Dual-Path Hybrid Synchronous Rectifierin Active Clamp Forward Converter
for Inductor Current Reduction
Katsuhiro Hata1, Sadanori Suzuki2, and Makoto Takamiya1
1
The University of Tokyo, Tokyo, Japan
2Toyota Industries Corporation, Aichi, Japan
E-mail: khata@iis.u-tokyo.ac.jp
Abstract— A dual-path hybrid synchronous rectifier (DPHSR) in active clamp forward (ACF) converters is proposed for
inductor current reduction to solve inductor cooling problems
under heavy load. In the proposed DPH-SR, the flying capacitor
also supplies current to the output, thereby reducing theinductor current. In the measurement, the peak efficiency of the
ACF converters with the proposed DPH-SR and conventional
SR was 90.9 % and 89.0 % at 10 AOUT, respectively, resulting in
the improvement in efficiency by 1.9 %. In addition, theinductor conduction loss of the proposed DPH-SR is reduced by
43.0 % by reducing the inductor current by 24.8 % at 20 A OUT.

Vx
VClamp
VIN

IO

nP

nS

+

CO
SS1,Φ1

+

RO

+

VO

SS2,Φ2IO

Vx1
VClamp
VIN

CClamp+

SP2,Φ2nP

nS++ VCFLY S

S3,Φ2

CFLY

IC

L

IL
Vx2

SP1,Φ1SS1,Φ1

CO

+

RO

+

VO

SS2,Φ2

Fig. 2. Active clamp forward converter with proposed dual-path hybrid
synchronous rectifier (DPH-SR).

Fig. 3 shows the two circuit states in the proposed DPHSR. Although the two switches on the primary side of the ACF
converter operate in opposition to apply the AC voltage to thesecondary side, the transformer supplies current to the load
only during State 1. Then, the flying capacitor CFLY isconnected to the inductor L in series. On the other hand, during
State 2, CFLY is in parallel with L and IO is supplied from thedual path, which helps to reduce the inductor DC current IL.
Fig. 4 shows the key operation waveforms in the proposed
DPH-SR. By considering the voltage-second balance in L andthe charge balance in CFLY and CO at the periodic steady state,
the voltage conversion ratio M of the ACF converter with theproposed DPH-SR is expressed as follows:

0 = VLII. PROPOSED DUAL-PATH HYBRID SYNVRONOUS RECTIFIER
A. Circuit operation and voltage conversion ratio
Fig. 1 shows the ACF converter with the conventional
synchronous rectifier (SR). In this topology, the output current
IO is supplied through the output inductor L, which can cause
inductor cooling problems under heavy load. To reduce theinductor losses, the dual-path hybrid synchronous rectifier(DPH-SR) is proposed as shown in Fig. 2. Compared to theconventional SR, the switch SS3,Φ2 and the flying capacitor
CFLY are added in the proposed DPH-SR.

978-1-6654-0688-8/22/$31.00 ©2022 IEEE

L

Fig. 1. Active clamp forward converter with conventional synchronousrectifier.

I. INTRODUCTION
Active clamp forward (ACF) converters [1–2] are suitable
for a high step-down conversion in applications such as
EV/HEV, servers, and data centers [3–5]. Due to the
conduction loss in the output inductor, however, the heatgenerated by the coil increases the cost of cooling. To solve
the problem, rectification circuits to reduce the inductor
current are beneficial for the ACF converter in high current
applications.

This paper describes the operation principle and basic
characteristics of the proposed DPH-SR. In addition, thefeasibility of the ACF converter with the proposed DPH-SR isdemonstrated by experiments.

+

SP2,Φ2

SP1,Φ1

Keywords— Active clamp forward converter, Synchronousrectifier, Hybrid DC-DC converter, Dual-path

Previous studies have proposed indirect hybrid step-down
DC-DC converters [6–10], where the inductor is indirectly
connected to the output, which can achieve the smaller
inductor current than the output current because the output
current is supplied from a dual path (inductor and capacitor
path) unlike the conventional buck converter and direct hybrid
converters [11–16]. Based on the circuit topologies of indirect
hybrid DC-DC converters, in this paper, a dual-path hybrid
synchronous rectifier (DPH-SR) for the ACF converter is
proposed to reduce the inductor conduction loss by reducing
the inductor current.

