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Designing A Flyback Converter with Si9113 for Feeding the TE from U-Interface In ISDN

Vishay Semiconductors 

AN728

Vishay Siliconix

POWER FEEDING TO NT

Figure 13 illustrates the method used to provide a power feed

across the U-interface from the local exchange to the NT1. The

two conductors on the U-interface are separated with a

capacitor to allow a dc voltage to be applied and to enable ac

signals to pass without attenuation. Protection circuitry is

added as needed.

POWER FEEDING TO TE

Power is fed from NT1 to TE in two configurations (Figure 14).

In the first configuration, power source 1 (PS1) feeds power to

the transformer at the NT, where the signal is injected. The

signal is then recovered from the transformer at the TE. A dc-dc

converter is used to power the TE circuitry.

Under normal conditions, the S-interface may be powered

locally from the NT1 using mains or batteries, and is backed up

with remote power from the network under emergency power

conditions where the local power source fails.

When active, the NT1 must consume no more than 500 mW

of power from the network, and in a deactivated state must

consume no more than 120 mW. Under emergency power

conditions when NT1 is expected to also power the user’s

designated TE across the S-interface, then the power

consumption of an active NT1 is allowed to rise to a maximum

of 1.1 W (although this varies between different ISDNs due to

different safety requirements and subscriber loop

configurations). The minimum voltage required for correct

operation at the NT1 is 28 V, while the feed voltage at the

exchange may vary among networks from 51 V to 115 V.

In the second configuration, power source 2 (PS2) feeds

power through a separate pair of conductors within the

S-cable. These conductors are independent of the transmit

and receive signal conductors. PS2 is capable of delivering

higher levels of power to TEs. Power feeding designs and

configurations vary among network operators and between

different countries and regions.

Table 1 shows how a dc-dc converter feeds power to the

S-interface on an NT1 board. Under normal conditions, the TE

power comes from an ac-dc converter via a relay. In a mains

failure condition, the relay switches the connection over to the

dc-dc converter and polarity reverses. Such polarity reversal

causes the TE to operate in restricted mode to minimize power

drawn from U-interface.

Signal

Transformer

TE

+

TX

–

Power

Source 1

RX

Power

Source 2

c

d

Power Feed

e

f

g

h

Power Feed

Signal

Transformer

NT

RX

+

Power

Source 1

+

TX

–

–

Power

Source 2

FIGURE 14. Power Feed Configurations at the S-Interface

Document Number: 71120

29-Feb-00

www.vishay.com S FaxBack 408-970-5600

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