Introduction
Hello, my name is Taro Washimiya, and I am a technical support engineer for Altera® FPGA products.
When you are selecting a device, I'm sure there are cases where you need to specify temperature characteristics. Especially in the case of Altera® FPGAs, you need to know the "Junction Temperature", but do you understand what temperature it refers to?
In this article, we will briefly explain the relationship between the ambient temperature and the junction temperature of semiconductors.
Are ambient temperature and junction temperature the same?
Junction temperature and ambient temperature are two completely different things. If you think that temperature characteristics are about ambient temperature, you are getting a funny story.
BGA Package and Temperature at Various Locations
- Ambient Temperature (Ambient Temperature/TA)
usually refers to the ambient air temperature at rest. (Sometimes it is also referred to as "local ambient air temperature around the device.) - Case Temperature (TC)
refers to the package temperature at the device surface. - Junction Temperature (TJ)
Refers to the temperature of the die surface inside the device package. It is the junction temperature specified based on the device's recommended operating conditions, which may be commercial, industrial, or automotive.- Commercial temperature range (Commercial): 0 to 85°C
- Industrial temperature range: -40 to 100°C
- Automotive temperature range: -40 to 125°C (-40 to 125°F)
What is the thermal resistance required to calculate junction temperature?
The next keyword to keep in mind is "thermal resistance.
When a device operates, heat generated on the die propagates to the surrounding area through the case (package). However, this heat is not completely transferred, and some of it is trapped inside the device. This causes the temperature of the case to rise.
The smaller the value, the better the heat transfer, the less heat is trapped inside the device, and the lower the internal temperature rises.
BGA Package and Thermal Resistance
The following three types of thermal resistance exist in devices. The unit is °C/W. (θ: lower case Greek letter theta)
- θJC: Thermal resistance when transferred from the die to the case
- θCA: Thermal resistance when transmitted from the case to the device surroundings
- θJA: Thermal resistance when transmitted from the die to the device surroundings
The relationship between these three thermal resistances is as follows.
θJA = θJC + θCA
Obtaining Thermal Resistance Information
Device manufacturers typically provide θJA and θJC values, which can be found in the device documentation. Information on the thermal resistance of Altera® FPGAs can be found here.
FPGA Package and Thermal Information for Altera® FPGAs
Step 1: Select the target FPGA
How to check the thermal resistance of a device Step 1.
Step 2: Click on Thermal
How to check the thermal resistance of a device Step 2
Step 3: Confirm the value of thermal resistance from device and package
How to check the thermal resistance of the device Step 3.
What is the temperature difference required to calculate the junction temperature?
Next, let's move on to the temperature difference. The temperature difference that occurs when heat is transferred through thermal resistance depends on the power consumption. The relational equation is as follows
Temperature difference [°C] = Thermal resistance [°C/W] x Power consumption [W]
This is where power consumption comes in. If the power consumption has not been verified on the actual device, an estimated value can be used. PowerPlay Early Power Estimator (EPE), a power estimation tool for Altera® FPGAs, can estimate power consumption based on FPGA resource information, as well as junction temperature and acceptable ambient temperature. This calculation includes the information described here.
Temperature Difference Examples
Let's take a simple example to better understand.
The temperature difference ΔTJA between the die surface and the device ambient for a device with θJA of 50 °C/W and power consumption of 600 mW is
ΔTJA = 50 × 0.6 = 30 °C
If the ambient temperature at this time was 25 °C, the junction temperature TJ would be
TJ = TA + ΔTJA = 25 + 30 = 55 [°C].
Do you understand?
Example: Temperature difference
Summary
Did you get an idea of the relationship between ambient temperature and junction temperature?
Many of you may have been uncomfortable with device heat, but now you know that it is not that difficult.
Please take a look at our articles onFPGA power consumption and how to estimate FPGA power consumption.
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