EP3644691A1

EP3644691A1 - Bleeder circuit and led driving circuit applying the same

Info

Publication number: EP3644691A1
Application number: EP19202845.4A
Authority: EP
Inventors: Wei Chen
Original assignee: Hangzhou Silergy Semiconductor Technology Ltd
Current assignee: Hangzhou Silergy Semiconductor Technology Ltd
Priority date: 2018-10-24
Filing date: 2019-10-11
Publication date: 2020-04-29
Also published as: CN109275230A; CN109275230B

Abstract

A bleeder circuit and an LED driving circuit applying the same. A bleeder circuit is controlled to generate a bleeder current at a moment of a dimmer being switched on, such that the dimmer can be switched on quickly. In a case that a driving current in a control circuit is sufficient to keep the dimmer on, the bleeder circuit stops generating the bleeder current. Thereby, the bleeder current can be maintained for a certain period, reducing a power loss of the bleeder circuit and improving system efficiency.

Description

FIELD

The present disclosure relates to the field of power electronic technology, particularly to a bleeder circuit and an LED driving circuit applying the bleeder circuit.

BACKGROUND

In conventional technology, an LED driving circuit uses a dimmer to adjust brightness of semiconductor light sources, such as light emitting diodes (LEDs), so as to generate a desired lighting effect. A conventional dimmer typically cuts a portion of each waveform of an input signal from a power source, and passes the remaining portion of the waveform to a light source. A widely used type of dimmers is TRIACs (bidirectional silicon controlled rectifiers), which have a simple circuit design and a low cost.
A latching current is a minimum main current capable to latch the TRIAC dimmer in an on-state, after the TRIAC dimmer is switched from an off-state to the on-state and a trigger signal is removed. A holding current is a minimum main current required to keep the TRIAC dimmer in the on-state. The holding current is generally less than the latching current. After a gate current is removed, a main current of the TRIAC dimmer should be immediately above the latching current and kept consistently above the holding current, so as to keep the TRIAC dimmer on throughout a whole dimming range.
The TRIAC dimmer provides an adjustable DC-bus voltage to an LED load, so as to control light outputted by the LED load. During a duty cycle, a bleeder current from the adjustable DC-bus voltage is provided to the TRIAC dimmer before the TRIAC dimmer is switched on. Thereby, a load current of the TRIAC dimmer is increased to switch on the TRIAC dimmer, avoiding flashing of the dimmer due to a small input current. In a case that the TRIAC dimmer is switched on, the bleeder current is always kept at a fixed value. Thereby, the bleeder current generated by the bleeder circuit is maintained at a high level for a long time, resulting in a large loss of the LED driving circuit and reduced system efficiency.

SUMMARY

In view of the above, a bleeder circuit is provided according to the present disclosure, aiming at providing a controllable bleeder current, shortening duration of the bleeder current, reducing a system loss, and improving system efficiency.
In a first aspect of the present disclosure, a bleeder circuit is provided for an LED driving circuit, including:

a bleeder current generating circuit, coupled to a DC bus of the LED driving circuit, and controlled to provide a bleeder current from the DC-bus to a dimmer; and
a controller, configured to control the bleeder current generating circuit to start generating the bleeder current in response to the dimmer being switched on, and switching off the bleeder current generating circuit when the driving current of the LED driving circuit is greater than a threshold current.

Preferably, the threshold current is equal to zero.
Preferably, the threshold current is not less than a holding current of the dimmer.
Preferably, the controller detects a transition of a DC-bus voltage, to determine a moment of the dimmer being switched on.
Preferably, the controller includes:

a detection circuit, configured to receive a sampling voltage characterizing the driving current; and
a logic circuit, configured to control the bleeder component to be switched between on and off based on the first comparison result and the second comparison result.

