A step-up Voltage Converter for powering LNBs from 12V

This article first appeared in 'P5' - March 1996

Most LNBs are designed to work on supply voltages greater than 12V. Older  ones prefer a fixed voltage of about 18V while newer ones use either 14V or 18V,  the change in voltage being used to switch from horizontal to vertical  polarisation or in some cases between low and high sections of the band. To  power them from a 12 battery, a voltage “booster” such as described here is  needed.

Several different types of DC-DC converters were tried with varying success  but in the end this design gave best value for money. Flyback inverters gave  higher efficiency under load but were more expensive and needed custom  transformers winding. They also had the disadvantage that their off-load voltage  would rise to unacceptable levels. Adding a voltage stabiliser would fix the  regulation problem but at the expense of cost and complexity.
This unit uses a voltage doubling technique. A pair of diodes is used to  steer the charging current to two capacitors. The first capacitor charges to  supply voltage then the voltage is made to “sit” on top of the supply so its  top end is at twice supply potential. The charge is then transferred through a  diode to the second capacitor which acts as a reservoir for the boosted voltage.  Theoretically, the output voltage is twice the supply voltage but in practice  there are drops between the collector and emitter junctions of the transistors  and across both the diodes. In the prototype the output was 22V off-load with a  12V supply and this dropped to 18V under a 150mA load. Input current at full  load is about 300mA which makes the unit a very respectable 75% efficient. Under  no load it draws about 15mA. The transistors do not need heat sinks, they run  quite cool, even under full load, but they run very hot if the output is shorted  out. I strongly suggest a 500mA fuse is wired in the supply line if there is any  chance of a short occurring.

Parts list:
R1, R2 1K
R3 100
C1, C2, C3 22nF Ceramic
C4, C5, C6 100uF 25V
TR1 BD131
TR2 BD132
D1, D2 MUR120 (Shottky)
IC1 NE555
None of the resistor or capacitor values are critical but the diodes must be  fast switching or schottky types, normal rectifiers will probably overheat and will  seriously reduce the units performance. The PCB layout accepts electrolytic  capacitors with 2.5 or 5mm pin spacing.
Click HERE for downloadable PCB files.