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Copyright © 2013 Microsemi Page 1
Rev. 0.1, Jan 2013 Analog Mixed Signal Group
One Enterprise Aliso Viejo, CA 92656 USA
PRELIMINARY/ CONFIDENTIAL
Technical Note
Sanjaya Maniktala,
Jan
201
3
Forward and Flyback Core
Selection using the LX7309 and
I
ndustr
y Recomm
end
ations
Introduction
In a Flyback topology, the selection of the transformer core is fairly straightforward. The Flyback transformer has a dual function: it not
only provides step-up or step-down ratio based on the Primary to Secondary turns ratio, but it also serves as a medium for energy
storage. The Flyback is a derivative of the Buck-Boost, and shares its unique property that not just part, but all, the energy that is
delivered to the output, must have previously been stored (as magnetic energy) within the core. This is consistent with the fact that
the Secondary winding conducts only when the Primary winding stops, and vice versa. We can intuitively visualize this as the windings
being “out of phase”. So we have an endless sequence of energy store-and-release, store-and-release…, and so on. The core selection
criterion is thus very simply as follows: the core must basically be capable of storing each packet of energy (per cycle) passing through
it. That packet is equal to P
IN
/ f = ΔƐ ≈ Ɛ
PEAK
/1.8 = (L × I
PEAK
2
)
/3.6, in terms of Joules. Here f is the switching frequency and Ɛ is energy
(see Figure 5.6 of Switching Power Supplies A-Z for a derivation of the above).
Equivalently, we can just state that the peak current,
I
PEAK
, should not cause “core saturation”, though that approach gives us no intuitive understanding of the fact that if we double the
switching frequency, the energy packets get reduced in half, and so in effect the same core can handle twice the input/output energy.
But that is indeed always true whenever we use an inductor or transformer as an energy-storage medium in switching power
conversion.
But coming to a Forward converter, at least two things are very different right off the bat.
a) All the energy reaching the output does not necessarily need to get stored in any magnetic energy storage medium (core) along
the way. Keep in mind that the Forward converter is based on the Buck topology. We realize from Page 208 of Switching Power
Supplies A-Z, that only 1-D times the total energy gets cycled through the core in a Buck topology. So, for a given P
O
, and a given
switching frequency, the Buck/Forward core will be roughly half the size of a Buck-boost/ Flyback handling the same power
(assuming D ≈ 1-D ≈ 0.5).
b) Further, in a Forward converter, the energy storage function does not reside in the transformer. The storage requirement,
however limited, is fulfilled entirely by the Secondary-side choke, not the transformer. So we can well ask: what does the
transformer do in a Forward converter anyway? It actually only provides “transformer action”, i.e., voltage/current step-up/down
function based on the turns ratio --- which is in a way, half the function of a Flyback transformer. Once it provides that step-
up/down ratio, there is an additional step-down function provided by simply running the Secondary-side choke in a chopped-
voltage fashion, as in any regular (non-isolated) Buck. That is why we always consider the output rail of a Forward converter, as
having been derived from the input rail, with two successive step-down factors applied, as shown
( )
S
O IN
P
N
V D V
N
Buck Transformer action
= × ×
⇑ ⇑
The perceptive will notice that the Forward converter’s transformer action could be such that we use the transformer turns ratio to
give an intermediate step-up instead of a step-down function, and then follow it up with a step-down function accruing from the
inherent Buck stage based around the Secondary-side choke. That could in effect give us another type of (overall) Buck-Boost
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