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 2006 Semtech Corp.

www.semtech.com

SC4911

POWER MANAGEMENT

48V TO 12V FLYBACK CONVERTER AS AUXILIARY

SUPPLY

Figure 2 shows a simple bias supply using the SC4911.

This 36 - 72V input flyback power supply employs primary

side regulation from the transformer bias winding, which

eliminates the need of opto coupler and secondary

reference error amplifier. This is best for low cost

applications requiring isolation and narrow load variation

range on the secondary.

During the start-up, the input source trickle charges the

capacitor C1 through start-up resistor R1. As long as

voltage across C1 reaches the Vcc turn-on threshold,

the SC4911 turns on abruptly and start driving the gate

of MOSFET Q1. After the SC4911 starts up the primary

bias winding supplies the primary circuitry.

Typical Applications

The output of the T1 bias winding is rectified, filtered by

C1 and is fed to the FB through feedback resistor R5

and R7. The output voltage on the secondary is

proportional to the bias voltage by the turns ratio of the

output to the bias winding. C7 and R9 are for frequency

compensation. An additional small capacitor could be

used from COMP to GND if it is needed. The LUVLO pin

monitors the input voltage via resistor R2 and R4 to

determine turn-on and turn-off of the converter. Capacitor

C5 helps filter out the noise from the input line.

Figure 2. 48V to 12V Flyback Converter as Auxiliary Supply

R1

100K

C7

47nF

Vin = 48V

Vpri = 12V

R4

100K

R2

2.7MEG

C5

100pF

R9

4.7K

C8

1000pF

D1 1N5819

Vo = 12V

Q1

Si2320DS

U1 SC4911

1

2

3

4

5

6

7

8

LUVLO

RT

FB

COMP

ILIM

GND

OUT

VCC

T1

3

5

8

10

2

1

R8

1.00

C3

10uF

D2 1N5819

R7

100K

R5

383K

R3

68

R6

51

C4

0.1uF

C6

150pF

C1

10uF

C2

100uF

R10

100K