Cost-Effective Single Switch Multi-Channel LED Driver

Abstract

In this paper, a cost-effective single switch multi-channel LED (light emitting diode) driver is proposed. While conventional LED drivers require as many non-isolated DC/DC converters as the number of LED channels, the proposed LED driver needs only one power switch and several balancing capacitors instead of expensive non-isolated DC/DC converters. Therefore, the proposed driver features a simpler structure, with a lower cost and a higher efficiency. Because its power switch can be turned off under the zero current switching condition, it has very desirable advantages such as improved electromagnetic interference characteristics and high efficiency. Moreover, it uses only a small number of DC blocking capacitors with no additional active devices for the current balancing of multi-channel LEDs. As a result, the proposed driver exhibits high reliability and is cost effective. To confirm the validity of the proposed driver, a theoretical analysis is performed, and design considerations and experimental results obtained from a prototype that is applicable to a 46" LED-TV are presented.

I. I NTRODUCTION

Recently, LCD (liquid crystal display) flat panel displays have become one of the fastest growing products in large screen displays due to various advantages such as their low power consumption, long lifespan, low profile, and high contrast ratio [1]-[2]. Because LCDs are non-emissive display devices, they usually require a BLU (back light unit) in monitor and TV applications. For conventional LCD TVs, a CCFL (cold cathode fluorescent lamp) is usually used as a BLU. However, restrictions on the use of mercury (Hg) as specified by the RoHS (Restriction of Hazardous Substances) directive, requires that environmentally friendly lamps must be employed as backlight sources. Moreover, it is very difficult to implement low profile and high definition TVs.

LEDs (light emitting diodes) are now considered to be more suitable for BLUs and other lighting applications. Although the high cost and low luminous efficiency of LEDs make it difficult for them to be widely adopted as a backlight source, LEDs can satisfy the RoHS directive. They also possess many desirable merits such as long lifespan, wide color gamut, and fast response. Moreover, since the luminous efficiency and production cost of LEDs have been improved, they have become among the most suitable solution for BLUs.

To realize sufficient luminance for large-sized LCD TVs, series-parallel configured multi-channel LEDs are usually adopted. In driving multi-channel LEDs, it is very important to obtain uniform and sufficient luminance despite the characteristic variations of each LED. However, because each LED has different properties depending on the voltage, current and temperature, there may be LED current variations even at the same driving voltage depending on the temperature of the p-n junction, the time variation and the differences in the characteristics of each array [3]-[5]. Therefore, individual LED arrays should not be individually placed in parallel, and should be driven by a constant current source instead of a constant voltage. Further, a single LED driver is generally used for each LED array to maintain a constant and balanced current through each of the multichannel LED arrays, where a non-isolated boost converter is usually used as the LED driver.

In the meantime, the power supplies for LED TVs are generally required to produce 12.8V for the audio amplifier, digital signal processing and auxiliary circuit. Additionally, they should also regulate the LED currents. However, most converters cannot obtain independently and tightly regulated dual outputs such as voltages or currents by themselves. Therefore, in recently developed LED TVs, one master output is tightly regulated by the main controller and the other slave outputs are regulated by an SSPR (secondary side post regulator) such as an extra non-isolated DC/DC converter, where the SSPR can also be implemented by the abovementioned non-isolated boost converter to balance the current in the multi-channel LED driver [6]-[7]. However, because the SSPR is composed of an inductor, capacitor, diode, switch, and control IC, it has several drawbacks such as a high production cost, cumbersome nature, and poor efficiency, due to its cascaded configuration.

To overcome all of the drawbacks faced by previous approaches, a new cost-effective single switch multi-channel LED driver is proposed in this paper. The overall power supply for an LED TV employing the proposed LED driver is constructed using a dual output LLC resonant converter, where the master output produces a tightly regulated 12.8 V voltage and the slave output supplies the input voltage of the proposed LED driver. The proposed LED driver then regulates all of the LED currents so that they are kept constant and balanced. The proposed driver is composed of only one power switch and several balancing capacitors without extra SSPRs. Therefore, it has a lower cost, simpler structure, higher efficiency, and higher power density.

