Journal of Power Electronics

The Korean Institute of Power Electronics (전력전자학회)
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Analysis, Design, and Implementation of a Soft-Switched Active-Clamped Forward Converter with a Current-Doubler Rectifier

  • Jang, Paul (Department of Electrical and Computer Engineering, Seoul National University) ;
  • Kim, Hye-Jin (Department of Electrical and Computer Engineering, Seoul National University) ;
  • Cho, Bo-Hyung (Department of Electrical and Computer Engineering, Seoul National University)
  • Received : 2015.08.25
  • Accepted : 2016.01.01
  • Published : 2016.05.20
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Abstract

This study examines the zero-voltage switching (ZVS) operation of an active-clamped forward converter (ACFC) with a current-doubler rectifier (CDR). The ZVS condition can be obtained with a much smaller leakage inductance compared to that of a conventional ACFC. Due to the significantly reduced leakage inductance, the design is optimized and the circulating loss is reduced. The operation of the ACFC with a CDR is analyzed, and a detailed ZVS analysis is conducted on the basis of a steady-state analysis. From the results, a design consideration for ZVS improvement is presented. Loss analyses of the converters shows that enhanced soft-switching contributes to an efficiency improvement under light-load condition. Experimental results from a 100-W (5-V/20-A) prototype verify that the ACFC with a CDR can attain ZVS across an extended load range of loads and achieve a higher efficiency than conventional ACFCs.

Keywords

I. INTRODUCTION

The forward converter has been one of the most extensively used topologies in low- and medium-power DC-DC converter applications (e.g., computer and telecommunication systems) because of its simple circuitry, low cost, and high efficiency. However, several issues remain such as high-voltage spikes across the MOSFET and resetting of the transformer. To solve these problems, forward converters have used several reset schemes. An active-clamp circuit is the most widely used scheme because it does not require an additional reset winding or energy dissipative component to minimize the voltage stress across the MOSFET [1]-[7]. In addition, an active-clamp circuit enables zero-voltage switching (ZVS) in a MOSFET.

In a conventional active-clamped forward converter (ACFC), the main switch can achieve ZVS by harnessing either the magnetizing inductance [8]-[10] or the leakage inductance [12]-[14]. The method of magnetizing inductance requires a gap in the transformer that increases the magnetizing current, discharging the output capacitor of the MOSFET and resulting in ZVS. However, a hard-switching operation is employed rather than a soft-switching one when a decreased switching loss cannot compensate for the increased conduction loss [10]. Consequently, this method is only suitable for converters with a small input current and a high-input voltage [11]. The method of the leakage inductance uses the resonance between the leakage inductance and the output capacitor of the MOSFET to discharge the stored energy. The ZVS condition is more easily met as the leakage inductance increases. However, large duty cycle losses result in an overall decrease in efficiency. Several methods have been proposed to improve the ZVS operation while using a relatively large magnetizing inductance and a small leakage inductance [15]-[17]. However, these methods either require additional components [15], [16] or only apply to ACFCs with an externally driven synchronous rectifier on the secondary side [17].

In order to overcome these limitations, this paper proposes an ACFC with a CDR to improve ZVS operation. CDR is widely used in applications with low-output voltage and high-output current because the root-mean-square (RMS) current on the transformer secondary is small and the output voltage ripple is reduced [18]-[20]. Many previous studies have reported on the general advantages of CDR-based topologies [21]-[25]. It has also been reported that CDR improves ZVS operation when used with a phase-shifted full-bridge converter [26], [27]. However, for a phase shifted full bridge converter, the primary current should decay rapidly during the zero state [27], and the output inductor current should become negative at the switching instant for ZVS improvement, which compromises the general advantages of CDR. While an ACFC with a CDR can inherently use the output inductor energy to improve ZVS operation, it has never been remarked before. Hence, an excessive resonant inductor has been used [12] or ZVS has been reported with only empirical results [21], [28]-[30].

Therefore, the present study rigorously analyzes an ACFC with a CDR to achieve enhanced ZVS operation. This paper demonstrates that an ACFC with a CDR improves the ZVS performance and presents a design consideration for further improvements. A quantitative comparison of the losses in an ACFC with a CDR and those in other ACFCs verifies that the enhanced ZVS performance is responsible for the improvement in light-load efficiency. The experimental results also show that enhanced soft-switching contributes to improvement in efficiency under light-load. Therefore, an ACFC with a CDR exhibits a high efficiency across all load conditions.

