A linear power supply is the correct topology when your load demands ultra-low output ripple (typically < 1mV RMS) and you can absorb the thermal penalty of 40–60% efficiency. While switching mode power supplies (SMPS) dominate high-current and portable applications, linear regulators remain the undisputed standard for precision analog front-ends, high-resolution ADCs, RF receivers, and professional audio preamplifiers where switching noise would destroy signal integrity.
Linear vs. Switching: The Noise vs. Efficiency Tradeoff
Choosing between a linear power supply and a switching converter comes down to your load's sensitivity to high-frequency noise versus your system's thermal and size constraints. Linear regulators operate by burning off excess voltage as heat, acting as a variable resistor. This means they inherently filter out input ripple, but they waste significant power. Switching converters use high-frequency PWM and inductors to step voltage up or down efficiently, but they inject switching noise (typically 20mV to 50mV peak-to-peak) into the output rail.
| Criteria | Linear Power Supply | Switching Power Supply (SMPS) |
|---|---|---|
| Efficiency | 40% – 60% (drops with high V_in - V_out) | 85% – 95% |
| Heat Generation | High (requires large heatsinks) | Low (minimal heatsinking needed) |
| Output Noise/Ripple | < 1mV RMS (virtually zero high-freq noise) | 20mV – 50mV+ (switching frequency spikes) |
| Cost & Weight | Heavy/Costly at >50W (due to iron transformer) | Light/Cheap across all power levels |
| Transient Response | Excellent (no LC filter delay) | Moderate (requires compensation tuning) |
When to choose which: Use a linear power supply for loads drawing under 2A that require pristine DC voltage, such as 24-bit audio DACs or 16-bit+ sensor arrays. Use an SMPS for motor drivers, LED lighting, microcontroller logic cores, and any battery-powered application where thermal waste directly reduces runtime.
Designing a 12V / 1A Linear Power Supply
Let's walk through a concrete design for a 12VDC, 1A linear power supply powered from a 120VAC mains input. This design example covers the exact input range, protection requirements, and headroom math necessary to ensure stable regulation.
Input Range and Primary Protection
North American nominal mains is 120VAC, but the acceptable utility range is 108VAC to 132VAC. Your transformer and downstream components must handle the 132VAC upper limit without saturating or exceeding voltage ratings.
- Primary Protection: Use a 0.5A slow-blow fuse on the 120VAC primary side to handle transformer inrush current without nuisance tripping.
- Secondary Protection: Place a bidirectional TVS diode (e.g., 1.5KE18CA) across the DC bus after the bridge rectifier to clamp inductive voltage spikes if the load disconnects suddenly.
Transformer and Rectifier Sizing
To get 12VDC out, you need a transformer secondary that provides enough peak voltage to cover the regulator's dropout voltage, even at the lowest AC input (108VAC).
- Transformer: 15VAC RMS secondary, rated for at least 1.5A (to account for capacitor charging pulse currents).
- Peak DC Voltage: 15VAC × √2 (1.414) = 21.2V peak.
- Bridge Rectifier Drop: Subtract ~1.4V for a standard silicon bridge (e.g., W10M), leaving 19.8VDC peak.
Filter Capacitor and Ripple Math
The bulk capacitor smooths the rectified DC. For a 1A load at 120Hz ripple frequency (full-wave rectified 60Hz), we target a peak-to-peak ripple of under 4V to maintain headroom.
Using the formula C = I / (2 × f × V_ripple):
C = 1A / (120Hz × 3.5V) = 0.00238 Farads.
Select a standard 2,200µF / 35V electrolytic capacitor. This yields a peak-to-peak ripple of roughly 3.7V. The minimum DC bus voltage is 19.8V - 3.7V = 16.1V.
Regulator Selection and Dropout Headroom
We will use the classic Texas Instruments LM317 adjustable linear regulator. The LM317 requires a minimum dropout voltage (headroom) of 2.0V to maintain regulation.
Headroom Check: V_in(min) - V_out = 16.1V - 12.0V = 4.1V. Since 4.1V > 2.0V dropout, the regulator will hold 12V steady even at the bottom of the AC ripple trough and at minimum mains voltage.
