MIC2171 Ver la hoja de datos (PDF) - Micrel

Número de pieza

componentes Descripción

Fabricante

MIC2171

100kHz 2.5A Switching Regulator

Micrel 

Micrel, Inc.

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 50%, the MIC2172 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

MIC2171

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 given here

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.

T1

D3

1:1:1

1N5819 L1 100µH

VIN

VO U T

5V, 1A

12V

R4

D4

C5 3.74k

R1*

C2*

1N5819 470µF 1%

C1

22µF

IN

SW

MIC2171

FB

GND COMP

D1*

D2

1N5819

R2

1k

C3

1µF

Q1† R 3†

C 4†

* Voltage clipper

† Duty cycle limiter

Figure 6. MIC2171 Forward Converter

R5

1.24k

1%

May 2007

10

M9999-051107

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