the voltage drop across the auxiliary winding, wherein the control unit, in the discontinuous operation, switches the switch back on at a time at which this voltage is zero after a minimum in the voltage curve.

[0015] The light source(s) may be light-emitting diodes.

[0016] The control unit may comprise a comparator for comparing the signal representing the current with a signal representing the minimum value.

[0017] The control unit can be designed such that the switch-off threshold in the limit operation and / or the time of re-switching the switch in the discontinuous operation is changed according to a dimming signal supplied to the control unit.

[0018] In the discontinuous operation, the control unit can determine a time for re-switching the switch according to the power to be transmitted and, if in one cycle the time at which the voltage across the auxiliary winding is zero after a minimum in the voltage curve and at which the switch was switched on is after the determined time, can re-switch the switch in a next cycle at a time at which the voltage across the auxiliary winding is zero after a minimum in the voltage curve and which is before the determined time.

[0019] Alternatively or additionally, during discontinuous operation, the control unit can continuously determine an average current delivered to the lamps based on the actual time of reactivation of the switch and regulate the average current to a predetermined current value corresponding to the power to be transmitted by changing the time of reactivation. A difference is calculated between the predetermined average current for a reactivation cycle and the actual average current of the reactivation cycle, and this difference is added to the predetermined average current for the next reactivation cycle. In this case, the reactivation time is checked for a minimum in each cycle, particularly at a zero crossing of the voltage curve.

[0020] An operating device for light-emitting diodes according to the present invention comprises one of the clocked flyback converter circuits described above.

[0021] According to the present invention, a method for controlling a switch-mode flyback converter circuit comprising a transformer, in which a primary winding of the transformer is coupled to a controllable switch and a secondary winding of the transformer is coupled to one or more lighting devices, comprises the steps of: controlling the switch; and detecting the current flowing through the switch and generating a signal representative of this current;wherein, in both marginal mode and discontinuous mode, the switch is turned off when the signal indicates that the current has reached a variable cut-off threshold, and the cut-off threshold is changed to change the power to be transmitted by the flyback converter circuit, wherein the cut-off threshold is only reduced to a minimum value and, in order to achieve a further reduction in the power to be transmitted by the flyback converter circuit, the switch is switched from marginal mode to discontinuous mode with the cut-off threshold fixed;

[0022] The invention is explained in more detail below with reference to the accompanying drawings. They show: Fig. 1 a clocked flyback converter circuit according to a first embodiment of the present invention. Fig. 2 a diagram with signal waveforms of the flyback converter circuit operated in limit operating mode according to an embodiment of the present invention, Fig. 3 a diagram with signal waveforms of the flyback converter circuit operated in discontinuous operating mode according to an embodiment of the present invention, Fig. 4 a diagram showing the course of control variables in marginal and discontinuous operation. Fig. 5 a clocked flyback converter circuit according to a second embodiment of the present invention. Fig. 6 an operating device for light-emitting diodes according to an embodiment of the present invention.

[0023] Components with the same functions are marked with the same reference numerals in the figures.

[0024] Fig. 1 shows a simplified circuit of a clocked flyback converter for the direct operation of one or more lighting devices according to a first embodiment of the present invention. A supply voltage, which can be a direct voltage or a rectified alternating voltage, is applied to the two input terminals 1, 2 of the illustrated flyback converter circuit 3.

[0025] The primary winding 4 of the transformer 5, the controllable switch 7, and a measuring resistor 8 are connected in series between the first input terminal 1 and the second input terminal 2. The second input terminal 2 is connected to ground. An LED section 11, formed from a series connection of, in the example, five LEDs, is connected to the two output terminals 9, 10 of the flyback converter circuit 3. The secondary winding 6 of the transformer 5 and a diode 12 are connected in series between the first output terminal 9 and the second output terminal 10. A capacitor 13 is coupled in parallel to the output terminals 9, 10. The primary and secondary windings 4, 6 of the transformer 5 have different polarities / winding directions.