Despite its advantages, the proposed converter needs an additional output inductor. If discrete magnetic components are used, three cores are required: one for the transformer and two for the output inductors. These can increase the cost and size of the converter. However, since previous studies [31]-[34] have reported that the three cores can be replaced by integrated magnetic structures, the present study focuses on the aspect of CDR efficiency.

The paper is structured as follows: Section II describes the circuit configuration of the ACFC with a CDR. Section III presents a ZVS analysis based on a steady-state analysis, and provides design considerations for ZVS improvement. Section IV presents a loss analysis to verify the role of the enhanced ZVS performance in improving efficiency. Section V experimentally verifies the assertions of Section IV using a 100-W (5-V/20-A) prototype, and Section VI concludes the paper.

 

II. CIRCUIT CONFIGURATION

The circuit configurations of the conventional ACFC and an ACFC with a CDR are shown in Figs. 1(a) and 1(b), respectively. Unlike the conventional ACFC, RL1, RL2, and Rt which are the equivalent series resistances (ESR) of L1, L2, and the transformer, are considered in Fig. 1(b) for the ZVS analysis as explained later. The auxiliary switch S2 and the clamp capacitor Cc are components of the active-clamp circuit and recycle leakage energy. The transformer is modeled with a magnetizing inductance Lm and an equivalent leakage inductance reflected on the primary side Llk. The main switch S1 is operated with the duty cycle D, and the auxiliary switch S2 is operated complementarily to the duty cycle of S1, with dead times preceding and following the auxiliary switch action. Both of the switches include body diodes Ds1 and Ds2, and output capacitors Cs1 and Cs2. The secondary side comprises two synchronous switches SR1 and SR2, two output inductors L1 and L2, and an output capacitor Co.

Fig. 1.Circuit configuration. (a) Conventional ACFC. (b) ACFC with CDR.

Active-clamp circuitry can be applied to either the high side or the low side. High-side clamps are applied across the primary side of the transformer and use an N-channel auxiliary switch on the clamp network. Hence, they are appropriate for high-input-voltage applications. However, additional high-side gate circuitry is needed to drive the auxiliary switch. A low-side clamp is applied across the drain-to-source of the main switch and a P-channel auxiliary switch is used on the clamp network. The drain-to-source voltage rating of the P-channel switch is lower than that of the N-channel switch and it cannot be used for off-line applications. However, it does not require any additional gate drive circuitry and improves the precision of the control over the delay timing for ZVS.

The level of the input voltage in most forward converter applications is lower than the line voltage. Therefore, the low-side clamp is adopted in this paper. The fundamental principles are exactly the same for both clamps. As a result, the following analyses can also be applied to high-side clamps.

 

III. ZVSANALYSIS

ZVS operation of S2 is guaranteed regardless of load variations [15] and does not need to be considered separately in the design consideration. Consequently, the ZVS analysis of S1 is conducted in the following.

A. Steady-state analysis

Steady-state waveforms of the ACFC with a CDR are shown in Fig. 2. The operation modes and analyses are complicated by the resonant operation. The circuit operation and ZVS analysis are simplified by the following assumptions:

Fig. 2.Steady-state waveforms.

Since these assumptions make scarcely any changes in the minimum value of iLlk(t), which is the most significant value for the ZVS analysis, they simplify the analysis without introducing any inaccuracies.

When the above conditions are applied, the total number of intervals in one switching cycle decreases from ten to two and D becomes equal to the effective duty Deff. Equivalent circuits for these two states are depicted in Fig. 3. The steady-state waveforms of the converter are shown in Fig. 4.

Fig. 3.The two operation states to calculate steady-state DC average values. (a) State 1: S1 is on. (b) State 2: S2 is on.

Fig. 4.Approximated steady-state waveforms.

