IEEE PEDS 2017, Honolulu, USA
12 – 15 December 2017

Dual-Port Output Control of
Isolated DC/DC Converter Focusing on
Duty Cycle and Frequency of Primary Inverter
Toshihiko Noguchi, Kazuki Shimizu, and Yoshinori Matsushita
Shizuoka University
noguchi.toshihiko@shizuoka.ac.jp, shimizu.kazuki.16@shizuoka.ac.jp

Abstract-This paper proposes a new approach of an isolated
DC/DC converter with dual-port outputs. The converter has
two outputs, i.e., a 12-V port and a 48-V port for automotive
applications. The dual-port output control can independently beachieved by using only a single primary inverter and a single
high-frequency transformer. The duty cycle and the frequencyof the inverter are used to control the two output voltages at thesame time. The both port voltages are regulated by means ofmanipulating the duty cycle and the frequency, respectively. In
the paper, a technique to achieve the dual-port output voltage
control of the isolated DC/DC converter is described and theircontrol characteristics are investigated to confirm validity of the
proposed technique.

describes the proposed circuit configuration and its operation
principle, and clarifies the operation characteristics throughsome computer simulations, which proves feasibility of the
proposed strategy.
CIRCUIT CONFIGURATION AND OPERATION OF PROPOSEDDUAL-PORT DC/DC CONVERTER
A. Circuit Configuration of Dual-Port DC/DC Converter
The circuit configuration of the isolated dual-port DC/DC
converter is shown in Fig. 1. The DC-bus voltage of the
primary inverter is assumed to vary from 140 to 260 V, andthe output frequency of the inverter is controlled around 400

INTRODUCTION
S3

S1

itr2

S2

C

V48

C

Load

vtr2

V

Load

V12

S4

ir

Lr

Cr

Fig. 1. Whole circuit configuration of isolated dual-port DC/DC converter.

E

C2
S1

S3

V

C

S2

2E

E

C1

S4

Load

(a) Isolated DC/DC converter with voltage doubler.
2I

I
L1
S1

I
L2

S3

V

C

S2

Load

As electric power consumption in a vehicle increases inrecent years, the power supply voltage feeding to many kindsof the auxiliary equipment is changing from 12 V to 48 V.
However, it is expected that the both voltages are redundantly
used for a while because it is very hard to replace entire
traditional 12-V components with 48-V components from theviewpoint of development cost; parts supply; maintenanceservice; market circumstances; and so forth. Therefore, adual-port isolated DC/DC converter is demanded, which has
capability to regulate two output voltages independently, andthe total output power rating up to 2 kW is required forpossibly increasing power consumption of a vehicle in thenear future. The simplest scheme to create the both output
ports is use of a non-isolated bidirectional buck-boost
chopper. However, the chopper is inserted between the two
output ports, so the cascade connection of power conversion
makes the total efficiency worse. Using the two sets of theisolated DC/DC converter also has a drawback of high costand large volume due to the increase of the electric devices
such as MOSFETs, diodes, inductors, capacitors, etc.
In this paper, a novel technique to achieve the independentdual-port output control of the isolated DC/DC converter is
proposed, which has only a single primary inverter and only asingle high-frequency transformer. The proposed circuit
features a series resonant circuit using as a variable
impedance adjuster, and is based on the pulse width control(PWC) and the pulse frequency control (PFC) to achieve theindependent control of the dual output ports. The paper

S4(b) Isolated DC/DC converter with current doubler.
Fig. 2. Classification of isolated DC/DC converters.

875
978-1-5090-2364-6/17/$31.00 ©2017 IEEE

ON
S1, S4t
OFF
tN

S2, S3

2

N1

Vtr 0V

E

t


IL1

N2
E
N1

t

0A
IL2

t
① ② ③ ④
Fig. 4. Operation waveforms of isolated DC/DC converter with current
doubler rectifier in secondary circuit.
0A

Fig. 3. Output voltage control of isolated DC/DC converter using PWC
inverter.

