MP6002 – MONOLITHIC FLYBACK/FORWARD DC-DC CONVERTER

MP6002 Rev. 1.0

www.MonolithicPower.com

10

12/4/2013

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© 2013 MPS. All Rights Reserved.

APPLICATION INFORMATION

Switching Frequency

selection of the transformer (Tr), the output cap,

(C2), and the input cap, (C1). The higher the

frequency, the smaller the sizes for Tr, C2, and

C1. However, a higher frequency also leads to

higher AC power losses in the power switch,

control circuitry, transformer, and in the external

interconnection. The general rule states that

lower the output power, higher the optimum

switching frequency. For low current (<10A)

synchronous rectifiers are used and 300KHz to

500KHz if Schottky rectifiers are used.

Fundamental Equations

The transformer turns ratio N is defined as:

S

P

N

N

N



primary

and

secondary

side

windings,

respectively.

N

V

D

1

D

V

IN

O







Where D is the duty cycle.

The steady-state drain to source voltage of the

primary power switch when it is off is estimated

as:

O

IN

DS

V

N

V

V







The steady-state reverse voltage of the

Schottky diode D2 is estimated as:

N

V

V

V

IN

O

2

D





The output current is calculated as:

)

D

1

(

I

I

D

O







Schottky diode when it is conducting.

The input current is calculated as:

D

I

I

S

IN





primary power switch when it is conducting.

Transformer (Coupled Inductor) Design

1. Transformer Turns Ratio

The transformer turns ratio determines the duty

cycle range, selection of the rectifier (D2),

primary side peak current, primary snubber loss,

and the current as well as voltage stresses on

the power switch (S). It also has effects on the

selection of C1 and C2. A higher transformer

turns ratio (N) means the following:

 Higher Duty Cycle

but higher secondary side RMS current

 Use of a smaller input capacitor but

bigger output capacitor.

and lower primary snubber loss.

 Lower main switch (S) turn-on loss

For

a

5V

power

supply

design,

with

stresses of the power switch (S) and the

rectifier (D2).

Table 1—Main Switch (S) and Rectifier (D2)

Voltage Stress vs. Transformer Turns Ratio

N

(V)

(V)

(V)

(V)

4

0.36

119

132

38

42

5

0.41

125

139

32

36

6

0.45

131

146

28

31

7

0.49

138

153

25

28

8

0.53

144

160

23

26

9

0.56

150

167

21

24

10

0.58

156

174

20

22

11

0.60

163

181

19

21

Note:

The voltage spike due to the leakage inductance of the

transformer and device’s voltage rating/derating factors were

considered. See power switch selection and snubber design for

more information.