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PDF AN702 Data sheet ( Hoja de datos )
| Número de pieza | AN702 | |
| Descripción | Efficient Isdn Power Converters Using The Si9100 | |
| Fabricantes | Vishay Intertechnology | |
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AN702
Vishay Siliconix
Efficient ISDN Power Converters Using the Si9100
INTRODUCTION
operation, as well as during equipment connections and
disconnections.
One of the latest technology revolutions, an integrated
worldwide telecommunications network, will be accompanied
by another advance in power conversion technology. The
integrated services digital network (ISDN) will allow different
forms of information (voice, computer data, video, facsimile,
etc.) to be transmitted over the telephone network. The
International Consultative Committee for Telephone and
Telegraph (CCITT) has proposed standards for the interfaces
required to implement ISDN. Although the standards have yet
to be formally adopted, telecommunications companies are
The Si9100 power IC facilitates compliance with these design
requirements with a minimum number of external parts. To
illustrate this capability, a discontinuous conduction mode
(DCM) flyback converter was built and tested. Measured
efficiency was greater than 80% for a wide range of loads, and
60% efficiency was achieved with only a 15-mW load. Before
describing the circuit concepts in detail, it is instructive to note
the main features of the ISDN power-feeding concept which
has been endorsed by the CCITT.
moving ahead with pilot test programs, and semiconductor
makers are developing chip sets to build ISDN hardware.
Every network terminator (NT), signal regenerator (RG), and
ISDN POWER FEEDING
terminal equipment (TE) unit used for the implementation of
ISDN will require a power converter.[1]
Figure 1 is a block diagram of the ISDN basic access
configuration. The two-wire transmission line defined at the
A major requirement of these telecom applications (due to the
U-interface provides a 192 k-bits-per-second (bps) digital data
need for emergency-mode operation from a high-impedance
path which connects subscriber equipment to the local
source) is high-efficiency energy conversion at fractional-watt
power levels. Minimization of parts count, another key factor
telephone exchange. Although ISDN permits many new
services to be offered, the basic service of voice transmission
for the design of these power converters, is sought to
remains a vital function. Therefore, the network power feeding
simultaneously achieve low cost and high reliability. DataSheet4frUom.cobmatteries in the local telephone exchange remains an
essential part of modern telephone system planning. The
BiC/DMOS integrated circuit technology is ideally suited for the
power requirements of ISDN. The analog and digital logic
functions needed for pulse-width modulation can be
implemented in CMOS to minimize quiescent current to the
controller. DMOS transistors provide high-voltage power
switching with both very low dynamic and gate drive losses.
network terminal (NT) connects the local loop, called the
S-bus, to the U-interface at the customer’s premises.
ISDN-compatible terminals (TE1) communicate at a standard
64 k-bps rate over the four-wire S-bus. Non-ISDN-compatible
terminal equipment (TE2), such as analog phones, must
connect to the S-bus via a terminal adapter (TA).
Integration of the CMOS controller on the DMOS power device
yields the best overall performance at the lowest cost and
component count.
To minimize noise-coupling problems, the S-bus must be
galvanically isolated from the two-wire U-interface. The CCITT
recommendations call for an off-line power converter in the NT
to supply 4 W at 40 V nominal to the S-bus during normal
DESIGN OBJECTIVES
operation (for up to four telephones with full features). Other
terminal equipment (e.g., fax terminals) would be fed solely
from local ac power lines. In the event of a power outage, one
While some differences exist between designs, there are
telephone at the customer premises must be fed from the
several requirements in addition to efficiency which are
central office battery. This procedure is accomplished by
common to ISDN power converter applications. These
reversing the voltage polarity on the S-bus. Non-priority
include:
terminals have a diode input which isolates them during
D reliable start-up and operation from the high source
impedance of telephone subscriber lines (U-interface
only)
emergency-mode operation. A single telephone terminal is fed
via a full diode bridge, allowing it to operate during the
emergency.