To ensure the ZVS of S1, the energy stored in Llk must be larger than the energy stored in Cs1 at the moment the switch is turned on, t1. Llk and iLlk(t1) are critical determinants of the ZVS of S1. ZVS is more easily achieved when both determinants increase. Large values of Llk degrade the converter efficiency and affect the voltage conversion ratio of the forward converter. Therefore, it is more desirable to increase iLlk(t1).

In the conventional ACFC, the leakage current iLlk(t) is equal to the magnetizing current iLm(t), and it discharges Cs1 at t1, as shown in Fig. 5(a). The ZVS operation of S1 is not easily achieved with a small leakage inductance because the magnitude of iLlk(t1) is not large enough to totally discharge Cs1.

Fig. 5.Current flow of transformer and Csl at t1. (a) Conventional ACFC. (b) ACFC with CDR.

Meanwhile, the transformer-secondary current isec(t) can flow bidirectionally on the ACFC with a CDR. In this case, iLlk(t) is not equal to iLm(t). It is determined from the sum of iLm(t) and the transformer-primary current ipri(t) at t1, as shown in Fig. 5(b). The increment of ipri(t1), reflected from iL2(t1), can contribute to the ZVS of S1. The steady-state and ripple values of iLm(t) and ipri(t) must be obtained to investigate how enhanced ZVS operation occurs in the proposed converter. The steady-state values of iLm(t), iL1(t), iL2(t), and vCc(t) can be obtained by solving the state-space equations (The detailed derivation is provided in the Appendix). The results are as follows [22]:

From Eqs. (1) and (3), it can be seen that ILm multiplied by the transformer turn ratio N is equal to IL2 This is due to the charge balance on Cc. Therefore, iLlk(t1) is determined by the ripple current of iLm(t) and iL2(t). The absolute value of iLlk(t1) is given by:

where VSR stands for the sum of the voltage drops across the ESRs and synchronous switches.

In the case of the conventional ACFC, |iLlk(t1)*|, which corresponds to |iLlk(t1)|, can be written as follow [3]:

where

B. ZVS advantageous area

For improved ZVS operation in the ACFC with a CDR relative to the conventional ACFC, |iLlk(t1)| has to be larger than |iLlk(t1)*|. If it is assumed that ECs = 0 for the worst case design and neglect VSR in Eq. (5), the ZVS condition for the extended load range can be described as follow:

When the condition in Eq. (8) is met, the ACFC with a CDR shows enhanced ZVS characteristics when compared with the conventional ACFC. From the small values of Llk, the duty cycle loss is reduced and the design of a forward converter becomes convenient. If the condition in Eq. (8) can be easily satisfied, the ACFC with a CDR represents an appropriate substitute for the conventional ACFC.

The advantageous ZVS conditions for the ACFC with a CDR and the conventional ACFC are investigated according to Eq. (8) using the converter system specifications given in Table I. The results for both converters are given in Fig. 6. The boundary condition line separates the region where the ACFC with a CDR most easily achieves ZVS from the corresponding region for the conventional ACFC. The ACFC with a CDR attains ZVS with a smaller leakage inductance than the conventional ACFC. Moreover, the difference between the minimum inductances for each converter increases as the load decreases.

Table ISYSTEM SPECIFICATION

Fig. 6.ZVS advantageous condition.

C. ZVS condition for the ACFC with a CDR

The main switch ZVS condition for the ACFC with a CDR is given by the following:

where Vg max max is maximum input voltage.

Using Eqs. (4) and (5) while assuming that VSR is negligible, Eq. (9) can be arranged as:

where Dmin is the minimum duty ratio.

Unlike the conventional ACFC, the leakage inductor for ZVS is not load dependent for the ACFC with a CDR. A leakage inductor that meets the condition in Eq. (10) guarantees the ZVS condition across the entire range of loads. Furthermore, the ACFC with a CDR can achieve ZVS while keeping Llk much lower than Lm if the output inductor is properly designed. The leakage inductance for ZVS is shown in Fig. 7 under the following conditions: Lm varies from 0 to 400 μH, L2 varies from 0.5 to 2 μH, and Vg is fixed at 48 V. The optimal design conditions can be easily identified from Fig. 7.

Fig. 7.Leakage inductance for ZVS operating ACFC with CDR.