[0026] The controllable switch 7 can be a power switch, a field-effect transistor, or a bipolar transistor. The controllable switch 7 can be an insulated-gate transistor.

[0027] A control unit 14 connected to the switch 7 controls the switch 7 to switch it on and off.

[0028] The control unit 14 is supplied with a dimming signal D for controlling the brightness of the LED section 11 (varying the output power) and a signal of the voltage drop across the measuring resistor for detecting the current flowing through the switch 7.

[0029] The control unit 14 may be a semiconductor integrated circuit or may comprise a semiconductor integrated circuit. The control unit 14 may be configured as a processor, a microprocessor, a controller, a microcontroller, or an application-specific integrated circuit (ASIC), or a combination of the aforementioned units.

[0030] In the clocked flyback converter 3, electrical energy is transferred between the input terminals 1, 2, which are galvanically isolated by the transformer 5, and the output terminals 9, 10. For this purpose, the control unit 14 repeatedly switches the switch 7 on and off. After switching on, current flows through the primary winding 4 of the transformer 5, and the diode 12 suppresses a current flow on the secondary side. After switching off (blocking phase), the energy stored in the primary winding 4 is released via the secondary winding 6 of the transformer 5 or forces a current flow on the secondary side through the diode 12. The capacitor 13 is charged, and the LED section 11 connected to the output terminals 9, 10 of the flyback converter circuit 3 lights up. The current flow on the secondary side decreases linearly and finally becomes zero in discontinuous (intermittent) operation and in limit operation before the control unit 14 switches the switch 7 on again.

[0031] According to the present invention, dimming or varying the output power in a first output power range, in which the flyback converter 3 is operated in limit mode, is achieved by varying the current value up to which, after the switch 7 is turned on, the current through the primary winding 4 of the transformer 5 and the switch 7 increases linearly. Upon reaching this predetermined current value, the switch 7 is turned off. The magnitude of the predetermined current value determines the brightness in this mode.

[0032] Fig. 2 shows in a simplified representation the time courses of the current through the primary winding 4 and the switch 7 (solid line) and the current through the secondary winding 6 (dashed line) of the flyback converter 3 operated in the limit mode. As in the diagram of the Fig. 2 As can be seen, the control unit 14 switches on the switch 7 at time t 1. The current flow through the switch 7 is determined by means of the voltage drop across the measuring resistor 8 and detected by the control unit 14.

[0033] After switching on at time t1, the current through the primary winding 4 and the switch 7 (solid line) increases linearly up to a predetermined current value IS1, which was set by the control unit 14 based on the dimming signal D (power request).

[0034] The control unit 14 compares the current switch current or the voltage drop across the resistor 8 representing the switch current with the threshold value I S1 and causes the switch 7 to be switched off as soon as the current through the switch 7 reaches the threshold value I S1 (switch-off threshold).

[0035] At time t 2 , the specified current value I S1 (threshold) is reached, and the control unit 14 turns off switch 7. The current flow through the secondary winding 6 (dashed line) begins and drops to zero at time t 3 , whereupon the control unit 14 turns switch 7 back on (limit operation). Reactivation can also occur only upon a positive edge (zero crossing) of the coil current.

[0036] For the cycle following cycle t 1 to t 3 , the control unit 14 lowered the switch-off threshold value to I S2 due to a reduced power requirement via the dimming signal D, so that after the switch 7 is switched on at time t 3 , the current through the primary winding 4 and the switch 7 (solid line) increases again, but only up to the predetermined current value Is 2 , since the control unit 14 switches off the switch 7 at time t 4 and switches it on again at time t 5 after the current through the secondary winding 6 (dashed line) has dropped to zero (limit operation).

[0037] According to the present invention, when the power requirement is reduced, the switch-off threshold is only lowered to the level shown in the diagram of the Fig. 2 shown minimum value I min . This minimum value I min can be selected such that below this minimum switch-off threshold value, reliable detection of the low current flow through switch 7 is not possible.