 

II. CONVENTIONAL LED DRIVER

Fig. 1 shows a schematic of a conventional LED driver for an n-channel LED TV. As shown in this figure, the overall system consists of three cascaded power conversion stages, namely a PFC (power factor corrector), an isolated DC/DC converter, and an LED driver. The PFC stage produces a constant DC link voltage from an AC input voltage source. Then, the isolated DC/DC converter converts this link voltage into the tightly regulated DC voltage (12.8 V) for auxiliary circuits and the unregulated DC voltage for the LED driver, where the LLC resonant converter is usually adopted as an isolated DC/DC converter due to its favorable advantages such as high efficiency and low EMI [8]-[9]. Then, to drive multi-channel LED arrays from the unregulated DC voltage, conventional LED drivers require one boost converter per LED channel to control the current through each LED. Although these boost converters can tightly control all of the LED currents, they have several disadvantages such as their high production cost, low power conversion efficiency, and cumbersome nature. Moreover, many of the active devices and driver ICs used in boost converters can deteriorate the system reliability.

Fig. 1.Schematic of the conventional LED driver.

 

III. PROPOSED LED DRIVER

Fig. 2 shows an example of the proposed n-channel LED driver. The proposed LED driver can tightly control one channel of LED current using only one switch. At the same time, other channel LED currents can be automatically balanced with each other by using DC blocking capacitors. Therefore, the proposed driver requires a small number of devices and has a low production cost. Moreover, because an overall power converter employing the proposed LED driver consists of two cascaded power conversion stages, its circuit structure and energy conversion efficiency are simpler and higher than those of conventional drivers. In addition, because its power switch can be turned off under the ZCS (zero current switching) condition, it has very desirable advantages such as improved EMI characteristics and heat generation.

Fig. 2.Schematic of the proposed LED driver.

As shown in Figure 2, the proposed LED driver consists of an ICR (initial current-control regulator) and a CE (current equalizer). The ICR controls only one channel of LED current, and the other channel LED currents are balanced with each other by the CE. Therefore, all of the LED currents can be controlled and balanced. The detailed operational principles are described as follows.

A. Initial Current-Control Regulator

Fig. 3 shows an example of a dual output LLC resonant converter, and it illustrates the operational principles of the proposed ICR. One output voltage Vo1 of the dual output LLC converter is controlled by varying the switching frequency, as in the case of a conventional single output LLC resonant converter, where Vo1 and Vo2 correspond to the master 12.8 V and slave driving voltages for one of the multi-channel LEDs, respectively. On the other hand, the other output voltage Vo2 can be regulated by varying the initial current Iini. The primary current ipri(t) and the output voltage Vo2 of the LLC resonant converter can be expressed as follows:

Fig. 3.Dual output LLC resonant converter with proposed ICR and its operational principles

Equations (1) and (2) show that the output voltage Vo2 depends on Iini. As a result, it can be regulated by varying the initial current Iini. In other words, because a larger Iini produces a larger resonant primary current i pri(t) , a larger secondary current isec2(t) = ipri(t) - iLM(t) is transferred to the output side. As a consequence, a larger output voltage can be achieved. Therefore, the output voltage Vo2 can be regulated by controlling the initial current Iini of ipri(t). Based on these operational principles, one master output voltage of the dual output LLC converter can be controlled by the switching frequency modulation and the other slave output voltage can be controlled by the initial current Iini as shown in Fig. 3.

B. Current Equalizer

Fig. 4 shows the proposed CE for a multi-channel LED with the exception of the abovementioned ICR. For the sake of simplicity, a 2-channel LED driver is used as an example. As seen in the figure, the proposed CE is composed of 1 transformer, 1 capacitor, 4 diodes and drive 2-channel LED arrays. The current balancing principles of the proposed CE are described as follows. When the switch M2 is on and the switch M1 is off during one half of the switching period of the driver, the current through the transformer’s primary side is the same as ipri_P. Therefore, all of the transformer secondary side currents are identical as follows:

Fig. 4.Current equalizer circuit and operation principles.

where < ● > is the average value of “●”, and n is the transformer turn ratio.