2I
I
L1
S1

L2

S3

V

C

S2

Load

S4

(a) Positive cycle (L1 charged and L2 discharged).
2I
I
L1
S1

I
L2

S3

V

C

S2

Load

S4

(b) Current holding period (both L1 and L2 discharged).
2I
I
L1
S1

I
L2

S3

V

C

S2

Load

S4

(c) Negative cycle (L1 discharged and L2 charged).
2I
I
L1
S1

I
L2

S3

V

C

S2

Load

kHz. The inverter has two degrees of freedom in controlling
its output voltage, i.e., a duty cycle and a frequency of theoutput voltage pulse. In the proposed technique, the 48-V
output port is controlled by using the duty cycle, which canbe achieved with the PWC of the inverter. On the other hand,the 12-V output port is regulated by using the frequency,
which can be controlled by the PFC of the inverter. Both of
the secondary rectifying circuits of the 12-V and the 48-V
output ports employ a double current topology because they
have low-voltage and high-current output specifications. In
addition, they introduce synchronous rectification using lowon-resistance MOSFETs to reduce their conduction losses.
B. Operation of Dual-Port DC/DC Converter
In general, power converters are classified into two
categories; i.e., voltage-source converters and current-source
converters. Fig. 2 shows the isolated DC/DC converters withthe voltage-source or the current-source secondary rectifier.
Fig. 2 (a) is a voltage doubling rectifier often used in a singlephase low-voltage AC/DC converter to boost the DC-bus
voltage. Assuming the winding turn ratio of the highfrequency transformer is unity; a peak voltage E of the AC
power source is applied across the capacitor C1 through D1 ina positive cycle, and is applied to C2 through D2 in a negative
cycle. Therefore, the total DC-bus voltage is the summationof the voltages across the both capacitor 2E. A dual circuit
topology of the voltage doubler is a current doubler shown in
Fig. 2 (b). When the inverter output is in a positive cycle, D2
is turned on and the current I flows through the inductor L1.
This current I is held at a constant value while D1 is turned on
to make the current I flow into another inductor L2 in anegative cycle of the inverter output. Therefore, the twocurrents flowing through L1 and L2 get together, and the total
DC-bus current becomes 2I.
Fig. 3 shows a block diagram of the output voltage control
system, where the control error between the actual output
voltage Vo and its command Vo* is given to a PI regulator todetermine the pulse width of the three-level inverter output
voltage. The output voltage control loop makes the DC/DC
converter robust against variations of the power source
voltage and the load power.

I

S4(d) Current holding period (both L1 and L2 discharged).
Fig. 5. Circuit operation modes of isolated DC/DC converter with current
doubler in secondary circuit.

Fig. 4 illustrates operating waveforms of the isolated
DC/DC converter with the current doubler, where the circuit
operation is divided into four modes indicated in Fig. 5. The
primary inverter outputs the three-level voltage waveform,
whose pulse width is determined by the PWC. The
transformer is excited in the positive direction in the mode (a),

876

2

  2 Lr Cr  1 
 ,(3)
Z r  Rr   Crwhere Lr, Cr, and Rr are an inductance, a capacitance, and a
resistance of the series resonant circuit, respectively. Fig. 7 is
the impedance characteristic with respect to the frequency,
assuming that the inductance and the capacitance values aresame as those used in the computer simulations and Rr = 10
. It is confirmed that the impedance is the minimum at the
point of the resonant frequency and that the impedance can be
2

Secondary transformer voltage vtr2 (V)

D/2

D/2

0

-V/a

Time (s)

30
25
20
15
10
5
0
Angular frequency 1000 (rad/s/div)

Fig. 7. Impedance characteristic of resonant circuit.