IL

CClamp1011

n
=  S VIN − VCFLY − VO  DTS + ( 0 − VO )(1 − D ) TS
n
 P

n

=  S VIN − 2VO  DTS − VO (1 − D ) TS nP
VO nS D
=
VIN nP 1 + D

(1)

IX,1

Vx1
nS
V
nP IN

IC,1 I
L
V
CFLY
+
L
CFLY

+

VL

IO

+

SP1
SS1

RO

+ CO

VO

+

+ VCFLY
CFLY

t

t

0
iL (t) ≈ IL, vCFLY (t) ≈ VO, vO (t) ≈ VO

vx1(t)

VO

IO

X,2

VL

+

+ CO

RO

t

0
vx2 (t)ILL(1-D)TSnS
nP VIN

Dual-Path I

IC,2

DTS

SP2
SS2,3

(a) State 1nS
VnP Clamp

State 2

0

+

Vx2

Vx1

State 1+nS
V -V
nP IN O

VO

t

0

Vx2

nS
( n VIN - 2VO)/LP

iL (t)(b) State 2

ΔiL

Fig. 3. Two circuit states in the proposed DPH-SR.

-VO /Lt0

M =VO nS D=
VIN nP 1 + D(IL - IO)/(CFLY + CO)
ΔvO(IL - IO)/CO

where VIN, VO, VCFLY, <VL>, nP, nS, TS, and D are the input
voltage, the output voltage, the flying capacitor voltage, theaverage inductor voltage, the number of the transformer
primary winding turns, the number of transformer secondary
winding turns, the switching period, and the duty cycle,
respectively.

VO

vO

0

t

Fig. 4. Circuit operation of the proposedosed DPH-SR.

Voltage conversion ratio M

Fig. 5 illustrates M as functions of D in the ACF converterwith the proposed DPH-SR and conventional SR. The
proposed DPH-SR can achieve a higher step-down ratio withthe same nP, nS, and D as compared with the conventional SR.
If a step-down ratio in the same range as that of theconventional SR is required, the proposed DPH-SR can beaccommodated by redesigning the transformer turns ratio.
B. Inductor DC current
By applying the charge balance in CFLY at periodic steadystate, the total charges QC,1 and QC,2 flowing to CFLY in eachstates satisfy the following equations.

0.2

Conv. (np:ns=4:1)
nP:nS = 4:1 M Conv. =Prop. (np:ns=4:1)
nP:nS = 4:1

0.15

VO nS
= D
VIN nP

Prop. (np:ns=3:1)
nP:nS = 3:1

0.1

Eq. (2)0.05
00

QC = QC,1 + QC,2 = 0
 QC,1 = −QC,2

IL /CFLY

vCFLY (t)vO (t)

(2)

vCFLY

0.1

0.2

0.3
0.4
0.5
Duty cycle D

0.6

0.7

Fig. 5. Voltage conversion ratio M vs. duty cycle D.

(3)

As shown in Fig. 3(a), QC,1 is determined based on the
inductor current IL because CFLY is connected to the inductor
L in series during State 1. Since QC,2 is given by eq. (3), QC,1and QC,2 can be described as follows:

QC,1 = QL,1 = I L DTS(4)

QC,2 = −QC,1 = − I L DTS .

(5)

parallel during State 2. Based on these assumptions, QO isobtained as follows:

QO = QX,1 + QX,2
= QC,1 + ( −QC,2 + QL,2 )= I L DTS + I L DTS + I L (1 − D ) TS

Additionally, by applying the charge balance in CO, theaverage output charge QO is equal to the sum of the total
charges QX,1 and QX,2 flowing to CO during each state. As
shown in Fig. 3(a), QX,1 is same as QC,1 because CFLY and L
are connected in series during State 1. On the other hand, QX,2
is given by QC,2 and QL,2 because CFLY and L are connected in(6)= (1 + D ) I LTS .
Therefore, the relationship between the output current IO
and the inductor DC current IL in the proposed DPH-SR isexpressed as follows:1012

I O = (1 + D ) I L(7)

Inductor DC current ratio KDC

1

C. Inductor Current Ripple
Additionally, in order to compare the inductor conduction
loss of the proposed DPH-SR and conventional SR, theinductor current ripple is also analyzed.

