Thermal Test Procedure for Module Power Supply

Module power supplies, which are known for their small size, are developing in the direction of low voltage input, high current output, and high power density. However, high integration and high power density will make the temperature rise per unit volume increasingly become the biggest obstacle to the reliable operation and performance improvement of the system. Statistics show that for every 2°C increase in the temperature of electronic components, their reliability decreases by 10%, and the life span at a temperature rise of 50°C is only 1/6 of that at a temperature rise of 25°C. Therefore, the purpose of thermal design is to discharge heat in a timely manner and keep the temperature of the product at a reasonable level to ensure that the thermal stress of the components does not exceed the specified value under the worst ambient temperature conditions. For module power supplies that attach great importance to reliability, heat treatment has become an indispensable part of their design.

Heat Generation

To explore thermal design methods, we must first understand how the temperature rise of the module power supply is generated. According to the law of conservation of energy, the total input power of the power supply should be equal to its total output power, that is, the energy conversion efficiency (η) is always 100%, but the actual situation is that the conversion efficiency (η=1-Ploss/Ptotal) is less than 100%, which means that some energy (Ploss) will be lost. So where does this lost energy go? Except for a small part that turns into electromagnetic waves and spreads into the air, the rest turns into heat energy, causing its temperature to rise. Excessive temperature will cause the internal components of the power supply equipment to fail and reduce the reliability of the entire equipment.

The parameter that links power loss and heat is thermal resistance, which is defined as the "resistance" of heat release from a heat-generating device to the surrounding area. It is precisely because of this "resistance" that a certain temperature difference is generated between the hot points and the surrounding area, just like a voltage drop occurs when current flows through a resistor. The thermal resistance of different materials is different. The smaller the thermal resistance, the stronger the heat dissipation. Its unit is ℃/W.

Heat generation treatment

1 Modeling analysis method

From the above analysis, we can get the first method to calculate the temperature rise: establish the power loss and thermal resistance models of each component separately, and then calculate the temperature rise value of the power device according to the following formula.

A basic expression for calculating temperature rise:

ΔΤ=RthJ-X·Рloss （1)

Where ΔΤ = temperature difference or temperature rise; RthJ-X = thermal resistance of the power device from junction to X.

It can be seen that: since the power loss of components is the root cause of heat generation, finding out the loss of each power device becomes the key to solving the heat treatment problem. Now let's take a 12W product from Goldensun with an efficiency of 91% as an example.

For a PWM-based self-driven synchronous rectification forward converter, the general application circuit principle is shown in FIG1 .

The losses of each power device are shown in Figure 2. In Figure 2, Pt is the primary transformer loss; Pl is the output filter inductor loss; Pmos is the MosFET loss; Pd1 is the rectifier diode loss; Pd2 is the freewheeling diode loss; Pother is the loss of other devices and

Now, some semiconductor device manufacturers can provide relatively detailed parameters related to losses, and power supply R&D personnel can also calculate the actual losses of power devices in actual projects, and then continuously correct these values ​​so that the losses of these components can be very close to the actual values. Therefore, in order to find out the actual temperature rise generated by each power device when consuming a certain power, the key now is to consider thermal resistance. However, the value of thermal resistance is generally greatly affected by the following factors, such as the loss of power components, the speed, direction, and level of disturbance of air flow, the influence of adjacent power components, the direction of the PCB board, etc. Therefore, the conditions for general thermal measurement are very strict. Now let's take a look at the thermal test method for a power component that is used for natural air cooling but is sealed on all sides and does not use a fan.