This paper proposes an LED driving circuit with a digital controller, power factor correct (PFC) function, and low light flicker. The key topology of the proposed circuit is a conventional Flyback combined with a prestage. As a result, there will be less light flicker than with other onestage PFC circuits. A digital controller, implemented using a lowcost microcontroller, dsPIC30F2020, will meet PFC and low light flicker. The experimental results validate the functionality of the proposed circuit.
1. Introduction
Digital controllers are currently popular for highfrequency, lowpower switching mode power supplies because of their advantages in terms of programmability, different implementation and low sensitivity to variations
[1]

[4]
. An LED driver controlled by a digital chip allows the control strategy to be userdefined, and therefore the circuit design becomes more flexible and better able to fulfill our target requirement. In the proposed circuit, we program a lowcost microcontroller to achieve closed loop control and PulseWidthModulation (PWM) functions. Compared with the complicated analog circuit that utilizes a comparator and optical coupler to close the voltage and currentcontrol loop, the proposed digital circuit only requires the user to define the voltage and current control by code, which is not only simpler but also reduces the material cost. LED drive circuits often contain power factor correction (PFC) functions. A singlestage topology such as Flyback or boost which is switched by the PWM signal can have PFC function with good power factor. PWM signal could control the switch to make the input current in phase with the input voltage. LED driving using a singlestage PFC circuit often has a huge light flicker, which can be harmful to eyes
[5

7]
. This flicker is triggered by the twicelinefrequency driving current. According to
[8]
and
[9]
, another switch added to the secondary side to control the power flow will reduce the capacitor’s value. However, this is not suitable for a condition that allows only one switch. Our circuit implements a prestage, which helps ensure that the drivingcurrent ripple is low, even with a smalloutput capacitor. The benefit here, in comparison to the conventional twostage PFC, is that it only uses one MOSFET. Meanwhile, the output current ripple is much smaller than that of the conventional Flyback with the same output capacitor. Therefore the proposed circuit will lead to a smaller light flicker.
A new LED driver circuit, as shown in
Fig. 1
, is proposed in this paper. The output current has a low ripple because of the energystorage capacitor on the primary side. The proposed circuit is compatible with a digital controller. The operating principle of the proposed circuit was analyzed, a theoretical analysis of the outputcurrent ripple was conducted, a design procedure for the proposed LED driver circuit was provided, and a prototype was built to verify performance.
Circuit topology of the proposed singlestage
2. Circuit Configuration and Analysis
Fig. 2
demonstrates the operating principles of the proposed circuit. From t
_{0 }
to t
_{1}
, the switch Q
_{1}
is on, the coupled inductor is charged, and the primaryside capacitor is discharged. The secondary side diode is biased, as is the current flow from the output capacitor to the LED load. From t
_{1}
to t
_{2}
, on the primary side, the coupled inductor is discharged, and the primaryside capacitor is charged. Current flows through the secondaryside capacitor and charges the output capacitor. From t
_{2}
to t
_{3}
, no current flows through the primary side. The situation on the secondary side is the same as that of t
_{1}
to t
_{2}
. In the last period, the secondaryside diode is reverse biased. The load current comes only from the output capacitor.
Operating principles of the proposed circuit
3. Digital Control of the Proposed Driver Circuit
Digital control of the proposed circuit is implemented using the dsPIC30F2020 chip
[10]
. The system diagram is shown in
Fig.3
.
System diagram of the proposed circuit
The circuit works with a dualloop control, with both the voltage regulation and current regulation. Here the dual loop control is different with the common series connected one. The traditional one is taking the voltage error as the reference of the current loop. We use a parallel structure, as shown in
Fig.3
. The advantage is that implementation is simple. The voltage and current loop will not have interference with each other. The algorithm of control is presented in
Fig. 4
.
PI control algorithm flow chart
The PI controller for the voltage loop can be represented as
in the continuous domain. Through the bilinear ztransformer
[11

17]
, it can be represented as
Since T=10
^{5}
s, we can get
Therefore, the control algorithm of voltage loop can be written as
The currentcontrol loop can also be represented in this way. A similar equation can be derived, such as
Then the outputs of the voltage loop and the current loop are compared. The one with the larger value has a larger error, and it should therefore be chosen as the input for the PWM.