 

IV. LOSS ANALYSIS

To verify that the enhanced ZVS performance was responsible for the improved efficiency, a loss analysis was conducted. The ACFC with a CDR was designed according to the system specifications in Table I, and the results are shown in Table II. For the conventional ACFC, the design specifications are the same as those of the ACFC with a CDR, except that a 1-μH output inductor was used in the conventional ACFC.

Table IIDESIGN RESULTS OF THE ACFC WITH CDR

The loss factors considered in this analysis are as follows: 1) the conduction loss in the FETs and ESRs (Pcond); 2) the switching loss in the FETs (Psw); and 3) the transformer core loss (Pcore). Numerical expressions for each of the loss factors are derived below.

First, the conduction loss is calculated from the resistance and RMS current through each resistor. The conduction loss is expressed as:

Next, the switching loss is estimated from the output capacitor loss and power loss during the switching transition period. The switching loss can be expressed as:

where Vi and Ii are the switch voltage and current, and ton and toff are the turn-on and turn-off switching periods of the corresponding power MOSFET, respectively. In the case of synchronous switches, only the turn-off losses are accounted for because they always attain ZVS by the load current and turn-on losses are negligible.

Finally, the core loss is given by

where ρm, Ve, and Bm are the core material density, the core volume, and the maximum flux density, respectively. Kc, α, and β are constants that can be determined by fitting the core data provided by the manufacturer [35].

The total efficiency and loss component are calculated for the four converters:

All of the device parameters needed in the loss calculation are obtained from the datasheet provided by the manufacturer. The results of the loss analysis are shown in Fig. 8.

Fig. 8.Comparison of estimated efficiency and loss components. (a) ACFC with CDR and hard-switching ACFC. (b) ACFC with CDR and soft-switching ACFCs.

In Fig. 8(a), the estimated efficiency and loss components of the ACFC with a CDR and the hard-switching ACFC are presented. The ACFC with a CDR has the lowest switching loss due to its outstanding ZVS characteristics. It exhibits a higher efficiency than the hard-switching ACFC at light-load conditions, when switching losses dominate. Due to its small RMS current on the secondary, the ACFC with a CDR also exhibits a higher efficiency in the heavy-load range. Therefore, the ACFC with a CDR achieves a high efficiency when compared to the hard-switching ACFC throughout the whole load.

In Fig. 8(b), a comparison of the ACFC with a CDR and two soft-switching ACFCs is presented. The Llk-ZVS ACFC has the advantage of soft-switching due to its large leakage inductance, thereby attains ZVS from medium-load conditions. However, the duty cycle loss and auxiliary switch turn-off loss increase with the load. Consequently, the Llk-ZVS ACFC exhibits the worst heavy-load efficiency of all of the converters. The Lm-ZVS ACFC achieves soft-switching due to its reduced Lm and it shows a high efficiency at light-load conditions. However, the heavy-load efficiency is worse than the ACFC with a CDR because of the increased conduction loss on the primary side.

The loss analysis shows that the ACFC with a CDR exhibits the most outstanding ZVS performance. The light-load efficiency is expected to improve with soft-switching. At heavy loads, the advantages of a small RMS current on the secondary contribute to a high efficiency. The two soft-switching ACFCs also show high efficiency at light-loads. However, heavy-load efficiency get worse because of the duty cycle loss, switch turn-off loss and primary side conduction loss.

 

V. EXPERIMENTAL RESULTS

The improved efficiency of the ACFC with a CDR was experimentally verified. Prototypes of the ACFC with a CDR (Lm = 200 μH & Llk = 3 μH), a hard-switching ACFC (Lm = 200 μH & Llk = 3 μH), a Llk-ZVS ACFC (Lm = 80 μH & Llk = 5 μH), and a Lm-ZVS ACFC (Lm = 40 μH & Llk = 3 μH) were built and tested.

The waveforms for the leakage inductor current (iLlk), transformer secondary current (isec), magnetizing inductor current (iLm), and two output inductor currents (iL1 and iL2) are shown in Fig. 9 for the ACFC with a CDR at a 10% load. The function for iLm is calculated from iLlk and isec divided by the transformer turn ratio. As seen in Fig. 9, the average is 226 mA, whereas the average is 982 mA. This is nearly N times larger than . Therefore, iLlk(t1) is determined by the ripple current of iLm and iL2, as shown in Fig. 4.