Resistor Network: To set V_out = 12.0V, use the formula V_out = 1.25V × (1 + R2/R1). Setting R1 = 240Ω (standard LM317 value), R2 calculates to 2,064Ω. Use a 2kΩ fixed resistor in series with a 200Ω trimmer potentiometer to dial in exactly 12.00V.
Ripple and Noise Expectations
The LM317 has a Power Supply Rejection Ratio (PSRR) of roughly 80dB at 120Hz. This means it attenuates the 3.7V peak-to-peak input ripple by a factor of 10,000. Your expected output ripple will be an exceptionally clean ~0.37mV peak-to-peak, making it ideal for noise-sensitive analog circuitry. For deeper insights into linear regulator PSRR and transient behavior, refer to Analog Devices Tutorial MT-085 on Linear Regulator Fundamentals.
Thermal Management and Derating
The fatal flaw of linear power supplies is heat. You must calculate the exact thermal dissipation to select a proper heatsink and prevent the regulator from entering thermal shutdown.
Calculating Heatsink Requirements
First, find the average power dissipated by the regulator:
- Average Input Voltage: ~18V (midpoint between 19.8V peak and 16.1V trough).
- Power Dissipation (P_D): (V_in_avg - V_out) × I_load = (18V - 12V) × 1A = 6 Watts.
Next, calculate the required heatsink thermal resistance (θ_SA). The LM317 maximum junction temperature (T_J) is 125°C. Assuming a worst-case ambient temperature (T_A) of 40°C inside an enclosure:
θ_SA = ((T_J - T_A) / P_D) - θ_JC - θ_CS
- θ_JC (Junction-to-Case) = 3.0°C/W
- θ_CS (Case-to-Sink with thermal paste) = 1.0°C/W
- θ_SA = ((125 - 40) / 6) - 3.0 - 1.0 = 14.16 - 4.0 = 10.16°C/W
Derating Note: While a 10°C/W heatsink is the mathematical minimum, always apply a 30% safety margin for enclosed spaces with poor airflow. Select an extruded aluminum heatsink rated for 7°C/W or lower (such as the Aavid Thermalloy 530002B02500G). Furthermore, keep your 2,200µF electrolytic capacitor physically separated from the heatsink; electrolytic caps lose half their lifespan for every 10°C rise in ambient temperature.
Frequently Asked Questions
Why does my linear power supply transformer hum or buzz loudly?
Transformer hum is caused by magnetostriction—the physical expansion and contraction of the iron core laminations as the magnetic field alternates at 60Hz (or 120Hz for the harmonic). If the hum is excessive, it usually indicates loose core laminations, an overloaded secondary winding causing core saturation, or a DC offset on your AC mains line. Potting the transformer in epoxy or using a toroidal core (which confines the magnetic field more tightly) will drastically reduce acoustic noise.
Can I use a linear power supply directly for battery charging?
Yes, but not without modification. A standard constant-voltage linear supply will attempt to push infinite current into a deeply discharged battery, acting as a dead short and triggering the regulator's internal short-circuit protection (or destroying it). To use it for charging a 12V lead-acid or LiFePO4 pack, you must add a constant-current (CC) limiting circuit—often implemented with a secondary LM317 configured as a current limiter upstream of the voltage regulator—and a reverse-polarity Schottky diode on the output to prevent the battery from back-feeding and destroying the regulator when AC power is removed.
What is the expected lifespan of a linear power supply compared to an SMPS?
A well-designed linear power supply will easily outlast an SMPS, often running for 20 to 30 years. Because it lacks high-frequency switching MOSFETs, optocouplers, and complex PWM controller ICs, the component count is low and stress is minimal. The primary point of failure is the electrolytic filter capacitor drying out. If you use high-quality, 105°C-rated, low-ESR capacitors (like Panasonic FR or Rubycon ZL series) and keep them thermally isolated from the regulator heatsink, the power supply will likely outlive the equipment it powers.
