Thermal testing of module power supplies

Module power supplies , which are known for their small size , are developing towards 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 their lifespan 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 the voltage drop caused by current flowing 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.

Figure 1 12W self-driven synchronous rectification forward converter schematic

For a PWM-based self-driven synchronous rectification forward converter, the general application circuit principle is shown in Figure 1.

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; and Pother is the sum of the losses of other devices.

Figure 2 Power device losses

Now, some semiconductor device manufacturers can provide relatively detailed parameters about loss, and power supply R&D personnel can also calculate the actual loss of power devices in actual projects, and then continuously correct these values ​​so that the loss of these components can be very close to the actual value. Therefore, the actual temperature rise generated by each power device when consuming a certain power is required. Now the key 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 disturbance level of air flow, the influence of adjacent power components, and the direction of the PCB board. 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. The cross-section diagram of the power component thermal test is shown in Figure 3.

Figure 3: Power device structure during thermal testing

Figure 4 2R model

In this way, the thermal resistance RthJA from the junction to the environment can be calculated according to the formula RJX=(TJ-TX)/Ploss (RthJA=RthJS+RthSA). Regarding the calculation of RthJA, here we only introduce a simple thermal model (Compact thermal model) 2R model, namely Two-Resistor Model. Its theoretical basis is shown in Figure 4.

(2)

But for module power supply, we usually package the semi-finished product in the shell, and its simplified diagram is shown in Figure 5.

Figure 5: Power device structure in the product

The shaded area in Figure 5 is potting material such as silicone and resin, which has two main functions: on the one hand, it is used to fix the semi-finished product; on the other hand, it is used to conduct the temperature of the surface of the power device (heat dissipation). Therefore, the thermal resistance RthJA from the node to the environment can be expressed as:

RthJA=[(RthJC1+RthC1E+RthEI+RthIC2+RthC2A)·(RthJT+RthTS+RthSB+RthBA)]/ [(RthJC1+RthC1E+RthEI+RthIC2+RthC2A)+
(RthJT+RthTS+RthSB+RthBA)] ( 3)

Then the temperature rise of the node corresponding to the power consumed Ploss can be calculated:

TJ=TA+Рloss·RthJA (4)

Among them, TA is the temperature value corresponding to the projection point of the geometric center of the power component on the upper surface.

However, the following conditions must be met for equation (4) to be true: the product has only one hot point or the effect of heat conduction between multiple hot points is small or negligible; the heat of the power device is only transferred upward or downward without considering other directions, that is, the 2R method is satisfied.

When there are multiple hot spots and the temperature distribution is uneven, empirical formulas are more important. The empirical formulas also need to be corrected and improved by the following methods.

2 Direct measurement method

There is another method for measuring temperature rise, which is relatively simple and commonly used: direct measurement method, which is to measure the temperature difference before the power device starts working and after it reaches thermal equilibrium.

In theory, we only need to ensure that the ambient temperature (TA) near the chip does not exceed the junction temperature (TJ) to make the chip work properly. But this is not the case in reality. The TA parameter is tested according to the JEDEC standard. In fact, it is almost impossible for the product to meet this test condition. Therefore, TA is meaningless to us here. In this case, the conservative approach is to ensure that the chip case temperature Tc is less than TA-max, so that the chip can still work properly. But from the perspective of reliability, we better require Tc to be less than the value of Tj-max after a certain level of derating. There are three common methods for measuring Tc.

(1) Temperature indicators: directly use a thermopaper to stick on the case of the power device, and read the corresponding Tc value based on the color of the thermopaper surface. This method is relatively simple, but for products with natural air cooling, sticking a thermopaper is not conducive to heat dissipation, and the actual measured value should be higher.

(2) Infrared imaging method (Thermal Imagine): Use the principle of infrared imaging to directly measure the surface temperature rise of components under thermal equilibrium conditions, such as Fluke's Ti20 or FL IR Systems' products.

Figure 6 Isothermal surface

Figure 7: Front thermal image

Figure 8 Thermal image of the reverse side

Figure 9 Shell surface temperature diagram

Figures 6 to 9 are thermal images of 12W products of Jinshengyang Company taken by Ti20. Through these pictures, we can not only clearly see the overall thermal distribution (the same temperature, the color used is consistent), but also use the software provided to analyze the temperature value of each component at this time. For example, the temperature values ​​of several components with relatively high temperatures are shown in Table 1.

Table 1 Power device loss table

Component Name Temperature Value (℃)

Transformer 88.3

Output filter inductor 84.6

Freewheeling diode 90.5

MOSFET 76.9

This method can intuitively analyze the temperature rise of each power device and the regional distribution of temperature. Through the overall temperature distribution diagram on the PCB board, we can adjust the distribution of different components according to the hot points. For example, the layout of components with high heat generation on the PCB board should be as far away as possible from temperature-sensitive components, such as electrolytic capacitors , etc., and there should be a certain distance between components with high heat generation, so as not to form new hot points.

(3) Thermocouple method. In practice, the power device of the product is not directly exposed to the air, but is potted or plastic-sealed in a metal or plastic shell. In this way, the temperature rise value of the component cannot be measured by the above two methods. At this time, we can use the thermocouple method. The specific steps are as follows: Use point temperature glue to fix the thermocouple on the shell close to the node of the power device, but do not touch the metal shell. Then package the semi-finished product together with the thermocouple , and measure T1 (temperature before operation) and T2 (temperature after thermal equilibrium) values ​​respectively. This method can directly measure the actual temperature value of the power device inside the module power supply, but because of the use of point temperature glue, the thermocouple and the shell (c1) of the power device form a new thermal resistance, and the glued thermocouple will conduct part of the heat of the shell (c1), eliminating the measurement error of the instrument, and the measured temperature value will be smaller than the true value.

These three temperature measurement methods each have their own advantages and disadvantages, and specific problems must be analyzed specifically during actual use. However, the direct measurement method is most helpful in improving the areas that are not well considered in the modeling analysis method.