Explaining lower efficiency of regulator at lighter loads

While working on most of the power supply designs with regulators, designers keep in mind the efficiency. For example, if we take the case of TPS5431 buck regulator, the efficiency is as below.

Considering the above characteristics, the designers do take an assumption of  efficiency around 90% and move ahead with the design. Unless it is a battery powered device, there isn't a serious look into the efficiency. If the regulator works thermally which is decided through thermal calculations, designer goes ahead with the selected regulator. The current used for such calculations are the maximum current drawn by the regulator. Most design engineers do not even bother about the lower side of the current drawn by the load connected to regulator output. We understand that the load current is not constant and it varies depending on load conditions. 

Consider the below graph for the same TPS5431 regulator, where the efficiency has dropped nearly to 80% at lower loads. 

While the efficiency dependency on input voltage is considered, the efficiency dependency on load current is ignored. This cannot be ignored in battery based designs.

For example, the TPS5431 has a quiescent current as shown below in the datasheet.

Quiescent current is the minimum current drawn by the regulator under no load conditions. If we see the above table, a current of Maximum 4.4mA is drawn under not switching conditions, that means output is disconnected and no current is drawn by the load. So, this 4.4mA in consumed entirely by the IC. Now considering this scenario, let us assume that a load current drawn is only 25mA. Under this scenario, for an input of 12V and output of 5V, the efficiency is calculated as,

% Eff = (Pout/Pin) * 100  = ((5*0.25)/(12*0.294))*100 = 35.4%

We can see that the efficiency is down drastically. To overcome these scenarios at light load conditions, there are several techniques used:

• Frequency Fold-back
• Burst mode