LED driver circuit

Abstract

The LED driver circuit uses a first current source circuit controlled by pulse width modulation to set the output current and a second current source circuit to charge the storage capacitor. A compensation device is used to increase the current delivered by the second current source circuit during the pulse width modulation's off-cycle time. This compensates for the current drawn from the input and enables better timing control of the input current pulses.

Description

Technical Field

[0001] This invention relates to LED driver circuits. Background of the Invention

[0003] LED incandescent bulbs are becoming increasingly popular due to their similar appearance to incandescent bulbs.

[0004] These lamps have high LED string voltages and are also subject to size limitations because the lamp driver must be housed within an Edison lamp cap. To meet these requirements, linear low power factor current source drivers are used to power the filament LED strips. The current in these linear drivers can be amplitude-controlled or pulse-width modulation-controlled (PWM dimming). PWM control is preferred in these drivers due to its dimming accuracy and color point control, although it can cause distortion of the mains input current.

[0005] However, the input current waveform needs to meet specified requirements, which is difficult to achieve when implementing PWM dimming. In particular, the PWM dimming signal is a high-frequency signal, such as a 1kHz signal.

[0006] Known linear drivers include a current source circuit connected in series with the LED string to drive a fixed current through the LEDs. A storage capacitor is connected in parallel, for example, with the combined LED string and the series-connected current source circuit. When the voltage of the storage capacitor is higher than the rectified AC input voltage, the capacitor discharges to the LED string (with a fixed PWM control current), and when the rectified AC input voltage is higher, the capacitor charges from the rectified AC input. During the charging phase, current from the rectified AC also flows to the LED device. Any modulation of the LED current will cause distortion of the AC current.

[0007] It is known that using another (fixed) current source to control the charging of the storage capacitor reduces current distortion and improves the power factor. However, current distortion is still a consequence of the rapid switching frequency of the PWM control signal. As a result, the phase angle of the input current relative to the rectified mains power is not well controlled, and in particular, the timing of the peak value in the input current (“peak angle”) may lead to failure to meet specified requirements. Summary of the Invention

[0008] This invention is defined by the claims.

[0009] According to an example of one aspect of the present invention, an LED driver circuit is provided, comprising:

[0010] Input, used to receive rectified mains power input signals;

[0011] Storage capacitors;

[0012] A first current source circuit is used to set the output current, wherein the current source circuit is controlled by pulse width modulation having a conduction period and a turn-off period, and wherein the first current source and the storage capacitor form a capacitor discharge circuit.

[0013] A charging circuit for charging a storage capacitor includes a second current source circuit, wherein the second current source circuit and the storage capacitor form a capacitor charging circuit; and

[0014] A compensation device for increasing the current delivered by the second current source circuit during the off-cycle time of the pulse width modulation.

[0015] In this way, the compensation device compensates for the time when the output does not draw any current by increasing the charging current of the second current source circuit.

[0016] This invention solves the problem of the received rectified mains signal not being synchronized with the PWM control signal. The aim is to provide a circuit in which the current peak occurs at a fixed timing point, independent of the timing offset between the mains frequency signal and the PWM signal.

[0017] The compensation device is used, for example, to increase the current delivered by the second current source circuit during the pulse width modulation off-cycle time only when current is drawn from the input.

[0018] During the operation of the capacitor charging circuit, current is drawn from the input, and no input current is drawn during the capacitor discharging circuit. Therefore, the input current includes pulses at the rectified frequency. These pulses suffer from high-frequency distortion caused by pulse width modulation of the first current source. A compensation device ensures that the peak of these pulses occurs at the beginning of each pulse.

[0019] The compensation device may include the coupling between the first current source circuit and the second current source circuit.

[0020] This coupling allows the charging current of the second current source circuit to be variable and to depend on the state of the first current source circuit.

[0021] In one example, the first current source circuit includes a first control input for setting a current level based on the voltage across a first resistor between a ground terminal and the first control input, and the second current source circuit includes a second control input for setting a current level based on the voltage across a second resistor between a dummy terminal and the second control input, wherein the compensation device includes coupling between the dummy ground terminal and the first control input.

[0022] Therefore, the compensation function can be simply implemented as a coupling between the first current source circuit and the second current source circuit, in particular by adapting the control input to the second charging source circuit, so that the delivered current depends on the output current (which is the LED current).