[0038] If, upon a reduction in the power requirement and a corresponding lowering of the shutdown threshold, the shutdown threshold would be below the minimum value I min , the control unit 14 switches from the limit operating mode to the discontinuous operating mode according to the present invention. To do this, the control unit 14 compares the shutdown threshold IS, which is to be set according to the dimming signal D, with the minimum value I min . In the discontinuous operating mode, the required further reduction in the power delivered by the flyback converter 3 is then achieved by extending the off-time of the switch 7 with the shutdown threshold fixed at the minimum value I min .

[0039] Fig. 3 shows in a diagram the time courses of the current through the primary winding 4 and the switch 7 (solid line) and the current through the secondary winding 6 (dashed line) of the flyback converter 3 operated in discontinuous mode. As in Fig. 3 As shown, the control unit 14 switches off the switch 7 as soon as the current flowing through the switch 7 (solid line) has reached the minimum value I min , the fixed switch-off threshold, and does not switch the switch 7 back on immediately after the current through the secondary winding 6 (dashed line) has dropped to zero, but only after a time period t dcm set according to the power to be delivered, after the current in the secondary winding 6 (dashed line) has dropped to zero. In the discontinuous operating mode, the switch 7 is always switched off when the fixed switch-off threshold is reached; an increase in the output power occurs with a corresponding decrease in the time period t dcm , and a decrease in the output power occurs with a corresponding increase in the time period t dcm .

[0040] If, upon increasing the power demand and correspondingly decreasing the time period t dcm , this would be below a minimum value or zero, the control unit 14 switches from the discontinuous operating mode to the limit operating mode according to the present invention. To do so, the control unit 14 compares the time period t dcm to be set according to the dimming signal D with the minimum value or checks whether the time period t dcm is zero or almost zero.

[0041] Fig. 4 illustrates the transition between the different operating modes. In the Fig. 4 The diagram shown shows the threshold value I s to be set by the control unit 14 and the time period t dcm to be set by the control unit 14 as a function of the level of the dimming signal D supplied to the control unit 14. As in the diagram shown in Fig. 4 As can be seen in the diagram shown, above a level D, the switch-off threshold value I s to be set increases linearly with an increase in the level of the dimming signal D, while the time period t dcm is zero. Below the level D x, the switch-off threshold value I s to be set is constant or fixed to the minimum value Imin, while the time period tdcm increases non-linearly with a reduction in the level of the dimming signal D below D x. The curves shown can be calculated by the control unit 14 or stored in advance in the form of a table in the control unit 14.

[0042] In discontinuous operation, after switch 7 is turned off, voltage oscillations across the primary coil 4 may occur due to parasitic effects. To avoid switching losses of switch 7, such oscillations should be taken into account when selecting the restart time, or the restart time should be selected such that, at the restart time, the voltage oscillation falls to a zero crossing after a voltage minimum.

[0043] Fig. 5 shows a clocked flyback converter circuit according to a second embodiment of the present invention, in which the transformer 5 has a primary-side auxiliary winding 15 for detecting this voltage waveform. The auxiliary winding 15 is connected to the second input terminal 2 and the control device 15. In discontinuous operation, the control unit 14 uses the voltage signal generated by the auxiliary winding 15 to determine the time at which the voltage waveform exhibits a zero crossing after a minimum in order to switch the switch back on at this time.

[0044] In the Fig. 3 In the diagram shown, the oscillation of the voltage signal after switching off is shown as a dotted line. However, if switch 7 is to be switched on again taking the detected oscillation into account, this means that switch 5 cannot be switched on again at an arbitrary point in time, but only at discrete time intervals, namely whenever the voltage oscillation crosses zero.

[0045] If, due to these voltage oscillations and the resulting re-switching time at zero crossing, the switch 7 is not re-switched on exactly at the time determined by the control unit 14 according to the load requirement, the specified power, the specified time average of the current I avg delivered on the secondary side, cannot be achieved exactly for this re-switching cycle and will be lower if, for example, it is only switched on again at the next possible discrete time later.

