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Rev. 0.1, Jan 2013 Analog Mixed Signal Group

One Enterprise Aliso Viejo, CA 92656 USA

PRELIMINARY/ CONFIDENTIAL

Technical Note

Sanjaya Maniktala,

Jan

201

Forward and Flyback Core

Selection using the LX7309 and

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 the energy that is all,

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

/ f = ΔƐ ≈ Ɛ

PEAK

/1.8 = (L × I

PEAK

)

/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,

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

, 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

( )

O IN

V D V

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

Rev. 0.1, Jan 2013 Analog Mixed Signal Group

One Enterprise Aliso Viejo, CA 92656 USA

PRELIMINARY/ CONFIDENTIAL

Technical Note

Sanjaya Maniktala,

Jan

201

Forward and Flyback Core

Selection using the LX7309 and

ndustr

y Recomm

end

ations

converter --- but not based on the classic inductor-based Buck-Boost anymore. And that is what we, in effect, usually do in the LLC

resonant topology.

The Secondary-side choke selection criterion is straightforward too: it is simply sized so that it does not saturate with the peak current

passing through it (typically 20% more than the load current). We see that it is the same underlying criterion as in a Flyback, Buck, a

Buck-Boost, and even a Boost. So that does leave us the basic question: how do we pick the Forward converter ? What transformer

does its size depend on? What is/are its selection criteria?

There are two major factors affecting the Forward converter transformer selection. First we need to understand that the Primary and

Secondary windings conduct . So they are intuitively “in phase”. The observed “transformer action”, i.e., the simple at the same time

turns-ratio based current flow of the Secondary winding, is in fact just a direct result of induced EMF (electromotive force, i.e., voltage

based on Faraday’s/Lenz’s law). The induced EMF in the Secondary, in response to the changing flux caused by the changing current in

the Primary, tries to oppose the change of flux, and since both windings can conduct current at the same time in a Forward converter,

the two voltages (applied and induced) lead to simultaneous currents in the windings, which create equal and opposite flux

contributions in the core, cancelling each other out. Yes, completely so! In effect, the “core” of the Forward converter’s transformer

does not “see” of the flux associated with the transfer of power across its isolation barrier. Note that this flux-cancellation “magic” any

was physically impossible in a Flyback, simply because, though there was induced EMF in the Secondary, the output diode was so

pointed, that it blocked any current flow arising from this induced voltage --- so there was no possibility of having two equal and

opposite flux contributions occurring (at the same time).

This leads to the big question: if the “core” of the Forward converter’s transformer does not see any of the flux related to the ongoing

energy transfer through the transformer, can we transfer limitless energy through the transformer? No, because the DC resistance of

copper comes in the way. This creates a based on the available window area “Wa” of the core. We just cannot stack physical limitation

endless copper windings in a restricted space to support any power throughput. Certainly not if we intend to keep to certain thermal

limits….because though the core may be totally “unaware” of the actual currents in the windings (because of flux cancellation), the

windings themselves do see I

R (ohmic) losses. So eventually, for thermal reasons, we have to keep to within a certain acceptable

current density. That in effect, restricts the amount of power we can transfer through a Forward converter transformer. We intuitively

expect that if we have double the available window area Wa, we would be able to double the currents (and the power throughput)

too, for a given (acceptable) current density. In other words, p2-we expect roughly (intuitively)

P Wa

∝

Truth does in fact support intuition in this case. But there is another key factor too: a transformer needs a certain excitation

(magnetization) current to function to be able to provide transformer action in the first place. So there is a certain relationship to the

core itself, its “ferrite-related” dimensions, not just the window area (air dimensions) that it provides. A key parameter that

characterizes this aspect of the core is the area of its center limb, or Ae (often just called “A” in this chapter). Finally we expect the

power to be related to both factors: the air-related component Wa and the ferrite-related component Ae:

P Wa Ae

∝ ×

The product Wa × Ae is generically called “AP”, or area product of the core. See Figure 1.

As indicated, we intuitively expect that doubling the frequency will allow double the power too. So p2-we expect

Rev. 0.1, Jan 2013 Analog Mixed Signal Group

One Enterprise Aliso Viejo, CA 92656 USA

PRELIMINARY/ CONFIDENTIAL

Technical Note

Sanjaya Maniktala,

Jan

201

Forward and Flyback Core

Selection using the LX7309 and

ndustr

y Recomm

end

ations

P AP f

∝ ×

Or better still, since in the worst-case (losses after the transformer), the transformer is responsible for the entire power, it incoming

makes more intuitive sense to write

P AP f

∝ ×

Figure 1: Basic definition of Area Product

Finer Classes of Window Area and Area Product (finer terminology)

As we can see from Figure 2 and Figure 3, we can actually break up the window area into several windows (with associated Area

Products). We should actually try to distinguish between them for the subsequent analysis, since typically, this becomes a source of

major confusion in literature, with innumerable equations and fudge factors abounding (fudge factors rather generically called “Kx”

usually), being apparently used to fit equations somehow to empirical data, rather than deriving equations from first principles then

seeing how they match data. So p3-we are creating some descriptors here.

a) Wac: This is the core window area. Multiplied by Ae, p3-we get APc.

b) Wab: This is the bobbin window area. Multiplied by Ae, we get APb.

c) Wcu: This is the window available to wind copper in (both Primary and Secondary windings). Multiplied by Ae, p3-we get APcu.

Note: In a safety approved transformer for AC-DC applications, we typically need 8 mm creepage between Primary and

Secondary windings (see Fig. 2), so a 4 mm margin tape is often used (but sometimes 2.5 to 3 mm nowadays). For telecom

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Varumärke:	Microchip
Kategori:	Inte kategoriserad
Modell:	AAC243

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