LT3574 Просмотр технического описания (PDF) - Linear Technology

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LT3574

Isolated Flyback Converter Without an Opto-Coupler

Linear Technology 

LT3574

Applications Information

Overdriving the BIAS Pin with a Third Winding

The LT3574 provides excellent output voltage regulation

without the need for an opto-coupler, or third winding, but

for some applications with higher input voltages (>20V),

it may be desirable to add an additional winding (often

called a third winding) to improve the system efficiency.

For proper operation of the LT3574, if a winding is used as

a supply for the BIAS pin, ensure that the BIAS pin voltage

is at least 3.15V and always less than the input voltage.

For a typical 24VIN application, overdriving the BIAS pin

will improve the efficiency gain 4% to 5%.

Loop Compensation

The LT3574 is compensated using an external resistor-

capacitor network on the VC pin. Typical values are in the

range of RC = 50k and CC = 1nF (see the numerous sche-

matics in the Typical Applications section for other possible

values). If too large of an RC value is used, the part will be

more susceptible to high frequency noise and jitter. If too

small of an RC value is used, the transient performance will

suffer. The value choice for CC is somewhat the inverse

of the RC choice: if too small a CC value is used, the loop

may be unstable, and if too large a CC value is used, the

transient performance will also suffer. Transient response

plays an important role for any DC/DC converter.

Design Example

The following example illustrates the converter design

process using LT3574.

Given the input voltage of 20V to 28V, the required output

is 5V, 0.5A.

VIN(MIN) = 20V, VIN(MAX) = 28V, VOUT = 5V, VF = 0.5V

and IOUT = 0.5A

1. Select the transformer turns ratio to accommodate

the output.

The output voltage is reflected to the primary side by a

factor of turns ratio N. The switch voltage stress VSW is

expressed as:

N = NP

NS

VSW(MAX) = VIN + N(VOUT + VF ) < 50V

or rearranged to:

N < 50 − VIN(MAX)

(VOUT + VF )

On the other hand, the primary-side current is multiplied by

the same factor of N. The converter output capability is:

IOUT(MAX)

=

0.8

•

(1−

D)

•

1

2

NIPK

D = N(VOUT + VF )

VIN + N(VOUT + VF )

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

0.34

16 ~ 22

2:1

39

0.57

28~ 35

3:1

44.5

0.73

37~ 45

4:1

50

0.84

44~ 52

3574f

15

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