MIC2172/3172

Micrel

4-26

1997

Finally, recalculate the transformer turns ratio to insure that

it is less than the value earlier found in equation (9).

(12)

a

≤

L

PRI

L

SEC

Then:

a

≤

1.8

2.54

a

≤ 0.84 Use 0.8 (same as 1:1.25).

This ratio is less than the ratio calculated in equation (9).

When specifying the transformer it is necessary to know the

primary peak current which must be withstood without satu-

rating the transformer core.

(13)

I

V

L

PRI

So:

I

4.0

18

µH

I

Now find the minimum reverse voltage requirement for the

output rectifier. This rectifier must have an average current

rating greater than the maximum output current of 0.25A.

(14)

V

V

V

()

F

Where:

V

reverse voltage rating

a = transformer turns ratio (0.8)

F

Then:

V

6.0 + 5.0

× 0.8

()

0.8

× 0.8

V

A 1N5817 will safely handle voltage and current require-

ments in this example.

Forward Converters

Micrel’s MIC2172/3172 can be used in several circuit con-

figurations to generate an output voltage which is less than

the input voltage (buck or step-down topology). Figure 13

shows the MIC3172 in a voltage step-down application.

Because of the internal architecture of these devices, more

external components are required to implement a step-down

regulator than with other devices offered by Micrel (refer to

the LM257x or LM457x family of buck switchers). However,

for step-down conversion requiring a transformer (forward),

the MIC2172/3172 is a good choice.

A 12V to 5V step-down converter using transformer isolation

(forward) is shown in figure 14. Unlike the isolated flyback

converter which stores energy in the primary inductance

during the controller’s on-time and releases it to the load

during the off-time, the forward converter transfers energy to

the output during the on-time, using the off-time to reset the

transformer core. In the application shown, the transformer

core is reset by the tertiary winding discharging T1’s peak

magnetizing current through D2.

For most forward converters the duty cycle is limited to 50%,

allowing the transformer flux to reset with only two times the

input voltage appearing across the power switch. Although

during normal operation this circuit’s duty cycle is well below

C2

2.2µF

C1*

100µF

D2

R4

10

Ω

1.2k

C4

1µF

C5

330µF

L1

100µH

3.7k

D1

1N4148

D3

1N4148

* Locate near MIC2172/3172 when supply leads > 2

"

†

R3/R2 sets output voltage

5V, 0.1A to 1A

MIC3172

FB

R3

470

C3

1µF

EN

GND

P1 P2

S

COMP

Figure 13. Step-Down or Buck Converter