2

R
I
Ploss  2 R   I 2 .
(1)
2
2
Therefore, it is possible to halve the power loss by using thecurrent doubling rectifier, which is significantly effective forlow-voltage and high-current applications.
C. Dual-Port Output Control with PWC and PFC
The 48-V output port of the proposed DC/DC converter isregulated by means of the PWC, while the 12-V output port isdone by the PFC. Since the secondary rectifier employs acurrent doubling synchronous rectifying technique to reducethe conduction loss, the steady state output voltage of the 48V port V48 is controlled by the duty cycle D as follows:
DV
V48 
,
(2)
2a
where V represents the DC-bus voltage of the primary
inverter and a is the winding turn ratio of the high-frequency
transformer. Because the primary side is constituted with afull-bridge inverter, the adjustable range of D is 0 ≤ D ≤ 0.5
as shown in Fig. 6.
As for the 12-V output port, a series and parallel resonant
circuits are inserted in front of the secondary rectifier. The
parallel resonant circuit is inserted for the purpose of
decoupling the series resonant circuit from the inductors of
the current doubler. The output voltage of the 12-V port V12
is controlled by manipulating an impedance of the series
resonant circuit with respect to the inverter output frequency.
The impedance of the series resonant circuit Zr can be
expressed as

V/aFig. 6. Secondary voltage waveform of high-frequency transformer.
Impedance Z ()where the diode D1 is off and D2 is turned on. Therefore, thecurrent 2I having flown into the diode D2 is shunted to L1 andL2. Since the energy is stored in L1 in the mode, the current
flowing into L1 gradually increases, while the current of L2
decreases because its stored energy is released to the load.
The output voltage of the inverter becomes zero in the next
mode (b), so the current 2I flowing into D2 diverges to L1 andL2 with decreasing the amplitudes and the energies of theboth. The operation mode (c) is the inverse of the mode (a),and L1 stores its energy and L2 releases its energy to the load
because D1 is turned on and D2 is turned off. Hence, thecurrent flowing into L1 increases and the current of L2
gradually decreases. The operation mode (d) is basicallysame as the mode (b).
Assuming that the total resistnce of the MOSFETs and theinductors on the secondary side is R and that the load current
is I, the power loss of the current doubler is calculated as

V48* +
-

PI
V48

V12*

+
-

D

PWC
Angular Frequency
Controller

InverterV12Fig. 8. Block diagram of dual-port voltage controller.

controlled by manipulating the frequency. The output voltageof the 12-V port V12 is expressed as the following equation;
DVZ
,
(4)
V12 
2aZ r  Z 
where the total impedance of the load and the smoothing
capacitor across the 12-V output port is Z and the parallel
resonant circuit is assumed to be ideal.
It is found from (2) and (4) that V48 can be controlled by D
and that V12 can be done by D and . Because 48-V output
voltage control is independent of the frequency , the both
output voltages are controllable by manipulating D and  atthe same time. Fig. 8 shows a block diagram of the controller
described above.
D. Soft Switching Operation of Primary Inverter
It is possible to utilize a soft switching technique owing to
the resonant current in the primary inverter caused by theseries resonant circuit. The primary current includes not only
the resonant current but also the current of the current
doubling rectifier. A condition to achieve the soft switchingis to make the current lagging. In other words, it is required
to make the frequency higher than the resonant frequency,
resulting in a zero-current switching (ZCS) during turning onof the switching devices. On the other hand, a zero-voltage
switching (ZVS) during turn off process is possible byconnecting capacitors to the switching devices in parallel.
These soft switching techniques possibly reduce theswitching losses and EMI/EMC noises because they restrict
the dv/dt and/or di/dt.

877

itr2 (A)V12 (V)

V48 (V)

50
40
30
20
10
0
14
12
10
8
6
4
2
0
100
50
0
-50
-100
200
100
0
-100
-200

vtr2 (V)TABLE I. SIMULATION PARAMETERS.
Parameters
ValuesDC-bus voltage
200 V
Resonance frequency
410 kHz
48-V port output power
1 kW
12-V port output power
1 kW
Turn ratio of transformer
7:4
Resonant inductor
6.8 H
Resonant capacitor
22 nF
48-V port smoothing capacitor
4.7 F
12-V port smoothing capacitor
10 F
48-V port inductor
10 H
12-V port inductor
5 H

60
30
0
-30
-60

t 50 (s/div)