Preferably, the detection circuit comprises a first comparison circuit, configured to compare the sampling voltage with a first threshold voltage, and n output signal of the first comparison circuit is coupled to the logic circuit.
Preferably, the first threshold voltage is set in accordance with a holding current of the dimmer.
Preferably, the detection circuit further includes:

a second comparison circuit, configured to compare a DC bus voltage with a second threshold voltage, and an output signal of the second comparison circuit is coupled to the logic circuit; and
a third comparison circuit, configured to compare the DC bus voltage with a third threshold voltage, and an output signal of the third comparison circuit is coupled to the logic circuit.

Preferably, the logic circuit is configured to control the bleeder current generating circuit to be switched between on and off based on the output signals of the first comparison circuit, the second comparison circuit, and third comparison circuit.
Preferably, the dimmer is a leading-edge phase-cut dimmer including a bidirectional triode thyristor.
Preferably, the bleeder current is kept constant when the bleeder current generating circuit is enabled.
Preferably, the controller includes:

a detection circuit, configured to detect a DC-bus voltage and the driving current, and generate a first comparison result and a second comparison result; and
a logic circuit, configured to control the bleeder current generating circuit in response to the first comparison result and the second comparison result.

In a second aspect of the present disclosure, an LED driving circuit is provided, including:

any bleeder circuit according to the first aspect;
a rectifying circuit, configured to receive an adjustable voltage signal generated by the dimmer, to generate the DC-bus voltage; and
a control circuit, configured to receive the DC-bus voltage and to generate the driving current to drive an LED load.

Preferably, the bleeder circuit and the control circuit are in parallel connection.
Preferably, the control circuit includes a power component coupled in series with the LED load which is controlled to be operated in linear mode.
The technical solutions according to embodiments of the present disclosure controls the bleeder circuit to generate the bleeder current in response to the dimmer being switched on, such that the dimmer can be switched on quickly. In a case that the driving current in the control circuit is sufficient to keep the dimmer on, the bleeder circuit stops generating the bleeder current. Thereby, the bleeder current is maintained for a certain period, reducing a power loss of the bleeder circuit and improving system efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter embodiments of the present disclosure is described in conjunction with drawings, to make the aforementioned and other objectives, characteristics and advantages of the present disclosure clearer. The drawings are as follows.