Fig. 9.Current waveforms of the ACFC with CDR at 10% load condition.

Waveforms for the drain-to-source voltage of S1 (Vds1), the gate-to-source voltage of S1 (Vgs1), the gate-to-source voltage of S2 (Vgs2), and the leakage inductor current (iLlk) are shown in Fig. 10 for the ACFC with a CDR. Vds1 is zero before S1 is turned on, which confirms the ZVS of S1 with a low electromagnetic interference (EMI) despite a small leakage inductance under all load conditions. The primary conduction loss is greater than that of the conventional ACFC because of the increased ipri(t1). Nevertheless, a decrease in the primary-switching loss compensates for the increased primary conduction loss. As a result, the efficiency of the ACFC with a CDR is higher than that of the hard-switching converter at light load.

Fig. 10.Waveforms of the ACFC with CDR (Lm = 200 μH & Llk = 3 μH). (a) 10% load. (b) 50% load. (c) 100% load.

Waveforms for the hard-switching ACFC at a 10% load are shown in Fig. 11(a). When S1 is turned on, a hard-switching operation is observed with EMI noise before Vds1 becomes zero. This diminishes the light-load efficiency of the hard-switching ACFC, as seen in Fig. 14(a). Waveforms of the hard-switching ACFC at 50% and 100% load are shown in Fig. 11(b) and Fig. 11(c), respectively. At the 100% load condition, S1 almost achieves ZVS due to the increased load current. However, the ACFC with a CDR still shows a high efficiency.

Fig. 11.Waveforms of conventional ACFC (Lm = 200 μH & Llk = 3 μH). (a) 10% load. (b) 50% load. (c) 100% load.

Waveforms for the Llk-ZVS ACFC at a 10% load are shown in Fig. 12 (a). Due to its large leakage inductance, hard-switching operation is observed with less EMI noise below 50% load condition. From a 50% load, S1 almost achieves ZVS as seen in Fig. 12(b). However, an increased duty cycle loss is observed at the heavy load condition, which deteriorates the efficiency.

Fig. 12.Waveforms of Llk–ZVS ACFC (Lm = 80 μH & Llk = 5 μH). (a) 10% load. (b) 50% load. (c) 100% load.

Waveforms for the Lm-ZVS ACFC are shown in Fig. 13. Due to its reduced magnetizing inductance, soft-switching operation is observed with less EMI noise under all load conditions. However, an enlarged primary current is observed relative to the other ACFCs. This decreases efficiency at heavy-load.

Fig. 13.Waveforms of Lm–ZVS ACFC (Lm = 40 μH & Llk = 3 μH). (a) 10% load. (b) 50% load. (c) 100% load.

In conclusion, the ACFC with a CDR achieves ZVS of S1 more easily than the conventional ACFC, even with a small leakage inductance. Improvements in efficiency are most evident in the light-load conditions, especially, when the switching loss is the dominant factor in the total loss. The conventional ACFC is also able to perform ZVS with an increased resonant inductance or a reduced magnetizing inductance. However, its heavy-load efficiency is worsened by its large duty cycle loss, auxiliary switch turn-off loss and primary side conduction loss. Consequently, the ACFC with a CDR has a higher efficiency with less EMI noise than the other ACFCs for all load conditions (see Fig. 14).

Fig. 14.Efficiency measurement. (a) ACFC with CDR and hard-switching ACFC. (b) ACFC with CDR and soft-switching ACFCs

 

VI. CONCLUSIONS

In this paper, the ZVS operation of an ACFC with a CDR is studied. Placing the CDR in the transformer secondary, the ZVS condition can be obtained with a much smaller leakage inductance compared to the conventional ACFC. A detailed ZVS analysis is conducted on the basis of a steady-state analysis. The design consideration for ZVS improvement is presented. A loss analysis of the converter shows that the enhanced ZVS performance contributes to improved efficiency under light-load conditions. Experimental results with a 100-W (5-V/20-A) prototype verified that the ACFC with a CDR can attain ZVS of the main switch more efficiently in spite of a small leakage inductance and that it can achieve a high efficiency compared to other ACFCs throughout the whole load range.

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