49.0
48.5
48.0
47.5
47.0
13.0
12.5
12.0
11.5
11.0
100
50
0
-50
-100
200
100
0
-100
-200

vtr2 (V)itr2 (A)V12 (V)

V48 (V)

Fig. 9. Operating waveforms of isolated dual-port DC/DC converter.

ir (A)

A. Control Characteristics in Steady State
Computer simulations have been conducted to examine thecontrol characteristics of the isolated DC/DC converter basedon the proposed strategy. Various parameters used in thesimulations are listed in TABLE I. As the frequency
response of the resonant circuit impedance has a steep
characteristic, the primary inverter is operated in the lowerfrequency range than the resonant frequency. This implies
that the adjustable range of the operating frequency can be
narrow to control the output port V12 although the soft
switching technique cannot be applied. It is assumed that theload power of the both output ports is 1 kW; hence, the 48-V
output port current and the 12-V output port current are 20.8
A and 83.3 A, respectively.
Figs. 9 and 10 show the simulation results of the operating
waveforms under the above condition. From the top to thebottom, the 48-V output port V48, the 12-V output port V12,
the secondary current of the transformer itr2, the secondary
voltage of the transformer vtr2, and the series resonant circuitcurrent ir are indicated in the figures. It is confirmed from
Fig. 9 that the voltages V48 and V12 of the both output ports
are stably regulated at their command values for the long
period. Fig. 10 is expanded waveforms. As can be seen in
the figure, the output voltages V48 and V12 are following theircommands with small tolerance, and their ripple frequencies
are double of the primary inverter output frequency. There isa phase difference between the secondary current and thesecondary voltage of the transformer, which is caused by the
inverter operating frequency lower than the resonant
frequency. Since the 12-V output port is controlled by meansof the PFC as described previously, the phase difference isdependent of the load power of the 12-V output port. The
operating frequency range determined by the PFC must be
restricted because the low-operating-frequency causes
magnetic saturation in the transformer and the inductors of
the current doublers. Therefore, the operating frequency
range is limited within ±250×103 rad/s.
Fig. 11 (a) shows static load regulation characteristics of
the isolated dual-port DC/DC converter. The graph lines withir (A)

VERIFICATION OF CIRCUIT OPERATION THROUGH COMPUTERSIMULATIONS

60
30
0
-30
-60

t 1 (s/div)

Fig. 10. Expanded operating waveforms.

square symbols are average voltages of the 48-V output portwith respect to the load power under a 100-% fixed loadcondition of the 12-V output port. The other graph lines withcircle symbols are the 48-V characteristics under thecondition of a 10-% fixed load on the 12-V output port. In
the similar way, the triangle symbols represent the 12-V
characteristics with a 100-% fixed load condition on the 48-V
output port, while the inversed triangle symbols are the 12-V
characteristics with keeping 48-V port load at 10 %. As canbe seen in the figure, both of the output port voltages arecontrolled properly at 48 V and 12 V regardless of the load
power variation. In addition, independent load regulation
characteristics can be confirmed between the two output ports.

878

■:48V output (V)V48 (V)

50
45•⁕12V is 10% load 40
35▲:12V output (V) 30⁕48V is 100% load 25▼:12V output (V) 20
15⁕48V is 100% load
10
⁕12V is 100% load
:48V output (V)(a) Steady-state output voltage characteristics to load variations.
1300
1200
1100
frequency•:Angular
1000(rad/s)⁕48V is 10% load 900
800■:Angular frequency(rad/s)V12 (V)

Load Factor 10 (%/div)⁕48V is 100% load

Time 1 (ms/div)

0

200

400

600

800

1000

Vpp (V)V12 (V)

Load Factor 10 (%/div)

V48 (V)(a) Step disturbance response of 48-V output port from 20 % to 100 %.

(b) Relationship between load power and operating frequency.
Fig. 11. Load regulation characteristics.
3000
2500
2000
1500
1000
500
0

52
50
48
46
44
42
15
14
13
12
11
10
9

L/C

52
50
48
46
13
12
11
10
9
8
7

Time 1 (ms/div)

Fig. 12. Voltage across capacitance in resonant circuit.