Figure 1 is a block diagram of an LED driving circuit according to an embodiment of the present disclosure;
Figure 2 is a circuit diagram of an LED driving circuit according to an embodiment of the present disclosure;
Figure 3 is a circuit diagram of a detection circuit according to an embodiment of the present disclosure; and
Figure 4 is an operation waveform diagram of the LED driving circuit according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present disclosure are described hereinafter. The present disclosure is not limited by the described embodiments. Hereinafter specific detailed parts are fully described in the description of the present disclosure. Those skilled in the art may thoroughly understand the present disclosure without such specific detailed parts. Methods, processes, elements and circuits that are well known by those skilled in the art are not fully described to prevent confusing substantial contents of the present disclosure.
In addition, those skilled in the art should appreciate that the provided drawings are for illustration, and dimensions shown in the drawings may not be drawn to scale.
In addition, it should be appreciated that the wording "circuit" in following description may refer to a conductive loop formed by at least one element or sub-circuit connected electrically or electromagnetically. In a case that an element or a circuit is referred to "connect" to another element or an element/circuit is referred to be "connected" between two nodes, it may be directly coupled or connected to another element, or there may be an intermediate element. Connections between elements may refer to a physical connection, a logical connection, or a combination of the physical connection and the logical connection. In a case that an element is referred to be "directly coupled" or "directly connected" with another element, it means that there is no intermediate element connected between them.
Unless explicitly defined otherwise in context, the terms "include", "comprise" or other similar terms in the whole specification and claims should be interpreted to be inclusive instead of being exclusive or exhaustive. Namely, they should be interpreted to be "including but not being limited to".
It should be appreciated in the description of the present disclosure that the terms "first" and "second" in the descriptions are merely for description, and should not be interpreted as indication or implication of relative importance. In addition, unless defined otherwise, the term "multiple" refers to a quantity of two or more than two in the description of the present disclosure.
Figure 1 is a block diagram of an LED driving circuit according to an embodiment of the present disclosure. As illustrated in Figure 1, an LED driving circuit 100 includes a dimmer 102 connected to an input voltage 101. In multiple implementations, the input voltage 101 may be an unrectified AC input voltage, for example, a 220V AC voltage. The dimmer 102 is a leading-edge phase-cut dimmer including a TRIAC. The dimmer 102 cuts off a leading edge of a waveform of a voltage signal from the input voltage 101, according to user's settings of a system, in order to provide an adjustable voltage signal and achieve dimming. A rectifying circuit 103 receives and rectifies the adjustable voltage signal, to generate a DC-bus voltage VBUS along a DC bus. A control circuit 104 receives the DC-bus voltage VBUS, and outputs a corresponding DC voltage to power a light source. In one embodiment, the light source is a light emitting diode (LED). The LED driving circuit 100 further includes a bleeder circuit 105 in parallel with the LED load. The bleeder circuit 105 provides a bleeder current from the DC bus to the dimmer 102, thereby increasing a load current of the dimmer 102, and enabling the dimmer 102 to maintain normal operation.
In one embodiment, the bleeder circuit 105 includes a bleeder current generating circuit and a controller. The bleeder circuit 105 starts to provide the bleeder current I_BLD in response to the dimmer 102 being switched on, and stop generating the bleeder current I_BLD in response to a driving current Id provided by the LED driving circuit. In one embodiment, the bleeder circuit 105 stop generating the bleeder current I_BLD when the driving current Id is greater than a threshold current. When the bleeder circuit 105 detects that the DC-bus voltage VBUS jumps, the dimmer 102 is switched on, and the bleeder circuit 105 controls a bleeder component to start generating the bleeder current. Thereby, the load current of the dimmer 102 is immediately greater than a latching current thereof in response to being switched on, so as to achieve fast switching-on. The bleeder circuit 105 samples a sampling voltage Vs characterizing the driving current Id, to determine magnitude of the driving current Id. In a case that the sampling voltage Vs is greater than a threshold voltage characterizing the threshold current, the bleeder circuit 105 switches off the bleeder component and stops generating the bleeder current. The threshold current is not less than a holding current of the TRIAC dimmer.
In one embodiment, the bleeder circuit does not start to generate the bleeder current until the dimmer is switched on, which can reduce a power consumption of bleeder in case of a large conduction angle of the dimmer. In a case that the driving current is greater than the threshold current, the bleeder circuit stops generating the bleeder current to reduce duration of the bleeder current, thereby reducing a power loss of the bleeder circuit and improving system efficiency.
