WO2013120682A1

WO2013120682A1 - Single stage led driver and illuminating device having the led driver, low ripple by input boost unit

Info

Publication number: WO2013120682A1
Application number: PCT/EP2013/051457
Authority: WO
Inventors: Zeke WEI; Michele Menegazzi; Xihe ZHUANG; Dan LIN
Original assignee: Osram GmbH
Current assignee: Osram GmbH
Priority date: 2012-02-16
Filing date: 2013-01-25
Publication date: 2013-08-22
Anticipated expiration: 2014-08-16
Also published as: CN103260289A

Description

SINGLE STAGE LED DRIVER AND ILLUMINATING DEVICE HAVING THE LED DRIVER, LOW RIPPLE BY INPUT BOOST UNIT Technical Field

The present invention relates to an LED driver for an illuminating device. In addition, the present invention further relates to an illuminating device having such LED driver.

Background Art An LED illuminating device as a cold light source has the ad^¬ vantages such as pure color of light, concentrated light beam, high color rendering index, good temperature properties, low electrical energy consumption, stable performance, long service lifetime, no pollution and simple utilization. In order to drive an LED light source, a large number of various types of AC-DC LED drivers are available in the market, which include single-stage LED driver, two-stage LED driver. However, they have different advantages and disadvan^¬ tages, for example, the single-stage LED driver has a simple structure and low cost, and can provide a good power factor, but generates a big ripple current; otherwise some problems will appear in some applications. For instance, in indoor ap^¬ plication, a big ripple current is easily caught by a camera because the ripple current is 100Hz/120Hz which is coupled from a main input. The two-stage LED driver can provide a high power factor and a relatively low ripple current, but has a complex structure and a high cost.

A single-stage LED driver is disclosed in the prior art. As shown in Fig. 1, the primary terminal of the single-stage LED driver has a quite simple structure and only has a bypass ca- pacitor C3 connected downstream from the rectifier bridge for reducing the ripple current. However, the capacitance value of the bypass capacitor C3 is usually small and cannot effectively reduce the ripple current in the primary terminal. In order to reduce the ripple current in the current output from the secondary terminal, a plurality of capacitors C2, C4 and C5 having a big capacitance value should be connected in parallel at the output terminal, which obviously will increase the cost. A two-stage LED driver is shown in Fig. 2. This two-stage LED driver is connected with a boost input unit and an additional filter capacitor CI downstream from the rectifier bridge. This boost input unit is directly connected to the filter ca^¬ pacitor CI for maintaining the voltage in the filter capaci- tor CI, but an additional control IC and control switch are needed. Though this solution well reduces the ripple current in the main output, it has a high cost.

Summary of the Invention

In order to solve the above technical problem, the present invention provides an LED driver that can well reduce the ripple current in the main output and has a relatively low cost and simple structure. In addition, the present invention also provides an illuminating device having such LED driver.

The first object of the present invention is accomplished via an LED driver. The LED driver comprises a rectifier bridge; a convertor connected to the rectifier bridge and comprising a primary winding and a secondary winding; a primary filter unit that has a first end connected between a first terminal of the primary winding and the rectifier bridge and a second end connected to ground; a switch unit connected to a second terminal of the primary winding to control connection between the primary winding and the ground; and a secondary output unit connected to the secondary winding, wherein the LED driver further comprises a boost input unit that has a first end connected between the rectifier bridge and the first terminal of the primary winding and a second end connected to the second terminal of the primary winding, wherein when the switch unit is enabled, the boost input unit and the primary winding are charged, and the primary filter unit discharges via the primary winding; and when the switch unit is disenabled, the boost input unit discharges via the primary winding so as to provide a charge current to the primary filter unit. In the solution of the present invention, the ripple current on the primary rectifier unit can be well maintained by means of the boost input unit, so that the ripple current in the discharge current of the primary winding is relatively small, and the ripple current in the main output also can be well reduced without using capacitors having a big capacitance value. In addition, it completely does not need addi- tional control IC for controlling the boost input unit to charge the primary filter unit, thus further reducing the cost .

