MIC2171 datasheet(14/24 Pages) MICROCHIP

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MIC2171

DS20006355A-page 14



2022 Microchip Technology Inc. and its subsidiaries

EQUATION 5-22:

A 1N5817 dioded will safely handle voltage and current

requirements provided in this example.

5.7

Forward Converters

The MIC2171 can be used in several circuit

configurations to generate an output voltage which is

lower than the input voltage (buck or step-down

topology). Figure 5-4 shows the MIC2171 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 Microchip

(refer to the LM257x or LM457x family of buck

switchers). However, for step-down conversion

requiring a transformer (forward), the MIC2171 is a

good choice.

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

isolation (forward) is shown in Figure 5-4. 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 depicted in Figure 5-4, 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 50%, the MIC2171 has a

maximum duty cycle capability of 90%. If 90% was

required during operation (start-up and high load

currents), a complete reset of the transformer during

the off-time would require the voltage across the power

switch to be ten times the input voltage. This would limit

the input voltage to 6V or less for forward converter

applications.

To prevent core saturation, the application presented in

this section uses a duty cycle limiter consisting of Q1,

C4 and R3. Whenever the MIC2171 exceeds a duty

cycle of 50%, T1’s reset winding current turns Q1 on;

this action reduces the duty cycle of the MIC2171 until

T1 is able to reset during each cycle.

FIGURE 5-4:

12V to 5V Forward Converter.

VBR

6.0

5.0 1.8







0.8 1.8



----------------------------------------



VBR 10.4V



1N5819

MIC2171

COMP

22μF

1μF

VOUT

5V, 1A

3.74k

1.24k

470μF

L1 100μH

VIN

12V

GND

1:1:1

1N5819

* Voltage clipper

†

Duty cycle limiter

D1*

C2*

Q1†

C4†

R3†

R1*

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