Bipolar transistor boosts switcher ' s current by

The circuit 1 uses a minimal number of external parts to raise the maximum output current of a 0.5A buck Switchi Ng-regulator IC to more than 6A. The circuit accommodates input voltages of 60V and delivers output voltages of 3.3, 5, or 12V, depending on your Cho Ice of ICS. Figure 2 provides a graph of conversion efficiency for the three standard output voltages, plotted over a range of input V Oltages extending to 60V. The circuit is useful in applications requiring higher input voltage, higher current, or both than are available from stand ARD ICs. The LM2594HVN is a buck regulator, switches an internal 0.5A device at. This current suffices to feed the base of Q_{1 and the bias resistor, r2. The function of r2 is to quickly turn off q1, a fast NPN switch With a beta greater than at 6A. The purpose of R1 may not being obvious without some knowledge of the internal workings of th E 2594. Its value was such to produce sufficient voltage drop at peak current so q1 begins to Satur Ate. The saturation causes q1 ' s beta to drop, and, as the transistor ' s base current rises to more th An 0.5A, the 2594 drops to its pulse-by-pulse limited-protection mode, followed by a reduction in clock frequency if the Overload is severe.}

This design example uses Through-hole components, because LOW-ESR capacitors and inductors in through-hole form is INEXPE Nsive and easy to find. Worst-case line and load conditions cause Q_{1 and D1 to dissipate 3W each, so you must choose a heat-sink size to keep the temperature rise within acceptable limits. A Heat-sink rating of 6 to 7°C/W can accommodate both devices for operation to 85°c ambient temperature. The capacitors is LOW-ESR types from Nichicon ' s PL series (http://www.nichicon-us.com). R2 dissipates less than 0.25W, but @ full load current, R1 can dissipate 1W at + V in + morethan 5W at l ow V in. You should locate R1 away from the regulator IC to minimize heating. The DIP version of the 2594 dissipates as much as 0.5W at high V on; you should solder leads 1, 2, 3, and 6 to a ground -plane area greater than 2 in.^{2 to avoid thermal shutdown. If you don ' t need the IC ' s on/off feature, then you should also solder Pin 5 to the ground plane.}}

Bipolar transistor boosts switcher ' s current by