Preferably, the boost input unit comprises an inductor and a first diode, the inductor has a first terminal connected be- tween the rectifier bridge and the first terminal of the primary winding and the second terminal connected to an anode of the first diode, and a cathode of the first diode is connected to the second terminal of the primary winding. When the switch unit is turned on, the current from the rectifier bridge is charged the inductor, so that when the switch power supply is turned off, the inductor provides a charge current to the primary filter unit. Advantageously, the LED driver according to the present invention further comprises a second diode that has an anode connected between the first terminal of the inductor and the rectifier bridge and a cathode connected to the first termi- nal of the primary winding. The second diode is used to provide a charge circuit to the primary filter unit when the LED driver is started, preventing an impulse current from flowing through the boost input unit.

Further preferably, the primary filter unit is a first ca- pacitor, an anode of the first capacitor is connected between the cathode of the second diode and the first terminal of the primary winding, and a cathode of the first capacitor is connected to the ground. In one solution of the present inven^¬ tion, the first capacitor is designed to be an electrolytic capacitor.

According to one preferred solution of the present invention, the boost input unit further comprises an RCD snubber that comprises a first resistor, a second capacitor and a third diode, wherein the first resistor has a first end connected between the second terminal of the inductor and the anode of the first diode and a second end connected to a cathode of the third diode, an anode of the third diode is connected to the ground, and the second capacitor has a first terminal connected between the second terminal of the inductor and the anode of the first diode and a second terminal connected be^¬ tween the second end of the first resistor and the cathode of the third diode. When the switch unit is disenabled, the first diode enters into reverse recovery, and a current will reversely flow into the inductor. After reverse biased of the first diode, the inductor needs a continuous current loop, otherwise, the first diode possibly will be reverse broken down. The RCD snubber provides this continuous current loop Preferably, the LED driver further comprises a third capacitor that has a first end connected between the first terminal of the inductor and the rectifier bridge and a second end connected to the ground. The third capacitor has a relatively small capacitance value and has a filtering function.

According to the present invention, the convertor is designed to be a flyback convertor. According to the principle of flyback convertor, the primary winding of the flyback convertor is charged when enabled, and discharges when disenabled to transfer energy to the secondary winding.

Further preferably, the switch unit comprises a controller and a transistor, wherein an output end of the controller is connected to a control Electrode of the transistor, a working Electrode of the transistor is connected to the second termi- nal of the primary winding, and a reference Electrode of the transistor is connected to the ground. The controller controls on or off state of the transistor, so that the primary winding and the ground are allowed to be turned on or turned off. In one solution of the present invention, the transistor can be a MOS transistor.

According to the present invention, the secondary output unit comprises a fourth diode, a fourth capacitor and a second re^¬ sistor, wherein a first terminal of the secondary winding is connected to an anode of the fourth diode, a cathode of the fourth diode is connected to an anode of a LED, the second terminal of the second winding is connected to the cathode of the LED through the second resistor, an anode of the fourth capacitor is connected between the cathode of the fourth diode and the anode of the LED, and a cathode of the fourth ca- pacitor is connected between a second terminal of the secon^¬ dary winding and the second resistor. When the switch unit is enabled, the primary winding and the inductor charge, and the fourth diode is reverse biased. When the switch unit is disenabled, the primary winding discharges, the fourth diode is forward biased, so as to realize energy transfer. In one so- lution of the present invention, only one fourth capacitor having a small capacitance value is used in the secondary input unit, and this fourth capacitor is designed to be an electrolytic capacitor that is sufficient to reduce the rip^¬ ple current in the main output.

According to the present invention, the LED driver further comprises a detecting unit that comprises a first detection input, a second detection input and a detection output, wherein the first detection input is connected between the cathode of the fourth diode and the anode of the LED for detecting a voltage in the main output, the second detection input is connected between the second resistor and the cath^¬ ode of the LED, and the detection output is connected to an input of the controller. In one solution of the present invention, the detecting unit, on one hand, detects the voltage in the main output to provide overvoltage protection, and on the other hand, detects the current in the main output and generates a control signal for the switch unit. The controller in the switch unit controls frequency of on and off of the transistor according to this control signal so as to ob^¬ tain expected output voltage.

