ICSET2008
Buck converter Associated with
Active clamp Flyback converterti2冠军
for PV Power System
S. -Y. Tng, Y. –J. Li and, Y. –J. Wu
GreenPower Evolution Applied Rearch Lab (G-PEARL)
Department of Electric Engineering
Chang Gung University
Kwei-Shan Tao-Yuan,Taiwan, R.O.C
Tel: 03-2118800
Email: u.edu.tw
Abstract--This paper prents an LED lighting system with a PV source. The circuit structure of the propod system parately adopts buck converter and flyback converter as the charger and discharger. With this cir-cuit structure, buck converter can vary its duty ratio to regulate charging current of battery, while flyback converter with active clamp circuit can recover energy stored in leakage inductor of transformer to drive LED lighting. To simplify the circuit structure, switches of two converters are integrated with a synchronous switch technique. With this approach, the propod system has veral merits, which are a less component counts, lighter weight, smaller size and higher conver-sion efficiency. As compared with the conventional converter with hard-switching circuit, the propod one can improve conversion efficiency of 7% and achieve efficiency about 83% under full load when the propod system is operated in the driving LED condition. Expe-rimental results obtained from a prototype with the output voltage of 10 V and output power of 20 W have been implemented to verify its feasibility.
Keywords: LED lighting, buck converter, flyback con-verter, active clamp circuit, synchronous switch tech-nique.
I.I NTRODUCTION
n the past century, rious greenhou effect and environmental pollution caud by overusing fossil fuels have disturbed the balance of global climate. In particular, effluent gas emissions and increas in CO2 levels in the atmosphere have affected global surface temperatures which increa at a rate 0.6˚C/century [1]-[2]. To reduce emission of ex-hausted gas, zero-emission renewable energy sources have been rapidly developed. One of the sources is photovoltaic (PV) cell, which is clean, quiet and an efficient method for generating electric-ity. The PV power system has been widely ud in power processing technologies, such as solar power generation for grid connection, solar vehicle con-striction, battery charge, water pump, satellite power system, traffic signal, electronic sign, and so on.
Due to the fast development of light emitting dio-des (LED S) technology in the past few decades, the
luminescence efficiency of ones has been incread
around veral times [3]. It is becoming more preva-lent in a wide variety of applications, which include
automotive taillights, LCD backlight, traffic signals
and electric signs [4]-[5]. As mentioned above, the
paper propos an LED lighting system for traffic
signal in which PV array is adopted as the power
source.
In traffic signal using PV energy, the propod
LED lighting system will inevitably need batteries
for storing energy during the day and for releasing
energy during the night. Therefore, the propod one
needs a charger and discharger, as shown in Fig. 1.
Since the propod one belongs in the low power level applications, some converters, such as buck,
boost, buck-boost, flyback and forward converters,
are adopted due to their simple circuit structure.
The propod lighting system consists of charger
and discharger. Since power source of charger us PV array, its voltage is usually higher than that of
battery (≈6 V). Therefore, a step-down converter, such as buck or buck-boost converter, can be adopted
[6]-[8]. Additionally, since buck-boost converter
posss step-up and-down voltage ratio simulta-understood
neously, its ulitization ratio of switch duty for con-verter as a step-down converter is only equal to half that of buck converter. Therefore a buck converter is chon as the charger. For choosing circuit structure of the discharger, a step-up converters can be adopted becau output voltage V0 is greater than battery vol-tage V B. Among step-up converter, flyback converter has many merits which are simpler circuit structure and wider range of step-up voltage ratio. Therefore, the propod lighting system is parately adopted buck converter as the charger and flyback converter as the dis
charger, as shown in Fig. 2.
Since leakage inductor of transformer in the fly-
back converter will induce spike voltage across
switch and energy loss, an active clamp circuit can be
introduced into flyback converter to recover the
energy trapped in leakage inductor, as shown in Fig.
I
3(a). Furthermore, buck converter can u a syn-chronous rectification manner to reduce forward loss of diode in low voltage level applications [9]-[11]. Since buck and flyback converters are oper-ated in complementary, two pairs of switches (M 1,M 4)and (M 2,M 3) are respectively operated in synchron-ous. Therefore, each pair of switches can u syn-chronous switch technique [12] to integrate them with a single switch, as shown in Fig. 3(g). From Fig. 3(g), it can be en that the propod system can u less component counts to achieve the same functions. As mentioned above, the propod lighting system can reduce cost, weight and size, and can increa
华迈净水器怎么样conversion efficiency, significantly.
Fig. 1 Block diagram of
PV power system for LED lighting.
Fig. 2 Schematic diagram of the conventional PV power system.
II.D ERIVATION O F T HE P ROPOSED S YSTEM Since the propod power system using PV array as the power source, the propod one consists of a charger and discharger, as shown in Fig. 1. Al-though the charger and discharger can supply energy to battery during the day and drive LED lighting during the night, they need more component counts and driving circuit, resulting in higher cost, and larg-er volume and size. Furthermore, to increa conver-sion efficiency of the LED lighting, a soft-switching circuit is introduced into the charger or discharger. As mentioned above, a buck converter with synchronous rectification manner as a charger and an active clamp flyback converter as a discharger are propod. To further simplify circuit structure of the propod one, switches of two converters are integrated with syn-chronous switch technique [12]. In the following, the propod LED lighting system with synchronous switch technique is derived.
