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:
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.
Notes: 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.
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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 |
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.
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