A Novel Forward AC/DC Converter With Input
Current Shaping and Fast Output Voltage
Regulation Via Reset Winding
Lon-Kou Chang, Member, IEEE, and Hsing-Fu Liu, Member, IEEE
Abstract—This paper presents a novel simple forward ac/dc con-verter with harmonic current correction and fast output voltage regulation. In the proposed ac/dc converter, a transformer incorpo-rating reset winding provides two main advantages. First, the bulk inductor used in the conventional boost-based power-factor-cor-rection cell is omitted in the proposed converter, allowing signifi-cant volume and weight of magnetic material to be saved. Second, the voltage across the bulk capacitor can be held under 450 V by adjusting the transformer winding ratio, despite the converter op-erating in a wide range of input voltages (90 265 V/ac). This new converter complies with IEC 61000-3-2 under a load range of 200 W and has fast output voltage regulation.
Index Terms—AC/DC converter, IEC 61000-3-2, input current shaper, power-factor correction (PFC).
I. INTRODUCTION
I
N modern electronic products, including personal com-puters, computer peripherals, and test instruments, ac/dc converters have become the primary power supplies. The ac/dc converters use switching circuits to achieve high-power transfer efficiency and ac/dc converter controllers can be designed flexibly. Improving power quality considerations requires two things: achieving high power factor and low high-frequency harmonics. Many studies have examined the relevant issues and numerous topologies have been proposed. The proposed solutions of these studies can be classified into two groups: those designed such that the input line current is sinusoidal and those designed such that it is nonsinusoidal [1], [2]. The group of topologies with the sinusoidal line current almost achieves the requirement of unity power factor but requires a complex topology or control circuit [3], [4]. Thus, the sinusoidal line current topologies are more costly to implement. Fig. 1 shows the block diagram of the ac/dc converter with sinusoidal input line current.The ac/dc converter with nonsinusoidal line current employs a simple topology, such as single-stage–single-switch, and costs less in practical applications [6]–[9]. Although the cir-cuits [7]–[9] lack a unity power factor, they comply with IEC 61000-3-2 [4]. A family of such circuits was described in [1]
Manuscript received December 14, 2003; revised February 9, 2004. Abstract published on the Internet November 10, 2004. This work was supported by the National Science Council, Taiwan, R.O.C., under Grant NSC 93-2213-E-009-128.
The authors are with the Electrical and Control Engineering Department, National Chiao Tung University, Hsingchu 300, Taiwan, R.O.C. (e-mail: [email protected]).
Digital Object Identifier 10.1109/TIE.2004.841103
Fig. 1. Classical ac/dc converter with power-factor-correction (PFC) function.
and [2]. In this family a boost circuit accompanied by a dc/dc converter was introduced to form the so-called single-stage single-switch ac/dc converters. The family circuits have PFC function, as illustrated in Fig. 2. This concept successfully simplifies a conventional power-factor corrector by changing it from two stages to one stage. However, this concept employs a bulk inductor in the boost section, which occupies significant volume and weight.
This study proposes a new converter. Fig. 3 shows the topology of this new converter. The new converter satisfies the input harmonic current limits required by IEC 61000-3-2 and also has fast output response. A multiwinding transformer is employed in the proposed converter. The additional winding in the primary side is known as a reset winding in the forward-type converter. The reset winding of the transformer replaces the boost inductor presented in [6]–[9]. Moreover, the proposed converter design reduces the volume and weight of the mag-netic material by almost half compared to existing boost-based single-stage PFC converters. Furthermore, the voltage across the bulk capacitor can be reduced to a reasonable value by adjusting the turns ratio of the windings and . Therefore, this design can adapt to significant line voltage variation. The structure and operating principles of the proposed converter are presented in Sections II and III, and the practical experimental results are presented in Section V.
II. PROPOSEDCIRCUIT
Fig. 4 illustrates the proposed forward ac/dc converter with harmonic current elimination and fast output regulation speed. The proposed circuit is a single-switch single-stage ac/dc con-verter, which comprises a single switch , an input filter , bulk capacitor , soft-switching inductor , and a transformer with two primary windings and . The reset winding , inductor , diodes and , switch , and bulk capacitor form a boost circuit. Moreover, the windings and , a bulk capacitor , switch , diodes and , inductor
Fig. 2. Prior single-stage ac/dc PFC converter.
