Do you know why thermal design is the primary problem with the AC-DC power supply? In an AC-DC power supply, thermal issues are not occasional malfunctions, but rather are determined by the unavoidable power losses that occur during the energy conversion process. When the power supply is towards higher power density and smaller size, the heat increases per unit volume, making the thermal design become the key to affect the reliability and lifespan of the AC-DC power supply. Even if the feature of critical normal, if the thermal energy is not released efficiently, maybe lead to parameter drift, part olding, and failure when working for a long time.
AC-DC power supply loses energy way
In the process of an AC-DC power supply, converting the alternating current to direct current, the energy is not all transferred load. The part energy will be lost by conduction losses, switching losses, and magnetic losses. These losses are all transferred to heat in all, and gather in the device and space. If you don’t control it, the part temperature will rise higher than the temperature of the environment and become a potential reliability risk.
The main heating device
1️⃣Primary side MOSFET[1](Metal-Oxide-Semiconductor Field-Effect Transistor) is the main heat source of the AC-DC power supply, where the main heat comes from conduction losses and switching losses[2]. In the condition of high input voltage or high frequency switching that switching losses will be a large proportion. The MOSFET becomes the complete machine in the heating device.
The function of the primary side MOSFET in the power supply
1. High frequency switching: MOSFET fast switch conduction and cutoff.
2. Power transform: through the fast switch action, let the direct current cutoff high-frequency pulse.
3 . Matching transformer: The pulse driver transformer works to make the voltage increase or decrease.
4. Highly efficient adjustable: ensure high efficiency voltage adjustment and power transmission.
In all say, the primary side MOSFET is like a high-speed “electronic switch”. In the input of the power supply, the system rapidly “switches” the electricity on and off according to instructions that make the current in a high-frequency pulse form enter the transformer, and high efficiency finishes the energy conversion.
2️⃣Rectifier bridge and secondary rectifier devices have function continuous conduction function in the power supply. These losses express is power consumption caused by the forward voltage drop. In a low-power supply, the device is a simple structure, but operating in the conducting state for extended periods will result in sustained and concentrated heat.
3️⃣Transformers and magnetic devices, heat source of copper loss, and core loss. The copper loss and current magnitude are directly related, and the magnetic losses are related to the work frequency and magnetic flux density. Due to the magnetic device volume being relatively concentrated, if the internal temperature rises, it is not easy to quickly release, and may be an invisible heat source in long-term work.
4️⃣Electrolytic capacitors and power resistors are not the highest power loss devices, but it extremely sensitive to temperature. The electrolytic capacitors are related to lifespan and work temperature. High temperature will speed up the old of the inter-electrolyte and shorten the lifespan.
Common temperature indicators used in thermal design
In thermal design, the temperature is not just a concept. Junction temperature is usually used to describe inter higher temperature of the semiconductor, which the key to judging the reliability of devices. The temperature of the shell lets you know the hot state of the device shell and the heat dissipation path. Moreover, the environmental temperature determines the basic cooling conditions of the power supply. The core of thermal design is to ensure that the temperature of critical components does not exceed their allowable range under the most unfavorable operating conditions.
Temperature and lifespan relation
Many engineering practices express, the temperature affects the lifespan of the power supply factor one. Such of electrolytic capacitors, it lifespan is usually defined by temperature, making the definition. In actual application, when the temperature increases, the amplitude and lifespan are affected. So even if the power supply test works normally, long-term high temperatures will lead to a loss in advance.
How to set the thermal design target
In actual design, the thermal design target usually satisfies the simple higher rated temperature of devices, which more enough reserve temperature margin for working long time. The margin needs to cover the changes in ambient temperature, load fluctuations, and device aging. That makes sure to keep reliable performance in the lifecycle of the power supply.
Thermal design targets in AC-DC power supplies usually include:
🔹Ensuring that the junction temperature of key semiconductor devices remains well below the maximum rated limit under worst-case conditions
🔹Limiting the temperature rise of critical components to maintain long-term reliability
🔹Providing sufficient thermal margin to accommodate ambient temperature variation, load fluctuation, and component aging
🔹Supporting stable electrical performance throughout the intended service life
AC-DC power supply thermal design does not have an independent existence in the design steps. It builds on the system of full understanding loss source, and the temperature effect. Only understand the heat source, temperature index, and design target in advance. The PCB layout and heat dissipation structure design can achieve the true function.
[1]Understanding MOSFETs is crucial for grasping their role in power supplies and heat management.
[2]Exploring switching losses will help you understand efficiency challenges in AC-DC power supplies.