[0046] To compensate for this error, the control unit 14 can (prematurely) re-enable the switch 7 in the next cycle at a discrete time before the time determined according to the load request, so that an early re-enablement and a re-enablement at a discrete time after the time determined according to the load request alternate.

[0047] The control unit 14 can be configured to determine the magnitude of the fault, i.e., the deviation between the time determined according to the load demand and the actual time of reclosure, and to trigger the early reclosure when the magnitude of the fault reaches a certain level. The fault can be determined for only one reclosure cycle or accumulated for multiple cycles, with early reclosure being triggered for the next reclosure cycle as soon as the accumulated error (total error) of the consecutive cycles reaches a certain value.Alternatively or additionally, the control unit 14 can continuously determine the actual, current average current value I avg on the basis of the actual time of the restart and regulate the average current value I avg by a corresponding adjustment / change of the time period t dcm , wherein a difference can be formed between the predetermined average current value for the last restart cycle and the actual average current value I avg of the last restart cycle and this difference can be added to the predetermined average current value for the next restart cycle.

[0048] According to the present invention, alternating between the discontinuous operating mode and the limit operating mode can occur when the load formed by the LED(s) is so low that the upper shutdown threshold I s resulting in a limit operating mode would be below the predetermined minimum value I min This alternating can lead, particularly in a transition region, to a more accurate average current value I avg over two restart cycles / periods, since an excessively low average current value I avg resulting from the discontinuous operating mode (restart at the next possible discrete, later time) can be compensated for by the excessively high average current value I avg in the subsequent limit operating mode.

[0049] The end of the alternating operation or the transition from the alternating operation to a continuous discontinuous operating mode can be triggered by means of a received control signal or can depend on the level of the dimming signal D or the length of the time period t dcm of the discontinuous operating mode, wherein an end of an alternating operation or the transition from the alternating operation to a continuous discontinuous operating mode occurs when the length assumes a maximum value.

[0050] Similarly, a transition from another continuous discontinuous operating mode to alternating operation can be triggered by a received control signal or can depend on the level of the dimming signal D or the length of the time period t dcm of the discontinuous operating mode, whereby a start of alternating operation or the transition from a continuous discontinuous operating mode to alternating operation occurs when the length assumes a minimum value. The minimum value and the maximum value can be the same.

[0051] Fig. 6 shows an operating device for light-emitting diodes according to an embodiment of the present invention. The operating device has the Fig. 5 shown and a rectifier consisting of a diode circuit 16 and a charging capacitor 17 for rectifying an AC voltage supplied to the input terminal 18. The operating device may include a power factor correction circuit (not shown) arranged between the diode circuit 16 and the charging capacitor 17.

[0052] The LEDs of the LED array 11 connected to the output terminals 8, 10 can be inorganic or organic LEDs. The LEDs can be connected in series or parallel. The multiple LEDs can also be connected in more complex arrangements, for example, in multiple series circuits connected in parallel. While five LEDs are shown as an example, the light source or LED array 11 can also have more or fewer LEDs.

[0053] Alternatively, the operating device can Fig. 1 shown flyback converter circuit.

Claims

1. Clocked flyback converter circuit for operating one or more lamps (11), comprising a controllable switch (7), a transformer (5) having a primary winding (4) coupled to the controllable switch (7) and a secondary winding (6) to which the lamps (11) can be coupled, a control unit (14) for actuating the switch (7), and means (8) for directly or indirectly detecting the current through the switch (7) in the switched-on state and for supplying a signal representing this current to the control unit (14), characterized in that the control unit (14) is configured to actuate and switch off the switch (7) in limit operation and in discontinuous operation when the signal representing the current has reached a variable switch-off threshold (IS), to modify the switch-off threshold (IS) in limit operation in order to modify the power transmitted by the flyback converter circuit (3), to reduce the switch-off threshold (IS) for the current signal only to a minimum value (Imin), and to change from limit operation into discontinuous operation in order to achieve a further reduction of the power transmitted by the flyback converter circuit (3) when the switch-off threshold (IS) is fixed at the minimum value (Imin), wherein a dimming signal (D) can be supplied to the control unit (14), and the control unit (14) is designed, in limit operation, to modify the switch-off threshold (Is) according to the dimming signal, and, in discontinuous operation, to modify the time of switching the switch back on according to the dimming signal (D) when the switch-off threshold (IS) is fixed at the minimum value (Imin), and wherein the transformer (5) has a primary-side auxiliary winding (15) to which the control unit (14) is coupled in order to detect a voltage curve across the auxiliary winding (15), and in discontinuous operation, the control unit (14) switches the switch back on at a time at which a voltage signal of the voltage curve generated across the auxiliary winding (15) is zero after a minimum in the voltage curve.

