Peak power vs average power – How to choose the right converter

An important figure here is P_P – the peak output power. The RAC05-SK/277 has a nominal output power of 5W although it can actually deliver 6W without the overload protection being triggered.

For overloads less than 120% nominal load, the limiting factor is the internal temperature of the components inside the converter. If the converter is given enough time to cool down in between overload events, it can then withstand multiple or cyclic overloads and still deliver a stable output voltage.

For very short, very severe overload events, an external output capacitor can be installed to provide the required peak current and stop the converter from going into overload protection. This is useful for applications such as wireless-connected microcontrollers wherein the current peaks during the transmission bursts are very short, high power events, but the average power consumption is much lower (Figure 4). In this case, the power supply can be dimensioned to provide the average power rather than the peak power.


Up to this point, we have considered AC/DC converters, but DC/DC converters can also be given the same analysis. The difference between them is that DC/DC converters are designed to be continuously operated in the 80–100% output power range, and their efficiency curve drops even more quickly if used at much lower loads. Therefore, operating them at low output current does not always mean that they run much cooler. In general, using a 10W DC/DC converter at a 5W load should be avoided, unless there is no other way to meet the required operating temperature range specification except by power derating.

For example, the RS12-Z series delivers an impressive 12W of isolated power in a compact SIP8 case (21.8mm x 9.6mm). With natural convection cooling and a nominal 24V supply, the RS12-Z converter can operate at full power at up to 75°C, while by derating the load to 50%, it can operate in the industrial temperature range of -40°C to +85°C. Thus, halving the load provides only an +10°C additional ambient temperature range because the converter is no longer operating at its highest efficiency. Nevertheless, even 6W in a SIP8 case operating in the full industrial temperature range with only free-air convection cooling is still significantly better than the competition, which have to resort to forced air cooling to deliver the same output power.

Many low-cost AC/DC and DC/DC converters have a very simple output over-current protection circuit based on detecting the voltage drop across an internal shunt resistor (Figure 5).

Such internal protection circuits, while simple to implement and highly effective as a short circuit protection, have a wide overcurrent limit variation as the trigger point is heavily dependent on the tolerance of the shunt resistor and the VBE threshold voltage of the NPN transistor. The component values are therefore determined so that the overcurrent protection does not nuisance-trigger above the entire ambient operating temperature range at 100% load. This gives the converter a very wide overload tolerance at room temperature – typically up to 140% of the nominal output power. Such converters can be reliably operated at continuous full load and still have significant headroom to cope with any overload conditions.

An exception to this generalisation are the DC/DC switching regulators that typically operate at a higher switching frequency to minimise the size of the components (for a buck converter, increasing the frequency reduces both the output inductor AND the output capacitor) and thus have less stored internal power reserves if subjected to sudden peak overload conditions. The shunt resistor is typically integrated onto the same wafer die as the main controller IC and has a much tighter resistor value tolerance, giving less variation in the overcurrent limit.

Additionally, most switching regulator controllers also use a cycle-by-cycle current limit monitor based on a precise comparator output rather than relying on an imprecise Vbe junction threshold voltage. As a result, they will shut down almost instantly if the overcurrent or short-circuit protection limit is reached. Therefore, DC/DC switching regulators should be dimensioned to handle the worst-case peak load conditions rather than the average load.