[0023] In this way, the output current PWM distortion is effectively compensated by adding the output current PWM distortion to the control circuit (specifically the sensing resistor) of the second current source circuit. Thus, the input current distortion is compensated.

[0024] The sensed current used to control the first current source circuit is provided to the control input of the second current source circuit (i.e., the charging loop) in this way, and thereby provides compensation in the analog control domain.

[0025] In another device, the compensation device includes a compensation circuit for setting the current of the second current source according to the pulse width modulation setting of the first current source circuit.

[0026] Therefore, the pulse width modulation signal can be used to control the second current source circuit. This is easy to implement because it avoids any coupling between the outputs of the first and second current source circuits, utilizing only the same PWM signal.

[0027] The compensation circuit may include a transistor for pulling the control terminal of the second current source high or low according to the pulse width modulation settings of the first current source circuit.

[0028] In another example, the first current source circuit includes a first control input for setting a current level based on the voltage across a first resistor between a ground terminal and the first control input, wherein the compensation device includes a compensation capacitor connected in parallel with the first resistor.

[0029] The addition of a compensation capacitor creates a filter circuit, particularly a differentiator circuit, which shifts the peak value of the input current to the beginning of the current waveform. Therefore, the effect is to shift and increase the peak value of the input current distortion, making it possible to meet the peak angle requirements.

[0030] The first resistor and the compensation capacitor can form a circuit with a cutoff frequency twice as large as the PWM frequency.

[0031] The present invention also provides a lighting device, the lighting device comprising:

[0032] The driver circuit as defined above; and

[0033] The output current is delivered to the LED device.

[0034] These and other aspects of the invention will become apparent from the following description of several embodiments. Attached Figure Description

[0035] To better understand the invention and to more clearly illustrate how to implement it, reference will now be made to the accompanying drawings by way of example only, wherein:

[0036] Figure 1 A known linear LED driver is shown;

[0037] Figure 2 It shows the result of Figure 1 The circuit produces input current distortion;

[0038] Figure 3 A first example of a linear LED driver according to the present invention is shown;

[0039] Figure 4 It shows the result of Figure 3 The circuit produces input current distortion;

[0040] Figure 5 It shows Figure 3 Other waveforms of the circuit;

[0041] Figure 6 A second example of a linear LED driver according to the present invention is shown;

[0042] Figure 7 It shows the result of Figure 6 The circuit produces input current distortion;

[0043] Figure 8 It shows Figure 6 Other waveforms of the circuit;

[0044] Figure 9 A third example of a linear LED driver according to the present invention is shown;

[0045] Figure 10 It shows the result of Figure 9 The circuit generates input current distortion; and

[0046] Figure 11 It shows Figure 9 Another waveform of the circuit. Detailed Implementation

[0047] The invention will be described with reference to the accompanying drawings.

[0048] It should be understood that while the detailed description and specific examples indicate exemplary embodiments of the apparatus, system, and method, they are 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 become more readily understood from the following description, the appended claims, and the accompanying drawings. It should be understood that the drawings are merely schematic and not drawn to scale. It should also be understood that the same reference numerals are used in all the drawings to denote the same or similar parts.

[0049] This invention provides an LED driver circuit that uses a first current source circuit controlled by pulse width modulation to set the output current and a second current source circuit to charge a storage capacitor. A compensation device is used to increase the current delivered by the second current source circuit during the off-cycle time of the pulse width modulation. This compensates for the current drawn from the input and enables better timing control of the input current pulses.

[0050] Figure 1 A known linear current driver circuit is shown.

[0051] The input is the rectified AC signal Vrect. This input is provided to the LED string D3, D4, and D5 connected in series with the first current source circuit 1 (which will simply be referred to as the first current source). The first current source drives a constant current through the LED string, but pulse width modulation (PWM) is used to turn the current source on and off.

[0052] Storage capacitor C1 is connected in parallel with the LED string and the current source circuit. One end of storage capacitor C1 is connected to the input Vrect, and the other end is connected to node N1. This node is grounded through a reverse diode D2, such that the series combination of the storage capacitor and diode D2 is connected in parallel with the LED string and the first current source 1. Node N1 is connected to the second current source circuit 2 through a forward diode D1, which will be simply referred to as the second current source. This current flows through the current sensing resistor Rsense.

[0053] The purpose of the storage capacitor C1 is to smooth the mains frequency ripple, and the purpose of the pulse width modulation of the first current source 1 is to provide dimming control.