Shown in Figure 2 is a circuit diagram of an LED driving circuit according to an embodiment of the present disclosure. As illustrated in Figure 2, an LED driving circuit 200 includes an input voltage source 201, a dimmer 202, a rectifying circuit 203, a control circuit 204, and a bleeder circuit 205. The control circuit 204 includes a power component Q2 and a sampling resistor R1 that are connected in series. A first end of the sampling resistor R1 is grounded, and a second end of the sampling resistor R1 is connected to a first end of the power component Q2. The driving current Id flows through the sampling resistor R1, and the sampling voltage Vs characterizing the driving current Id is generated at the second end of the sampling resistor R1. A second end of the power component Q2 is coupled to a common point B at which an output capacitor C1 and an LED load are connected in parallel. A control terminal or a gate of the power component Q2 is coupled to an output of an error amplifier 2041. The error amplifier 2041 receives the sampling voltage Vs and a reference voltage Vref, to control the power component Q2 to generate a current. It should be understood that although the power component Q2 shown in Figure 2 is an N-type MOSFET, the power component Q2 may be a field effect transistor of any type, and may include transistors of other types in the art without departing from the scope of teachings according to the present disclosure. Additionally, the control circuit is not limited to the mode in this embodiment, and another circuit having a similar structure or function is also suitable for implementation.
The control circuit 204 may further include a diode D1 configured to prevent the output capacitor C1 from discharging an input port. It should be understood that diode D1 may be replaced with another circuit capable to conduct uni-directionally. An anode of the diode D1 receives the DC-bus voltage VBUS, and a cathode of the diode D1 is connected to a common point A at which the output capacitor C1 and the LED load are connected in parallel.
The bleeder circuit 205 includes a controller 21 and a bleeder current generating circuit including a bleeder component Q1. The bleeder current generating circuit is connected to the DC bus and controlled to provide the bleeder current I_BLD from the DC bus to the dimmer 202. The controller 21 includes a detection circuit 2051 and a logic circuit 2052. In one embodiment, the logic circuit 2052 is an RS flip-flop. In multiple implementations, the bleeder component Q1 may be a field effect transistor of any type, such as a metal-oxide semiconductor field effect transistor (MOSFET) or a bipolar transistor. In a case that the dimmer 202 is switched on, the rectifying circuit 203 generates the DC-bus voltage VBUS that increases rapidly. Therefore, the detection circuit 2051 can determine the dimmer 202 being switched on by detecting a transition of the DC-bus voltage VBUS. When the DC-bus voltage VBUS jumps, the dimmer 202 is switched on, and the detection circuit 2051 outputs an active signal to set the RS flip-flop 2052. An output signal Q of the RS flip-flop 2052 controls the bleeder component Q1 to be switched on. The bleeder component Q1 pulls the bleeder current I_BLD down from the DC-bus, to increase the load current of the dimmer 202. In a case that the dimmer 202 is turned on, the driving current Id is zero since the DC-bus voltage VBUS is lower than a driving voltage of the LED load, that is, Id=0 and I_in=I_BLD. The bleeder current I_BLD is configured to latch the dimmer on. In a case that the DC-bus voltage VBUS reaches the driving voltage of the LED load, there is a current flowing through the LED load. Further, in a case that the driving current Id in the control circuit 204 is sufficiently large, the detection circuit 2051 generates an active signal according to the sampling voltage Vs to reset the RS flip-flop 2052. The output signal Q of the RS flip-flop 2052 controls the bleeder component Q1 to be switched off, and the bleeder component Q1 stops pulling down the bleeder current I_BLD. In such case, the driving current Id provided by the control circuit 204 is not less than a holding current of the dimmer 202, and can keep the dimmer 202 operating normally. Therefore, the bleeder circuit 205 does not need to provide additional load current to the dimmer 202. It should be understood that, since the control circuit 204 is controlled in a linear driving mode in this embodiment, it is convenient to use the sampling resistor R1 to detect the driving current Id flowing through the control circuit. Detection of the driving current Id is not limited to the implementation in this embodiment, another circuit having a similar structure or function is also suitable for implementation. As an alternative embodiment, the detection circuit determines the magnitude of the driving current by detecting a current flowing through the diode D1.
Shown in Figure 3 is a circuit diagram of a detection circuit according to an embodiment of the present disclosure. A detection circuit 300 acquires a moment of the dimmer being switched on, by detecting a transition of the DC-bus voltage VBUS. In a case that the DC-bus voltage VBUS is greater than both a first threshold voltage Vref1 and a second threshold voltage Vref2, which indicates that the DC-bus voltage VBUS jumps, the dimmer is switched on, and the bleeder circuit controls the bleeder component Q1 to generate the bleeder current I_BLD. When the sampling voltage Vs characterizing the driving current Id flowing through the control circuit is greater than a third threshold voltage Vref3, the driving current Id provided by the control circuit is not less than the holding current of the dimmer and can keep the dimmer on. The bleeder circuit switches off the bleeder component Q1 and stops generating the bleeder current I_BLD.
