Design of an Energy Efficient Buck Based LED Driver in DCM and CCM

It is the design criteria of driving LEDs at constant current and keeping the input current’s harmonics low, attaining a high power factor, and maximizing efficiency. For low power general purpose lighting buck converter can be chosen for simplicity and reduced cost. It is a difficult to improve power quality parameter like, the THD (total harmonic distortion) and power factor of ac mains. In the proposed topology, the operation for high power factor and low THD at the AC mains’ input is accomplished through buck converter operation in both discontinuous conduction mode (DCM) and continuous conduction mode (CCM) with a suitable control circuit. This paper assures constant current in both cases for suitable design of control circuit. Over voltage protection has been incorporated. To validate the LED driver circuit, simulation studies are carried out.

Introduction

Due to its many advantages over other lighting sources, lighting using LED (light emitting diode) has been the primary source of lighting in recent decades. Since LEDs can tolerate shock and tremor, that is why they are used in traffic signals, automobile and airplane lights, indoor and outdoor lighting, and more. Compared to tiny fluorescent lamps, LEDs have the following advantages: they don’t release any dangerous UV rays, they switch on promptly to full brightness, frequent usage doesn’t diminish their lifespan, and a high ignition voltage isn’t needed [1], [2].

PFC (power factor corrected) LED drivers that are both isolated and non-isolated are utilized in general-purpose LED lighting. The development of improved power quality single-stage PFC converter-based topologies was favored by several researchers. The utilization of single-stage single switch PFC ac-dc converters can mitigate the several shortcomings of conventional two-stage PFC-based LED drivers, including their large component count, high cost, and low efficiency [3], [4].

Flyback and single-stage buck PFC converter topologies are the most appropriate for low-power applications due to their minimal cost and component count. To lessen the ringing effect in flyback converter architecture, a well-designed snubber circuit is needed [5]–[8]. In addition, this results in extra costs and power loss in the snubber circuit. Therefore, in this case, the buck PFC converter is recommended due to its minimal cost and smaller component count. Discontinuous conduction mode (DCM) is the mode of operation for the buck converter [9]–[11]. With an efficient design of control circuit of the universal ac mains, power factor can be nearly one using the PFC buck converter [12]–[15].

The most often utilized method for driving numerous LED lights for universal voltage applications is PWM dimming [16]. For medium- and high-power LED bulb driving, single- and two-stage half bridge LLC resonant converters with soft switching are good choices [17]–[21].

The literature reports on a unique output voltage regulated ac-dc converter that uses reset winding to reduce stress on the dc-link capacitor [22]. The literature [23] discusses the use of a buck-boost converter to lessen ripples in the output voltage.

This work proposes a LED driver based on buck converter that has constant current and enhanced power quality features, such as low THD and high PF working in both CCM and DCM. To meet the required parameters, we proposed a double loop control circuit that will work well. The most straightforward, economical, and effective switching converter for low power LED lighting applications is the suggested buck converter. For the PFC circuit to be appropriate for universal ac mains, it must be properly designed. With a crest factor (CF) of 1.42 and a smaller than the total harmonic distortion (THD) of the ac mains current 17.27%, the suggested LED driver shows nearly unity power factor.

Section 2 of this article is titled Buck PFC Converter. Section 3 is titled Design of LED Driver Circuit. Section 4 is titled Harmonic Reduction. Feedback control and transfer function in 5, Section 6 contains the results and discussions, whereas Section 7 contains the conclusion.

Buck PFC Converter

The proposed power system’s schematic circuit for the LED comprises a PFC buck converter with a single switch, as depicted in Fig. 1.

Fig. 1. Fundamental BUCK using LED load with input filter (L_f, C_f).

Let the input voltage vin, let the voltage of the rectifier’s output,Vg and the inductor current, iL passes via an inductor L. (1)Input voltage,vin=VmSinωt

For the working principle of BUCK converter, when the inductor voltage vL, output voltage vo: (2)VL=Vg−vo=LdiLdt

If the efficiency of the Buck converter η: Iin=VoIoηVin =Vo2ηR Vin[Vo=IoR] =D2Vin2ηR Vin (3)Iin=D2ηRVin=D2ηRVmsinωt

The aforementioned formula demonstrates how, in a Buck DCM, input voltage and input current follow one another, ensuring unity power factor. Fig. 2 shows the waveforms. However, the bridge rectifier diode contaminates harmonics and distorts the wave pattern of the input current. Therefore, reducing harmonics are necessary for reaching a high-power factor.

Fig. 2. Waveform of gate pulse vp, inductor current iL(t), switching current I_SW diode current iD and capacitor current ic.

