April 2005

13

M9999-042205

MIC2172/3172

Micrel, Inc.

Then:

2 × 1.25

1.25 × 4

δ ≥ 0.5 (50%) Use 0.55.

The slightly higher duty cycle value is used to overcome

circuit inefficiencies. A few iterations of equation (8) may be

required if the duty cycle is found to be greater than 50%.

Calculate the maximum transformer turns ratio a, or

N

3172 power switch.

(9)

a ≤

V

V

SEC

Where:

a = transformer maximum turns ratio

V

maximum voltage

F

commercial and industrial applications)

V

V

For the practical example:

V

F

V

Then:

a ≤

65 × 0.8 ± 6.0

5.6

a ≤ 8.2143

Next, calculate the maximum primary inductance required to

store the needed output energy with a power switch duty

cycle of 55%.

(10)

L

0.5 f

ON

2

P

OUT

Where:

L

f

V

T

Then:

L

()

2

1.25

L

Use an 18µH primary inductance to overcome circuit ineffi-

ciencies.

To complete the design the inductance value of the second-

ary is found which will guarantee that the energy stored in the

transformer during the power switch on time will be com-

pleted discharged into the output during the off-time. This is

necessary when operating in discontinuous-mode.

(11)

L

0.5 f

OFF

2

P

OUT

Where:

L

T

Then:

L

()

2

1.25

L

Figure 12. MIC3172 5V 0.25A Flyback Converter

C4

470µF

D2

1N5818

C1

22µF

C2

1µF

R3

1k

R4*

5V, 0.25A

* Optional voltage clipper (may be req’d if T1 leakage inductance too high)

C3*

D1*

R1

3.74k

1%

R2

1.24k

1%

4V to 6V

T1

Enable

Shutdown

1:1.25

MIC3172

FB

EN

GND

P1 P2

S

COMP