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Considerations when designing power management circuitry

Posted: 19 Oct 2009 ?? ?Print Version ?Bookmark and Share

Keywords:power management design? power circuitry? power inductor?

A highly efficient power management circuitry not only improves battery life and reduces the total energy requirement, but also ensures that power dissipated in the circuitry doesn't cause excessive temperature rise and eventual device failure. However, efficiency has its limits and consequently, the more output power required results in more power dissipated within the power supply and the associated external components. As a result, even with highly efficient devices, proper component selection and PCB design are critical in ensuring the junction temperatures and component temperatures do not exceed their maximum limits.

The focus of this article is to highlight a switch mode power supply and a typical power inductor and their performance during high temperature conditions. Additionally, methods for measuring thermal resistance and thermal capacitance to ambient are discussed. Examples include an inductive boost with a high-current white LED (WLED) current source and a typical power inductor.

Excessive heat from high ambient temperatures or from internal power dissipation can alter the characteristics of electronic components and cause them to shutdown, operate outside specified operating ranges or even fail. Power management devices (and their associated circuitry) run into this problem quite frequently since any power loss between the input and load results in device heating. This heat must be dissipated away from the device, either into the PCB and nearby components, or the surrounding air. Even in switching power supplies, with traditionally high efficiency, heat must be accounted for when designing the PCB and choosing external components.

Before investigating thermal considerations when designing power management circuitry, a basic understanding of heat transfer is helpful. First, heat is the energy transferred between two systems due to the temperature difference between them. Heat transfer takes place via three mechanisms: conduction, convection and radiation.

Conduction occurs when a device with a high temperature makes contact with a device of low temperature. The high vibration amplitudes of high temperature atoms collide with atoms of the low temperature material and increase the kinetic energy of the low temperature material. This increase in kinetic energy results in the increase in temperature of the high temperature material and a decrease in temperature of the low temperature material.

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