[0054] PWM dimming control, for example, operates at 1 kHz, causing the first current source 1 to turn on and off at 1 kHz. The duty cycle determines the average current flow.

[0055] This circuit has a charging cycle and a discharging cycle.

[0056] A charging cycle occurs when the instantaneous voltage Vrect is greater than the smoothed voltage stored in the storage capacitor C1. During this period, the first current source 1 draws current from the input Vrect, and the second current source 2 draws charging current from the storage capacitor through diode D1. Diode D2 is off. The charging current is indicated by the solid arrow.

[0057] Therefore, the storage capacitor is charged by the constant current from the second current source 2.

[0058] A discharge cycle occurs when the instantaneous voltage Vrect is less than the smoothed voltage stored in the storage capacitor C1. During this period, the first current source 1 draws current from the storage capacitor C1, and the charge flow is indicated by the dashed arrow. Diode D2 is turned on during this phase, and diode D1 is turned off. The diode thus automatically (passively) and effectively turns off the second current source 2 based on the voltage at the input during the discharge cycle. The first current source is effectively turned on and off.

[0059] The charging and discharging cycle of the storage capacitor C1 is at a mains frequency of 50Hz or 60Hz, and it smooths out the ripple at 50Hz or 60Hz. A 1kHz ripple still exists in the LED current (because there is zero current when the first current source is off). This ripple also appears in the mains current without compensation.

[0060] Figure 2 It shows Figure 1 The mains voltage Vmains (before rectification) and the current Imains drawn from the mains in the circuit (before rectification).

[0061] Each peak of the Imains signal occurs when the input voltage Vrect is greater than the voltage across the storage capacitor C1. During the remaining time, the storage capacitor provides the current drawn from the first current source 1.

[0062] High-frequency ripples are visible in the input current. This is due to a specific problem arising from the rapid switching frequency of the PWM control signal not being synchronized with the mains frequency. As a result, the phase angle of the current peak relative to the rectified mains is not well controlled, and in particular, the timing of the current peak ("peak angle") may fail to meet specified requirements.

[0063] Figure 3 It shows Figure 1 One possible implementation of the circuit and a first modification according to an example of the present invention.

[0064] Figure 3 The diagram shows a non-rectified AC input 10, a rectifier 12, and an EMI filter 14. The filter output is... Figure 1 The rectified mains signal Vrect is shown. The input diode D0 is also shown.

[0065] The circuit includes a first integrated circuit that implements a first current source 1 and a second integrated circuit that implements a second current source 2.

[0066] As shown at the first current source 1, this example has two LED strings, such as a string of LED 1 containing warm white (WW) LEDs at 2200K and a string of LED 2 containing cool white (CW) LEDs at 3000K. They are connected between the LED power supply voltage VLED+ and the corresponding output pins of the integrated circuit. The two different color temperatures are mixed to provide control over the desired color temperature. The LED strings together, for example, form an LED filament bulb.

[0067] For this purpose, each string has its own PWM settings. These PWM settings are shown as WW and CW, and these settings are provided to the dimming input of the first current source 1.

[0068] Figure 3 The modification is to use PWM settings in the control of the second current source 2.

[0069] For this purpose, the second current source 2 has an input transistor Q1 that is turned on when either the PWM input CW or WW is high. Therefore, transistor Q1, together with input diode 16, forms a turn-off port. When Q1 is turned on, the dimming input to current source 2 is pulled low. The resistor Rdrain, connected in series with the drain of Q1, is pulled to ground via Q1, providing a lower (default) charging current setting.

[0070] The effect is that input current distortion compensation is ineffective. The charging current of the input capacitor has a normal charging setting.

[0071] When Q1 is turned off, the dimming input to current source 2 is pulled high to the supply voltage 18. Since the resistor Rdrain, which is in series with the drain of Q1, is not pulled low to ground, the higher current is effective for the charging current source.

[0072] Its effect is that input current distortion compensation is effective. The charging current of the input capacitor is the sum of the normal setting and the LED current (WW+CW).

[0073] WW and CW are switched in a complementary manner, so that in combination, CW and WW have a 100% duty cycle at full brightness. For example, at full brightness, WW = 70% and CW = 30%. When dimmed to 90%, WW = 63% and CW = 27%, and this introduces distortion in the input current.

