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.