4. Analysis of the Current Ripple and Flicker
Methods for analyzing the twicelinefrequency ripple for a PFC circuit are introduced in
[18

20]
. In this paper, we found and solved the relationship between the twicelinefrequency ripple and the circuit parameters
[18]
. We first analyzed the outputvoltage ripple. We know the current ripple of the LED is related to the voltage ripple of the LED; therefore, as long as we know the voltage ripple, we can analyze the current ripple based on the IV curve of the LED. The averaged statespace model over
T_{s}
can be built based on the voltagetime balance of the coupled inductor and the Flyback transformer. Eqs. (6) and (7) are derived from the averaged statespace model:
Furthermore, we decomposed each state and input into DC and ripple components. Then, we gathered the first order ripple terms in the differential equations. We did not consider the perturbation of the duty ratio because the circuit was designed to operate with a constant duty ratio and a constant switching frequency. Finally, we get Eq. (8), which describes the perturbations in the primaryside capacitor’s voltage and in the output voltage:
The twicelinefrequency ripple components of the output voltage can be solved according to the following steps
[18]
. First, the equations above can be represented as
Therefore, the ripple dynamic equations of a certain ripple can be written with a superposition of sinusoidal functions on the righthand side:
where (
ω
∈{
ω_{d}
,2
nω_{L}
;
n
=1,2,...},i=1,...,M)
The solution to Eq. (10) is assumed to have the form
and can be solved as
where A is
The inverse of the matrix
can be found using MATLAB. We neglect the trivial terms of the parameters and retain the dominant ones. Finally, we get the approximation equation for
Because
x
_{S2}
is much smaller than
x
_{C2}
, the twicelinefrequency ripple’s waveform can be approximated as
Where
ω
is 2
π
times the twiceline frequency. According to the energy balance of the Flyback transformer, we can derive the equation
Based on this equation, Eq. (14) can be rewritten as
According to the IV curve for the LED, as shown in
Fig.5
, we can derive the relationship between the current ripple and the voltage ripple as
LED VI Characteristics
Where
r_{c}
is the current ripple rate and
r_{v}
is the voltage ripple rate.
Once the current ripple has been estimated from the parameters of the circuit, the intensity of the light flicker can also be estimated. The following is the procedure for deriving the relationship between the current ripple and the two measurements of the light flicker, which are the flicker percentage and the flicker index.
Fig. 6
shows the luminous flux waveform of the LED. The flicker index is defined as
Waveform of the luminous flux from the LED
Because the output luminous flux of the LED has an almost sinusoidal shape,
So, Eq. (19) can be expressed as
Because
we get the relationship
5. Design Procedures of the Proposed Circuit
Fig. 7
demonstrates the design procedures for the proposed circuit. In this paper, we mainly discuss the guidelines for choosing the coupledinductor value, the transformer’s magnetizing inductance value, and the primaryside capacitor’s value. We also analyze the voltage and the current stress for switching devices to acquire the knowledge needed for device selection. First, we calculated the Flyback magnetizing inductance. In order to achieve a good power factor, we needed to guarantee that the coupled inductor operates in a discontinuous conduction mode (DCM), as shown in
Fig. 8
.
Design procedures for this proposed circuit
Current waveform for the coupled inductor
Therefore,
On the other hand, we can get
where R
_{load}
is the equivalent resistance of the LED, V
_{1}
(t) is the primaryside capacitor’s voltage, and T
_{s}
is the period of the switching cycle.
Substituting Eqs. (24) and (25) into Eq. (26), we get the Flyback magnetizing inductance:
where P
_{o}
is the output power. To simplify the calculation, we approximate V
_{1}
=V
_{1}
(t). Therefore, to guarantee that L
_{B}
operates in the DCM for a universal input range, we demand that the constraint
be satisfied. Finally, we get
where V
_{in_peak@88v}
is the maximum value of V
_{in}
when its RootMeanSquare (RMS) value is 88 V. In our prototype design, we found that
L
≤ 0.77mH . Here, we choose 0.3 mH. Next, we calculated the coupled inductance. The maximum voltage of the primary capacitor should be set to a certain value. Here, we chose a maximum voltage of 450 V. Through derivation, the primary side capacitor’s voltage, V
_{1}
, can be expressed as
where V
_{in_peak}
is the maximum value of V
_{in}
. Therefore, we got L/L
_{B}
=0.75. Third, we calculated the maximum duty ratio. For a Flyback transformer, the maximum duty ratio cannot exceed 0.5. For a universal input range, the maximum duty ratio occurs at 88 V. Because
we found that D=0.25. Fourth, we calculated the turns ratio of the Flyback transformer. To guarantee that L operates in the DCM, we can use
Therefore, we calculated the turns ratio as
n
= 3 . Fifth, we decided on the capacitance of the primary side capacitor. We fixed the output capacitor at a certain capacitance. In our prototype, considering the package size, we chose the value to be 1880 uF, and we aimed to achieve a current ripple of less than 15%. According to Eq. (18), we can estimate that a current ripple under 15% means a voltage ripple under 3.93%. On the other hand, with averaged statespace modeling, we derive
Where
ω
is two times the twiceline frequency. Based on Eq. (33), we established that the primaryside capacitor should have a capacitance of at least 10 uF. As to the existence of a high switchingfrequency ripple, the value of C
_{1}
should be chosen to be larger than the same critical value. Sixth, we found the voltage and current stress of the switching device. Within the analysis, we obtain
where V
_{sw_max}
, V
_{d1_max}
, V
_{d2_max}
, and V
_{d3_max}
are the maximum voltages of the switch Q
_{1}
and of the diodes D
_{1}
, D
_{2}
, and D
_{3}
; i
_{sw_max}
, i
_{d1_max}
, i
_{d2_max}
, and i
_{d3_max}
are the maximum currents of the switch Q
_{1}
and of the diodes D
_{1}
, D
_{2}
, and D
_{3}
; and V
_{1@264V}
shows the voltage of V
_{1}
when the input voltage is 264 V.
