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LTC4278

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APPLICATIONS INFORMATION

A final note—the susceptibility of the system to bistable

behavior is somewhat a function of the load current/

voltage characteristics. A load with resistive—i.e., I =

V/R behavior—is the most apt to be bistable. Capacitive

loads that exhibit I = V2/R behavior are less susceptible.

Secondary Leakage Inductance

Leakage inductance on the secondary forms an inductive

divider on the transformer secondary, reducing the size

of the flyback pulse. This increases the output voltage

target by a similar percentage. Note that unlike leakage

spike behavior, this phenomenon is independent of load.

Since the secondary leakage inductance is a constant

percentage of mutual inductance (within manufacturing

variations), the solution is to adjust the feedback resistive

divider ratio to compensate.

Winding Resistance Effects

Primary or secondary winding resistance acts to reduce

overallefficiency(POUT/PIN).Secondarywindingresistance

increases effective output impedance, degrading load

regulation. Load compensation can mitigate this to some

extent but a good design keeps parasitic resistances low.

Bifilar Winding

A bifilar, or similar winding, is a good way to minimize

troublesome leakage inductances. Bifilar windings also

improve coupling coefficients, and thus improve cross

regulation in multiple winding transformers. However,

tight coupling usually increases primary-to-secondary

capacitance and limits the primary-to-secondary

breakdown voltage, so is not always practical.

Primary Inductance

The transformer primary inductance, LP, is selected

based on the peak-to-peak ripple current ratio (X) in the

transformer relative to its maximum value. As a general

rule, keep X in the range of 20% to 40% (i.e., X = 0.2 to

0.4).Highervaluesofripplewillincreaseconductionlosses,

while lower values will require larger cores.

Ripplecurrentandpercentagerippleislargestatminimum

duty cycle; in other words, at the highest input voltage.

LP is calculated from the following equation.

LP =

VIN(MAX) •DCMIN

(

fOSC • XMAX •PIN

VIN(MAX) •DCMIN

(

)2 •Eff

fOSC • XMAX •POUT

where:

fOSC is the oscillator frequency

DCMIN is the DC at maximum input voltage

XMAX is ripple current ratio at maximum input voltage

Using common high power PoE values, a 48V (41V < VIN

< 57V) to 5V/5.3A converter with 90% efficiency, POUT=

26.5W and PIN = 29.5W. Using X = 0.4 N = 1/8 and fOSC

= 200kHz:

DCMIN =

N • VIN(MAX)

VOUT

+ 1

• 57

= 41.2%

LP =

57V • 0.412

(

)

200kHz • 0.4 • 26.5W

=260µH

Optimization might show that a more efficient solution

is obtained at higher peak current but lower inductance

and the associated winding series resistance. A simple

spreadsheet program is useful for looking at tradeoffs.

Transformer Core Selection

Once LP is known, the type of transformer is selected.

High efficiency converters use ferrite cores to minimize

core loss. Actual core loss is independent of core size for

afixedinductance,butdecreasesasinductanceincreases.

Sinceincreasedinductanceisaccomplishedthroughmore

turns of wire, copper losses increase. Thus, transformer

design balances core and copper losses. Remember that

increasedwindingresistancewilldegradecrossregulation

and increase the amount of load compensation required.

The main design goals for core selection are reducing

copper losses and preventing saturation. Ferrite core

material saturates hard, rapidly reducing inductance

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