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 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1990