6. Simulation Verification
Our simulation verified that the currentripple rate in the simulation matched our value calculated on the derived equations very well. The simulation results are shown in
Fig. 9
. Our simulation was designed for various input AC voltages. The simulation parameters were as follows: output voltage V
_{o}
=33 V, output power P
_{o}
=33 w, C
_{1}
=10 uF, C
_{2}
=1880 uF, L=0.3 mH, and L/L
_{B}
=0.75.
Simulation result of output current ripple versus input AC voltage
7. Experimental Verification
Fig. 10
shows a picture of the hardware prototype we built based on the design guidelines.
Fig. 11
shows details of the circuit board.
Fig. 12
and
Fig. 13
present the experimental results of the conventional Flyback LED driver and proposed LED driver with 110V input voltage.
Hardware prototype of the proposed circuit
Details of the proposed circuit board
Experimental results of the conventional Flyback with 110V input voltage
Experimental results of the proposed circuit with 110V input voltage
Fig. 14
and
Fig. 15
present the experimental results of the conventional Flyback LED driver and proposed LED driver with 220V input voltage . From the results, we can conclude that the proposed LED driver circuit has a much lower current ripple than that of the conventional circuit. Meanwhile, the power factor of the proposed circuit is above 0.90, which is considered to be good.
Experimental results of the conventional Flyback with 220V input voltage
Experimental results of the proposed circuit with 220V input voltage
8. Conclusion
We provide a design guideline for the digitalcontrolled LED driver. An experimental prototype was built based on the design guideline. The experimental results verified that the proposed circuit had a lower currentripple rate than the conventional Flyback circuit and that it had a power factor above 0.90.
Acknowledgements
This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20132020101530).This work was supported by the Energy Efficiency & Resources Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20132010101950).
BIO
Yi Li received the B.S degree in control engineering from Harbin Institute of Technology, China, in 2010, and the M.S. degree in electrical engineering from the Ohio State University, United States in 2012. She is currently working toward the Ph.D. degree in electrical engineering with Hanyang University, Ansan, Korea. Her research interests are modeling for power converters and power factor correction.
JaeWoo Lim received the B.S. degree in electronic and communication technology from the Mokpo National Maritime University, Mokpo, Korea, in 2012, and He is currently pursuing the Integrated Master and Ph.D. degree in electrical engineering with Hanyang University, Ansan, Korea. His current research interests include DC/DC converters for EV and DC/DC converter with GaN Transistors.
HeeJun Kim received the B.S and M.S. degree in electronics engineering from Hanyang University, Seoul, Korea, in 1976 and 1978, respectively, and the Ph.D. degree from Kyushu University, Fukuoka, Japan, in 1986, all in electronics engineering. Since 1987, he has been a Professor with Hanyang University, Ansan, Korea. His current research interests include switching power converters, softswitching techniques, and analog signal processing. Prof. Kim is the President of the Korean Institute of Electrical Engineers and a senior member of IEEE.
Guo S.
,
Gao Y.
,
Xu Y.
,
LinShi X.
,
Allard B.
“Digital PWM controller for highfrequency lowpower DCDC switching mode power supply”
Power Electronics and Motion Control Conference, 2009. IPEMC ’09. IEEE 6th International
1720 May 2009
1340 
1346
Liu Y.
,
Sen P.C
2005
“Digital control of switching power converters”
Proceedings of the 2005 IEEE Conference on Control Applications
Toronto, Canada
635 
640
Maksimovic D.
