Some subscriber line interface cards (SLICs) require power-supply voltages isolated from the local supply. The circuit here generates three such isolated voltages from a 5V input: +3.3V @ 100mA, -24V @ 100mA, and -72V @ 25mA. It features a boost controller (U1) operating in a transformer flyback topology, and an optocoupler for isolating the feedback signal. To provide this feedback (from the --24V output to the boost controller) the optocoupler (U4) is driven by a shunt voltage regulator (U3) acting as an error amplifier.
Multi-Tap Transformer Helps
Figure 1. This SLIC power supply isolates the three outputs (+3.3V, -24V, and -72V) with a transformer, and isolates the feedback signal with an optocoupler.
The wide range of output voltages requires a custom transformer. Its core is a standard-geometry Coiltronics SG3-0138 with AL = 138nH/T2. The primary inductance is 6.7 mH, and the peak primary current is 4A. The primary-to-secondary turns ratio is 1:5, so for best efficiency the converter operates with a duty cycle near 50%. The unregulated --72V output is derived from the regulated --24V output via three identical secondary windings connected in series. Also available is a tap for a --48V output, and a low-voltage winding that drives a linear voltage regulator (U2) to provide the +3.3V output. The transformer's winding specs are:
Primary is 7 turns of #28 bifilar.
Tapped secondary is three layers of 35 turns each of #32 per tap.
Low voltage secondary is 5 turns of #32.
At maximum specified load with an input of 5.0V @ 1.138A, the circuit yields 80% efficiency while delivering +3.28V @ 103.9mA, -24.0V @ 100mA, and --73.2V @ 25.2mA.
Table 1. Output Regulation