Mitigating High Junction Temperature to Optimize PCB Performance

October 3, 2018 Altium Designer

Thermal image of a circuit board with scale on the side

 

I consider myself a great multitasker, especially when it comes to preparing meals for my son. I’ll have fish frying in one pan, soup boiling in the other, and potatoes in the oven. Things usually go as well as planned except on rare occasions when one of the dishes burns while I’m absorbed in other tasks. Fortunately, these temperature problems won’t cost me much more than a few dollars and some time.

 

In electronics, your hardware can literally heat up if you fail to correctly calculate the potential rise in the junction temperature of specific components. A worse fate awaits your PCB if you’re totally clueless about what junction temperature is, as I was when I was a newbie designer.

Junction Temperature and Why it Matters

Most electronic components are made of a silicon die. The silicon die is covered in a case, which is what you usually solder onto the PCB. The junction temperature indicates the temperature of the silicon die when the component is powered on and current flows through it.

 

There are good reasons for you to be concerned about the junction temperature of the components. Besides scalding clueless technicians troubleshooting or servicing the PCB, a skyrocketing temperature that breaches the maximum junction temperature can compromise the reliability of components.

 

Don’t make your design susceptible thermal mismanagement due to measurement misdoings or unaccounted for voltage and power necessities. Your device, whether it’s using LEDs or a diode or a p-n junction ought to have thermal control and regulation. So how do you move your device into production without getting your assembly team off your wavelength?

How to Calculate Junction Temperature From the Datasheet

If you’re dealing with high current or current levels known to dissipate a substantial amount of power, you can estimate the increase in the junction temperature from the datasheet. The datasheet specifies the maximum junction temperature allowed for each component. It also describes the related temperature coefficients that can be used to project the increase in temperature.

 

The first step in getting an accurate calculation is figuring out the power that is being dissipated by the component. For instance, power dissipation for a linear voltage regulator is derived from the difference between the input and output voltage multiplied by the current passing through.

 

Calculating junction temperature

Be sure of the junction temperature before you start designing.

 

When the value of the dissipated power is obtained, look for the junction-to-ambient temperature coefficient. It is usually expressed in °C/W which means the increase in temperature for every single Watt dissipated. To get the exact temperature measurement of the component, you’ll need to add the calculated temperature with the ambient temperature of the PCB.

 

For instance, the LM7805 by Texas Instruments has a junction-to-ambiance of 23.9 °C/W for the TO-220 package. If the component dissipates 1 Watt in a room ambiance of 27 °C, the temperature of the component will increase to 50.9°C.

Tips to Deal With High Junction Temperature

It’s unavoidable for certain components to heat up when the hardware is being used. Calculating the junction temperature gives you a good picture of how hot it can get. As a hardware designer, you do have some control over preventing the temperature from getting out of control. Here are some measures you can take to lower the heat:

Heat Sink

Some components are available in packages like TO-220 and can be screwed onto an external heat sink. In your design, these components are placed vertically and spaces are allocated for the heatsink that is installed.

 

Heat sink for overheating components

Handy for reducing the heat of a component.

 

 

 

Air Flow

Ventilation is an important feature that you must communicate with the product designer or mechanical engineer responsible for the enclosure for the PCB. It prevents the heat from building up at a single spot.

Keep Critical Components Away From The Heat

It is commonly stated that heat of 10 °C shortens the lifespan of components by half. To avoid this catastrophe, you may want to keep other components away from the heated zone of your PCB.

Thermal Vias

Using (Lara is a cost-efficient way to dissipate excessive heat coupled from the component to the PCB. These are a series of vias placed in a matrix-like pattern that enables the heat to disperse to the opposite side of the PCB. Often, the vias are placed on conductive copper planes for better dissipation.

 

Don’t find resistance from your design software when mitigating high junction temperatures. Let your CAD tools take the measurement for you, orchestrate the phosphor, place the LED and diode, and let you achieve true thermal management. With Altium Designer, thermal vias can be placed in a matter of clicks within an easy-to-integrate schematic-to-layout unified design environment.  

 

While there are various ways to address high junction temperature, lowering the chances of overheating a PCB requires foresight. With great PCB layout software like CircuitStudio, you can mitigate heat-related issues and optimize your design. Need more help in dealing with high junction temperature? Talk to an Altium expert today.

About the Author

Altium Designer

PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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