The other object of the present invention is realized by an illuminating device having the LED driver of the above type. The frequency of light output from the illuminating device is higher, and flicker of the light will not be easily captured by the camera. Besides, the cost of the illuminating device of the present invention is lower. It shall be understood that both the above general description and the following detailed description are for illustrative and explanative purposes in order to provide further description of the claimed present invention. Brief Description of the Drawings

The accompanying drawings constitute a part of the present Description and are used to provide further understanding of the present invention. Such accompanying drawings illustrate the embodiments of the present invention and are used to de- scribe the principles of the present invention together with the Description. In the accompanying drawings the same components are represented by the same reference numbers. As shown in the drawings :

Fig. 1 is a circuit diagram of a single-stage LED driver in the prior art;

Fig. 2 is a circuit diagram of a two-stage LED driver in the prior art;

Fig. 3 is a principle block diagram of an LED driver according to the present invention; Fig. 4 is a circuit diagram of a first embodiment of the LED driver according to the present invention;

Fig. 5 is a circuit diagram of a second embodiment of the LED driver according to the present invention;

Fig. 6 is a waveform diagram of current of the LED driver of the present invention; Fig. 7 is a waveform diagram of ripple current in a main output of an LED driver in the prior art; and

Fig. 8 is a waveform diagram of ripple current in a main output of an LED driver according to the present invention. Detailed Description of the Embodiments

Fig. 3 is a principle block diagram of an LED driver accord^¬ ing to the present invention. It can be seen from Fig. 3 that the LED driver comprises a rectifier bridge 1; a convertor 2 connected to the rectifier bridge 1 and comprising a primary winding PI and a secondary winding SI; a primary filter unit 4 that has a first end connected between a first terminal of the primary winding PI and the rectifier bridge 1 and a second end connected to GND; a switch unit 3 connected to a second terminal of the primary winding PI to control connection between the primary winding PI and the ground GND; and a secondary output unit 5 connected to the secondary winding SI. It also can be seen from Fig. 3 that the LED driver further comprises a boost input unit 6 that has a first end connected between the rectifier bridge 1 and the first terminal of the primary winding PI and a second end connected to the second terminal of the primary winding PI. In one solution of the present invention, the convertor 2 is designed to be a fly^¬ back convertor that is charged when the primary winding PI is enabled and discharges when the primary winding PI is disen- abled to transfer energy to the secondary winding SI.

Fig. 4 is a circuit diagram of a first embodiment of the LED driver according to the present invention. It can be seen from Fig. 4 that the boost input unit 6 comprises an inductor LI and a first diode Dl . The inductor LI has first terminal connected between the rectifier bridge 1 and the first termi- nal of the primary winding PI and the second terminal connected to an anode of the first diode Dl, and a cathode of the first diode Dl is connected to the second terminal of the primary winding PI. In addition, in order to provide a charge circuit to the primary filter unit 4 and prevent an impulse current from flowing through the boost input unit 6 when the LED driver is started, the LED driver further comprises a second diode D2 that has an anode connected between the first terminal of the inductor 11 and the rectifier bridge 1 and a cathode connected to the first terminal of the primary winding PI .

In the circuit diagram shown in Fig. 4, the first capacitor CI constitutes the primary filter unit 4. An anode of the first capacitor CI is connected between the cathode of the second diode D2 and the first terminal of the primary winding PI, and a cathode of the first capacitor CI is connected to the ground GND.

Besides, it can be seen from Fig. 4 that the LED driver also comprises a third capacitor C3 that has a first end connected between the first terminal of the inductor LI and the rectifier bridge 1 and a second end connected to the ground GND.

The switch unit 3 controlling the primary winding PI comprises a controller 3a and a transistor Ql, wherein an output of the controller 3a is connected to a control Electrode of the transistor Ql, a working Electrode of the transistor Ql is connected to the second terminal of the primary winding PI, and a reference Electrode of the transistor Ql is connected to the ground GND.

The secondary output unit 5 of the LED driver according to the present invention is shown on the right of the secondary winding SI, which comprises a fourth diode D4, a fourth capacitor C4 and a second resistor R2, wherein a first terminal of the secondary winding SI is connected to an anode of the fourth diode D4, a cathode of the fourth diode D4 is con- nected to an anode of a LED, he second terminal of the second winding SI is connected to the cathode of the LED through the second resistor R2, an anode of the fourth capacitor C4 is connected between the cathode of the fourth diode D4 and the anode of the LED, and a cathode of the fourth capacitor C4 is connected between a second terminal of the secondary winding SI and the second resistor R2.

