A Multi-Channel Dc/Dc LED Driver with Inductor-Less Series-Parallel Auto-regulated Rectifier

Chen Hu, Yuanjun Zhang, Xinke Wu College of Electrical Engineering

Zhejiang University Hangzhou 310027, China

wuxinke@zju.edu.cn

Fang Z. Peng Department of Electrical and Computer Engineering

Michigan State University East Lansing, MI 48824, USA

fzpeng@egr.msu.edu

ABSTRACT—An inductor-less series-parallel auto-regulated rectifying circuit is proposed for multi-channel Dc/Dc LED driver. Utilizing the charge balancing principle in capacitors and the proposed series-parallel auto-regulated secondary rectifying circuit, accurate current sharing among output LED strings is achieved. By using resonant converter as primary side inverter circuit, a resonant Dc/Dc LED driver for four LED strings is constructed. The operating principle and AC circuit equivalent are presented. The current sharing capability for each output is also discussed. A four-channel output prototype with 0.35A/channel is built up to verify the theoretical analysis.

I. INTRODUCTION In recent years, high-brightness LED (HBLED) has been

widely used in many applications, such as street lighting, automobiles and backlighting, for its high efficacy, long lifetime etc. The maximum current rating of individual LED module is limited by packaging technology and thermal management, most applications require a large number of LEDs in a single system to achieve required expected luminance value [1, 2]. Because the forward current of LED luminous quantity determines its luminance, it is important to keep the current in each LED identical for the same luminance and thermal performance [3]. The series structure is the easiest way to balance the current in LEDs. But this will result in high output voltage and poor reliability. Therefore, using LED strings in parallel has been a common practice [4]. But the current sharing ability is poor in LEDs because the LED’s exponential voltage-current characteristic and the negative temperature coefficient of its forward voltage drop [5], thus current balancing technique is necessary in multi-string LEDs.

One method to achieve current sharing of paralleled LED strings is using a linear or switching circuit to form a current regulator in each string. With linear regulator [6-9], the power loss is a major problem even employing bus voltage adaptive control because the voltage differences of the LED strings are high. With the switch-mode regulator [10], the efficiency is high but the cost increases a lot. Another method is using a passive component like capacitor or inductor to attain current

sharing. The current sharing transformer based method is proposed in [11], but the magnetic component is usually bulky, furthermore it introduces additional power loss and needs custom design for output conditions. The capacitor impedance based method [12] needs high impedance capacitor which causes high voltage drop across the capacitor and high circulating energy in LED driver, moreover, the current sharing accuracy is limited when the LED’s voltage drops are different. The methods [3, 13-15] utilizing charge balance in capacitor is simple, precise and low-cost. However, one capacitor can only realize current sharing between two channels, and an additional magnetic component has to be introduced for more LED strings. This paper presents a multi-channel LED driver based on charge balance in capacitors without additional magnetic components. The structure inherits the advantages of the capacitor charge balancing method and can be easily to extend to multi-channel output applications and modularization.

Figure 1. The proposed multi-channel LED driver

This work is supported by National Technology Support Project of China under Grant NTSP2011BAE01B01 & Public Technology Industrial Research Project of Zhejiang Province (C31111)

978-1-4673-4355-8/13/$31.00 ©2013 IEEE 829

II. OPERATING PRINCIPLES The proposed four-channel isolated LED driver with

precise capacitive current sharing is depicted in Fig.1. LLC topology is adopted in the primary side. Blocking capacitors (Cb1, Cb2, Cb3) are in series with the secondary windings of the transformer (S1, S2, S3).

The operating principle of the primary side is the same as common LLC topology. To simplify the analysis, the primary side is modeled as a high-frequency current source ip. According to the relationship of switching frequency and resonant frequency, the primary current source can be continuous or discontinuous. A discontinuous mode (DCM) is assumed, the proposed topology can be divided into four modes in one switching cycle. When ip is in the continuous mode (CCM), the equivalent modes become simpler due to only mode1 and mode3 are left.

The theoretical steady-state waveforms and corresponding equivalent circuit are shown in Fig.2 and Fig.3. The LED load is modeled as voltage source. The capacitances of 3 blocking capacitors (Cb1, Cb2, Cb3) and turns of 3 secondary windings (S1, S2, S3) are the same. All the devices are ideal. Detailed circuit operation of each mode is presented as follows.

MODE 1(t1-t2): At this stage, ip>0, is1, is3>0, is2<0, D11, D13, D22, D31 and D33 are conducting. S2 and S3 are in series therefore is2=is3 in this mode. is1 and is3 charge Cb1 and Cb3 respectively, is2 discharges Cb2. is2 (is3) provides energy to LED3, meantime is1 provides energy to LED1.

