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MUR1100E-MUR1100ERL
ULTRAFAST RECTIFIERS 1.0 AMPERE 900-1000 VOLTS
AN1679/D
How to deal with Leakage
Elements in Flyback
Converters
Prepared by: Christophe BassoON Semiconductor
14, rue Paul Mesplé – BP1112 – 31035 TOULOUSE Cedex 1, France
33 (0)5 34 61 11 54
e–mail: christophe.basso@
INTRODUCTIONIt is difficult to find a good tradeoff between cost and
performance when designing offline Flyback power
supplies. The basic problem is the presence of the various
leakage inductances of the transformer. Those leakage
inductances cause voltage overshoot (which, in turn, calls
for power switches with increased voltage rating), divert a
portion of the primary current to a clamp, and degrade the
voltage regulation in power supplies with primary
regulation or with multiple outputs.
To evaluate the effect of the leakage inductances on the
circuit performance in the design phase is a challenging task.
Here a simulation program can be quite helpful. This paper
reviews the magnetic and electric models of the
two–winding and three–winding transformers, discusses
how to extract the inductance values of the models by
measurements and calculations, and how to feed a SPICE
model with. Furthermore, the paper analyzes the influence
of the primary leakage inductance on the switch voltage
waveform, provides guidelines on how to protect the switch
from the voltage overshoot, and considers the effect of the
secondary leakage inductances on the cross regulation of
multiple–output Flyback power supplies.
The Reluctance ConceptSince the transformer, or coupled inductances for a
Flyback, is the key player in our setbacks, it is important to
spend some time to understand the way the inductive
leakage elements are spread to finally measure them and
feed our simulator model with.
Several ways exist to depict how the magnetic variables
are organized within the transformer. The reluctance method
keeps the analogy with our electric circuits and therefore
appears easier to us non–magnetic experts. Remember that
the electrical resistance of a metallic piece tends to brake the
current flow. That is to say, we impose a current variation dI
(Amps) through the resistance R (�) of the metallic part by
applying a voltage variation dV (Volts) across it, or
With magnetic materials, we could compare the current with
the flux � (Weber) which is constrained to circulate in the
magnetic material by an external force F, the magnetomotive
force (mmf) expressed in Amps–turns. This force is
elaborated by a winding of N turns in which a current I flows.
The resulting force F is equal to N x I. The obstacle which
brakes the circulation of the flux is the reluctance ℜ of the
magnetic medium and can be explained by:
That is to say, how much of magnetomotive force F shall I
apply to the magnetic material in order to obtain a change in
the flux �. The analogy between electricity and magnetism
is depicted by Figure 1.