Fig. 3(a) shows schematic diagram of the conven-tional LED lighting system. Since the propod charger and discharger are operated in complemen-tary in which the operational states are controlled by switch S 1, switch pairs of (M 1,M 4) and (M 2,M 3) can be parately operated in synchronous. It will not affect the operation of the original LED lighting sys-tem. From Fig. 3(a), it can be en that switches M 2and M 3 are operated in synchronous and they share a common node C. According to synchronous switch technique [12], switches M 2 and M 3 can be integrated and replaced with switch M 23, as shown in Fig. 3(b). Due to the complementary operation between the charger and discharger, voltage across switches M 2and M 3 are the same value in their turn-off states. Therefore, diodes D B231 and D B232 can be shorted, and D F231 and D F232 can be removed, as shown in Fig. 3(c). From Fig. 3(c), it can be obrved that leakage and magnetizing inductors L K and L m in ries are connected in parallel with inductor L 1. Since leakage inductor L K is much less than magnetizing inductor L m , inductors L 1 and L m can be regarded as a parallel circuit. Therefore, inductors L 1 and L m can be com-bined as inductor L 1m , as shown in Fig. 3(d). Similar-ly, switches M 1 and M 4 are combined and replaced with switch M 14, as illustrated in Fig. 3(e). According to switch integration principle and the same voltage values between switches M 1 and M 4 during switches turn-off interval, diodes D B141 and D B142 can be shorted, and D F141 and D F142 can be removed, as shown in Fig. 3(f), sings of Since capacitors C 1 and C 2 are conne
菱角怎么煮
cted in parallel, they can be integrated with capacitor C C . Furthermore, in order to simplify component sign shown in Fig. 3(f), switches M 14 and M 23 are parately replaced with switches M 1 and M 2,while that of inductor L 1m is also changed with in-ductor L m , as shown in Fig. 3(g). From Fig. 3(g), it can be found that the propod LED lighting system can u a less component counts to achieve the same functions. Thus, the propod lighting system can reduce cost, weight and size, and increa conversion
efficiency significantly.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Fig. 3 Derivation of the propod lighting power for LED lighting.
III.
DESIGN OF THE PROPOSED LIGHTING
SYSTEM
The propod lighting system is compod of buck converter and an active clamp flyback converter. Since buck and flyback converters share the same inductor L m as the stored component, design of in-ductor L m must meet requirements of each converter. When the propod two converters are designed, their design procedures are the same two conven-tional converters, buck and flyback converters, ex-cept that design inductor L m must meet requirements of each converter simultaneously. Since design of buck converter with synchronous rectification tech-nique is similar to the conventional one, design of the active clamp flyback converter is only derived in this paper. In design of the active clamp flyback converter, determination of duty ratio D 2, transformer T f , active clamp capacitor C c and output capacitor C o are pre-nted.
A.Duty Ratio D 2
To determine duty ratio D 2, it needs to first obtain input to output voltage transfer ratio M 2 of the pro-pod flyback converter. That is, transfer ratio M 2will be the same as the conventional one. According to volt-cond balance principle, the following equa-tion can be obtained by
022()0,B S S V
V D T D T N影调
−+−= (1)
where D 2 is the duty ratio of switch M 2,N is the turns ratio of transformer T f and V 0 is the output voltage. From (1), it can be found that transfer ratio M 2 can be expresd as
0222
.1B V ND M V D ==
− (2) According to (2), a larger duty ratio D 2 corresponds to a smaller transformer turns ratio N , which results in a lower current stress impod on switches M 1 and M 2, as well as lower voltage stress on freewheeling diode D 2. However, in order to accommodate varia-tions of load, input voltage and component value, it had better lect an operating range between D 2=0.35~0.4.
B.Transformer T f
Once duty ratio D 2 is specified, turns ratio N of transformer T f can be determine from (2), which yields
202(1).
B D V N D V −= (3) By applying the Faraday ′s law, the number N 1 of the turns at the primary winding can be determined as
21,B S c D V T
N A B
=+ (4)
where A C is the effective cross-ction area of the transformer core and +B is the working flux density. According to (3) and (4), N 2 (=NN 1) can be therefore determined.
For the flyback converter, magnetizing inductor L m of transformer T f is determined by taking into a
c-count the current down slop, which corresponds to the off-time of switch M 2, and the inductance must be large enough to maintain CCM operation. The in-ductance of L mf must satisfy the following inequality:
0222(max)
(1),
s mf D V D T L N I −≥+ (5) where I D2(max) is the maximum ripple of the condary winding current of transformer T f and it is equal to +I Lm(max)/N in which +I Lm(max) is the maximum val-ue of the primary winding current. When the maxi-mum current ripple +I D2(max)is specified, the mini-mum magnetizing inductance L m can be determined. When the propod lighting system is operated in the charging state, buck converter is in working. If buck converter is operated in CCM, the inductance of L mb must satisfy the following inequality:
1(max)
(1),B s mb m V D T L L −≥+ (6)
where D 1 is the duty ratio of switch M 1 and V B is the voltage of battery. When D 1 is equal to D 2, (6) can be rewritten by
2(max)
(1).