D current limiting to prevent failure of other network
terminals when one power converter output is shorted
(S-interface only)
D a free-running internal oscillator for start-up as well as
independent operation, which can be synchronized to an
external clock signal
D electromagnetic interference (EMI) filtering to limit
DataSheet4U.ccoonmducted emissions during both start-up and normal
A signal regenerator may be required for long loops
(U-interface). The Deutsche Bundespost (DBP) proposes to
increase the feeding voltage from 60 V to 93 V to compensate
for voltage drops on long lines requiring signal regeneration.
The standard telephone line voltage used in many other parts
of the world is 48 V. Whatever the voltage, the problem for
power converters connected to telephone subscriber lines
remains the same--they are fed from a high-impedance
source.
DataShee
Document Number: 70576
DataSheet4 U .com
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1 page  
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AN702
Vishay Siliconix
et4U.com
FB
14 (20)
COMP DISCHARGE
13 (18)
9 (12)
OSC OSC
IN OUT
8 (11)
7 (10)
VREF 10 (14)
1 (2)
BIAS
VCC 6 (9)
+VIN 2 (3)
Error
Amplifier
–
+
4 V (1%)
2V
Ref
Gen
–
+
+
–
Current
Sources
1.2 V
To
Internal
Circuits
Current-mode
Comparator
OSC
Clock (1/2 fOSC)
R
Q
S
C/L
Comparator
3 (5) DRAIN
5 (8)
–VIN
(BODY)
4 (7) SOURCE
VCC
8.1 V
Undervoltage Comparator
–
+
–
+ 8.6 V
S
Q
R
11 (16) SHUTDOWN
12 (17) RESET
DNOaTtaE:SFhigeuerets4inUp.acroenmthesis represent pin numbers for 20-pin package.
FIGURE 6. Si9100 Functional Diagram
DataShee
CONVERTER PERFORMANCE
Measured efficiency data for the flyback converter is given in
the last column of Table 1. Most notable is the 60% efficiency
at a load of only 15 mW, which is allowed by the low quiescent
current requirement of the CMOS control circuitry in the
Si9100. Although power converters can operate at much
higher frequencies, the dynamic losses incurred reduce the
efficiency during the power-down state. The switching speed
(30-ns typical) of the DMOS output transistor in the Si9100
permits operation above audible frequencies with very low
dynamic and drive losses. Such performance cannot be
achieved with bipolar transistors. A single resistor, R3, sets the
oscillator frequency at approximately 34 kHz. A positive sync
pulse (5-V amplitude and 0.5-ms pulse width) at 40 kHz was fed
through R7 and C9 to pin 8 to demonstrate the principle of
synchronization with an external clock. Typically, the
free-running frequency should be set at 10 to 20% below the
external clock frequency (note that the switching frequency is
½ of the oscillator frequency).
Start-up characteristics were verified by connecting a 600-W
resistance from a dc power supply to the converter input
terminals. Reliable start-up was demonstrated at maximum
DataSheet4loUa.dcofomr supply voltages as low as 44 V. With zero source
Document Number: 70576
DataSheet4 U .com
resistance inserted in the line, the converter maintained
regulation down to an input voltage of 23 V. In both cases, the
maximum operating voltage is 70 V for the Si9100 . The
inductor, L1, was wound with 540 turns of #32 magnet wire on
a #55206 molypermalloy powder core.
The relatively high series resistance of this inductor (6 W)
provides series damping of the input filter. This damping
reduces peaking of the filter output impedance, preventing
degradation of the control loop response at the filter resonant
frequency when the supply is operated from a low-resistance
source.
Measured ripple on both outputs was less than 50 mV peak to
peak, and regulation was better than 5% over line and load.
The –5-V output increases from –5.05 V to –5.75 V when
totally unloaded.
The current-mode controller of the Si9100 provides fast
current-limiting response in the event of a shorted output. With
either output shorted to ground, the measured value of
short-circuit current drawn at the converter input was 30 mA.
Any output terminal can be shorted for an indefinite period with
no resulting high stress condition on the Si9100. Normal
operation resumes when the short circuit is removed.
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