Prop. (np:ns=4:1)
nP:nS = 4:1

Prop. (np:ns=3:1)
nP:nS = 3:1

0.9
0.8

From the inductor voltage VL in state 2, which is shown in
Fig. 3(b), the inductor current ripple of the proposed DPH-SR∆IL,Prop. is obtained as follows:

Eq. (10)0.7

VOTS(1 − D )L
V T M= O S 1 −
.
L  ( nS,Prop. / nP,Prop. ) − M 



0.6

I L,Prop. =

0.5
0

0.02

0.04 0.06 0.08 0.1 0.12
Voltage conversion ratio M

0.14

Similarly, the inductor current ripple of the conventional
SR ∆IL,Conv. is also described as follows:

Fig. 6. Inductor DC current ratio KDC vs. voltage conversion ratio M.
Inductor current ripple ratio Kripple

(11)

1.1

VOTS
(1 − D )L
V T M= O S 1 −
.
L  ( nS,Prop. / nP,Prop. ) I L,Conv. =1
0.9

Eq. (13)0.8
0.7

Since the voltage applied to L is reduced thanks to CFLY,
the inductor current ripple in the proposed DPH-SR can be
reduced as compared with that in the conventional SR asfollows:

0.6
Prop. (np:ns=4:1)
nP:nS = 4:1

0.5

Prop. (np:ns=3:1)
nP:nS = 3:1

0.4
0

0.02

0.04 0.06 0.08 0.1 0.12
Voltage conversion ratio M

0.14

K ripple =Fig. 7. Inductor current ripple ratio Kripple vs. voltage conversion ratio M.

=


1
M 
I L,Prop. =I O = 1 −
 IO .
1+ D
 ( nS / nP ) 

I L,Conv. = I O

(8)

(9)

From eqs. (8) and (9), the inductor DC current in the
proposed DPH-SR IL,Prop. can be reduced thanks to thecontribution of CFLY to the output current in State 2, ascompared with the inductor DC current in the conventional SR
IL,Conv. as follows:
I L,Prop.
I L,Conv.

= 1−

(n

M

S,Prop.

/ nP,Prop. ).

I L,Prop.
I L,Conv.

(n

( n

S,Conv.

S,Conv.

On the other hand, the inductor DC current in theconventional SR IL,Conv is as follows regardless of thetransformer turns ratio (nS/nP).

K DC =(12)(10)

where nP,Prop. and nS,Prop. are the number of the primary andsecondary winding turns of the transformer in the proposed
DPH-SR, respectively. Therefore, the inductor DC current
ratio KDC depends only on the step-down ratio and thetransformer turns ratio of the proposed DPH-SR.
Fig. 6 shows KDC vs. M, indicating that the proposed DPHSR can reduce IL. Although the effect differs depending on thetransformer turns ratio, KDC is always less than 1 under thecondition of step-down operation. Assuming the conditions of
M = 0.05 and nP,Prop. : nS,Prop. = 4:1, the inductor DC current canbe reduced by 20% because KDC is 0.8.

 .
) − M

/ nP,Conv. ) ( nS,Prop. / nP,Prop. ) − 2M

/ nP,Conv. ) − M

( nS,Prop.

/ nP,Prop.

(13)

where nP,Conv. and nS,Conv. are the number of the primary andsecondary winding turns of the transformer in theconventional SR, respectively. Therefore, the inductor current
ripple ratio Kripple depends not only on the transformer turns
ratio of the proposed DPH-SR but also on that of theconventional SR.
Fig. 7 shows Kripple vs. M, indicating that the inductor
current ripple in the proposed DPH-SR can be reduced at any
step-down ratio when using a transformer with the same turns
ratio as the ACF converter with the conventional SR.
Assuming the conditions of M = 0.05 and nP,Prop. : nS,Prop. =nP,Conv. : nS,Conv. = 4:1, the inductor current ripple of the
proposed DPH-SR is slightly improved compared to the thatof the conventional SR because Kripple is 15/16 = 0.9375. In
addition, even if the transformer winding ratio of the proposed
DPH-SR is redesigned to 3:1, the current ripple of the inductor
does not deteriorate significantly.
III.

EXPERIMENTAL VERIFICATION

To demonstrate the feasibility of the proposed DPH-SR,
the experiments were conducted using the ACF converter andadditional circuit board shown in Fig. 8. The experimental
conditions and selected components are listed in Table I.
Fig. 9 shows the measured waveforms of the proposed
DPH-SR at the output current IO of 10 A, indicating that theoperation shown in Fig. 4 was successfully implemented. In

1013

DC-to-DC efficiency [%]Transformer

SP1 x2

SS2

SP2

SS1

L
CFLY

Value and Design Selection

Input voltage VIN

100 V

Output voltage VOUT

5V

Switching frequency

200 kHz

Transformer turns ratio nP : nS

4:1
600 V, 29 A, RDS(on),max: 99 mΩ,FMC60N099S2A, Fuji Electric
100 V, 180 A, RDS(on),max: 2.3 mΩ,

Output capacitor CO

STH315N10F7-6, STMicroelectronics
264 μF (22 μF x 12), X7R,
GRM32ER71E226KE15, Murata
264 μF (22 μF x 12), X7R,
GRM32ER71E226KE15, Murata

Inductor L

10 μH

Flying capacitor CFLY

89.0 %

88.6 %
Conv.
Prop.

