Switching and Linear Power Supplies and Filtering
Power supply design and filtering often dictate the ultimate performance of an electronic circuit. In RF and sensitive audio applications, the power supply is practically part of the signal path. If the DC power isn’t perfectly clean, that noise will inevitably mix with your signals, causing hum, instability, or masked signals. Here is a breakdown of how power supplies generate DC and the filtering strategies used to keep that power clean across different stages of a build.
1. The Basics: Rectification and Bulk Smoothing
The primary job of a standard AC-to-DC linear power supply is to step down the mains voltage, convert the alternating current (AC) into pulsating direct current (DC) using a rectifier, and then “smooth” those pulses into a flat line.
The workhorse of this smoothing process is the reservoir capacitor. It charges up when the rectified voltage peaks and discharges into the load when the voltage dips. The slight drop in voltage between pulses is known as ripple.
For a full-wave rectifier, the peak-to-peak ripple voltage can be approximated using:
Vr ≈ I/(2fC)
Where:
- Vr = Ripple voltage (Peak-to-Peak)
- I = Load current in Amps
- f = AC line frequency in Hz (e.g., 50Hz or 60Hz)
- C = Smoothing capacitance in Farads
When pulling high current—such as the 15 to 20 amps required to drive a 100W linear amplifier—the load current (I) is massive. To keep the ripple voltage (Vr) low enough to prevent distortion or hum on voice peaks, you need an exceptionally large capacitance (C), often tens of thousands of microfarads.
2. RF and High-Frequency Filtering
Bulk smoothing only handles low-frequency (50/100 Hz) mains hum. It does very little to stop high-frequency noise from switching regulators, digital logic boards, or stray RF from entering the power lines.
When dealing with low-level RF signals—like measuring millivolt VFO outputs or running a digital SDR setup—even millivolts of high-frequency noise can introduce severe instability.
- Bypass (Decoupling) Capacitors: A large electrolytic capacitor is physically too slow (due to internal inductance) to react to high-frequency noise. Therefore, smaller ceramic capacitors (typically 0.1 μF and 0.01 μF) are placed as close to the power pins of active components as possible. They act as a dead short to RF, shunting high-frequency noise directly to ground before it can enter the component.
- RF Chokes and Inductors: While capacitors shunt noise to ground, inductors (chokes) block it from traveling down the wire. Placing a molded RF choke or winding the power cable through a ferrite toroid creates a high-impedance barrier to RF, keeping transmitter energy out of the power supply and keeping digital “hash” away from the receiver.
3. Linear vs. Switching Power Supplies (SMPS)
Choosing the right type of supply heavily impacts your filtering strategy:
- Linear Supplies: They use a heavy iron transformer, a rectifier bridge, and large capacitors. They are physically heavy and inefficient, but they are electrically “quiet.” They are the gold standard for analog radio transceivers because they generate almost zero high-frequency noise.
- Switching Supplies (SMPS): These are lightweight, highly efficient, and can deliver high current (ideal for powering single-board computers like a Raspberry Pi). However, they operate by switching power on and off at high frequencies (often 50 kHz to over 1 MHz). This generates significant switching noise. If an SMPS is used near sensitive radio equipment, it usually requires aggressive external LC (inductor-capacitor) filtering and proper shielding to prevent it from acting like a broadband jammer.