‘Cool’ design for power supplies

Modern mains power supplies are extremely compact, but dissipate a substantial amount of power as heat while operating. This very often leads to overheating of internal components, which in turn has an adverse effect on reliability and the lifetime of the power supply. Typically, the heat dissipation is so high that, to keep the power supply components at a safe operating temperature, the desi...
By Dr. Werner Woelfle, TracoPower August 1, 2008

Modern mains power supplies are extremely compact, but dissipate a substantial amount of power as heat while operating. This very often leads to overheating of internal components, which in turn has an adverse effect on reliability and the lifetime of the power supply.

Typically, the heat dissipation is so high that, to keep the power supply components at a safe operating temperature, the design engineer has no other choice but to use a fan to cool the components. The use of mechanical fans, however, often represents a technically unsatisfactory solution because of their relatively low reliability and irritating noise.

Group 1: Top and bottom thermal images of the 65 W flyback power supply. Source: TracoPower
Group 1: Top and bottom thermal images of the 65 W flyback power supply.

Flyback vs. half-bridge

The most used topology for switch-mode power supplies is the flyback converter. The rectified input voltage in a flyback converter is connected to the primary coil of the inductor during the on phase of the switching transistor.

While the transistor is switched off, the stored inductor current flows toward the output from the secondary coil to the output capacitor via the output rectifier diode. The output diode is idle while the input transistor is switched on, and the switching transistor is idle while the output rectifier diode is switched on. This leads to high effective currents in all devices.

This topology is cost-efficient, but it incorporates such disadvantages as high peak and RMS (root mean square) currents in power semiconductors, transformers, and capacitors. This leads to high temperatures in all circuit components, including the printed circuit board.

Poor energy conversion efficiency, as a result of power losses, causes high temperature stress to the electronic components, leading to a significantly higher failure rate of the power supplies. These failures are not evident during the initial testing and performance verification of the power supply within the application, but become very marked in field use over time.

Unlike the flyback converter, the half-bridge converter comprises two primary switching transistors, two output diodes, and a separate storage inductor on the output side of the transformer. The switching transistors connect the transformer alternatively to the rectified mains voltage. Therefore, the energy is transferred through the transformer for a much longer duty cycle, which leads to much lower current pulses in all components.

Temperature distribution

Thermal pictures taken of a typical 65 Watt flyback power supply and a 100 Watt half-bridge power supply (image groups 1 and 2), both with dimensions of 2 x 4 x 1.2 inch, show the temperature distribution between the two power supplies.

In image group 1, large areas on the component side of the 65W flyback power supply are very hot. In particular, semiconductors mounted on heat sinks near the mains input side and on the output side appear white on the image, indicating that they have already reached around +100 °C at ambient room temperature. It is easy to imagine what will happen if such a power supply unit were used in an application with little installation space and low air circulation. The components would inevitably become overheated.

In image group 2, areas with extremely high operating temperatures on the 100 W half-bridge power supply are visible, particularly in the area around the transformer. This PCB area is intensely heated by the surrounding semiconductor devices, and the printed circuit board is nearly +100 °C at ambient room temperature. In the region around the output diodes, the temperature on the PCB is also very high.

Group 2: Top and bottom thermal images of the 1100W half-bridge power supply. Source: TracoPower
Group 2: Top and bottom thermal images of the 1100W half-bridge power supply.

On both the component side and the solder side, the 65 W flyback power supply unit is considerably hotter than the 100 W unit under same load conditions. The 100 W power supply remains not only significantly cooler at 65 W output power, but can deliver 100 W output power in the same 2 x 4 in. format without any overheated components.

The calculated MTBF (mean time between failure) value for a power supply unit with a flyback converter, in accordance with IEC 61709 (taking into account stress factors for voltage, current, and temperature) is approximately 200,000 hours. In contrast, the power supply unit with a half-bridge converter, with the same physical volume and load conditions, provides a 5 to 10 times better MTBF.

Author Information
Dr. Werner Woelfle is head of engineering for TracoPower. TracoPower offers the Top-100 power supply unit based on a resonant pulsed half-bridge circuit.