In order to detect a voltage of a main output for overvoltage protection, and for the sake of detecting a current in the main output to generate a control signal for the switch unit, the LED driver also comprises a detecting unit 7. The detecting unit 7 comprises a first detection input 7a, a second de^¬ tection input 7b and a detection output 7c, wherein the first detection input 7a is connected between the cathode of the fourth diode D4 and the anode of the LED, the second detec- tion input 7b is connected between the second resistor R2 and the cathode of the LED, and the detection output 7c is connected to an input of the controller 3a.

Fig. 5 is a circuit diagram of a second embodiment of the LED driver according to the present invention, which has the only difference from the first embodiment of the LED driver ac^¬ cording to the present invention shown in Fig. 4 in that the boost input unit 6 further comprises an RCD snubber. The RCD snubber comprises a first resistor Rl, a second capacitor C2 and the third diode D3. The first resistor Rl has a first end connected between the second terminal of the inductor LI and the anode of the first diode Dl and a second end connected to a cathode of the third diode D3, an anode of the third diode D3 is connected to the ground GND, and the second capacitor C2 has a first terminal connected between the second terminal of the inductor LI and the anode of the first diode Dl and the second terminal connected between second end of the first resistor Rl and the cathode of the third diode D3. When the switch unit 3 is disenabled, the first diode Dl enters into reverse recovery, and a current will reversely flow into the inductor LI. After reverse cut-off of the first diode Dl, the inductor LI needs a continuous current loop, otherwise, the first diode Dl possibly will be reverse broken down. The RCD snubber provides this continuous current loop.

Next, the actual working condition of the LED driver according to the present invention will be described according to the waveform diagram of current shown in Fig. 6. It can be seen from Fig. 6 that in tO-tl period of time, the transistor Ql is enabled, the inductor LI and the primary winding PI store energy, at which time, the fourth diode D4 is reverse biased, and the primary winding PI does not transfer energy to the secondary winding SI. In tl-t2 period of time, the transistor Ql is disenabled, the inductor 11 releases stored energy, which energy flows through the primary winding PI and enables the first capacitor CI, at which time, the fourth diode D4 is forward biased, and after release of the energy of the inductor LI is completed, the circuit enters oscillation. Thereafter, the circuit again enters the tO-tl period of time. In this solution of the present invention, the controller 7 is a fixed on time type, thus, the current on the inductor LI will follow up the input voltage, and the power factor also will be relatively high. Fig. 7 is a waveform diagram of ripple current in a main output of an LED driver in the prior art; and Fig. 8 is a waveform diagram of ripple current in a main output of an LED driver according to the present invention. Upon comparison of the two waveform diagrams, it can be seen that the ripple current in the main output of the LED driver according to the present invention is obviously smaller than the ripple cur- rent in the main output of the LED driver in the prior art.

The above is merely preferred embodiments of the present invention but not to limit the present invention. For the person skilled in the art, the present invention may have various alterations and changes. Any alterations, eguivalent sub- stitutions, improvements, within the spirit and principle of the present invention, should be covered in the protection scope of the present invention.