To obtain good power quality at the input side, the suggested driver circuit works in conjunction with a filter circuit and a PFC buck converter. The PFC buck converter pulls sinusoidal alternating current and upholds almost unity power factor by employing the double loop feedback control circuit and inner loop (multiplier). To maintain appropriate illumination, the outer-loop regulates a high frequency, continuous LED current. 100 kHz is the operational switching frequency. Utilizing high frequency helps to minimize component size.

Design of LED Driver

The single-phase ac voltage source, inductor, capacitor, diode, MOS switch, and LED module make up the Buck topology. Fig. 3 displays the schematic diagram for the LED driver circuit. When switch M is turned off, the inductor supplies power to the load by storing energy.

Fig. 3. BUCK converter based LED driver Circuit with double loop feedback.

Control Circuit Design

The multiplier unit is utilized to obtain the sinusoidal shape of the buck converter’s input current, as seen in Fig. 1. Through the use of a buck converter, this control approach achieves approximately unity power factor by regulating the input voltage and the input current in phase. A voltage amplifier, current amplifier, multiplier, and PWM generator make up this control method. The current is sent through the inductor (IL) is compared to the reference.

Proportional and integral function is performed in the current error amplifier. The current amplifier’s output is multiplied with the input voltage templet in order to obtain the sinusoidal replica.

A 2-1 MUX selects and feeds the over voltage signal and the regular PWM signal to the MOSFET at startup. The 2-1 MUX selector establishes normal functioning and over voltage. Fig. 4 shows the overvoltage protection. The MOS gate receives regular PWM pulses during typical operation.

Fig. 4. Over voltage protection system.

The circuit’s overall control circuit is utilized to improve performance by boosting circuit stability, lowering THD, and increasing power factor.

Harmonics Reduction

An LED driver supplies an LED bulb with a steady current. A full bridge rectifier is utilized before Buck in order to obtain the required DC output from the AC input. These MOSFET switches and diode rectifiers produce harmonics and input current distortion. THD is a crucial issue of LED drivers and must be maintained at a minimum. Reduced peak currents, improved power factor, and increased efficiency are all caused by lower THD in LED drivers. Harmonics arises also from LED nonlinearity. The inductor L and capacitor C, which transmit energy between them, are the factors that have an impact on the harmonics in the input current.

THD and Voltage Ripple Reduction of Output Current

The relationship between input inductor L and input ripple current △IL is inversely proportional. By utilizing a larger value of L, the input inductor current ripple △IL can be lowered; otherwise, the harmonics will be higher [24], [25].

The value of C will be such that the output current ripple is minimized. L and C ought to be maximized to lower THD. In the absence of optimization, the input current harmonics rise. LED voltage and current become smoother after optimization. Therefore, by optimizing Buck settings in conjunction with a well-designed feedback control circuit, a high-performance LED driver can be created. The simulated plot in Fig. 5 is obtained in absence of optimization of CandL. Here C=3000 μF,L=100 μH. It shows P.F.=0.94, THD=31.39% and as a result, THD and P.F. are not acceptable. The efficiency is 67%. It is discovered that the input current THD is dominated by the effect of L.

Fig. 5. Simulated plot of input voltage and input current without optimization. Here, power factor =0.94,THD=31.39% when L=100 μH,C=3000 μF.

Transfer Function and Feedback Control

One capacitor C and one inductor L, with corresponding series resistors rc and rd, make up the Buck. Achieving stability also requires a MOSFET on resistance rms and a diode on resistance rd. During control operation, output voltage fluctuates in tandem with changes in the duty cycle. Below is a presentation of the transfer function analysis for the suggested BUCK based LED driver.

Transfer Function of Buck with Feedback Control

By using MATLAB for the system with the compensator, the overall open loop transfer function is obtained as follows: (4)Tcomp=5.624e4s2+2.11s+2.778

From the step response in Fig. 6a, it is found that for the compensated model, it takes only 4 seconds for the system to reach a steady state. The root location of the BUCK converter in Fig. 6b shows that the compensated system is operating steadily.

Fig. 6. Step response (a) and Root locus (b) with compensator in the of BUCK based LED driving system.

Results and Discussions

THD is considerably lower in DCM operation following L adjustment, but it is still high, as seen in Fig. 7. Here, PF = 0.974 and THD = 14.08% were observed with L = 1000 μH and C = 3000 μF. There is 90.48% efficiency. The LED voltage and current are constant in a steady state, as seen in Fig. 8.

Fig. 7. Input voltage and input current simulation when L and C are optimized. Here we obtained P.F.=0.974 and THD=14.08% when, L=1000 μH and C=3000 μF.

Fig. 8. Simulation of output voltage, output current when L=1 mH, and after optimization of C (when C= 3mF).

