TJSPC 2012, Taipei, Taiwan

Design of Predictive Digital Current Control for Series-Parallel Compensation
Type DC-DC Converter
Koji Orikawa＊a), Jun-ichi Itoh＊, Po-tai Cheng＊＊
This paper describes an implementation of a predictive digital current control for the series-parallel compensation type DC-DC
converter in order to control the current and the output voltage at improved voltage loop dynamic responses. A dynamic responseof the current control and the voltage control are compared between the predictive current control and a PI control. As a result ofsimulations, it is confirmed that the predictive current control can achieve high dynamic responses compared with that of the PIcontrol.
Keywords : Series compensation, Parallel compensation, PI control, Predictive current control

1.

Introduction

Recently, hybrid power supplies using both a fuel cell and abattery have been researched. A series-parallel compensation type
DC-DC converter have been proposed by authors for one ofhybrid power supplies(1). When the fuel cell voltage is closed tothe output voltage, the output power is directly provided by thefuel cell without any switching operation; therefore, highefficiency is obtained by the series compensation circuit. Whenthe load condition has changed, the parallel converter compensates
the quick variation of the fuel cell current.
On the other hand, in DC-DC converters, a reduction of output
smoothing capacitors is required in order to achieve a downsizingof DC-DC converters. However, it results in a large fluctuation of
the output voltage. Therefore, a high speed response of a current
regulator and an output voltage regulator are required.
In this study, the predictive digital current control isimplemented for the proposed circuit in order to suppress thefluctuation of the fuel cell current and the output voltage when theload is changed. This paper shows simulation results with a PI
control and the predictive current control.

2.

the next switching cycle is calculated based on the sensed orobserved state and input/output information.

4.

Simulation result

Fig. 2 shows block diagrams of the proposed circuit. As in Fig.
2, the feedback control consists of an output voltage outer loop
and the current inner loop. The outer output voltage loop is basedon PI regulator with output voltage command.
In the proposed circuit, the series converter and the parallel
converter work at the same time. Therefore, the predictive duty
cycle of two converters include the predictive duty cycle ofanother converter. The predictive duty cycle dS1[n+1] anddS4[n+1] are expressed by (1) and (2) in the boost mode, and thenfrom m1 to m6 are slopes of the current shown in Fig. 3.
d S 1 n  11

i

 i fcm3  m2 Ts fc _ command1
2m1  m2  m1 d S 4 n  d S 4 n  1 d S1 n
m3  m2.. (1)d S 4 n  1
1

i



Series-parallel compensation type DC-DCconverterm6  m5 Ts comp _ commandFig. 1 shows the circuit configuration of the proposed circuit.
The series converter generates a positive and negative voltage
according to the boost mode and buck mode, respectively. Whenthe load condition is changed, the series converter compensates
the differential voltage between the fuel cell voltage and theoutput voltage, and the parallel converter compensates the quickvariation of the fuel cell current.

1
2m5  m4  m6 d S1 n  d S1 n  1  d S 4 n
m6  m5

3.

Predictive current control

A predictive current control is a kind of digital controls inpower converters(2)-(3). In one switching period, the duty cycle fora) Correspondence to: Koji Orikawa. E-mail:orikawa@stn.nagaokaut.ac.jp＊Nagaoka University of Technology
1603-1, Kamitomioka-machi, Nagaoka, Niigata, Japan 940-2188＊＊
National Tsing Hua University
101, Section 2, Kuang-Fu Road, Hsinchu, Taiwan 30013

 icomp

Series converter
S2

Vfc
Ifc

Lfc

LoutVLS1Isb
Vsb
Parallelconverter

ILout

Fig. 1.

Cout

S3
S4.. (2)Vout*: 48 (V)Vsb: 60 (V)Vfc: 40 (V)Ic Iout Ts: 10 (ms)Lout: 100 (mH)
Lfc: 100 (mH)C : 470 (mF)
Vout fout
sw: 100 (kHz)

Icomp

Series-parallel compensation type DC-DC
converter.