[0074] The circuitry surrounding Q1 performs simple inverse modulation of the current delivered by the second current source 2 via PWM control of the LED string. When no LED current flows through the mains input (i.e., WW and CW are zero), the charging current source is modulated by the additional LED current because terminals Dim1 and Dim2 are pulled high by source 18.

[0075] In this way, the input current distortion is compensated.

[0076] Therefore, transistor Q1 adapts the control of the second current source 2 to the dimming level defined by CW and WW. It provides additional current when there is no LED current.

[0077] At full brightness, transistor Q1 is turned on and provides the normal charging current setting. Compensation is activated only when both signals WW and CW are zero.

[0078] Storage capacitor C1 and diodes D1 and D2 are used in conjunction with Figure 1 The same configuration method is shown.

[0079] The current source has a current sensing input, one for each LED string. Current sensing resistors RS1 and RS2 are shown for the first current source 1. The LED current flowing through each LED string passes through its respective current sensing resistor, such that the resulting voltage is a measure of the current flow. Current sensing resistors RS3 and RS4 are used to monitor the charging current, such as... Figure 1 As shown in Rsense.

[0080] Each current source circuit has a ground pin. Figure 3 In the example, the ground pin of each current source circuit is connected to the actual ground.

[0081] Figure 4 It shows Figure 3 The effect of the circuit on the input current Ines.

[0082] Figure 5 The charging current Icharge and the (total) LED current ILED are also shown. The charging current is no longer a constant value, but has a ripple of 1kHz. In particular, the charging current Icharge increases when the LED current is zero (therefore the low portion of the PWM current flows through the LED string).

[0083] Figure 6 The following is shown with modifications according to the second example of the invention. Figure 1 The implementation of the circuit.

[0084] Figure 6 The second modification shown is to couple the ground pin of the first current source 1 to one of the current sensing inputs of the second current source 2. Therefore, coupling exists between the first current source circuit and the second current source circuit.

[0085] Its effect is that the sensed charging current and the sensed LED current are added together via RS1 and RS2 through the sensing resistor RS4 of the charging current source.

[0086] In this implementation, the total current through RS3 and RS4 is monitored and used to control Icharge, so the sensed LED current can be added to either current sensing input of the second current source 2.

[0087] When the LED current through sensing resistors RS1 and RS2 is zero, the charging current source detects this low sensing current, and therefore the feedback control mechanism implemented by current source 2 automatically adds an additional current (corresponding to the LED current).

[0088] Therefore, the charging current source delivers additional current when the LED current is zero. In this way, the input current distortion is further compensated.

[0089] Figure 7 It shows Figure 6 The effect of the circuit on the input current Ines.

[0090] Figure 8 The charging current Icharge and the (total) LED current ILED are also shown. The charging current is again no longer a constant value, but has a ripple of 1kHz. Specifically, the charging current Icharge increases when the LED current is zero (therefore, the low portion of the PWM current flowing through the LED string).

[0091] Figure 9 Another alternative method is shown.

[0092] The first current source 1 remains normal, such as Figure 3 As shown, there is no coupling between the current sensing input of the second current source 2 and the ground pin of the first current source 1. Therefore, the first current source is not shown.

[0093] As described above, the second current source circuit 2 includes a control input for setting the current level (in the example shown, there are two, CS1 and CS2) based on the voltage across a resistor between the ground terminal and the control input (there are two in the example shown, RS3 and RS4).

[0094] Figure 9 The modification is to provide a compensation capacitor in parallel with the resistor, and in this example there are therefore two compensation capacitors C2 and C3.

[0095] The addition of a compensation capacitor creates a filter circuit, particularly a differentiator circuit, which shifts the peak value of the input current to the beginning of the current waveform. Therefore, the effect is to shift and increase the peak value of the input current distortion, making it possible to meet the peak angle requirements. Each current sensing resistor and its associated compensation capacitor, for example, form a circuit with a turn-off frequency greater than twice the PWM frequency.

[0096] This is achieved by providing a peak pulse injection into the charging current source via analog control. As the charging current source begins charging capacitor C1, the initial current setpoint is increased via differentiating capacitors C2 and C3. In this way, input current distortion is not improved, but the peak angle is shifted towards the start of the input current to meet specified requirements.

[0097] Figure 10 The effect on the input current Ines is shown.

[0098] Figure 11 The charging current Icharge and the (total) LED current ILED are also shown. The charging current is no longer a constant value, but has an initial peak value.

