All heat produced by the electronics within a product is wasted energy, unless of course the product is a heater! Electronics design engineers should be aware of the causes of waste heat generation and the lifetime issues this can cause.
Depending upon the application, the useful work of electronic circuits is to generate sound, light, wired and wireless communications, motion and perform calculations and measurements. However, no process is 100% efficient. Energy used that doesn’t contribute to the desired output is converted to other forms of energy, most commonly heat.
There are many sources of heat in electronics. The more common ones come from ohmic heating in resistive elements such as resistors, inductors, printed circuit board traces and transistors. These can be discrete components on a printed circuit board or elements within an integrated circuit (IC).
Resistors are a fixed value and their power consumption is related to the current through them (P=I2R). The resistance of a MOSFET (metal on silicon field effect transistor) changes with applied gate-source voltage. This can have a wide range of channel resistances. When used in high speed switching circuits the MOSFETs are repeatedly switched from high to low resistance but each time transition through a range of resistances. Slower transition speeds high switching frequencies result in the MOSFET being resistive and burning more power and getting hot.
Heat can be bad for electronic components
Heat can be bad for electronic components, for HALT (Highly Accelerated Life Testing), elevated heat is one of the environmental stresses placed upon components and products together with temperature cycling, random vibrations, power cycling and power margining.
Within aluminium electrolytic capacitors, temperature has the largest impact on life. A good rule of thumb is:
the lifetime halves every 10 degrees Celsius increase in temperature.
This relationship follows a chemical reaction formula known as Arrhenius Law of Chemical Activity.
Elevated temperatures can also have a significant effect on batteries. Batteries are typically specified to achieve an optimum service life when used at or slightly below 20°C. Operating batteries above this can improve their short-term performance at the expense of cycle life. Operating at 30°C instead of 20°C can reduce the cycle life by 20% and at 45°C this is only half of that expected at 20°C.
As temperatures rise further there can be problems within integrated circuits due to electromigration. This is less of a problem with modern devices but is still a physical phenomenon that can degrade devices and reduce their operating life.
When temperatures rise even further:
- melting or de-soldering of components
- damaging printing circuit boards
- melting internal bond wires
can occur. This can happen very quickly if thermal management is not considered. Such failures instantly stop a product working.
Temperature can also affect high speed signals. The electrical parameters of conductors, insulators and semiconductors changes with temperature. This can change the timing of critical signals, resulting in units that work at ambient temperature, but which begin to fail at higher temperatures. This can be a cause of the classic “no fault found” loop where the product heats up over time when used in the field, is returned under warranty as faulty yet when tested after being posted back to the manufacturer. When the unit arrives back at the manufacturer it has cooled and is therefore fully working when tested by technicians. This unit may then be used as a warranty replacement and sent out to another customer only for the same thing to reoccur a few weeks later.
It is therefore important to consider power dissipation and thermal management during electronics design and pcb layout.
Using linear regulators where there is a significant difference between input and output voltage can cause a large power dissipation within the regulator through the main transistor. A 12V input, 5V output linear regulator, with a load of just 500mA, can get very hot unless it has adequate heatsinking as it is dissipating 3.5 Watts in a package with only a few millimetres of area. A switched mode power supply may be a better choice, followed by a low dropout regulator if the supply needs to be very clean for audio or radio purposes.
Devices often feature a dedicated thermal ground pad which is used to connect to an area of copper within the printed circuit board to spread the heat and act as a heatsink.
Testing your design for wasted energy
It is imperative that when the design is being tested during electronics design proving activities that the board is checked for excessive heat production. As heat builds up over time this test is best performed repeatedly until the board temperatures have stabilised. One excellent way of efficiently and safely performing this test is to monitor the board(s) using a thermal camera. This allows the full temperature profile to be observed and allows the test to be aborted if any concerning hot spots are observed.
When this has been performed and some confidence gained it is wise to repeat the tests with the power supplies loaded to their maximum rated loads to ensure the board remains within permitted temperature ranges when fully loaded.
The final activity for the electronics engineer to perform is to subtract the highest temperature achieved on the board from the ambient temperature to understand the temperature rise above ambient. That temperature rise is then added on to the products maximum operating temperature to predict the highest temperature that will occur. This then needs to be checked against component and material datasheets to ensure all are operating within their rated temperature range.
If your electronics are producing wasted energy in the form of heat, or if you are concerned that the reliability or lifetime of your electronics are unnecessarily shortened, get in touch and let’s see how we can resolve this.