TJSPC 2012, Taipei, Taiwan

Controller
LPF
Vout*
Vout

1+1  sTv -PI

Physical circuit

LPF+ ILout

PI= Kp_v*+11  sT

Iout

HPF

dS1(n+1)*

Ifc +Ifc

Vsb--+sTv  1sTv

Ifc-+Vsb

Iout-+

1/sLout+
Vfc
Vout

Vconv2dS4(n+1)

ILout

Vconv1-+

Currentcontrol(b) or (c)Icomp* +
Icomp

Vsc

1/sLfc1/sCout++-

Icomp(a) Whole control diagram.

Ifc*-Ifc

Kp_i

sTi  1sTi

dS1(n+1)

Icomp*-Icomp

Kp_i

sTi  1sTi

dS4(n+1)(b) PI control.

dS4(n)
dS4(n+1)+(According to Eq. (1))
dS1(n)

Feedforward+
Ifc*-Ifc+Kpfc

Proportional gain
(According to Eq. (1))-dS1(n+1)
Z

dS1(n+1)+(According to Eq. (2))+
Feedforward

Icomp*-Icomp-1

Kpcomp+

Proportional gain
(According to Eq. (2))

dS1(n)

dS4(n+1)-Z-1

dS4(n)(c) Predictive current control.

Fig. 2.

Block diagrams of series parallel compensation type DC-DC converter.

m1  m3  V fc / L fc ........................................................... (3)Carrier&Duty command

m2  V fc  Vsb / L fc ......................................................... (4)



dS4
dS1

m4   V fc Lout  Vsb  Vout L fc / L fc Lout .......................... (5)m5   V fc Lout  Vout L fc / L fc Lout ..................................... (6)m6  Vsb  V fc Lout  Vsb  Vout L fc / L fc Lout ................... (7)where Vfc is the fuel cell voltage, Vsb is the battery voltage, Voutis the output voltage, ifc is the fuel cell current, ifc_command is thecommand of the fuel cell current, icomp is the compensation current,
icomp_command is the command of the compensation current, Lfc is aninductance that is connected to the fuel cell in series, Lout is an
output inductor, Ts is a sampling time, dS1[n] is a previous duty
cycle for the switch S1 and dS4[n] is a previous duty cycle for theswitch S4.
Fig. 4 shows comparisons of the output voltage and eachcurrent with the PI and the predictive current control when theload condition is changed. In Fig. 4(a), the voltage drop of over
4.1% occurs at the point of the load change. On the other hand, inFig. 4(b), the voltage drop is suppressed within 1%. Therefore, it
is confirmed that the predictive current control can achieve highdynamic response compared with that of the PI control.
References
(1)

(2)

(3)

Koji Orikawa and Jun-ichi Itoh : “A comparison of the series-parallel
compensation type DC-DC converters using both a fuel cell and a battery”,
Energy Conversion Congress and Exposition (ECCE), pp.1414-1421 (2010)
A. H. Mitwalli, S. B. Leeb, G. C. Verghese, and V. J. Thottuvelil : “Anadaptive digital controller for a unity power factor converter”, IEEE
transaction on Power Electronics, Vol.11, No.1, pp.374-382 (1996)
Jingquan Chen, Aleksandar Prodic, Robert W. Erickson, and Dragan
Maksimovic : “Predictive Digital Current Programmed Control”, IEEE
transaction on Power Electronics, Vol.18, No.1, pp.411-419 (2003)tGate signal of S1dS1(n)Tst

Gate signal of S4(1-dS4(n))TsifctdS4(n)Ts

m1
m2
m3
iLout
(dS4(n)-dS1(n))Tst
t

icomp

m4

m6

m5t

Fig. 3.
50
48
46

Current waveforms and gate signals.

Vout, Vout* (V)Vout, Vout* (V)Vout*: 48 (V)Ts: 10 (ms)ILout, ILout* (A)1050Ifc, Ifc* (A)1050I , I * (A)10 comp comp2 (ms)
50
Load change-5Time (ms)Fig. 4.

ILout, ILout* (A)Ifc, Ifc* (A)Icomp, Icomp* (A)
2 (ms)
Load changeTime (ms)(a) PI control (b) Predictive current control
Simulated voltage and current waveforms.

Vout