MODE 2(t2-t3): ip=0 in this mode because discontinuous mode is assumed for ip. is1, is2, is3 are all zero and all diodes are off. This mode disappears if the current source ip is CCM.

MODE 3(t3-t4): In contrast to mode1, ip<0 from t3, is1, is3<0, is2>0, D12, D21, D23, D32, and D34 are conducting. S1 and S2 are in series thus is1=is2 from t3 to t4. is1 and is3 discharges Cb1 and Cb3 respectively, is2 charges Cb2. is1(is2)

provides energy to LED2 and is3 provides energy to LED4 simultaneously.

MODE 4(t4-t5): This mode is the same as mode3.

Due to the charge balance in blocking capacitors, the charge and discharge in mode1 and mode3 must be equal, (1) can be derived. Besides, as mentioned in the operating modes, is2=is3 in mode1, is1=is2 in mode 3, (2) can be deduced. The current in LED1, LED2, LED3, LED4 is the average value of Q1, Q2, Q3, Q6 respectively as shown in (3). According to (1), (2) and (3), the output current of each channel is the same and current sharing is achieved.

The proposed structure can also be extended to n-channel application without additional magnetic components as shown in Fig.4.

Figure 2. Steady-state waveforms of the proposed structure

mode1(t1-t2) mode2(t2-t3)&mode4(t4-t5) mode3(t3-t4)

Figure 3. Equivalent circuits of modes during one switching cycle

830

2 4

1 32 4

1 32 4

1 3

1 1 2 1

3 2 4 2

5 3 6 3

t t

t tt t

t tt t

t t

Q is dt Q is dt

Q is dt Q is dt

Q is dt Q is dt

⎧= = =⎪

⎪⎪ = = =⎨⎪⎪ = = =⎪⎩

∫ ∫

∫ ∫

∫ ∫

(1)

2 2

1 1

4 4

3 3

3 2 5 3

2 1 4 2

t t

t t

t t

t t

Q is dt Q is dt

Q is dt Q is dt

⎧= = =⎪

⎪⎨⎪ = = =⎪⎩

∫ ∫

∫ ∫ (2)

1 1/ ; 2 2 /3 3 / ; 4 6 /

Iled Q Ts Iled Q TsIled Q Ts Iled Q Ts

= == =

(3)

Figure 4. Extension of the proposed structure

III. DESIGN CONSIDERATIONS

A. AC equivalent circuit and circuit parameters selection The average voltage across Cb1, Cb2 &Cb3 can be derived

in (4) according to the voltage-second balance of the transformer secondary windings. They can be defined as the DC component of the voltage across Cb1, Cb2 &Cb3, indicated by DC voltage sources in Fig.5, where the AC component is the voltage ripple on Cb1, Cb2 &Cb3. Vs1’, Vs2’&Vs3’ are the combinations of the rectifiers’ voltage and VCb1, VCb2 &VCb3. They are square-waves, the amplitudes of Vs1’, Vs2’&Vs3’ is equal as shown in (5). Since Cb1=Cb2=Cb3, Vs1’=Vs2’=Vs3’, the three secondary windings can be merged into one in Fig.6.

Circuit parameters selection is the same as a two-channel LLCC LED driver because the three secondary windings in AC equivalent circuit can be merged into one [14,15].

5 1 2 3 416

3 4 1 222

5 4 1 2 336

Vo Vo Vo VoVcbVo Vo Vo VoVcbVo Vo Vo VoVcb

− − −= −+ − −=− − −=

(4)

1 2 3 461'( 2 ', 3')

1 2 3 46

Vo Vo Vo Vo

Vs Vs VsVo Vo Vo Vo

+ + +⎧⎪⎪= ⎨ + + +⎪−⎪⎩

(5)

D12

D13

D11

S1

Vo1

- VS1' +

D22

D23

D21

+VCb2-S2

D32

D33

D31

+VCb3-S3 D34

Co2

Co3

Co4

Co1

ip

Cb1

Cb2

Cb3

+VCb1-

+ VS2' -

- VS3' +

Vo2

Vo3

Vo4

iS1

iS2

iS3

Figure 5. Equivalent circuit when blocking capacitor divided into DC and AC component

831

Figure 6. Simplified equivalent circuit

B. Current sharing region As mentioned in section 2, transformer windings S2&S3

connect in series in mode1, but there is a restricting fact that Vx1shown in Fig.7 (a) in mode1 should meet (6), otherwise D21 or D34 would conduct and is2≠ is3. Similarly, Vx2 shown in Fig.7 (b) in mode2 should meet (7). Based on KVL in two modes, Vx1&Vx2 can be solved as (8). Then the restrictions of the output voltage can be concluded as (9).

