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 , 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. Thus, typical linear power supplies are inefficient and physically large and heavy.
Switch-mode Power Supplies
Switch-mode Power Supplies (SMPS) are an alternative to linear supplies, typically used when size and efficiency are important criteria. An SMPS uses a rectifier to convert the line voltage to a DC bus, with a filter capacitor. This DC voltage is then chopped using a switching device (usually a MOSFET), and passed through a high-frequency step-down transformer. The resulting voltage is rectified and smoothed to obtain the final DC output. A feedback loop senses the output voltage and uses it to adjust the duty cycle of the chopper, resulting in a regulated DC output.
A typical SMPS operates at a switching frequency of several tens to several hundreds of kilohertz, which allows the inductors to be much smaller, without saturation. The ripple at the output is primarily at multiples of this switching frequency, and can be very difficult to filter out due to the rising impedance of filter capacitors at these frequencies.
Filtering the Output of an SMPS
Several approaches are used to filter the output of an SMPS. Some of these are listed below.
Filter Capacitor
The simplest approach to filtering the output of an SMPS is to use large capacitors in parallel with the output. Aluminum electrolytic capacitors, popular in this role, typically have equivalent series resistance (ESR) of several ohms, and will not efficiently filter out the "spikes" due to the switching frequency.
