Background

Recently, the miniaturization, high power density and high temperature of the usage environment for the automotive devices have advanced. And requests for resistors to conform the high temperature is increasing. Figure 1 is the derating curve based on the terminal part temperature and this is introduced to realize these requests for the surface mount resistors safely.
Rated terminal part temperature is the maximum terminal part temperature of the surface mount resistor at which the rated power may be applied continuously including the temperature rise by self heat generation.

The derating curves based on the terminal part temperature is already used in the metal plate type ultra-low resistance value resistors for current sensing (PS series etc…). It is because these resistors are used in sensing of large currents such as inverters and converters which the terminal part temperature rise irrelevantly from the ambient temperature because of the generated heat from the nearby switching elements or the large current applied to the copper pattern. This point of view was deployed to the general resistors as well.

Overview of the establishment of the derating curves based on ambient temperature

The idea of the traditional derating curve based on the ambient temperature defined in the JIS and IEC standard was established in the vacuum tube era, far back from the appearance of the surface mount resistors. There were no printed boards in those days and the cylindrical shaped resistors with lead wires were wired in the air to lug terminals as shown in Figure 2.

The Joule heat that is generated in the resistor is dissipated in three pathways regardless of the shape of the resistor. The first path is conduction to the connected parts such as the terminal. The second path is convection including the heat transfer to the atmosphere by natural convection and airflow. The third path is radiation by infrared. The larger the area connected to the resistor becomes, the larger the heat conduction will be. And the larger the surface area of the resistor becomes, larger the convection and radiation will be.

When the cylindrical shape resistor with lead wire is mounted on the lug terminal, the lead wire which is the heat path by conduction is thin and long so the heat resistance is large and the heat dissipation will be small. In the contrast, the heat dissipation ratio of the convection and radiation becomes large since the area will be large. It is determined in the simulation that 80% to 90% of the heat of the cylindrical shape resistor with lead wire is dissipated directly into the ambient air. The temperature of the resistor can be calculated by adding the ambient temperature and the temperature rise caused by the self-heating, so the ambient temperature will be most sufficient for the usage environment temperature standard of the resistors. This is why the derating curve based on ambient temperature was provided to the customers as the index of design.

Heat dissipation of surface mount resistors

The Figure3 shows the main heat path of the present surface mount resistors. The surface mount resistors have only small surface area so the convection and radiation will be small. On the other hand, they are connected to the print board patterns in a large area so the ratio by conduction will be very large. In estimation, the ratio by conduction through the terminal to the board takes over 90% even when the convection and radiation is presumed at the maximum level. Therefore, the control point of the surface mount resistor should be the temperature of the terminal part which is the connection point of the board and the main heat path.

Derating curve suitable for the surface mount resistor

The temperature of the resistor will increase the same ΔT from the standard terminal part temperature regardless of the ambient temperature when the same power is applied as shown in Figure 4. This is because there is hardly any heat dissipation from the resistor surface to the ambient air.

Even when the same power is applied to the surface mount resistor under the same ambient temperature, the temperature will not be the same if the printed board which the resistor is mounted is different. It is because the terminal part temperature changes. There is a possibility that the temperature of the resistors becomes higher than the endurance test of ambient temperature 70℃ which is defined in the JIS and IEC standard and implemented at our site when they are mounted closely to each other or there are other heat generating devices mounted on the board as shown in Figure 5.

The traditional derating curve based on the ambient temperature is established from the endurance test of ambient temperature 70℃. There will be no problem if the resistors are used with electrically and thermally sufficient margins, but it is inferable that the recent requests for miniaturization, high power density and high temperature usage environment would lead to reducing the margins at the device designing phase.

Using the derating curve based on the terminal part temperature will be a rational method to reduce the margin. We will provide the derating curve suitable for the surface mount resistors based on the tests implemented under the conditions that the terminal part temperature becomes the rated terminal part temperature (refer to Terms and definitions).

How to use the derating curve based on the terminal part temperature

Some examples of rational method to reduce the margins or reduce the number of resistors or replace with a smaller size will be shown. The prior conditions will be the following.　Be aware that the terminal part temperature does not always become 120℃ when the ambient temperature is 100℃

1. Ambient temperature of the board: 100℃
2. Terminal part temperature of the surface mount resistor: 120℃
3. Actual load power: 0.05W
4. Margins defined by the customer internal regulations: 50%

The necessary rated power for the resistor will be calculated from the conditions (1), (3), (4) using the derating curve based on the ambient temperature. The result is shown in Figure 6. For the RK73B products, one piece of 2A size, or two pieces of 1E size will be necessary.

However, when the resistor is selected from the conditions (2), (3), (4) using the derating curve based on the terminal part temperature, one piece of 1E size will be sufficient.

As seen above, the number of resistors and the mounting area can be reasonably reduced by using the derating curve based on the terminal part temperature and this will lead to cost saving.

Derating curve suitable for the surface mount resistor

As shown in Table 1, for the surface mount resistors, there are products that have 2 rated powers for the same type in the rating column. The high rated power is basically available and applicable only to boards with adequate heat dissipation design for example multilayer boards, DCB (direct copper bonding) boards and single layer boards with wide heat dissipation area land. Therefore, the horizontal axis of the derating curve for high rated power is only defined with the terminal part temperature and please be careful that the conventional derating curve defined by the ambient temperature cannot be used in this case. For these products, "-" will be shown in the rated ambient temperature column which means "Not Applicable". In addition, we implement load life tests for the products with high rated power by using a test board that can specially control the terminal part temperature.

【Table1. Rating column of products with 2 rated power】

In the case of Table1, there will be 3 derating curves as shown from Fig.8 to Fig.10.

How to use each derating curve is shown as the following.

When 0.25W is the rated power

When the terminal part temperature can be measured:
The derating curve in Figure 8 can be applicable and it can be used with rated power 0.25W up to terminal part temperature 125°C. The derating curve with the horizontal axis based on the terminal part temperature supersedes the conventional derating curve with the horizontal axis based on the ambient temperature.
Therefore, even when the ambient temperature exceeds 100°C, it can be used with rated power 0.25W as long as the terminal part temperature is below 125°C.
When the terminal part temperature is not measured and only the ambient temperature is measured:
The product may be used by derating the load power from the ambient temperature 70°C according to the conventional derating curve shown in Figure 9. However, as mentioned in the past descriptions, the temperature of the resistor differs according to the wiring patterns and heat generating components nearby, even when the ambient temperature is the same, so it is not a derating method with good precision.

When 0.5W is the rated power

Managing the terminal part temperature is the requirement to apply the rated power 0.5W. Only the derating curve with the horizontal axis based on the terminal part temperature as shown in Fig.10 can be used but it can assure up to the high power. The product can be used with 0.5W if the terminal part temperature is below 100℃.