DC-DC Voltage Booster
by Don Nelson, NØYE (ex-NØUGY)
There are mobile radios that do not work properly when the battery voltage drops just a little bit. For these radios, a DC-DC power supply that delivers a reliable 13.8 volts is a highly desirable piece of gear to have. The circuit presented here is for those cases where a relatively stable voltage is needed. The heart of the circuit is a switching IC that contains most of the DC-DC power supply circuitry. The IC is the LT1270A made by Linear Technology. It is not unique and comparable ICs are made by other manufactures. This IC works well and requires a small number of additional parts to complete the circuit. The circuit as presented is what I have built and used. The field test of this unit delivered about one amp on average for 7 hours at 7000 feet during a recent hot July day. The power supply was only a few degrees above ambient after 7 hours.
I chose to drive a 7 amp-hour battery with this DC-DC power supply. The battery holds the voltage for the radio over the short term and the power supply keeps the battery charged to 13.75 volts. As long as the load requires less current than the power supply can deliver, the battery will stay at or near 13.75 volts. What happens if the input power to the DC-DC power supply fails? I disconnected the power supply from its source for 10 seconds and nothing failed. I did not do a long term test where the power supply is not powered for an extended period of time. I am projecting that power supply will not be harmed.
The inductor, L1, is an important component to chose carefully. L1 is key to the transfer of energy from the input to the output by the switcher and has to carry up to 10 amps of current with no more loss than necessary. The LT1270A is limited to 10 amps. Chose L1 carefully. If L1 is too small, then L1 could saturate under load and cause the input current to jump to a high value without any increase in output power. The design value for L1 is 10 micro henries. I wound my own L1 by removing the wire on a larger inductor and rewinding with heavier wire. D1 can be any Schotky diode than can carry up to 10 amps. The voltage drop across D1 is one source of power loss in the power supply. C2 and C5 are good RF bypass caps and are put very close to the LT1270A and D1 respectively to keep the RF noise to a minimum. I built the circuit on a thin sheet of copper as the ground plane. Solder all grounded components to this copper and close to the same place to keep ground currents to a minimum. C3-L2-C4 and C6-L3-C7 ar pi networks for reducing RF generated by the switching circuit. L2 and L3 could be made larger for better filtering as long as L2 and L3 can carry the current without much loss. (Schematic Page 7)
Linear Technology Corporation, “Linear View Applications,” a CD based software package, version 2.0 April 1997.
D. Nelson, “DC-DC Switching Power Supplies for Amateur Radio Applications,” Microwave Update 1998 Proceedings, page 112-118.