Power Supply design calls for advanced thermal engineering

Stacey Vinther Uncategorized

When it comes to reliability in electronics heat is the enemy, and that is as true of power supplies as the rest of the system. The Arrhenius equation shows how much excess thermal energy affects lifetime: each 10°C rise in temperature reduces the average lifetime of an electronic device roughly by half.

Anything you can do to mitigate heat production and to channel it away from the system will pay off in product lifetime. So the power supply and how it is designed into the system plays an important role in minimising thermal problems.

Thermal mitigation strategies range from system-level design and integration techniques down to the circuit topology used by the power supply itself. The topology matters because it feeds directly into heat generation. A 250W power supply that operates at full load with an efficiency of 85 per cent will dissipate more than 44W in heat. A power supply just 5 per cent more efficient will waste 16W less.

One of the most effective ways to improve efficiency is to move to resonant or quasi-resonant topologies. Resonant topologies take advantage of the resonant filtering effect of passive components introduced into the circuit to smooth out peaks and troughs in the supplied current. The resonance makes it possible to switch active components when little or no current or voltage is present, which not only reduces stress but also switching losses.

There is no one-size-fits-all answer for deploying resonant topologies: it depends on the application at hand. Zero-current switching (ZCS), for example, tends to be used in high-power applications. At the start of each cycle, charge flows into a capacitor in the resonant filter and its voltage rises towards a maximum. The circuit is then switched to allow the energy stored in the capacitor to transfer to an inductor in the output stage until the current drops to zero and the switch can be turned off again ready for the next cycle.

During the switching phase, the voltage can change rapidly and this can cause coupling to gate-drive circuitry through the power transistor’s Miller capacitance, which slows down the switching process. However, high-power circuits based on insulated-gate bipolar transistors (IGBTs) rather than field-effect transistors (FETs) suffer more from tail currents when switched off – which favours the ZCS strategy.

A zero-voltage switching (ZVS) supply rearranges the passive components, better suiting FET-based circuits. The topology allows current to flow first into an inductor then, when the switch feeding the inductor is turned off, the energy flows into the resonant capacitor. The switching events occur as the voltage approaches zero.

Although the use of high-efficiency power supply design reduces the effect, some heating is inevitable and needs to be directed out of the system. In most applications, convection conveys much of the excess thermal energy. Forced-air cooling – usually driven by fans – helps the convection process but it typically adds noise and take up more space within the design. Without forced-air cooling, careful design is required through the use of heatsinks and component placement to ensure good convection performance.

Conduction provides a secondary avenue for heat removal and is a primary consideration in highly space-constrained designs. The high copper content of a PCB, and the metal of an enclosure, can both be harnessed to provide good paths for heat flow.

Thermal simulation provides much-needed information on how heat will build up in a system design and can be directed away from critical components. Using techniques derived from computational fluid dynamics, the power supply and its components are represented as a 3D mesh of elements that generate and absorb heat from the PCB or from the air.

The job of mesh generation and simulation requires expertise and experience. The mesh needs to be tuned to concentrate on critical areas of the design and on components that are most likely to affect thermal behaviour. It is also important to perform calibration to real-world conditions to make sure the simulation is accurate. But the effort pays off in its ability to home in on critical parts of the design, such as components that are known to suffer more from high temperatures, such as electrolytic capacitors.

Conversely, heat can be used productively in some parts of the power supply. In many diodes that handle high power levels the forward resistance drops with an increase in temperature. If heat is directed over these diodes, efficiency will go up. Channelling heat away from capacitors will improve lifetime.

Heating effects also play into component selection. The effective series resistance has an impact on power-handling capacitors, leading to the production of excess waste heat. Those parts of the circuit will favour low-ESR capacitors or using multiple capacitors to reduce the overall ESR. Armed with information from thermal simulation and accurate models of heat generation within the supply, an engineering team can perform intelligent tradeoffs.

Simulation cannot account for every eventuality. But reliability can still be maintained during thermal excursions caused by external factors. Digital monitoring and control provide a way to ensure that the supply continues to operate safely and reliably. PMbus standard gives power-supply designers the ability to make power supplies smarter. Individual components and subsystems can report their real-time status to a system manager. The system manager can respond by altering output levels or perform a safe shutdown process if it senses dangerous conditions emerging. By providing active control over the power supply, the PMbus is a suitable complement to the digital control techniques now used in high-efficiency power supplies.

Guided by advanced techniques such as thermal simulation, power engineers are making use of advanced topologies and smart thermal design to deliver cost-effective, compact and efficient power-supply subsystems that promise longer operational life. These tools and techniques require experience but that expertise is available from specialist, outsourced power-supply design and manufacturing providers.

To find out more about Stadium Group’s power products and technology visit www.stontronics.com