Many engineers tend to choose a power supply with sufficient “surplus”. For example, if an application consumes 5W of power, then they will choose a 10W power supply to cope with the worst case. The reason behind this is that in addition to the need for a certain safety factor to obtain high reliability, it is also necessary to ensure that there is enough power supply capacity to cope with the additional load when adding functions to the application circuit in the future. These are powerful arguments that are difficult to refute, but they are not always the most effective method for power applications.

Take the typical efficiency/load diagram of a 10W AC/DC power supply (such as RAC10-12SK/277) as an example:

Figure 1: Efficiency/load graph of 10W AC/DC converter

The efficiency graph shows that it remains flat when the load exceeds 20% and performs well. But at 50% load (5W), the efficiency varies from 77% to 81% depending on the power supply voltage (Figure 1, orange line). At 100% load, the efficiency remains unchanged at 83% regardless of the input voltage (Figure 1, blue line). This difference may not seem obvious, but an efficiency of 77% means that 30% of the supplied energy is wasted as heat, while an efficiency of 83% means that only 20% is wasted, which greatly reduces the power dissipation. If the power supply is replaced with an equivalent 5W power supply, such as RAC05-12SK/277, the efficiency will remain at 83% regardless of the power supply voltage (Figure 2).

Figure 2. Efficiency/load graph of 5W AC/DC converter

In addition, it not only works more efficiently, the size of 5W power supply is only half of 10W, but it is also cheaper: this is a win-win!

Peak power and average power

The key word here is “worst case”. During normal operation, the load is usually lower than the power demand. If the converter works continuously under the worst load conditions, it can still easily handle such power levels, but the actual load will not be that high. This provides some “thermal headroom” for the converter to handle short-term peak overloads that are higher than continuous workloads.

For example, the RAC05-SK/277 specification provides the calculation formula for peak load capacity (Figure 3):

Figure 3. Peak load calculation formula (from the data sheet)

An important value here is PP-peak output power. The nominal output power of RAC05-SK/277 is 5W, but in fact it can provide 6W without triggering the overload protection. If the overload is less than 120% of the nominal load, the temperature of the device in the converter is the limiting factor. If the converter has enough time to cool down after the overload, it can withstand multiple overloads or cyclic overloads while continuing to provide a stable output voltage.

To cope with a very short and severe overload, an external output capacitor can be installed to provide the required peak current and prevent the converter from starting the overload protection. This is very practical for applications such as wirelessly connected microcontrollers. Although the current peak during the transmission burst occurs for a short time and high, the average power consumption is much lower (Figure 4). In this case, the power supply can be designed to provide average power rather than peak power.

Figure 4: Typical current consumption curve supporting WLAN microcontroller

We have discussed AC/DC converters so far, but DC/DC converters can also be analyzed in the same way. The difference between them is that the DC/DC converters are designed for continuous operation in the range of 80-100% of the output power. Their efficiency curves will drop faster at low loads, so low output currents are not Represents low operating temperature. In general, you should avoid using a 10W DC/DC converter with a 5W load, unless there is no other way than derating to meet the required operating temperature range. For example, the RS12-Z series uses a compact SIP8 housing (21.8mm x 9.6mm) to provide excellent 12W isolation power.

The RS12-Z converter uses natural convection cooling and a nominal 24V power supply to work at full power at temperatures up to 75°C, and the operating temperature is -40°C to +85°C when the load is derated by 50%. The load is halved but only an ambient temperature range of +10°C is provided. This is because the converter no longer works at maximum efficiency. Even so, the SIP8 packaged 6W converter, which only uses natural convection cooling to work in the full industrial temperature range, is still significantly better than the competition, because the latter must resort to forced air cooling to provide the same output power.

Overcurrent protection

Many low-cost AC/DC and DC/DC converters have very basic output overcurrent protection circuits to detect the voltage drop of the internal current sampling resistor (Figure 5).

Figure 5: Basic overcurrent protection. When the voltage across the current sampling resistor exceeds 0.7V, the NPN Transistor is turned on and the gate drive of the power FET is turned off.

Although this protection circuit is simple and very effective as a short-circuit protection, the trigger point is largely determined by the tolerance of the current sampling resistor and the VBE threshold voltage of the NPN transistor, resulting in a large change in the overcurrent limit. Therefore, it is necessary to determine the device value so that the over-current protection will not be falsely triggered when the ambient operating temperature is above the ambient operating temperature range under 100% load. This gives the converter a very wide overload capacity at room temperature—typically up to 140% of the nominal output power. This kind of converter can work reliably under continuous full load, while still having a large margin to cope with any overload conditions.

There is one exception to this overview. DC/DC switching regulators usually work at higher switching frequencies to reduce device size (increasing the frequency of the buck converter reduces the output inductance and output capacitance), so if you encounter sudden peak overloads There will be less power reserve. The current sampling resistor is usually integrated with the main controller IC on a chip, and has a stricter resistance value tolerance to reduce the variation of the overcurrent limit. In addition, most switching regulator controllers also use accurate comparator outputs to monitor the cycle-by-cycle current limit, instead of relying on the inaccurate Vbe junction threshold voltage, so they will be turned off immediately when the limit of overcurrent or short circuit protection is reached. . It can be seen that the worst-case peak load condition of the DC/DC switching regulator should be considered instead of the average load.

in conclusion

Excessive specification of AC/DC or DC/DC converters to cope with transient peak loads, as if they are in a continuous state, will not only reduce efficiency, but may also cause the power supply to exceed the necessary range. By understanding the average, worst-case, and peak load conditions of the application, the most appropriate solution can be selected to ensure a reliable supply voltage at a lower cost. Our technical support engineer or technical sales team can provide the best advice for your application.