[0099] All three examples above involve increasing the current delivered by the second current source circuit during the pulse width modulation (PWM) off-cycle (relative to the current during the PWM on-cycle). Therefore, they all have the desired effect of controlling the peak timing of the mains current pulse. An example of a current source circuit is the IC BP5578EJ.

[0100] The example above uses two LED strings with two PWM signals and two current sensing feedback loops. Of course, this circuit can be implemented with a single LED string, such as... Figure 1 As shown.

[0101] By studying the accompanying drawings, the disclosure, and the appended claims, those skilled in the art can understand and implement variations of the disclosed embodiments in 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.

[0102] The fact that certain measures are described in mutually different dependent claims does not imply that combinations of these measures cannot be used advantageously.

[0103] If the term “suitable” is used in the claims or specification, it should be noted that the term “suitable” is intended to be equivalent to the term “configured as”.

[0104] Any reference numerals in the claims should not be construed as limiting the scope.

Claims

1. An LED driver circuit, comprising: Input (Vrect) is suitable for receiving rectified mains input signals; A storage capacitor (C1) is coupled to the input (Vrect). A first current source circuit (1) is adapted to set an output current, wherein the first current source circuit (1) is controlled by a pulse width modulation having a turn-on period and a turn-off period, and wherein the first current source circuit (1) and the storage capacitor (C1) form a capacitor discharge circuit. A charging circuit adapted to charge the storage capacitor (C1), the charging circuit including a second current source circuit (2), wherein the second current source circuit (2) and the storage capacitor (C1) form a capacitor charging circuit; and The compensation device is adapted to increase the current delivered by the second current source circuit (2) during the off-cycle time of the pulse width modulation. The compensation device includes a coupling between the first current source circuit and the second current source circuit. The first current source circuit (1) includes a first control input adapted to set the current level based on the voltage across a first resistor (RS1) between the virtual ground terminal and the first control input, and the second current source circuit (2) includes a second control input adapted to set the current level based on the voltage across a second resistor between the actual ground and the second control input, wherein the compensation device includes coupling between the virtual ground terminal and the second control input.

2. An LED driver circuit, comprising: Input (Vrect) is suitable for receiving rectified mains input signals; A storage capacitor (C1) is coupled to the input (Vrect). A first current source circuit (1) is adapted to set an output current, wherein the first current source circuit (1) is controlled by a pulse width modulation having a turn-on period and a turn-off period, and wherein the first current source circuit (1) and the storage capacitor (C1) form a capacitor discharge circuit. A charging circuit adapted to charge the storage capacitor (C1), the charging circuit including a second current source circuit (2), wherein the second current source circuit (2) and the storage capacitor (C1) form a capacitor charging circuit; and The compensation device is adapted to increase the current delivered by the second current source circuit (2) during the off-cycle time of the pulse width modulation. The compensation device includes a compensation circuit adapted to set the current of the second current source circuit according to the pulse width modulation setting of the first current source circuit, wherein the compensation circuit includes a transistor adapted to pull up or pull down the control terminal of the second current source circuit according to the pulse width modulation setting of the first current source circuit.

3. The driver circuit according to claim 1, wherein the compensation device is adapted to increase the current delivered by the second current source circuit (2) during the off-cycle time of the pulse width modulation only when drawing current from the input.

4. The driver circuit according to claim 1, wherein the compensation device includes a compensation capacitor connected in parallel with the second resistor.

5. The driver circuit of claim 2, wherein the second current source circuit includes a second control input adapted to set the current level based on the voltage across a second resistor between actual ground and the second control input, wherein the compensation device includes a compensation capacitor connected in parallel with the second resistor.

6. The driver circuit according to claim 4 or 5, wherein the second resistor and the compensation capacitor form a circuit having a cutoff frequency that is twice the size of the PWM frequency.

7. The driver circuit according to any one of claims 1, 2, 3, 4 or 5, comprising a first diode (D1) in the charging circuit and a second diode (D2) in the discharging circuit.

8. The driver circuit of claim 7, wherein the first diode and the second diode are connected to a node (N1) having opposite polarities.

9. A lighting device, comprising: The driver circuit according to any one of claims 1 to 8; as well as The output current is delivered to the LED device.

10. The lighting device according to claim 9, wherein the LED device comprises an LED filament bulb.

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