When M1 is on and M2 is off during the other half of the switching period of the driver, the current through the transformer primary side is also the same as ipri_N. Therefore, all of the transformer secondary side currents are identical as follows:

At this point, because the blocking capacitor (CB) is serially connected to the transformer’s secondary side, the DC offset current of the transformer’s secondary side can be eliminated. In other words, the average currents through the transformer’s secondary side during each half of the switching period are equal to one another as follows:

Finally, from equations (3), (4) and (5), all of the LED currents can be perfectly balanced as follows:

Accordingly, the proposed driver can keep the currents through all of the LED arrays constant and balanced using only passive devices such as transformers and capacitors. Moreover, if only the current through one LED array is controlled, all of the currents can be controlled by the abovementioned principles. Therefore, the proposed LED driver can control the current through the LED channel using the ICR. At the same time, it can equalize all of the LED currents using the CE.

 

IV. OPERATING PRINCIPLES OF PROPOSED LED DRIVER

The key waveforms of the proposed converter are presented in Fig. 5, and its operation modes in five modes according to the conduction state of each switch are shown in Figure 6. For the convenience of the mode analysis in the steady state, the following assumptions are made:

Fig. 5.Key waveforms of the proposed converter.

Fig. 6.Operation modes of the proposed converter. (a) Mode 0 (~t0). (b) Mode 1 (t0~t1). (c) Mode 2 (t1~t2). (d) Mode 3 (t2~t3). (e) Mode 4 (t3~t4). (f) Mode 5 (t4~t5).

Before t0, it is assumed that the switch M1 is initially conducting, and a transformer magnetizing current iLM flows through M1, as shown in Fig. 6(a).

Mode 1 [t0-t1]: When M1 is turned off and M2 is on at t0, mode 1 begins, as shown in Fig. 6(b). Because the voltage VIN - VCR is applied to the transformer primary side, the dot of the transformer is positive. As a result, D2, D4, and D5 are conducting. Therefore, ipri flows due to the resonance between CR and LR. Because the reflected output voltage (NP/NS2)Vmaster is applied to LM, the magnetizing current iLM increases linearly.

Mode 2 [t1-t2]: When the transformer primary current ipri becomes equal to iLM and isec1, and iD5 reach zero at t1, mode 2 begins, as shown in Figure 6(c). At this moment, because all of the rectifier diodes are blocked and VIN -VCR is applied to LM+LR, ipri is still increasing because of the resonance between CR and LM+LR. However, because the resonant frequency between CR and LM+LR is very low when compared with the switching frequency, ipri can be assumed to have increased linearly.

Mode 3 [t2-t3]: When M2 is turned off and M1 is on at t2, mode 3 begins, as shown in Fig. 6(d). Although M1 is conducting and -VCR is applied to the transformer primary side, the turned-off M3 prevents the input power from being transferred to the output side. Therefore, there is no resonance between CR and LR, isec2 is maintained at zero, and the initial current Iini decreases with a slope of -VCR/(LR+LM). As shown in Figure 5, ipri decreases by the resonance between CR and LM+ LR. Similarly, because the resonant frequency between CR and LM+ LR is very low, ipri can be assumed to have decreased linearly.

Mode 4 [t3-t4]: When M3 is turned on at t3, mode 4 begins, as shown in Figure 6(e). When M3 is turned on with M1 conducting, D1 and D3 are turned on. At the same time, the resonant current between CR and LR is transferred to the output side. Moreover, because the reflected output voltage -(Np/Ns2)Vmaster is applied to LM, iLM decreases linearly.

Mode 5 [t4-t5]: When ipri becomes equal to iLM, and isec2 reaches zero at t4, mode 5 begins, as shown in Figure 6(f). At this time, because all of the rectifier diodes are blocked and -VCR is applied to LM+LR, ipri is still decreasing due to the resonance between CR and LM+ LR. In particular, if only M3 is turned off between t4 and t5’, M3 can be turned off under zero current switching. When M1 is turned off at t5, mode 5 ends. Subsequently, the operation from t0 to t5 is repeated.

 

V. ANALYSIS OF THE PROPOSED LED DRIVER

For the convenience of the analysis, it is assumed that the dead time between M1 and M2 is zero, and that iLM increases or decreases linearly with a slope of Vo1/n1 LM or -Vo2/n2 LM, respectively, where Vo1 and Vo2 correspond to the master 12.8 V and slave driving voltages, respectively, for one of the multi-channel LEDs.

As shown in Fig. 7, the steady-state offset current through LM can be obtained as:

Fig. 7.Transformer primary current ipri and resonant capacitor voltage VCR.

where n1=Ns1/Np and n2=Ns2/Np.

