DN31 - Isolated Power Supplies for Local Area Networks

Isolated Power Supplies for Local Area Networks
Design Note 31
Sean Gold
Introduction
Local Area Networks such as Ethernet or Cheapernet,
require low cost isolated power supplies with modest line
and load regulation. Table 1 summarizes the objective
design specifications based on IEEE 802.3 and ECMA
200-V. The LT®1072 high efficiency switching regulator
can be used in isolated flyback mode to satisfy these
requirements with minimal support circuitry.1
Circuit Design
Figure 1 illustrates the design approach. In isolated
flyback mode, the LT1072 has no electrical connection
to the load; instead, the regulator obtains a feedback
signal from the transformers flyback voltage during the
switch off-time. The voltage sense occurs after a 1.5μs
delay, which prevents the internal error amplifier from
regulating the voltage spike due to transformer leakage
inductance. The LT1072 compares the feedback signal
with a reference voltage, which is set at the feedback
pin with a resistor to ground. The primary voltage
is regulated to 16V + (VFB/RFB)7k. The feedback pin
voltage VFB, clamps to about 400mV, and the term
(VFB/RFB)7k is nominally set to 2V, making the total
flyback voltage 18V. The circuit is programmed for
–9V output by setting the transformer turns ratio to 2
to 1. The feedback resistor RFB, includes a 500Ω trim
to take into account variations in the clamp voltage and
gain within the LT1072.
Table 1. Power Supply Specifications for Figure 1
PARAMETER
VALUE
COMMENTS
VOUT
–9V
Ripple
Vn<10mVp-p
ILOAD
150mA
Load Reg
5%
Line Reg
5%
Efficiency
e>70%
Isolation
3000V
500V
Ethernet 11.4<VIN<12.6V
Cheapernet 4.55V<VIN<5.45V
40mA Min, 250mA Max
Ethernet
Cheapernet
Note 1: LTC’s Application Note 19, the LT1070 Design Manual,
presents a detailed discussion of isolated flyback mode and
general information on switching regulator design.
L1
2:1
VIN
D1
D2
VIN
C1
22μF
L2
18μH
D3
+
C2
22μF
TANT
VOUT
–9V
+
C3
47μF
TANT
VSW
+
DN031 F01
LT1072
GND
VFB
VC
CC RFB
0.01μF
500Ω
500Ω
L1 = PE-65342 (CHEAPERNET), L1 = PE-65329 (ETHERNET)
L2 = 77F180K J.W. MILLER RF CHOKE
D1 = 1N5936
D2 = MUR120
D3 = 1N5819
36<RL<225Ω
*TRIM FOR –9V OUTPUT
= SYSTEM GROUND
= FLOATING COMMON
Figure 1. Isolated Switching Regulator for LAN
02/90/31_conv
RL
A snubber network consisting of a fast turn-on, high
breakdown diode and a 36V Zener diode, limits the
magnitude of the leakage inductance spike. This snubber
configuration improves efficiency because it minimizes
the duration of the inductance spike. A Schottky diode
in the secondary reduces the voltage loss to the output
and increases efficiency.
Specifications for power supply filters are application
dependent. When noise levels of 150mV are tolerable,
a single 100μF tantalum capacitor is a suitable supply
filter. When output noise below 10mV is required, the
use of large output capacitors is often impractical. An
LC filter is an appropriate recourse. The optional LC
filter in Figure 1 contains an RF choke L2, and tantalum
filter capacitors C1 and C2. These components have low
effective series resistance (ESR) which helps maintain
5% load regulation.
Figure 2 shows the voltage on the switch pin, trace A,
and the current flowing through the inductor, trace B.
Trace C is a magnified view of trace A, which more
clearly shows regulation of the primary voltage after
the switch off-time. Figure 3 shows the voltage and
current noise at the output.
Transformer Design
The circuit design for 12V to – 9V (Ethernet) and 5V to
–9V (Cheapernet) circuits are identical except for the
transformer specifications. Both circuits develop a
regulated 18V primary voltage, but the available input
voltage determines the required primary inductance.
LPRI =
=
VIN
( ∆I)( f )(1+ VIN / VPRI )
5V
(0.3A )( 40kHz )(1+ 5 / 18)
Increased isolation also mandates a larger core to
accommodate additional insulation. The transformers
used in both applications are shown in Figure 4. The
PE-65329 for Ethernet (right) achieves 3700V isolation,
while the PE-65342 for Cheapernet (left) provides 500V
isolation.2 These transformers are constructed with low
loss core material and low resistance wire, to further
improve efficiency.
A. 20V/DIV AT
5μS/DIV
B. 400mA/DIV AT
5μS/DIV
C. 10V/DIV AT
1μS/DIV
DN031 F02
Figure 2. Switching Waveforms
A. 5mV/DIV
B. 4mA/DIV
10μS/DIV
DN031 F03
Figure 3. Voltage and Current Noise
= 326μH (Minimum)
Where,
ΔI = Magnetizing Current
f = Switching Frequency
VIN = Input Voltage
VPRI = Primary Voltage
Ethernet requires a larger primary inductance than
Cheapernet, which implies a larger transformer.
Note 2: A 500V version of the Ethernet transformer (PE-65330) is
available in the 0.5 inch package in Figure 4.
Data Sheet Download
www.linear.com
Linear Technology Corporation
Figure 4. LAN Transformers, PE-65342 (Left),
PE-65329 (Right)
For applications help,
call (408) 432-1900
dn31f_conv IM/GP 0290 165K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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© LINEAR TECHNOLOGY CORPORATION 1990
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