2. Clocked flyback converter circuit according to claim 1, wherein the control unit (14) is configured, in discontinuous operation, to modify the duration (tdcm) between a time at which the current of the secondary winding (6) has dropped to zero and the time of switching the switch (7) back on in order to modify the power transmitted by the flyback converter circuit (3), to reduce the duration (tdcm) only to a minimum duration, and to change from discontinuous operation to limit operation in order to achieve a further increase of the power transmitted by the flyback converter circuit (3).

3. Clocked flyback converter circuit according to either of claims 1 to 2, wherein the control unit (14) has a comparator for comparing the signal representing the current with a signal representing the minimum value (Imin).

4. Clocked flyback converter circuit according to any of claims 1 to 3, wherein the control unit (14) is configured, in discontinuous operation, to determine a time for switching the switch (7) back on according to the power to be transmitted, and if, in a cycle, the time at which the voltage across the auxiliary winding (15) is zero after a minimum in the voltage curve and at which the switch (7) has been switched on is after this determined time, the switch (7) is switched back on in a subsequent cycle at a time at which the voltage across the auxiliary winding (15) is zero after a minimum in the voltage curve and which is before the determined time.

5. Clocked flyback converter circuit according to any of claims 1 to 4, wherein the control unit (14) is configured, in discontinuous operation, to continuously determine a mean current value that is output to the lamp (11), on the basis of an actual time of switching the switch (7) back on, and to control the mean current value with respect to a mean current value specified according to the power to be transmitted, by modifying the time of switching back on, wherein a difference is formed between the specified mean current value for a cycle of switching back on and the actual mean current value of the cycle of switching back on, and this difference is added to the specified mean current value for the subsequent cycle of switching back on.

6. Operating device for light-emitting diodes, comprising a clocked flyback converter circuit (3) according to any of claims 1 to 5.

7. Method for controlling a clocked flyback converter circuit (3) comprising a transformer (5) in which a primary winding (4) of the transformer is coupled to a controllable switch (3), and a secondary winding (2) of the transformer (5) is coupled to one or more lamps (11), comprising the steps of: actuating the switch (7); and detecting the current flowing through the switch (7) and generating a signal representing this current; characterized in that in limit operation and in discontinuous operation, the switch (7) is switched off when the signal indicates that the current has reached a variable switch-off threshold (Is), and in limit operation, the switch-off threshold (IS) is modified in order to modify the power to be transmitted by the flyback converter circuit (3), wherein the switch-off threshold (IS) is reduced only to a minimum value (Imin) and is transitioned from limit operation into discontinuous operation in order to achieve a further reduction of the power to be transmitted by the flyback converter circuit (3) when the switch-off threshold (IS) is fixed at the minimum value (Imin), wherein a dimming signal (D) is supplied to the control unit (14) and, in limit operation, the switch-off threshold (Is) is modified by the control unit (14) according to the dimming signal, and, in discontinuous operation, the time of switching the switch back on is modified according to the dimming signal (D) when the switch-off threshold (Is) is fixed at the minimum value (Imin), and wherein the control unit (14) is coupled via a primary-side auxiliary winding (15) of the transformer (5) in order to detect a voltage curve across the auxiliary winding (15), and in discontinuous operation, the control unit (14) switches the switch back on at a time at which a voltage signal of the voltage curve generated across the auxiliary winding (15) is zero after a minimum in the voltage curve.