Things to keep in mind when designing power management circuitry
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 effcient 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.
In convection, heat transfer occurs from the air surrounding the device. In natural convection, an object heats the surrounding air, which expands as it is heated, creating a vacuum that causes cool air to replace heated air.
This results in a cyclic air flow that continually transfers the heat of the device to the ambient temperature. On the other hand, forced convection would, for example, be a fan intentionally blowing cool air across the device, forcing the displacement of the warm air.
Radiation occurs when electromagnetic waves (thermal radiation) are emitted from an object to the surrounding environment. Radiated heat does not need a medium (heat can radiate through empty space). In PCBs, the primary method of heat transfer is conduction and to a lesser degree convection.
The mathematical model for conducted heat transfer is given by the equation
H = K x A x (TH - TL)/d,
where H is the rate of heat transfer in J/s; K is the thermal conductivity of the material; A is the area; (TH - TL) is the temperature difference; and d is the distance.
Heat conducts faster as contact area between interfaces increases, the temperature difference increases, or the length between interfaces decreases.
Heat transfer can be made analogous to an electrical circuit by equating the power (source of heat or H term in the previous equation) to a current source, the difference in temperature between the high temperature and low temperature devices, a voltage drop and the (K x A) /d term as a thermal conductivity, or the inverse d/ (K x A) as a thermal resistance in °C/W.
Often thermal resistance is given the symbol theta or just RA-B, where A and B are the two devices from which heat transfer takes place. Re-writing the rate of heat transfer equation using the electrical analogy results in P>sub>D = (TH - TL) /RH-L.
This analogy can be carried one step further to describe another thermal attribute of a device called thermal capacitance. As thermal resistance is analogous to electrical resistance, thermal capacitance (CT with units in J/°C) is analogous to electrical capacitance.
The electrical analogy of heat transfer using the thermal capacitance in parallel with the thermal resistance gives a thermal impedance (ZT). The simplified RC model of conducted heat transfer is shown in Figure 1 below.
 |
| Figure 1: Shown is a simplified thermal impedance model. |
The power source is modeled as a current source and the thermal impedance is CT in parallel with RT. In electrical circuits, every thermal interface has a thermal impedance. The thermal impedance varies with material, geometry, size and orientation.
The thermal impedance of a system (or circuit) has an overall thermal impedance to the ambient temperature, which can be broken up into parallel and series combinations of the thermal impedances for each component in the circuit.
For instance, in a semiconductor device, the total thermal impedance between the die (also commonly called the junction) to the surrounding air (called the thermal impedance), from junction to ambient (ZJ-A), would be the sum of the individual thermal impedances of each separate material in the structure.
Consider a discrete MOSFET mounted on a PCB. The steady state thermal impedance (or thermal resistance RJ-A) is the sum of the thermal resistances from the junction to the device case (RJ-C), the case to the heat sink (RC-S) and the heat sink to the air (RS):
RJ-C = RJ-C + RC_S + RS-A
Additionally, there can be a parallel heat path from the MOSFET junction through the case and into the PCB, and then from the PCB to ambient temperature.
Normally the junction to case would be given by the semiconductor manufacturer. The RC-S and RS-A, on the other hand, are mainly dependent on the properties of the heat sink and PCB. Many factors influence the thermal resistances RC-A or RC-S, including the number of PCB layers, number of via's to secondary planes, proximity to other devices and rate of air flow.
Often, RJ-A is listed in device datasheets, but this number is given under specific test board conditions and would only be applicable for comparison between devices measured under the same conditions.
The thermal resistance (RJA) is an important parameter for electronic components since it is a measure of how much heat a device can dissipate, based on ambient conditions and PCB layout. In other words, RJ-A will help us estimate the operating junction temperature, based on ambient conditions and power dissipation.
Loading comments... Write a comment