Fig. 3. Proposed single-stage ac/dc converter with ICS.
, and output capacitor form a forward converter. The cir-cuit connection of the reset winding differs from that in the classical forward converter. In the proposed design, the reset winding has two functions: to recycle the magnetic current generated by the winding and, also, to form a magnetic feed-back for shaping line current.
A turns ratio of can determine not only the corner angle of the line current, but also the voltage across a bulk ca-pacitor . More detailed effects of turns ratio of are discussed in the following section. The inductor provides a soft-switching function for diodes and . The current of gradually reducing to zero, as illustrated in Fig. 6, results in (in mode ) or (in mode ) turning off switching loss. Additionally, the inductance and volume of are signifi-cantly smaller than the primary windings or of the trans-former.
The control circuit can be designed using either a simple fixed-frequency voltage-mode control or a conventional peak-current-mode control. The experimental results have demon-strated that even if a simple control method is used, the line cur-rent of the proposed ac/dc converter can comply with the stan-dard IEC 61000-3-2 and the converter also can exhibit a fast dynamic response to the load.
III. ANALYSIS OFCIRCUITOPERATION
The operating principle of the proposed converter resembles the boost-based ac/dc single-switch single-stage isolated PFC power supply . The energy, stored in winding while switch turns on, is delivered to bulk capacitor via
when switch turns off. The energy stored in winding comes from the magnetizing current when switch turns on. Furthermore, windings and are based on the same operating principle as the conventional forward converter. The current gives more magnetizing current to charge
to have the same value used in conventional ac/dc forward converter to maintain almost constant in a line cycle. Since the capacitance of is large, remains almost constant during the whole line period. Furthermore, the inductance of the regulation inductor in secondary side is set sufficiently large to keep working in the continuous conduction mode, and also to keep the duty cycle almost constant during these two operation modes.
Fig. 5 shows that the proposed circuit has two operation modes and where and are mirror symmetric to and . Fig. 6 illustrates the relative voltage and current waveforms in a single switching cycle in two operation modes.
A. Operation Modes or (During or )
Within this mode, the converter acts as the conventional for-ward converter. However, the magnetizing current gener-ated by winding is transferred to winding and the ca-pacitor is charged when is switched off. The current linearly reduces to zero when is turned off. denotes the voltage across the bulk capacitor . If is sufficiently large, then can approximate a constant during a line cycle in the steady state, and can be calculated as follows:
(1) At time , the magnetic flux energized by winding re-leases to zero via winding . Thus, the time bound of mode
can be obtained as follows:
(2) or
(3) where is a rectified power source, .
Fig. 7 illustrates the current loop in three intervals in mode or . The winding currents and voltages are calculated as follows:
(4) (5) where
Fig. 4. Proposed forward ac/dc converter with ICS.
Fig. 5. Operation modes in the half of line cycle.
where and is a load ripple current
(7)
(8)
(9)
(10)
B. Operation Modes or (During or )
In this mode, is sufficiently large to turn on when is turned on. Current flows through the winding , ,
, and . Moreover, the capacitor supplies current which flows through winding and . Therefore, the induced voltage across winding forces to linearly decrease to zero. Simultaneously, turns on and the transformer delivers the power to the output circuit. When turns off, the mag-netizing current induces and charges capacitor via winding , , and . The induced current linearly de-creases to at the end of the duty off period of switch
Fig. 6. Voltage and current waveforms in two modes.
, where both and are nonzero in this mode. Fig. 8 shows the current loops for three operating stages in .
The corresponding currents and voltages are obtained as fol-lows:
(11) For is employed, will be continuous at the time ; that is, . Since proper is used,
Fig. 7. Current loops whenS (a) turns on (t t < t ), (b) turns off (t t < t ), and (c) turns off (t t < t ) in mode M =M .