As illustrated in Figure 3, the detection circuit 300 includes a first comparator COM1, a second comparator COM2, and a third comparator COM3. A first input terminal (e.g. a non-inverting input terminal) of the first comparator COM1 receives the DC-bus voltage VBUS. A second input terminal (e.g. an inverting input terminal) of the first comparator COM1 receives the first threshold voltage Vref1. An output terminal of the first comparator COM1 is coupled to a one-shot circuit OS1. An output terminal of the one-shot circuit OS1 is coupled to a first input terminal of an AND gate AND. A first input terminal (e.g., a non-inverting input terminal) of the second comparator COM2 receives the DC-bus voltage VBUS. A second input terminal (e.g. an inverting input terminal) of the second comparator COM2 receives the second threshold voltage Vref2. An output terminal of the second comparator COM2 is coupled to a one-shot circuit OS2. The second threshold voltage Vref2 is greater than the first threshold voltage Vref1. The second threshold voltage Vref2 may be selected to be N times the first threshold voltage Vref1, and N is greater than 1. An output terminal of the one-shot circuit OS2 is coupled to a second input terminal of the AND gate AND. An output terminal of the AND gate AND is coupled to a set terminal S of an RS flip-flop 301. The one-shot circuit OS1 and the one-shot circuit OS2 may generate a pulse having a predetermined temporal length in response to a rising edge or a falling edge of an input signal, such that the AND gate AND outputs a first comparison result L1 and sets the RS flip-flop 301. A first input terminal (e.g. a non-inverting input terminal) of the third comparator COM3 receives the sampling voltage Vs characterizing the driving current flowing through the control circuit. A second input terminal (e.g. an inverting input terminal) of the third comparator COM3 receives the third threshold voltage Vref3. An output terminal of the third comparator COM3 is coupled to a one-shot circuit OS3. The third threshold voltage Vref3 characterizes the holding current of the dimmer. The output terminal of the one-shot circuit OS3 is coupled to a reset terminal R of the RS flip-flop 301. The one-shot circuit OS3 may generate a pulse having a predetermined temporal length in response to a rising edge or a falling edge of an input signal, so as to generate a second comparison result L2 and reset the RS flip-flop 301. An output terminal Q of the RS flip-flop 301 controls the bleeder component Q1. The bleeder component Q1 is controlled to be switched between on and off by an output of the RS flip-flop 301. In a case that the DC-bus voltage VBUS rises above the first threshold voltage Vref1 and further to the second threshold voltage Vref2, the RS flip-flop 301 is set based on the first comparison result L1. The bleeder component Q1 is switched on, and the bleeder circuit generates the bleeder current I_BLD. In a case that the sampling voltage Vs characterizing the driving current rises to the third threshold voltage Vref3, the RS flip-flop 301 is reset based on the second comparison result L2. The bleeder component Q1 is switched off, and the bleeder circuit stops generating the bleeder current I_BLD. It should be understood that detection of the DC-bus voltage is not limited to the implementation in this embodiment, and another circuit having a similar structure or function is also suitable for implementation. As an alternative embodiment, the detection circuit can determine the jump of the DC-bus voltage by sampling a portion of the DC-bus voltage allocated to a divider resistor.
Shown in Figure 4 is an operation waveform diagram of the LED driving circuit according to an embodiment of the present disclosure. As illustrated in Figure 4, during an interval from t0 to t1, the dimmer is not switched on and the bleeder current I_BLD is zero. At such time, a capacitor inside the dimmer generates a leakage current that flows through the DC-bus, such that the DC-bus voltage VBUS is uncertain before the dimmer is switched on. In conventional technology, a designer can clamp the DC-bus voltage to a fixed value by using a specific control manner, to prevent an additional loss on the LED driving circuit due to uncertainty of the DC-bus voltage.
At the moment t1, the detection circuit detects the jump of the DC-bus voltage. The dimmer is switched on, and the bleeder circuit starts to generate the bleeder current I_BLD. Thereby, the dimmer is switched on quickly. During an interval from t1 to t2, the bleeder current I_BLD is kept constant to ensure the dimmer operating normally.
At the moment t2, the sampling voltage Vs characterizing the driving current is increased, and the detection circuit detects that the driving current in the control circuit is sufficient to keep the dimmer on. The bleeder circuit stops generating the bleeder current I_BLD, and the bleeder current I_BLD drops to zero.
The technical solution according to the embodiments of the present disclosure controls the bleeder circuit to generate the bleeder current in response to the dimmer being switched on, such that the dimmer can be switched on quickly. In a case that the driving current in the control circuit is sufficient to keep the dimmer on, the bleeder circuit stops generating the bleeder current. Thereby, the bleeder circuit operates for a certain period, reducing the power loss of the bleeder circuit and improving system efficiency.
Described above are only preferable embodiments of the present disclosure, and the present disclosure are not limited thereto. Those skilled in the art can make various modifications and variations to the present disclosure. Any modification, equivalent replacement, modification, or the like that is made within the spirit and principle of the present disclosure should fall within the protection scope of the present disclosure.

