Many design engineers working on automotive electrical systems ask for higher efficiencies of the power supply. This is not much different to what power supply designers aim for in virtually every industry. However, the special part of the request is that the efficiency at very low output loads is most critical.

As more and more electronic systems are put into automobiles the demand for electricity not only increases while a car is running, but also while the car is parked. Very often these electronic systems require a voltage rail to be up and running all the time to keep a clock running or save memory states.

In such battery powered systems it is very important to not drain the battery too much so that there is still enough energy left in the car battery to start the combustion engine after the driver returns to the car after letting it sit for a few weeks at an airport for example. For very high efficiencies at extremely low loads, special power management regulators were developed. The following paragraphs will show how a very high-efficiency step down system can be designed.

Often the focus is on current rather than efficiency. For very low output loads, the efficiency is usually calculated by summing up the average consumed micro amps. There are a few reasons that this makes more sense than talking about efficiency percentage numbers at low powers.

First, many guidelines, such as those from the automotive industry, give maximum current consumption requirements rather than efficiencies. One example from an automotive manufacturer is the maximum system current consumption of the central locking system in standby is limited to only 1000µA. Systems that permanently maintain memory values are only allowed to draw up to 100µA. So rather than demanding a certain efficiency, demanding a maximum amount of current ensures that both the systems power consumption as well as the power converters power consumption is low.

The second reason is that many energy consumers in a power supply are drawing fixed currents rather than fixed powers. In other words, current consumption is not reduced when the input voltage rises. Such consumers are the leakage current of a typical Schottky diode in a non synchronous converter and the part of the input current which is used by the switch mode power IC to generate the internal ICs bias rails.

Different current consumers in a typical step down switching application. Switching regulators which are optimized for low current consumption in low output power applications such as circuits built with the LM26001 from National Semiconductor, require power for various internal functions such as:

Supply current to the power supply control IC.

This current supplies the internal reference and basic control functions as well as protection functions for the power supply. In Figure 1 this is part of the current flowing into the Vin pin at point A.

Cboot current taken from the Vcc supply.

This current is taken from the IC's bias supply in step down regulators with a high side N-FET. This energy is necessary to supply the N-FET driver and it can be found at point B in Figure 1.

Leakage current of the schottky diode.

This current varies with the specific diode used. Some Schottky diodes have larger leakage currents then others. This leakage is typically a strong function of temperature. Figure 1 shows the diode at point C.

Feedback resistor divider current.

This current can be made small by selecting high impedance feedback resistors. If the impedance becomes too high, it is likely to cause noise issues in the power supply. The feedback opamp's input bias current may also cause offset errors if a high impedance is used. See point D in Figure 1.

Other support currents.

An example for other support currents is the current into an enable pin of the switch mode power IC. Enable functions always consume some current, either in the on-state or in the off-state. For high efficiency at low loads, an IC should be selected that consumes less current in the on state than in the off state. One example is the enable pin at point E in Figure 1.

Other leakage currents.

These include leakage currents of input and output capacitors as well as the power MOSFET and are typically very small. See points F in Figure 1.

When designing very high efficiency, low output power converters, all six groups of current consumption have to be considered. Like a diligent accountant or investigator all the micro amps of current need to be counted to get a realistic estimate of power consumption for a given power supply. Some of the power consumption is not constant, but periodic. High efficiency, low output power converter ICs such as the LM26001, usually operate in skip mode at light loads. This improves efficiency since the switch is only used for a few cycles to bring the output voltage up and then many internal IC functions are shut down until the output voltage is recognized as being too low. Such an operating mode is very efficient but it does not consume a constant average current. When taking an input current measurement, the measurement will need to be averaged over long periods of time. Some automotive manufacturers which specify a maximum current consumption of a system require this to be averaged over 24 hours. This usually allows higher peak currents as long as there are times when the system consumes less than the average specification limit.

Figure 2 shows a typical efficiency measurement setup. It includes current measurements on the input and output as well as voltage measurement very close to the input and output connections of the device under test. Such a measurement is typical for all kinds of efficiency measurements.

When counting the micro amps, it is important to know about the precision and limitations of the current meters. Often a current meter's manual will not precisely define how current is averaged over time. For this, a test setup will help in which a precise resistor, a signal generator and a scope is used. With the signal generator one can apply different waveforms at different frequencies and over a precise resistor to ground one can see different current waveforms. In such a setup the current meter's ability to average small currents in the micro amp region can be tested. Figure 3 shows the setup for evaluating how a current meter (the amp meter in the circle) handles current averaging over time. If a relatively large but precise resistor is used, both the precision as well as the averaging ability of the current meter can be tested.

Where will it all go? Low current consumption of power supplies will become more important in the future. In products such as mobile phones we've seen continuing efficiency improvements for many years now. Applications in the automotive industry as well as some off-line equipment which needs to comply with energy efficiency standards, see the need for high efficiency during low output load conditions. Power supply IC manufacturers are putting more focus on such low current consumption ICs to provide solutions for these demands.