5
10
15
Output current IO [A]

Conv.
Prop.

15

20

- 24.8 %10- 24.0 %

5
0

0

1.2

n
ΔVx1 = nS VIN - VO ( = 20 V)

vx1 (20 V/div)+ 1.4 %

51015Output current IO [A]20

Fig. 11. Measured inductor DC current comparison between the ACF
converters with proposed DPH-SR and conventional SR.

5 μs

vO (5 V/div), VO = 5 VPn
Vx1,Φ2 = nS VIN ( = 25 V)

1
0.8
0.6

0.4

Conv.

Prop.

- 42.2 %0.578- 43.0 %

0.570

0.2
0

P

10
20
Output current IO [A]nΔVx2 = nS VIN - VO ( = 20 V)P

vx2 (20 V/div)
Vx2,Φ1 = 0+ 1.9 %

20

Normalized inductor
conduction loss [-]Power MOSFETs SS1-3

Vx1,Φ1 = VO ( = 5 V)

Inductor DC current IL [A]

CIRCUIT COMPONENTS AND PARAMETERS.

Parameters90.0 %

Fig. 10. Measured efficiency comparison between the ACF converters
with proposed DPH-SR and conventional SR.

Fig. 8. Active clamp forward converter and additional circuit board forthe proposed DPH-SR.

Power MOSFETs SP1-2

90.9 %0CO

SS3

TABLE I.

92
91
90
89
88
87
86
85
84

n
Vx2,Φ2 = nS VIN - VO ( = 20 V)
P

iL (10 A/div)

Fig. 12. Normalized inductor conduction loss comparison between the
ACF converters with proposed DPH-SR and conventional SR.

ΔiL = 8.1 A
IL = 7.7 A

Fig. 9. Measured waveforms of the proposed DPH-SR at IO = 10 A.

addition, the inductor DC current IL is smaller than IO = 10 A,
confirming the feasibility of the proposed DPH-SR in the ACF
converter.
Fig. 10 shows the measured efficiency comparison
between the ACF converters with the proposed DPH-SR andconventional SR. The peak efficiency of the proposed DPHSR and conventional SR is 90.9 % and 89.0 % at IO = 10 A,
respectively, resulting in the improvement in efficiency by
1.9 %. Furthermore, at IO = 20 A, the efficiency is 90.0 % and88.6 % in the proposed DPH-SR and conventional SR,

respectively, where the efficiency improvement is 1.4 %.
These results demonstrate the superiority of the proposed
DPH-SR over the conventional SR.
Fig. 11 shows the measured inductor DC current
comparison between the ACF converters with the proposed
DPH-SR and conventional SR. Although the conventional SR
has the almost same IL as IO, the proposed DPH-SR can reduce
IL by 24.0 % at IO = 10 A and 24.8 % at IO = 20 A. As a result,the inductor conduction loss was drastically reduced by
42.2 % at IO = 10 A and 43.0 % at IO = 20 A, as shown in Fig.
12. Consequently, the proposed DPH-SR can improve theefficiency and help to solve the cooling problem of the output
inductor in the ACF converter.

1014

[5]

IV. CONCLUSIONSA dual-path hybrid synchronous rectifier (DPH-SR) in
active clamp forward (ACF) converters is proposed for
inductor current reduction to solve inductor cooling problems
under heavy load. In the measurement, the peak efficiency of
the ACF converters with the proposed DPH-SR andconventional SR was 90.9 % and 89.0 % at 10 AOUT,
respectively, resulting in the improvement in efficiency by
1.9 %. Furthermore, the efficiency at 20 AOUT was 90.0 % and88.6 % in the ACF converters with the proposed DPH-SR andconventional SR, respectively, indicating the efficiency isimproved by 1.4 %. Then, the proposed DPH-SR can reducethe inductor DC current by 24.8 % at 20 AOUT compared to theconventional SR. As a result, the inductor conduction loss ofthe proposed DPH-SR in the ACF converter was reduced by
43.0 % at 20 AOUT. Consequently, the proposed DPH-SR can
improve the efficiency and help to solve the cooling problemof the output inductor in the ACF converter.
As future works, detailed design guidelines for active andpassive components of ACF converters with the proposed
DPH-SR will be proposed. Additionally, parameter
optimization such as a dead time, a gate resistance, and an
active clamp capacitance will be conducted to improve theconverter performance.
ACKNOWLEDGMENT
This work was partly supported by JSPS KAKENHI Grant
Number 20K14710.
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