Fig. 8. Current loops whenS (a) turns on (t t < t ), (b) turns on (t t < t ), and (c) turns off (t t < t ) in mode M =M .
the current is almost constant. Considering the magnetic flux generated and applied between winding and we can yield
(12)
Substituting (12) into (11) yields , or . For the other winding currents, the following is obtained:
(13) where
(15) (16) (17) (18)
(19)
IV. DESIGNCONSIDERATIONS A. Line Current and Duty Ratio
The line current is a low-frequency component of flowing through the low-pass filter . Mathematically, the line current is the average current of within a switching cycle. Equation (4) demonstrates that the average current of
, namely, , varies slightly with in both modes and . Moreover, (11) demonstrates that the line current varies markedly with the line voltage in both modes and . Accordingly, the nonzero current , that is,
, also varies with . Therefore, the resultant line current is produced, as illustrated in Fig. 5. Moreover, according to Fig. 6, the current is discontinuous in mode and . Therefore, the current of the proposed converter operates in DCM.
In the control the duty ratio approximates a constant in modes through since the inductor is large and op-erates in a continuous current mode. Additionally, the voltage is almost constant because of the use of a bulk capacitor . Equation (1) shows the relationship between and duty ratio .
B. Corner Angle of Line Current
Line current corner angle (CA) is defined as . Equa-tion (3) demonstrates the relaEqua-tion between CA and parameters, duty ratio , , , and . Moreover, (1)–(3) demonstrate the relation of CA and in different and duty ratio . CA is larger in the high line voltage than the low line voltage given constant output power . Furthermore, the CA also infects the power factor. Low CA is associated with high power factor.
C. Voltage Across Bulk Capacitor
denotes the voltage across bulk capacitor . The influ-ences on the voltage include , duty ratio , corner angle CA, input voltage , and output voltage . The relation is illustrated in (20) and also demonstrates that increases slightly faster than the line voltage. Consequently, the corner angle produced in a high line voltage exceeds that produced in a low line voltage. Smaller CA is known to produce higher power factor and lower total harmonic distortion (THD). Furthermore, in practical applications should be below 450 V/dc with
wide range input, 90 V 265 V/ac, and parameters,
and , also must meet the requirement ( below 450 V/dc) to produce a given output
(20) Fig. 4 illustrates that the bulk capacitor voltage can be determined as follows:
where , and is a period of line voltage and
else.
The current is inversely proportion to , as shown in (4) and (11). Therefore, is also inversely proportion to . The value of thus decreases with increasing .
D. Inductor
Two reasons exist for using inductor . The first reason is to reduce the high-frequency harmonics of , while the second is to reduce the voltage across capacitor . Fig. 5 demonstrates that the slope of is zero at time . The total charge trans-ported by during the duty on period thus equals the total charge transported by . Thus, equating the integration of (4) and (5) gives
The equation above can give the ratio of
(21) or
(22)
E. Design Procedure
The circuit design methods, namely, the design of control loop and components voltage stress for the proposed converter, resembles the conventional forward converter. However, more design considerations must be performed in the proposed cir-cuit for determining the reset winding and magnetic inductance. Therefore, the design method for transformer is shown as fol-lows.
1) Windings turns ratio : Based on (1) and (3),
can be calculated via given , , , and ,. denotes the amplitude of
is recommended to be 2/1 in this prototype.
2) Magnetic inductance and : Since magnetizing
current stores energy to charge bulk capacitor; is recommended to be 20% of the primary load current
(23)
where .
The inductance can be simply obtained from (24) (24) where can be obtained by solving Faraday’s law
(25) where is the effective area of core and is flux den-sity change in the transformer core.
3) Series inductance : The inductance can be obtained from calculating (22).
V. EXPERIMENTRESULTS
The proposed structure has been tested under the specifi-cations of 85 265-V/ac input voltage range, 50-V/dc output voltage, and 100-W output power. The turns ratio of
is 27/23/12, and the inductance , where H and transformer core PQ32/20 is used. The transformer core used in the previous similar converter should be EER35 in [4] and [6]. Although numerous previous similar converters have a similar transformer core size to the proposed converter in given similar output power and switching frequency, the boost inductors sizes, 58 uH 240 H in [4] or 1.4 mH in [6], are several times greater than that of in the proposed converter. The sizes of the boost inductors employed in [4] and [6] are thus several greater than those of when flowing through a similar line current. Fig. 9 illustrates the line current in a full line cycle. Experiments have verified that the harmonic distribution complies with the standard IEC 61000-3-2. Table I demonstrates that the detailed harmonic distribution of the prototype design meets the requirements of class D.