Figure 7. Current sharing range derivation

0 1 1Vx Vo< < (6)

0 2 1Vx Vo< < (7)

1 2 3 2 413

2 1 2 2 3 423

Vo Vo Vo VoVx

Vo Vo Vo VoVx

+ + −⎧ =⎪⎪⎨ + − −⎪ =⎪⎩

(8)

1 2 3 2 42 1 2 4 2 32 1 2 2 3 4

1 3 4 2 2

Vo Vo Vo VoVo Vo Vo VoVo Vo Vo Vo

Vo Vo Vo Vo

+ + >⎧⎪ + > +⎪⎨ + > +⎪⎪ + + >⎩

(9)

In practical applications, the variation of four LED channels caused by parameter discreteness, temperature, aging degree etc. is random. In order to meet the randomness of LED voltage drops, the output voltage range of each channel should be the same. Assuming the output voltage range of each channel is Vledmin to Vledmax, the worst condition for (9) is the variables on the left are Vledmin and the variables on the right are Vledmax. Therefore the current sharing range of the driver is (10). It should be noticed that this range can guarantee the current sharing performance, but some groups of output voltage beyond this range could also realize balanced current among 4 channels.

3 min 2 maxVled Vled> (10)

The current sharing range of the proposed topology can be expressed as Fig.8. In Fig.8, the ratio of the maximum voltage drop Vledmax of the LED channels and the maximum output voltage Vomax of the LED driver is used as horizontal axial; the voltage drop of any LED channel is used as vertical axial. The dash line means the voltage drop of any LED channel is equal to the maximum voltage drop of the LED channels, the dot line means the voltage drop of any LED channel is equal to 2/3 the maximum voltage drop of the LED channels which is the current sharing range in (10). So once the maximum voltage drop of the LED string is determined, the range of the LED channels’ voltage drop is the vertical segment in the shaded area whose horizontal axial is the maximum voltage drop of the LED strings.

Figure 8. Current sharing range for the proposed structure

832

IV. EXPERIMENTAL RESULT A four-channel prototype is built up to verify the

theoretical analysis. The input voltage is 400V and the output is 50V-100V/0.35A for each channel. Fig.9 shows measured waveforms of primary side current ip and secondary side currents is1, is2&is3. The phase and magnitude of them are identical. Fig.10 shows the output current waveforms at different output voltages. Current of four channels are identical because the LED string voltage drops are in the designed region. When the voltages of LED strings are out of current sharing region, current balancing can be maintained as shown in Fig.11. Fig.12 shows the working points of load condition in Fig.10 and Fig.11 respectively. Table1 lists measured output currents and voltages of four outputs at some typical load conditions. Finally, the efficiency curve is given in Fig.13.

Figure 9. Primary side and secondary side currents

Figure 10. Output currents of LED strings in the current sharing range

Figure 11. Output currents of LED strings beyond the current sharing range

Figure 12. The working points in Fig.10&Fig.11 in the form of Fig.8

TABLE I. MEASURED OUTPUT CURRENTS AND VOLTAGES OF FOUR OUTPUTS AT SOME TYPICAL LOAD CONDITIONS

load condition Vo1 (V)

Vo1 (V)

Vo1 (V)

Vo1 (V)

Io1 (mA)

Io2 (mA)

Io3 (mA)

Io4 (mA) whether in current sharing region

I 99.7 99.6 99.6 99.8 340 338 340 340 yes II 49.8 49.7 49.6 49.7 340 338 340 340 yes III 79.1 55.1 55.1 55.0 340 338 340 340 yes IV 55.1 79.1 55.1 55.0 340 337 341 340 yes V 84.9 55.1 55.1 55.0 340 338 340 340 no VI 55.5 84.5 55.6 55.6 366 311 366 366 no

833

93

93.5

94

94.5

95

95.5

96

96.5

50 60 70 80 90 100

Figure 13. Efficiency curve of the prototype

V. CONCLSIONS An inductor-less multi-channel LED driver with charge

balance in capacitors is presented in this paper. This structure utilizes charge balance in the secondary resonant capacitors and series-parallel auto-regulated connection of secondary transformer windings to realize current sharing in different channels. The average currents of different LED strings are the same even the difference among the voltage drops of the LED strings are large. Because no additional magnetic components are introduced, the proposed structure is simple, lossless and convenient for modularization. The current sharing range is wide enough for conventional four LED strings application.

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