The simulated data of the Buck parameters (L, C) optimization for harmonic reduction is displayed in Table I. The primary component in energy transmission is the inductor, according to the data in Table I and the results of the simulation.

Table I. Buck Parameters Optimization for Harmonic Reduction

Inductor L (mH)	Input capacitor C (μF)	THD (%)	Power factor	Efficiency (%)
100.0	1000.0	31.39	0.94	67
100.0	3000.0	22.04	0.94	67
1000.0	3000.0	14.08	0.974	90.48, optimized

Table II displays the suggested LED driver’s performance metrics as determined in DCM simulation. We are aware that there is a trade-off between achieving high efficiency and high PF at the same time. By optimizing Buck parameters and creating a conveniently designed feedback control circuit, the suggested LED driver achieves a high power factor and comparatively high efficiency while maintaining a steady LED driving current.

Table II. The Simulated Parameters of the Proposed LED Drivers (DCM)

VS (V)	IS (mA)	V0 (V)	I0 (A)	PF	THD (%)	CF	Efficiency
90	214	44.86	0.397	0.967	44.95	1.41	91.3
120	236	46.25	0.565	0.920	37.75	1.40	92.0
150	251	47.34	0.729	0.953	24.09	1.39	91.3
180	311	48.82	1.000	0.963	17.65	1.41	86.0
220	375	50.92	1.466	0.974	14.08	1.40	90.5

Again, we have designed the LED driver at 220 V for operating in CCM mode. Table III shows the power parameters obtained in CCM operation.

Table III. The Simulated Parameters of the Proposed LED Drivers (CCM)

L (mH)	IS (mA)	V0 (V)	I0 (A)	PF	THD (%)	CF	Efficiency
5	340	49.89	1.246	0.934	34.23	1.41	91.37
3	333.49	49.94	1.254	0.944	27.24	1.40	91.7
10	364.27	50.50	1.28	0.877	46.99	1.39	91.8

In case CCM, L, and C are replaced by 5 mH and 1 mH. Since the second approach is the CCM control, by varying L, parameters are given in Table III, the circuit and results of simulation with Ltspice software are shown in (a) and (b) of Fig. 9.

Fig. 9. Simulation of input voltage and current when L and C are optimized. Here we obtained P.F.=0.944 and THD=27.24% when, L=1000 μH and C=3000 μF.

LED driver in CCM mode obtains THD = 27.24%, PF = 0.944, Efficiency = 91.7% at 220 V. Figs. 10a and 10b show the inductor current waveshapes in DCM and CCM, respectively. At 220 V, the simulation displays a total power loss of 9.52%, an efficiency of 90.48%, and a THD of 3.7%.

Fig. 10. Inductor current in (a) DCM (b) CCM.

The simulated power losses for DCM operation are shown component by component in Fig. 11. The rectifier and Buck diodes are represented by the diodes R and S, respectively, and the divider resistance is used in this instance to mimic the input voltage to the multiplier.

Fig. 11. The primary sources of power loss in the LED driver (simulated).

It can be observed that the MOS switch experiences the biggest power loss (2%), the divider resistance is just above 1%, the rectifier diode is below 1%, the Buck diode is next in line, and biasing results in the least amount of power loss.

The MOS switch uses the most power, as the simulated result in Fig. 11 shows. The parameter values of the components used in the simulation are shown in Table IV.

Table IV. Optimized Parameters Used in Simulated

Parameters	Values
Inductor, L	1mH
Capacitor, C	3 mF
Power diode	1000 V/10 A
MOS switch, M1	2sk1101−01 MR(450 V/10 A)
Operating voltage	220 volts
Switching frequency, fs	100 kHz

Fig. 12 shows that the power factor increases with input voltage increase. Power factor in CCM is higher. In Fig. 13 efficiency, efficiency stays almost constant. However, THD decreases with increasing input voltage, as shown in Fig. 13.

Fig. 12. Variation of PF with the variation of input voltage in DCM and CCM.

Fig. 13. Variation of efficiency with the variation of input voltage in DCM and CCM.

We find that PF increases with the increase of input voltage, and PF obtained in CCM is higher than in DCM. Efficiency is almost constant with the rise of input voltage and it is higher in DCM. Fig. 14 shows the THD variation in DCM and CCM. THD is lower in CCM, and with the increase of input voltage THD is decreasing. However, we can get a comparative study of DCM and CCM buck operation.

Fig. 14. Variation of THD with the variation of input voltage in DCM and CCM.

Conclusion

This study presents the design of an LED driver with good performance using Buck converter. The design of the LED driver’s feedback control circuit optimizes the use of Buck inductors and capacitors. With a straightforward control circuit and fewer components, the LED driver maintains DCM and CCM operation separately. We have done a comparative study on DCM and CCM. We can get both business and residential lighting benefits from these features.