Fig. 10 illustrates dynamic response switching between a half and a full load under 110-V/ac input voltage. The output voltage of the prototype displays a fast response and stable regulation. Moreover, Fig. 11 illustrates the voltages across the bulk capac-itor for different input voltages under a full load. The voltage of the bulk capacitor depends on and turn-ratio but it is almost independent of load current. The maximum voltage can be held below 450 V/dc, a popular commercial voltage in the market for electrolytic capacitors, by adjusting turns ratio
.
Table II displays the voltage stress in , voltage across bulk capacitor , and efficiency . The voltage stress in exceeds
Fig. 9. i and V waveforms at V = 110 V, I = 1:5 A. TABLE I
HARMONICMAINCONTENTS OF THELINECURRENTi
Fig. 10. Dynamic response waveforms forV , i , V , and I when V = 110 V, V = 50 V and I = 0:5 A=1 A.
500 V when input voltage surpasses 130 V. Therefore, an extended type forward converter with two switches [10] can be considered as a solution if the user wants to reduce the voltage stress in . The efficiency is penalized due to a part of the power processed twice. Moreover, it operates in DCM at the current
Fig. 11. Voltage stress of bulk capacitorV and line voltageV . TABLE II
VOLTAGESTRESS OFS , VOLTAGEACROSSBULKCAPACITOR
C ANDEFFICIENCY
VI. CONCLUSION
This paper has proposed a new ac/dc converter structure. The proposed converter has harmonic current correction and fast output voltage regulation. The proposed converter is imple-mented with a single-switch–single-stage topology and single control loop and, thus, is simple. Synchronously, the structure markedly reduces the volume and weight of magnetic material compared to the conventional converters by employing reset winding in transformer. The reset winding replaces the bulk inductor used in boost-based converters. The line current of the proposed converter complies with standard IEC 61000-3-2 and the voltage is tightly regulated under load change, as experimentally verified. The voltage across bulk capacitor can be held below 450 V by adjusting the turns ratio in full range input (85 265 V/ac). The proposed structure also can be extended to other types of converters.
However, this new converter bears a high voltage stress in and a penalized efficiency because a part of the power is processed twice. Alternatively, an extended type forward con-verter with two switches can reduce the high voltage stress. The proposed converter can operate in CCM at line current by in-creasing the inductance . Moreover, the efficiency can be im-proved in CCM operation.
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Lon-Kou Chang (M’87) received the B.S. degree in
electronics engineering from Chung Yuan Christian University, Chung-Li, Taiwan, R.O.C, in 1975, the M.S. degree in electronics engineering from the National Chiao Tung University, Hsingchu, Taiwan, R.O.C., in 1977, and the Ph.D. degree in electrical engineering from the University of Maryland, College Park, in 1995.
Since 1983, he has been with National Chiao Tung University, where he is currently an Associate Pro-fessor of Electrical and Control Engineering. During 1982–1985, he served as a part-time Electrical Supervisor for the Tri-Service General Hospital, Taipei, Taiwan, R.O.C. He was also an R&D Consultant with Sunpentown International Company, Taiwan, R.O.C., in 1996–1998. His re-search interests include circuit design and analysis of power electronics, chipset implementation of power circuits, CAD in circuit design, and related applica-tions.
Hsing-Fu Liu (M’00) received the M.S. degree in
electrical engineering from Chung Yuan Christian University, Chung-Li, Taiwan, R.O.C., in 1991. He is currently working toward the Ph.D. degree at National Chiao Tung University, Hsingchu, Taiwan, R.O.C.
From 1991 to 1995, he was with the Computer and Communications Research Laboratories, In-dustrial Technology Research Institute, Hsingchu, Taiwan, R.O.C., where his research focused on soft-switching techniques, high-density dc/dc con-verters, and ac/dc power-factor-correction circuits. In 1995, he joined Philips Electronics, Chung-Li, Taiwan, R.O.C., as Senior Designer, developing ac/dc and dc/dc power circuits for new model PC monitors. In 1998, he joined Delta Electronics, Chung-Li, Taiwan, R.O.C., where he was involved in developing a switching-mode rectifier for a telecom power system. From 2000 to 2004, he was with Analog Integrations Corporation, Hsingchu, Taiwan, R.O.C., as the Manager of the Application Engineering Department. His interests include high-frequency soft-switching techniques, passive and active snubber circuits, ac/dc power-factor-correction circuits, LEDs lighting power, and modeling simulation on power electronics.