Claims

A bleeder circuit for an LED driving circuit, comprising:
a bleeder current generating circuit, coupled to a DC bus of the LED driving circuit, and controlled to provide a bleeder current from the DC-bus to a dimmer; and

a controller, configured to:
control the bleeder current generating circuit to start generating the bleeder current in response to the dimmer being switched on, and

switching off the bleeder current generating circuit when the driving current of the LED driving circuit is greater than a threshold current.
The bleeder circuit according to claim 1, wherein the threshold current is equal to zero.
The bleeder circuit according to claim 1, wherein the threshold current is not less than a holding current of the dimmer.
The bleeder circuit according to claim 1, wherein the controller detects a transition of a DC-bus voltage, to determine a moment of the dimmer being switched on.
The bleeder circuit according to claim 1, wherein the controller comprises:
a detection circuit, configured to receive a sampling voltage characterizing the driving current; and

a logic circuit, configured to switch off the bleeder current generating circuit based on the sampling voltage.
The bleeder circuit according to claim 5, wherein:
the detection circuit comprises a first comparison circuit, configured to compare the sampling voltage with a first threshold voltage, and

an output signal of the first comparison circuit is coupled to the logic circuit.
The bleeder circuit according to claim 6, wherein the first threshold voltage is set in accordance with a holding current of the dimmer.
The bleeder circuit according to claim 6, wherein the detection circuit further comprises:
a second comparison circuit, configured to compare a DC bus voltage with a second threshold voltage, and an output signal of the second comparison circuit is coupled to the logic circuit; and

a third comparison circuit, configured to compare the DC bus voltage with a third threshold voltage, and an output signal of the third comparison circuit is coupled to the logic circuit.
The bleeder circuit according to claim 8, wherein the logic circuit is configured to control the bleeder current generating circuit to be switched between on and off based on the output signals of the first comparison circuit, the second comparison circuit, and third comparison circuit.
The bleeder circuit according to claim 1, wherein the dimmer is a leading-edge phase-cut dimmer including a bidirectional triode thyristor.
The bleeder circuit according to claim 1, wherein the bleeder current is kept constant when the bleeder current generating circuit is enabled.
The bleeder circuit according to claim 1, wherein the controller comprises:
a detection circuit, configured to detect a DC-bus voltage and the driving current, and generate a first comparison result and a second comparison result; and

a logic circuit, configured to control the bleeder current generating circuit in response to the first comparison result and the second comparison result.
An LED driving circuit, comprising:
a bleeder circuit according to any one of claims 1 to 14;

a rectifying circuit, configured to receive an adjustable voltage signal generated by the dimmer, to generate the DC-bus voltage; and

a control circuit, configured to receive the DC-bus voltage and to generate the driving current to drive an LED load.
The LED driving circuit according to claim 15, wherein the bleeder circuit and the control circuit are in parallel connection.
The LED driving circuit according to claim 15, wherein the control circuit comprises a power component coupled in series with the LED load which is controlled to be operated in linear mode.