14th International Conference on Optimization of
Electrical and Electronic Equipment OPTIM 2014
May 22-24, 2014, Brasov, Romania
Combined Isolated Forward-Flyback-Converter with only one Diode as Battery Charger
Yvonne Flicker1, F. Himmelstoss1, IEEE Member
1
University of Applied Science Technikum Wien, Austria
Abstract
Converters are very important, especially for power supplies, battery chargers and for coupling DC networks. The well-known
classical forward converter [2, 3] needs three windings (Fig. 1). Two are used to transform the energy (N1, N2) and the third one
(N3) is necessary to demagnetize the core. The new combined forward-flyback-converter (Fig. 2) [5] needs only two windings and
one diode. The transformer is therefore easier to manufacture, is smaller, has lower weight and is therefore cheaper. In this paper
the new forward-flyback-converter is analyzed. The weighted state differential equations of the converter model in continuous
inductor current mode are linearized in order to use the possibilities of the linear control theory. A basic analysis of the converter
topology is given. In addition some experimental results are presented.
The new combined forward-flyback-converter, which is treated here, needs only two windings and one diode instead
of three in the conventional forward converter. The converter consists of a transformer with the windings N1 and N2, a
capacitor C, an active switch S, and a passive switch D. To demagnetize the core now a capacitor is used to transport
the magnetizing energy to the output. The positive pole of the output voltage is the cathode of the diode. When used
as a battery charger, an additional inductor LB has to be switched in series to the battery. The battery is modelled by a
voltage source UB.
The mean value of the output voltage U2 (across the diode), with the duty ratio of the active switch d (the on-time of
N
d
the active switch referred to the switching period) and neglecting the losses is U 2 
(1)
 2 U1 .
1  d  N 1
The converter is a step-up step-down converter, which also enables us to drive a DC motor in one direction (onequadrant drive), controlled braking is not possible with this circuit.
The converter can be described as a three order system. A two-point controler with hysterese is useful to control the
output voltage or output current. In this way, the active switch is turned on and off. Therefore, it is possible to keep
the load current iLB in a defined range by stopping the rise of the coil current via the active switch. The converter is
especially useful for solar and small fuel cell applications. It can be designed also for medium power applications e.g.
for charging car batteries out of a solar generator. In the last years different combined forward-flyback-converters
were published and analyzed e.g. [4,1]
Fig. 1. Three winding forward converter
Fig. 2. Combined forward- flyback converter
References
[1] Flicker, Y., and Himmelstoss, F. A., Analysis of a Charger with Autotransformer, IEEE Industrial Electronic Conference
IECON, Vienna, Austria, Nov. 10-13, 2013, pp. 439-444. ISBN 978-1-4799-0223-1/13
[2] Zach, F., Leistungselektronik, 4.Aufl, Wien: Springer, 2010.
[3] N. Mohan, T. Undeland, and W. Robbins, Power Electronics, Converters, Applications and Design (2nd Ed.), New
York: W. P. John Wiley & Sons, 2003.
[4] Himmelstoss, F. A., and Votzi, H.L .: Combined Forward-Flyback-Converter with only two Diodes – Function and
Modelling, Communications – Scientific Letters of the University of Zilina Vol. 13, 2A-2011, pp. 6 - 12, Zilina, SK,
June 2011, ISSN: 1335-4205.
[5] F. Himmelstoss, “Kombinierter Sperr-Durchflusswandler mit nur einer Diode“, Austrian patent office AT 511846 B1
filed 2011.11.15.
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