Power Supply Basic Terms

Because power supplies have capacitive rectification circuits and smoothing circuits, the waveform of the input current will be distorted. The product of the input voltage (RMS) and the input current (RMS) is apparent power. The active power is the time-averaged product of the instantaneous voltage and current over a full cycle. Apparent power is the power that flows in the power supply, active power is the power consumed by the power supply. The gap between the apparent power and active power is the power not utilized, and is returned to the power grid.

Efficiency is the ratio of output power to input active power. This value in the specification is at the rated output power. Efficiency is typically highest near the rated load and decreases at very light loads.

Power factor is the ratio of active power to apparent power. In the case of a power supply with capacitive input, the power factor is normally approximately 0.6. A power supply with PFC (Power Factor Correction) is approximately 0.9.

As switching power supplies apply capacitive input circuits, the inrush current can be from multiples of ten to multiples of hundreds of times that of normal operation. Inrush current is the crest value of current that flows when applying an input voltage to a power supply. In series regulators, an inrush current, typically 5 to 10 times the rated steady-state current, occurs. This is primarily caused by the charging of the secondary-side filter capacitor and potential transformer core saturation at power-on.

Ripple is a wave synchronized with the switching frequency and/or input frequency added to the output voltage. Ripple is usually expressed in peak-to-peak terms. When a capacitor is added to the output of a switching power supply, the component of the ripple synchronized with the switching frequency becomes smaller, but the composition synchronized with the input frequency remains the same. For series regulators, the ripple is only the noise synchronized with the input frequency.

Output noise is the combination of periodic ripple and random, high-frequency noise components superimposed on the DC output. It is often specified as PARD (Periodic and Random Deviation). Most switching power supplies use rectangular waveform inverters, so noise from switching transistors or output rectifier diodes is added to the output. Below shows the relation of ripple, output noise, and ripple noise. Series regulators, which lack high-frequency switching components, exhibit significantly lower noise compared to switching power supplies. Their output noise is primarily composed of ripple at the line frequency and its harmonics.

Since power supplies convert electrical energy, energy loss manifests as heat. Effective heat dissipation is a critical component of power supply design to ensure reliability.

a. Natural Convection Cooling

Natural convection cooling dissipates heat via air movement caused by temperature differences, without using fans. Since this method relies on airflow, sufficient clearance around the unit is required. When installing inside an enclosure, ensure proper ventilation to prevent heat buildup. Effective exhaust ventilation is often more critical than intake size. Always install the power supply according to the specified orientation.

b. Forced Air Cooling

Forced air-cooled power supplies use internal fans to generate airflow across components. For these units, maintaining the ambient temperature within specifications is typically sufficient. However, ensure that the intake and exhaust vents are never blocked to maintain proper airflow.

Remote control functionality allows users to turn the power supply output on/off or adjust voltage/current settings using external signals. Control can be achieved via contact closures (relays, switches), transistors, or logic levels from integrated circuits. Logic polarity varies by model (e.g., LOW = ON, HIGH = OFF). Always verify the specifications in the datasheet before use.

Operating temperature refers to the ambient temperature range in still air (unless forced air cooling is specified) where the power supply meets its specifications. Avoid installing the unit vertically or at extreme angles unless permitted. For forced air-cooled units, the operating temperature is the temperature of the air entering the intake. As a general rule, the lifetime of an electrolytic capacitor is reduced by 50% for every 10°C rise in temperature; therefore, operating at lower temperatures prolongs product life.