Power Supply selection for RF Components (TN-005)

Selection of a suitable power supply for RF components has traditionally been limited to linear power supplies, in order to prevent noise from the supply from affecting the performance of the RF components in question. The alternative, linear supplies, can be bulky and inefficient, and in situations where weight and efficiency are important, switch-mode supplies are the only option. This article explores the use of switch-mode supplies to power RF components.

Linear Power Supplies

Linear power supplies are constructed by using a mains transformer to reduce the AC line voltage down to a suitable intermediate voltage. This voltage is filtered to obtain an unregulated supply. Some form of linear regulation is then used to control the voltage. The linear regulation stage dissipates heat proportional to $(V_{in}-V_{out})I_{out}$, resulting in inefficient operation, particularly with high load currents. The output has low ripple content, which may be lowered further through the use of large filter capacitors. The filter capacitors are particularly effective at suppressing ripple at the mains frequency (60 Hz) since the impedance due to the equivalent inductance of the capacitors at these frequencies is quite low.

The transformer used to reduce the line voltage down to an intermediate voltage must operate at the 60 Hz line frequency. To prevent core saturation at low frequencies, the magnetics must be physically large.