LT1105IN datasheet(21/32 Pages) LINER

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LT1103/LT1105

to zero or changing polarity. Therefore, the voltage on the

bias winding is only valid as a representation of the output

voltage while the secondary is delivering current.

Although the bias winding flyback voltage is a representa-

tion of the output voltage, its voltage is not constant. For

a brief period following the leakage inductance spike, the

bias winding flyback voltage decreases due to nonlinearities

and parasitics present in the transformer. Following this

nonlinear behavior is a period where the bias winding

flyback voltage decreases linearly. This behavior is easily

explained. Current flow in the secondary decreases lin-

early at a rate determined by the voltage across the

secondary and the inductance of the secondary. The

parasitic secondary leakage inductance appears as an

impedance in series with the secondary winding. In addi-

tion, parasitic resistances exist in the secondary winding,

the output diode and the output capacitor. These imped-

ances can be combined to form a lumped sum equivalent

and which cause a voltage drop as secondary current

flows. This voltage drop is coupled from the secondary to

the bias winding flyback voltage and becomes more sig-

nificant as the output is loaded more heavily. This voltage

drop is largest at the beginning of “switch off” time and

smallest just prior to either all transformer energy being

depleted or the switch turning on again.

The best representation of the output voltage is just prior

to either all transformer energy being used up and the bias

winding voltage collapsing to zero or just prior to the

switch turning on again and the bias winding going

negative. This point in time also represents the smallest

forward voltage for the output diode. It is possible to

redefine the relationship between the secondary winding

voltage and the bias winding voltage as:

VVf I • R

BIAS

OUT

()

where Vf is the forward voltage of the output diode, I is the

current flowing in the secondary, RP is the lumped sum

equivalent secondary parasitic impedance and N1 is the

transformer turns ratio from the secondary to the bias

winding. It is apparent that even though the above point in

APPLICATIONS INFORMATION

voltage, the secondary voltage, the bias voltage and the

winding currents are indicated in the figures found on the

following page for both continuous and discontinuous

modes of operation.

Simplified Flyback Converter

1:N

LT1103 AI01

VBIAS

L(lkSEC)

VOUT

COMMON

L(IkPRI)

VIN

1:N1

N = TURNS RATIO FROM SECONDARY TO PRIMARY.

N1 = TURNS RATIO FROM SECONDARY TO BIAS.

N2 = N/N1

L(lkPRI) = PRIMARY LEAKAGE INDUCTANCE.

L(lkSEC) = SECONDARY LEAKAGE INDUCTANCE.

R = PARASITIC WINDING, DIODE AND OUTPUT

CAPACITOR RESISTANCE.

When the switch “turns on,” the primary winding sees the

input voltage and the secondary and bias windings go to

negative voltages as a function of the turns ratio. Current

builds in the primary winding as the transformer stores

energy. When the switch “turns off,” the voltage across

the switch flies back to a clamp level as defined by a

snubber network until the energy in the leakage induc-

tance of the primary dissipates. Leakage inductance is one

of the main parasitic elements in a flux-sensed converter

and is modeled as an inductor in series with the primary

and secondary of the transformer. These parasitic induc-

tances contribute to changes in the bias winding voltage

and thus the output voltage with increasing load current.

The energy stored in the transformer transfers through the

secondary and bias windings during “switch off” time.

Ideally, the voltage across the bias winding is set by the DC

output voltage, the forward voltage of the output diode,

and the turns ratio of the transformer after the energy in

the leakage inductance spike of the primary is dissipated.

This relationship holds until the energy in the transformer

drops to zero (discontinuous mode) or the switch turns on

again (continuous mode). Either case results in the volt-

age across the secondary and bias windings decreasing

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