List of reference signs

1 rectifier bridge

2 convertor

3 switch unit

3a controller

4 primary filter unit

5 secondary output unit

6 boost input unit 7 detecting unit

7a first detection input

7b second detection input

7c detection output

PI primary winding

SI secondary winding

GND ground

Dl first diode

D2 second diode D3 third diode

D4 fourth diode

LI inductor

CI first capacitor

C2 second capacitor

C3 third capacitor

C4 fourth capacitor

Rl first resistor

R2 second resistor

Ql transistor

Claims

1. An LED driver for an illuminating device, comprising a rectifier bridge (1); a convertor (2) connected to the recti- fier bridge (1) and comprising a primary winding (PI) and a secondary winding (SI); a primary filter unit (4) having a first end connected between a first terminal of the primary winding (PI) and the rectifier bridge (1) and a second end connected to ground (GND) ; a switch unit (3) connected to a second terminal of the primary winding (PI) to control connection between the primary winding (PI) and the ground (GND) ; and a secondary output unit (5) connected to the sec^¬ ondary winding (SI), characterized in that the LED driver further comprises a boost input unit (6) that has a first end connected between the rectifier bridge (1) and the first terminal of the primary winding (PI) and a second end connected to the second terminal of the primary winding (PI), wherein when the switch unit (3) is enabled, the boost input unit (6) and the primary winding (PI) are charged, and the primary filter unit (4) discharges via the primary winding (PI); and when the switch unit (3) is disenabled, the boost input unit (6) discharges via the primary winding (PI) so as to provide a charge current to the primary filter unit (4) .

2. The LED driver according to Claim 1, characterized in that the boost input unit (6) comprises an inductor (LI) and a first diode (Dl) , a first terminal of the inductor (LI) is connected between the rectifier bridge (1) and the first terminal of the primary winding (PI) and a second terminal of the inductor (LI) is connected to an anode of the first diode (Dl), and a cathode of the first diode (Dl) is connected to the second terminal of the primary winding (PI) .

3. The LED driver according to Claim 1, characterized in that the LED driver further comprises a second diode (D2) that has an anode connected between the first terminal of the inductor (LI) and the rectifier bridge (1) and a cathode connected to the first terminal of the primary winding (PI) .

4. The LED driver according to Claim 3, characterized in that the primary filter unit (4) is a first capacitor (CI), an anode of the first capacitor (CI) is connected between the cathode of the second diode (D2) and the first terminal of the primary winding (PI), and a cathode of the first capaci- tor (CI) is connected to the ground (GND) .

5. The LED driver according to Claim 3, characterized in that the boost input unit (6) further comprises an RCD snub- ber that comprises a first resistor (Rl), a second capacitor (C2) and a third diode (D3) , wherein a first end of the first resistor (Rl) is connected between the second terminal of the inductor (LI) and the anode of the first diode (Dl) and a second end of the first resistor (Rl) is connected to a cath^¬ ode of the third diode (D3) , an anode of the third diode (D3) is connected to the ground (GND) , and the second capacitor (C2) has a first terminal connected between the second terminal of the inductor (LI) and the anode of the first diode (Dl) and the second terminal connected between the second end of the first resistor (Rl) and the cathode of the third diode (D3) .

6. The LED driver according to Claim 2, characterized in that the LED driver further comprises a third capacitor (C3) that has a first terminal connected between the first terminal of the inductor (LI) and the rectifier bridge (1) and a second terminal connected to the ground (GND) .

7. The LED driver according to any one of Claims 1-6, char- acterized in that the convertor (2) is a flyback convertor.

8. The LED driver according to any one of Claims 1-6, characterized in that the switch unit (3) comprises a controller (3a) and a transistor (Ql) , wherein an output end of the con- troller (3a) is connected to a control Electrode of the tran^¬ sistor (Ql), a working Electrode of the transistor (Ql) is connected to the second terminal of the primary winding (PI), and a reference Electrode of the transistor (Ql) is connected to the ground (GND) .

9. The LED driver according to Claim 8, characterized in that the secondary output unit (5) comprises a fourth diode (D4), a fourth capacitor (C4) and a second resistor (R2), wherein the first terminal of the secondary winding (SI) is connected to an anode of the fourth diode (D4), a cathode of the fourth diode (D4) is connected to an anode of LED, the second terminal of the second winding (SI) is connected to the cathode of the LED through the second resistor (R2), an anode of the fourth capacitor (C4) is connected between the cathode of the fourth diode (D4) and the anode of the LED, and a cathode of the fourth capacitor (C4) is connected between the second terminal of the secondary winding (SI) and the second resistor (R2) .

10. The LED driver according to Claim 9, characterized in that the LED driver further comprises a detecting unit (7) that comprises a first detection input (7a) , a second detection input (7b) and a detection output (7c), wherein the first detection input (7a) is connected between the cathode of the fourth diode (D4) and the anode of the LED, the second detection input (7b) is connected between the second resistor (R2) and the cathode of the LED, and the detection output (7c) is connected to an input of the controller (3a) .

11. An illuminating device, characterized in that the illuminating device comprises the LED driver according to any one of Claims 1-10.