Power Adapter Conversion Efficiency

A power adapter is essentially an integrated transformer composed of a transformer, an AC/DC converter, and corresponding voltage stabilizing circuits. In simple terms, this integrated unit contains two main components: the transformer and the current converter. Both of these components inherently consume electrical energy, and their affiliated stabilizing circuits are no exception. Therefore, the power adapter itself is also an energy-consuming device.

The energy input into the power supply cannot be 100% converted into usable energy for the various components within the host device. This is the issue of conversion efficiency we are discussing today.

Conversion efficiency is a critical indicator for power adapters. High efficiency means the adapter itself incurs smaller losses, leading to greater energy savings. The conversion efficiency of a power adapter is defined as the total output power divided by the total input power: Power Efficiency η = Po / Pi. In this formula, Po represents output power, and Pi represents input power.

The relationship between a power adapter's conversion efficiency and its temperature rise must be addressed. Since the adapter internally loses a certain amount of power, its conversion efficiency cannot be 100%. The power consumed by the adapter manifests as heat. The level of heat generated depends primarily on the adapter's conversion efficiency and its physical size. Under certain heat dissipation conditions, the adapter will have a specific temperature rise-the difference between its case temperature and the ambient temperature. The surface area of the adapter's case directly affects this temperature rise. A rough estimate can be made using this formula: Temperature Rise = Thermal Resistance Coefficient × Block Power Consumption. In high-temperature environments, the adapter must be derated to reduce its power consumption, thereby lowering the temperature rise and ensuring the internal components do not exceed their maximum temperature limits. Beyond meeting the operational requirements of electronic devices, the operating temperature rise significantly impacts the adapter's Mean Time Between Failures (MTBF) when output power is constant. High efficiency and low temperature rise result in longer product life, smaller size, and reduced weight. This discussion of size naturally leads us to the topic of power density.

The vast majority of power adapter manufacturers use power density as a standard to measure product effectiveness. Power density is typically expressed in watts per cubic inch (W/in³). If the adapter cannot be used within the specified maximum environmental temperature range, it may not achieve the stated maximum output power. The available average output power is the usable power density.

Usable power density depends on the following factors:
■ A. Required Output Power. This is the maximum average power required by the application.
■ B. Thermal Impedance. Defined as the temperature rise caused by power dissipation, usually measured in °C/W.
■ C. Maximum Case Operating Temperature. All power components have a specified maximum case operating temperature. This refers to the highest temperature the internal elements of the component can withstand during operation. To maintain reliability, operation must remain below this temperature.
■ D. Operating Ambient Temperature. This refers to the worst-case environmental temperature during the component's operation. If a power component generates too much heat and cannot dissipate it quickly enough to the surrounding medium, it may fail due to exceeding its guaranteed operating temperature. Therefore, selecting an appropriate heat sink is one of the essential conditions for reliable component operation.

The main parameters required for the thermal design of power components are as follows:
■ 1. Component Operating Junction Temperature: The maximum allowable operating temperature limit for the device, provided by the manufacturer or mandated by product standards.
■ 2. Component Power Dissipation: The average steady-state power consumed by the device during operation, defined as the product of the average RMS output current and the average RMS voltage drop.

■ 3. Power dissipation of power devices: refers to the heat dissipation capacity of a specific heat dissipation structure.

■ 4. Thermal Resistance (R): The temperature rise per unit of power dissipation as heat transfers between media.