,
Zane R.
,
Erickson R.
“Impact of digital control in power electronics”
EEE Power Semiconductor Devices and ICs, Proceedings 2004, 16th International Symposium
13 
22
Ma L.
,
Wang X.
,
Sun D.
,
Lv X.
2009
“Research and simulation on voltagefeedback type digital stabilized voltage switching mode power supply”
Information Engineering, 2009. ICIE ’09. WASE International Conference on
2
188 
191
Wang S.
,
Ruan X.
,
Yao K.
,
Tan SC.
,
Yang Y.
,
Ye X.
2012
“A flickerfree electrolytic capacitorless ACDC LED driver”
IEEE Transactions on Power Electronics
27
(11)
4540 
4548
DOI : 10.1109/TPEL.2011.2180026
Wilkins A.
,
Veitch J.
,
Lehman B.
“LED lighting flicker and potential health concerns: IEEE standard PAR1789 update”
Energy Conversion Congress and Exposition (ECCE), 2010 IEEE
1216 Sept. 2010
171 
178
Lehman B.
,
Wilkins A.
,
Berman S.
,
Poplawski M.
,
Miller N.J.
“Proposing measures of flicker in the low frequencies for lighting applications”
Energy Conversion Congress and Exposition (ECCE), 2011 IEEE
1722 Sept. 2011
2865 
2872
Nagao M.
2000
“A novel onestage forwardtype powerfactorcorrection circuit”
Power Electronics, IEEE Transactions on
15
(1)
103 
110
Zhang J.
,
Lu D.D.C.
,
Sun T.
2010
“Flybackbased singlestage powerfactorcorrection scheme with timemultiplexing control”
IEEE Transactions on Industrial Electronics
57
(3)
1041 
1049
DOI : 10.1109/TIE.2009.2028336
Microchip
2014
“28/44Pin dsPIC30F1010/202X enhanced flash SMPS 16bit digital signal controller”, DS 70000178D
Liu Y.F.
,
Meyer E.
,
Liu X.
2009
“Recent developments in digital control strategies for DC/DC switching power converters”
IEEE Transactions on Power Electronics
24
(11)
2567 
2577
DOI : 10.1109/TPEL.2009.2030809
Xie M.
2003
Digital control for power factor correction. M.S. Thesis
Center for Power Electronics Systems, Virginia Polytechnic Institute and State University
Blacksburg, Virginia
1 
109
Suul J.A.
,
Molinas M.
,
Norum L.
,
Undeland T.
2008
“Tuning of control loops for grid connected voltage source converters”
Power and Energy Conference, 2008. PECon 2008. IEEE 2nd Internationa
797 
802
Olayiwola A.
,
Sock B.
,
Zolghadri M.R.
,
Homaifar A.
,
Walters M.
,
Doss C.
2006
“Digital controller for a boost PFC converter in continuous conduction mode,”
Proceedings 1st IEEE Conf. Ind. Electron. Appl.
1 
8
Barai M.
,
Sengupta S.
,
Biswas J.
2009
“Dualmode multipleband digital controller for highfrequency DCDC converter”
IEEE Transactions of Power Electronics
24
(3)
752 
766
DOI : 10.1109/TPEL.2008.2008391
Corradini L.
,
Costabeber A.
,
Mattavelli P.
,
Saggini S.
2009
“Parameter independent timeoptimal digital control for pointofload converters”
IEEE Transactions on Power Electronics
24
(10)
2235 
2248
DOI : 10.1109/TPEL.2009.2022397
Krismer F.
,
Kolar J.W.
2009
“Accurate smallsignal model for the digital control of an automotive bidirectional dual active bridge”
IEEE Transactions on Power Electronics
24
(12)
2756 
2768
DOI : 10.1109/TPEL.2009.2027904
Choi J.Y.
,
Cho B.H.
1998
“Smallsignal modeling of singlephase powerfactor correcting AC/DC converters: a unified approach”
Power Electronics Specialists Conference, 1998. PESC 98 Record. 29th Annual IEEE
2
1351 
1357
Nagao M.
,
Nakakohara T.
,
Jinno M.
,
Harada K.
1994
“Analysis of high power factor ACDC boost converter”
T.IEE Japan
114D
1139 
1148
Erickson R.
,
Madigan M.
,
Singer S.
“Design of a simple highpowerfactor rectifier based on the Flyback converter”
Applied Power Electronics Conference and Exposition, 1990. APEC '90, Conference Proceedings 1990, Fifth Annual
1116 March 1990
792 
801