V12 (V)

47.6
47.8
48.0
48.2
48.4
14
12
10
8
6
4
2

Time 1 (ms/div)

V48 (V)(c) Step disturbance response of 12-V output port from 20 % to 100 %.
47.6
47.8
48.0
48.2
48.4

V12 (V)

Fig. 11 (b) shows relationships between the operating
frequency and the load power. Every symbol stands for thesame operating condition as described in Fig. 11 (a). It isinferred that the frequency variation is remarkably large inthe heavy load range of the 48-V output port because thecurrent flowing into the parallel resonant circuit is reduced
and is hard to damp in the heavy load range. When the load
power of the 12-V output port is low, the operating frequency
also decreases because the quality factor of the series
resonant circuit becomes low. Therefore, it is rather difficult
to operate the 12-V output port under condition less than 10% load power, which may require a dummy load sacrificing
the efficiency in the light load range.
It is definitely necessary to design the resonant circuit sothat the resonant characteristic has an appropriate quality
factor. If the impedance characteristic with respect to thefrequency is too steep, even a small frequency change may
cause a large output voltage variation. In order to make the
PFC stable, therefore, the quality factor of the resonant circuit
should not be too high. The voltage characteristic in theresonant circuit is shown in Fig. 12, where the quality factorof the resonant circuit is changed. The actual voltage is lower
than the characteristic shown in the figure because theresistive component is ignored in the resonant circuit. It isfound that the quality factor must be determined from theviewpoint of withstanding voltages of the resonant inductors
and the resonant capacitors, too.
B. Dynamic Response to Disturbance Load
Disturbance load tests have been conducted through thesimulations to confirm the dynamic responses of the proposed
DC/DC converter. From the top to the bottom in Fig. 13, thestep disturbance response of the 48-V output port is indicatedunder a condition of 100-% continuous load power on the 12V output port. The load of the 48-V output port has beenchanged stepwise from 20 % to 100 % and vice versa. The

V48 (V)(b) Step disturbance response of 48-V output port from 100 % to 20 %.

40
35
30
25
20
15
10

Time 1 (ms/div)(d) Step disturbance response of 12-V output port from 100 % to 20 %.
Fig. 12. Step disturbance responses of isolated dual-port DC/DC converter.

second waveforms show the step disturbance response of the
12-V output port with continuously making the 48-V output
port load 100 %. Although the convergence time back to thecommand voltages 48 V and 12 V takes approximately 2 ms,
both of the output port voltages asymptotically return to thecommand values. As (2) and (4) indicate, D starts to change
when the disturbance is applied to the 48-V output port,
resulting in the time variations of the both output port
voltages. However, 48-V output port is not affected by the
disturbance to the 12-V output port because 48-V output port
is controlled by only D and has nothing to do with thefrequency. Since the resonant circuit is employed in the 12-V
circuit, the dynamic behavior of V12 is much slower than that879

of V48, which is caused by a high-quality-factor design of theresonant circuit. Therefore, the resonant current is hard tochange owing to the high-quality-factor, and it takes a longtime for the command value to be effective to the DC/DC
converter, resulting in the long convergence tine. This
problem van be solved by reducing the quality factor, but thevolumes of the transformer and the inductors may be large
because the variation range of the frequency is wider due tothe low-quality-factor design. It is necessary to choose theappropriate quality factor of the resonant circuit by taking thewithstanding voltage of the components, frequency variation
range, and disturbance response into account.
CONCLUSION
In this paper, a dual-port output control strategy of theisolated DC/DC converter has been discussed, which iscapable to regulate the two output voltages simultaneously
and independently. The control is achieved by manipulating
the duty cycle and the frequency of the high-frequency PWC
inverter. It has been confirmed through several computer
simulations that the both output voltages are controlled
accurately and independently even if the step load change isapplied to one port, which proves feasibility of the proposed
strategy.
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