Shortly after I obtained my first mobile radio, I started thinking about solutions for using it as a base station in my house. I had already procured an antenna, and so was left to find a solution for power. Although being off grid is nice in times of emergency, I found most of my use was while commercial power was available. So I was met with the age-old question “linear or switching”?
The basic technology behind AC to DC power supplies is the transformer. As the rate of change of the magnetic field increases in the transformer (i.e., increase in switch frequency), the transformer can be made smaller with smaller cores and wires to produce the same output power. Lighter core materials such as ferrite can be used instead of laminated iron.
In linear supplies, commercial power is connected to the primary side, stepped down, rectified through a low voltage, high current bridge, filtered and regulated by a power transistor operating in its linear region — basically acting as a variable resistor in series with the load. A control circuit modifies the transistor bias to maintain a constant voltage output, regardless of changes in the load current.
By contrast, in a switching supply the incoming ac voltage is rectified and filtered to produce a high-voltage dc. In this case a low-current, high-voltage bridge rectifier (which may not require a heatsink) may be used. A power transistor is connected in series with the transformer and acts as an on/off switch at a preset frequency. The magnetic energy received by the secondary windings of the transformer is then full-wave-rectified and reconstructed into the proper dc level. Similar to linear supplies, a feedback circuit monitors output characteristics and is fed back to the primary side to serve as input to the PWM that controls the MOSFET adjusting its duty cycle for proper regulation.
Most hams swear by linear power supplies–and rightly so in a lot of cases. They have low cost, are relatively simple, and can’t be beat in their output ripple/noise.
However, despite the obvious weakness of switchers: high frequency switching which can be picked up, amplified and transmitted with your desired signal — switching power supplies have come a long way over the years and some even rival their linear counterparts in output ripple and load regulation. Whether we like it or not, “green” is in and switching power supplies are much more efficient, leading to cooler and physically smaller power supplies.
Another advantage is the ability of the switcher to be a buck, or step-down converter when the input voltage is higher than desired regulation voltage, and switch to boost, or step-up converter when the input voltage is below the desired regulation voltage — allowing you to drain the last of the energy out of batteries (which is not recommended as you will damage the capacity of your battery).
If you do choose a switcher, select one whose switching frequency is not a low order harmonic of the frequencies you intend to transmit at, thus reducing the likelihood of noise. Some manufacturers actually put an adjustment dial called “noise shift” which allows the user to slightly vary the switching frequency in an attempt to reduce noise at radio frequencies.
Of course, if funds are not a concern, you can buy low ripple switchers whose output characteristics are very close to those of linear power supplies.
Bottom line is for weekly nets, service events, or any decent amount of transmitting, a power supply is an essential part of the Ham radio operators’ equipment.
by Scott Chandler, KF7EPK