Power Supply Cooling: When and How to Add Fans

Power supplies convert AC mains to regulated DC — and in the process, generate significant waste heat. A power supply operating at 85% efficiency wastes 15W as heat for every 100W delivered. Proper power supply cooling prevents thermal shutdown, electrolytic capacitor degradation, and fire hazards. This guide covers when and how to add fans to your power supply projects.

Why Power Supplies Need Cooling

Every power supply has components with temperature-sensitive lifespans:

• Electrolytic capacitors: Lifespan halves for every 10°C increase above rated temperature. A capacitor rated for 5,000 hours at 105°C lasts only 2,500 hours at 115°C.
• Switching transistors: MOSFETs and IGBTs have maximum junction temperatures of 150-175°C. Exceeding this causes immediate failure.
• Transformers: Core losses generate heat proportional to switching frequency and flux density. Insulation degrades with heat.
• Rectifier diodes: Forward voltage drop × current = heat. A 20A Schottky rectifier drops 0.5V, generating 10W of heat.

In India, where ambient temperatures of 40-45°C eat into thermal margins, power supply cooling is especially critical.

Heat Sources Inside a Power Supply

Primary side (AC):

• Bridge rectifier diodes: 2-5W typical
• PFC boost MOSFET and diode: 3-10W at high loads
• Primary MOSFET/transistor: 2-8W depending on topology

Secondary side (DC):

• Output rectifier diodes: Often the biggest heat source. A 12V/20A output with Schottky rectifiers can waste 10W+ just in rectification
• Output filter inductors: Core losses at high current
• Voltage regulators: Linear post-regulators waste power proportional to dropout voltage

Transformer: Core and copper losses typically 2-5% of rated power. A 500W transformer generates 10-25W of heat.

Fan Placement: Intake vs Exhaust

Fan placement strategy depends on your enclosure design:

Exhaust (blowing hot air out): The most common approach. Mount the fan on the rear or top of the enclosure, pulling air through the power supply and out. Works well when the air intake has adequate venting.

Intake (blowing cool air in): Pressurises the enclosure, forcing air through all gaps. Reduces dust ingress (positive pressure) and directs cool air at specific hot components. Requires filtered intake.

Cross-flow (intake + exhaust): Intake fan at the bottom, exhaust at the top. Creates a defined airflow path. Best for large enclosures or rack-mount supplies.

Key rule: always direct airflow over the hottest components. Use baffles or ducts to ensure air does not bypass critical components through a path of lower resistance.

Selecting the Right Fan Size and Speed

Match fan size to your power supply heat output:

Temperature-Controlled Fan Circuits

Running fans at full speed continuously wastes energy and creates unnecessary noise. A temperature-controlled fan circuit activates cooling only when needed:

Simple thermistor circuit: An NTC thermistor in a voltage divider drives a comparator (LM393). When temperature rises, thermistor resistance drops, comparator output goes high, and a MOSFET turns on the fan. No microcontroller required.

Arduino-based: Use the DS18B20 circuit from our Arduino Fan Controller article for PWM speed control based on temperature. Place the sensor on the hottest component (usually the output rectifier heat sink).

Integrated fan controllers: Many ATX power supplies use a dedicated fan controller IC that monitors multiple temperatures and adjusts fan speed on a smooth curve. The TC620 and EMC2101 ICs provide similar functionality.

Fanless Power Supply Design

Fanless power supplies offer zero noise and zero dust ingestion — ideal for audio equipment, medical devices, and bedroom electronics. Design requirements:

• Use synchronous rectification (MOSFETs instead of diodes) to reduce rectifier losses by 50-70%
• Use larger heat sinks with wide fin spacing for natural convection
• Derate output power to 60-70% of active-cooled rating
• Use higher-quality (low ESR) capacitors rated for higher temperatures (125°C)
• Keep the enclosure open-topped or well-ventilated for natural convection

In India, fanless designs are challenging due to high ambient temperatures. A power supply rated for fanless operation at 25°C may need a fan at 45°C ambient.

Recommended Cooling Components

Safety Considerations

WARNING: Power supplies contain lethal voltages. Observe these safety rules:

• Never open or modify a mains power supply unless you are qualified
• Capacitors can hold charge for hours after disconnecting — always discharge before touching
• Fan wiring must be rated for the voltage present (mains-rated wire for primary-side fans)
• Fan failure must not create a fire hazard — use thermal fuses as backup protection
• Keep fan intake away from flammable materials
• Use proper earthing/grounding on metal enclosures