B s mb L V D T L I −≥+ (7)
As mentioned above, when inductance of inductor L m satisfies (5) and (7) simultaneously, it can meet the requirements of each converter.
C.Output Capacitor C o
The output capacitor C o is primarily designed for reducing ripple voltage of output voltage V 0. The ripple voltage across output capacitor C o is deter-mined as follows: 0(max)0(max)1
(),co rco s o o o
DI Q V I DT C C C ==
×=++ (8) where I O(max) is the maximum output current.
D.Active Clamp Capacitor C C
The active clamp C C is ud to achieve soft-switching feature. To achieve a ZVS feature, the
energy trapped in leakage inductor L K must satisfy
the following inequality:
22(8)(9)12(max)11()()22
K LK tv LK tv M M DS L I I C C V −≥+, (9) where I LK(tv8) is the leakage inductor current at time t 8,
I LK(tv9) is that at time t 9,C M1 and C M2 are parately
junction capacitors of switches M 1 and M 2, and
V DS2(max) is the maximum voltage across switch M 2
and its value is equal to (V B +V 0/N). Once C M1,C M2,I LK(tv8) and I LK(tv9) are specified, leakage inductor L K can be determined as
2122(max)(8)(9)().
()
M M Ds K LK tv LK tv C C V L I I +≥− (10) To achieve ZVS feature using active clamp circuit, one half of the resonant period formed by L K and C C should be greater than the maximum off-time of switch M 2. Thus, capacitor C C must satisfy the fol-
2(1).off s t D T =− (11) From (11), when L K is specified, the capacitance
ranges of clamp capacitor C C can be determined as
222(1).s C K
D T C L −≥ (12) IV. MEASURED RESULTS
To verify the performance of the propod lighting system, a prototype with the following spe-cifications was implemented.
历史教育A.buck converter for charging battery
z Input voltage V PV : DC 17 ~ 20 V (PV arrays), z Switching frequency f s : 300 kHz,
z Output voltage V B : DC 5~7 V (battery:
6V/2.3Ah), and
z Maximum output current I dc(max): 2 A.
B.Active clamp flyback converter for lighting sys-tem
z Input voltage V B : DC 5~7 V , z Output voltage V o : 10 V dc ,
z Switching frequency f s2: 300 kHz, and z Output current I o : 2 A.
Since the charging current I B of battery can be changed by duty ratio D 1 of switch M 1 in buck con-verter, it is proportional to duty ratio. Fig. 4 (a) and (b) respectively show the measured voltage V DS of
switch M 1and current I B waveforms under duty ratio of 0.2 and 0.3, from which it can be obrved that current I B is proportional to duty ratio D 1. When the propod lighting system is operated in t
he discharg-ing state, active clamp flyback is in working. Since the propod lighting system is ud to drive LEDs, it
does not need to dim light output in the traffic signal. Therefore, the propod lighting system can only regulate output voltage. In the lighting state, meas-ured voltage V DS and current I DS waveforms of switches M 1 and M 2 are shown in Figs. 5and 6. Fig. 5
shows tho of switches M 1 and M 2 at 20% of full
load, while Fig. 6 shows tho of switches M 1 and M 2
under full load. From Figs. 5and 6, it can be en that
switches M 1 and M 2can be operated with ZVS at
turn-on transition. Comparison of conversion effi-
ciency between flyback converter with hard-switching circuit and one with active clamp circuit from light load to heavy load is shown in Fig. 7, from which it can be found that efficiency of the propod converter is higher than hard-switching one. Its efficiency is 83% under full load. Fig. 8 shows the step-load change between 20% of the full load and the full load, illustrating that the voltage regulation of output voltage V o
has been limited within 1%.
海参的种类
(20V/div ,500m A/div , 1 μs/div)
(a)
(20V/div , 500m A/div, 1 μs/div)
(b)
Fig. 4. Measured voltage V DS waveform of switch M 1 and the charged current I B waveform operated in duty ratio of (a) 0.2, and
(b) 0.3 for working in the charging state.
刀鱼扒白菜的做法
(20V/div ,10 A/div , 1 μs/div)
(a)
(20V/div ,10 A/div , 1 μs/div)
(b)
Fig. 5. Measured voltage V DS and current I DS waveform of (a) switch M 1 and (b) switch M 2 for working in the discharged state
under 20% of full load.
(20V/div ,10 A/div , 1 μs/div)
(a)
(20V/div ,10 A/div , 1 μs/div)
(b)
Fig. 6. Measured voltage V DS and current I DS waveform of (a) switch M 1 and (b) switch M 2 for working in the discharged state
under full load.
al hard-switching flyback converter and the propod one from light load to heavy load for operating in the discharged state.