From equation (7), the values of Ip and Iv can be expressed as follows:

where TS is the switching period.

Moreover, iLM1 and iLM2 can be obtained from equations (8) and (9) as follows:

Meanwhile, when M2 is conducting, the difference between ipri and iLM1 is transferred to the output side as isec1. Similarly, when M1 is conducting, the difference between ipri and iLM2 is transferred to the output side as isec2. From equations (7), (10), and (11), isec1 and isec2 can be obtained as follows:

where ω is the angular frequency.

Therefore, the output load currents Io1 and Io2 can be calculated from the mean values of isec1 and isec2, respectively, as follows:

where D’ is the turn-off duty ratio of the switch M3.

From equations (14) and (15), the output voltages Vo1 and Vo2 can be obtained as follows:

where trs=tr - D’Ts and:

If it is assumed that the equivalent circuit of the LED is a serially connected equivalent resistor Req and a forward voltage drop VF, the current through the LED can be expressed as follows:

 

VI. EXPERIMENTAL RESULTS

To confirm the validity of the proposed LED driver, a prototype for a 46” 2-channel LED TV is constructed. Table I shows the design specifications and circuit parameters.

TABLE IDESIGN SPECIFICATIONS AND CIRCUIT PARAMETERS

Fig. 8 shows the key waveforms according to the master load conditions. As shown in this figure, the switching frequencies at no-load and full-load are measured as 130 kHz and 115 kHz, respectively, to regulate the master output voltage. Under this situation of frequency variation, the ICR can tightly regulate the LED current by varying the initial current. Moreover, because M3 is turned off after isec2 reaches 0A, the ZCS of M3 can be ensured along the entire load range. Fig. 9 shows both the LED current and master output voltage according to the master output load conditions. As shown in this figure, the proposed LED driver has excellent performance with respect to master voltage regulation, LED current regulation, and current balancing regardless of the master load conditions. Further, Figure 10 shows the LED currents and M3 gate signal of the proposed LED driver according to various dimming conditions. In addition, figure 11 shows that the current through each of the LED channels can be well controlled and balanced along the entire dimming range. Table II shows comparisons between the conventional and proposed LED drivers with respect to the number of devices. As shown in this table, because the proposed LED driver does not need an extra boost converter for current control and balancing, the proposed LED driver has fewer devices than the conventional driver. Therefore, the proposed LED driver features a simpler structure and a higher power density than the conventional driver. Moreover, the efficiency of the proposed LED driver is measured to be about 86%, which is higher than the 84% obtained by the conventional driver at Vac = 220 Vrms. In this case, the PFC is the same and the efficiency is about 97%. This is because the boost converter stage is removed in the proposed LED driver.

Fig. 8.Key waveforms according to master load conditions. (a) No load. (b) Full load.

Fig. 9.LED currents and master voltage according to master load conditions. (a) No load. (b) Full load.

Fig. 10.LED currents and M3 gate signal according to dimming conditions. (a) 100% Dimming. (b) 80% Dimming. (c) 20% Dimming. (d) 0.2% Dimming.

Fig. 11.LED currents according to dimming conditions.

TABLE IICOMPARISONS BETWEEN CONVENTIONAL AND PROPOSED 2-CHANNEL LED DRIVERS WITH RESPECT TO THE NUMBER OF DEVICES

 

VII. CONCLUSIONS

In this paper, a cost-effective single switch multi-channel LED driver is proposed. While the conventional LED driver requires as many boost converters as the number of LED channels, the proposed driver needs only one power switch and several balancing capacitors. In addition, it features a lower cost, simpler structure, higher efficiency, and higher power density. Meanwhile, because an overall system employing a conventional LED driver is composed of 3 power conversion stages, its power conversion efficiency is measured as 84% at Vac = 220 Vrms. On the other hand, because an overall system employing the proposed LED driver consists of only 2 power conversion stages and the ICR switch can be turned off under the ZCS condition, its overall power conversion efficiency is measured to be as high as 86% at Vac = 220 Vrms. Moreover, although the proposed driver is composed of only passive devices for current balancing, its CE can achieve excellent current balancing performance with a maximum of 1.7% deviation. Due to these advantages, the proposed LED driver is expected to be suitable for various LED applications such as displays and lighting.