AB-28

APPLICATION BULLETIN

2

7/18/02

Fig.4. Datacom AC Power Distribution

With the advent of the converged systems, these separate

approaches to power distribution will converge into new

architectures but the bottom line is that at the board or back-

plane level the usual voltages will need to be delivered,

namely 12V and 5V, as well as 0.9V, 1.8V, 2.5V, 3.3V with

more to come.

The delivery of such low voltages starting from DC or AC

power will be the focus of this document from here on.

DC-DC conversion

Fig. 5 shows the –48V to +Vout (+5, +12V etc.) with a

forward converter architecture based on the ML4823 high

frequency PWM controller:

Fig. 5. –48V to +Vout Conversion

Fig.6 shows the DC-DC conversion from 12V and 5V down

to a slew of typical low voltages required by modern

electronic loads.

Fig. 6. DC-DC Conversion Diagram

The conversion down to heavy loads is done with

synchronous rectification switching regulators of single or

multiphase interleaved type, while for lighter loads

linear regulators can be utilized.

AC-DC conversion

Fig. 7 shows the conversion of AC power directly down to

one of the DC voltages listed earlier, all low enough to be

safely distributed on the motherboard. The AC line is

rectified first by means of a full bridge diode rectifier that

converts the alternate AC voltage into a continuous but still

poorly regulated intermediate voltage. Next this voltage is

converted down to a low voltage usable by the electronics on

the motherboard by means of a “fly-back” converter

(KA5x03xx family) with minimum number of external

components. This conversion requires electrical isolation

between the high input and the low output voltages: this is

accomplished via the utilization of a transformer (T) in the

forward conversion path and an opto-coupler in the feedback

path.

Fig .7. AC/DC Conversion Diagram

DC/DC

AC LINE

AC UPS

AC

DC/AC

Rectifier

Battery

Backup

12V DC

-48V

120/208V AC

UF4005

Lreset

Rload

MBR

3060PT

MBR3060PT

Lbias

C1

C2

C3

+Vout

L

D1

D2

D3

T

ML4823

H11A817A

RC431A

Rlim

-48V

-48V

2

Φ FAN5092

TSSOP28

FAN5236 Dual

S024/28

LDO RC1587

LDO RC1585

D1

12V

5V

C1

C1

T1

N:1

Vout1: 1-5V, 30A

Vout2: 2.5V, 6A

Vout3: 1.8V, 6A

Vout4: 3.3V, 3A

Vout5: 1.5V, 5A

Rload

C2

Vout

1N5822

Vstart

FB

GND

GND

Drain

Vcc

GND

GND

AC Line

+

–

T

H11A817A

RC431A

KA5H0365

MB6S