Supply converts 5V to-48V

Kurk Mathews, Linear Technology Corp, Milpitas, CA -- 11/24/1999

As the demand for networking equipment grows, the need arises for a -48V supply that can power telecommunication lines. The circuit in Figure 1 delivers 24W at -48V from a 5V input. One of the biggest challenges in this design is choosing the input voltage. Although high-current, 5V sources are commonly available, lower input voltages generally mean high input currents with accompanying low efficiency. With a relatively simple topology and a 5V input source, the circuit in Figure 1 delivers greater than 85% efficiency (Figure 2). T₁ stores energy during the on-state time of Q₁. Energy transfers to two stacked 24V outputs to create -48V. C₁ charges to a dc value equal to the 24V input voltage while clamping T₁'s leakage-inductance spike and providing a path for input current during Q₁'s off-state time. This operation results in continuous input current and thus reduces capacitor ripple-current requirements

The reduced input ripple current, which is characteristic of this topology, demands sensing of the switch current rather than the input current. In this case, T₂ senses the switch current, eliminating 400 mW of resistor loss without using excessive board space (7x8 mm). Other additions improve the circuit's efficiency and performance. The LTC1624's Boost pin normally provides the internal output driver (the TG pin) with a 5.6V regulated supply, but TG produces only 4.2V with a 5V input. Bypassing the internal regulator by connecting the Boost and V_IN pins increases Q₁'s gate voltage, resulting in a gain of more than 3.2% in overall efficiency. R₁ and D₁ keep the Boost pin below its 7.8V rating in the event of an input overvoltage condition. The addition of Q₂ and Q₃ provides an additional 5.5% of efficiency by speeding transitions and increasing gate voltage from 5 to 5.3V. This voltage peaking results from excess emitter current as Q₂ turns off after charging Q₁'s gate capacitance. Q₄ and Q₅ translate the -48V output to the 1.2V that the feedback pin (V_FB) requires to regulate the output voltage. The LTC1624's switching frequency decreases with output voltage, thereby reducing input current during output short-circuit conditions. (DI #2440).

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