Isolated SLIC Supply

Some subscriber line interface cards (SLICs) require power-supply voltages isolated from the local supply. The Figure 1 circuit generates three such isolated voltages from a 5V input: +3.3V at 100mA, -24V at 100mA, and -72V at 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.


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 6.8µH, 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: 7T 28AWG bifilar       Secondary: 35T 32AWG   35T 32AWG   35T 32AWG   5T 32AWG

At maximum specified load with an input of 5.0V at 1.138A, the circuit yields 80% efficiency while delivering +3.28V at 103.9mA, -24.0V at 100mA, and -73.2V at 25.2mA.

VIN (V) Iin (A) Vo (V) Io (mA) Vo (V) Io (mA) Vo (V) Io (mA) Eff. (%) 4.50 1.27 24.01 100.2 73.2 25.2 3.28 103.9 80.3 5.01 1.138 24.01 100.2 73.2 25.2 3.28 103.9 80.5 5.50 1.038 24.01 100.2 73.2 25.2 3.28 103.9 80.4 Notes:

1. T1 is wound on a standard geometry core with 138nH/T².
   Primary inductance is 6.7µH.
   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.
   

A similar version of this article appeared in the September 24, 2001 issue of EE Times magazine.