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LT3573
(Rev.:RevB)

Isolated Flyback Converter without an Opto-Coupler

Linear Technology 

LT3573

Applications Information

The transformer turns ratio is selected such that the con‑

verter has adequate current capability and a switch stress

below 50V. Table 6 shows the switch voltage stress and

output current capability at different transformer turns

ratio.

Table 6. Switch Voltage Stress and Output Current Capability vs

Turns-Ratio

VSW(MAX) AT VIN(MAX) IOUT(MAX) AT VIN(MIN) DUTY CYCLE

N

(V)

(A)

(%)

1:1

33.5

0.53

16 ~ 22

2:1

39

0.88

28~ 35

3:1

44.5

1.12

37~ 45

4:1

50

1.30

44~ 52

BIAS winding turns ratio is selected to program the BIAS

voltage to 3V~5V. The BIAS voltage shall not exceed the

input voltage.

The turns ratio is then selected as primary: secondary:

BIAS = 3:1:1.

2. Select the transformer primary inductance for target

switching frequency.

The LT3573 requires a minimum amount of time to sample

the output voltage during the off-time. This off-time,

tOFF(MIN), shall be greater than 350ns over all operating

conditions. The converter also has a minimum current limit,

LMIN, of 250mA to help create this off-time. This defines

the minimum required inductance as defined as:

LMIN

=

N(VOUT +

IMIN

VF )

•

tOFF(MIN)

The transformer primary inductance also affects the

switching frequency which is related to the output ripple. If

above the minimum inductance, the transformer’s primary

inductance may be selected for a target switching frequency

range in order to minimize the output ripple.

The following equation estimates the switching frequency.

fSW

=

1

tON + tOFF

=

IPK

VIN

1

+

IPK

NPS(VOUT +

VF

)

L

L

Table 7.Switching Frequency at Different Primary

Inductance at IPK

L (µH)

fSW AT VIN(MIN)

(kHz)

fSW AT VIN(MAX)

(kHz)

25

236

305

50

121

157

100

61

80

Note: The switching frequency is calculated at maximum output.

In this design example, the minimum primary inductance is

used to achieve a nominal switching frequency of 275kHz

at full load. The PA2454NL from Pulse Engineering is

chosen as the flyback transformer.

Given the turns ratio and primary inductance, a custom‑

ized transformer can be designed by magnetic component

manufacturer or a multi-winding transformer such as a

Coiltronics Versa-Pac may be used.

3. Select the output diodes and output capacitor.

The output diode voltage stress VD is the summation of

the output voltage and reflection of input voltage to the

secondary side. The average diode current is the load

current.

VD

=

VOUT

+

VIN

N

The output capacitor should be chosen to minimize the

output voltage ripple while considering the increase in

size and cost of a larger capacitor. The following equation

calculates the output voltage ripple.

DVMAX

=

LI

2

PK

2CVOUT

4. Select the snubber circuit to clamp the switch

voltage spike.

A flyback converter generates a voltage spike during switch

turn-off due to the leakage inductance of the transformer.

In order to clamp the voltage spike below the maximum

rating of the switch, a snubber circuit is used. There are

many types of snubber circuits, for example R-C, R-C-D and

3573fb

16

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