# How to evaluate chip temperature rise and actual testing

25/08/2021 Seektronics

Nowadays, devices are becoming more and more compact in design, which means that the thermal power consumption per unit of volume becomes larger. But the biggest problem with electronic products is temperature. Most electronics get hot over time, and batteries become less durable the longer they are used.

High temperatures degrade the insulation properties of electronic products, components are gradually damaged and materials become aged. Generally speaking, high temperatures reduce resistance, which is why their lifespan becomes shorter. Other materials such as transformers in electronic products are affected by high temperatures and their performance decreases, mechanical strength decreases and electrode currents increase, causing temperatures to rise further and eventually rendering components ineffective.

LDO Chip Products

The LDO is a device that is used in many of our consumer electronics and other devices, so the analysis related to thermal calculations is done for this device.

Junction temperature estimation

Please note that we have used the LM1117 chip, in a SOT223 package, for our calculations. (The package affects the size of the thermal resistance of the LDO, which causes the device to generate a temperature rise).

RθJA: This refers to the magnitude of the thermal resistance from the wafer inside the chip to the air；

RθJC(top): This refers to the magnitude of the thermal resistance from the internal wafer of the chip to the upper surface of the device；

Calculation
Our estimates are made using RθJA as follows：
TJ-TA=RθJA* PD

In the above equation：
TJ is the chip junction temperature；
TA is the ambient temperature
PD is power consumption

For example, the input of the LDO is 9V, the output is 5V, the load current size is 200mA, the ambient temperature is 25 degrees Celsius, then the chip junction temperature at this time is estimated to be?

(1) Chip power consumption PD = (9-5) * 200 = 0.8w regardless of Iq current

(2)TJ = TA + RθJA*PD = 25 + 61.6 * 0.8 = 74.28 degrees Celsius.
Based on this, it is possible to calculate what the maximum power consumption of the chip is at 125 degrees Celsius or 150 degrees Celsius.

Junction temperature accurate test

Our estimate is calculated using RθJC as follows：
TJ-TC=RθJC* PD
In the above equation：
TJ is the chip junction temperature；
TC is the surface temperature on the chip
PD is power consumption

For example, the input of the LDO is 9V, the output is 5V, the load current size is 200mA, the ambient temperature is 25 degrees Celsius, then the chip junction temperature at this time is estimated to be?

(1) Chip power consumption PD = (9-5) * 200 = 0.8w regardless of Iq current

(2) TJ = TC + RθJC*PD = TC + 61.6 * 0.8 = junction temperature at current power.

Based on this, the current junction temperature of the chip can be calculated by using a thermocouple or infrared tester to test the surface temperature TC on the chip.

General Description

The need to calculate the junction temperature accurately requires the use of the upper surface temperature of the chip, as the error in calculating or estimating the chip junction temperature using air is high due to air disturbance, etc.