Power supply current efficiency

Current efficiency of the power supply is the maximum current the power supply should supply to the load, while maintaining all the parameters specified in the data sheet (output voltage, power etc.) and operating within the operating conditions (temperature, humidity etc.).

As per the SI system of units, the current intensity is expressed in amperes (A). The current can also be expressed in milliamperes (mA).
1 mA is 1/1000 A, i.e. 1000 mA = 1 A (e.g. 600 mA = 0.6 A).

Diagram (Fig. 1) shows the relationship between the power supply (1) and the load (2). The source of electric power (power supply), supplies the power which is absorbed by the load. The load shown in the diagram with a resistor symbol has a resistance (R) and generates a power (P). The load can convert the electric power into another form of energy, e.g. light (light bulb) or heat (radiator).

The current efficiency is the maximum current, which may be supplied by the power supply to the load and is related to the power generated by the load. It results from a fundamental law that governs all electrical circuits, i.e. the Ohm’s law. It defines the relationship between voltage (V), current (A), resistance (R) and power (R).

General formula of Ohm's law:

A more useful formula of Ohm’s law to calculate the maximum output current of the power supply:

After transposing, the formula for the maximum output power of the power supply is:

where:

I – output current in amperes (A),
P – power in watts (W),
U – output voltage in volts (V),
R – load resistance.

Below is an example calculation (Ohm’s law application) for the power supply with 12 V voltage and 5 A current efficiency.

Fig. 2. Ohm’s law applications (example calculations)

Power

Resistance

Current 

or

Voltage 

Other examples:

Power supply with 12 V output voltage and 10 A power efficiency. We will now calculate the maximum power:

Power supply with 12 V output voltage and 150 W power. We will now calculate the current efficiency:

If the power supply is intended for continuous operation (24h), the rated current should not exceed 80% maximum current. It is closely tied to temperature. The current efficiency (or power), must always relate to the operating temperature, i.e. ambient temperature of the power supply.

The temperatures exceeding 50°C (preferably 40°C) should be avoided, however, some manufacturers may allow operation at higher temperatures. Maximum temperature specified by the manufacturer for some devices is 70°C; please refer to the specification. There are, however, devices available designed for operation at up to 30°C.

Example 1.

High quality modular power supply 12 V/12.5 A/150 W. Operating temperature: -10°C to 70°C. The data sheet includes a (Fig. 3) load vs. operating temperature characteristic.

Fig. 3. Load vs operating temperature characteristic

The power supply may supply the load at full power up to 50°C. At an ambient temperature of 70°C, the device can be supplied at 50%, i.e. at half of the maximum current. It means that at an ambient temperature of 70°C, the current efficiency of the power supply is halved. In this case it is reduced from 12.5 A to 6.25 A.

Example 2.

Modular power supply 12 V/5 A/60 W. Operating temperature: -10°C to 40°C. The data sheet includes an allowable output current (I_N) of the power supply vs. ambient temperature (t_amb) characteristic (Fig. 4) (for momentary load).

Fig. 4. Allowable output current (I_N) of the power supply vs. ambient temperature (t_amb) (for momentary load).

The power supply may supply the load at maximum current, but up to 30°C. At an ambient temperature of 40°C, the device can be supplied at 70%.
In that case:
– ambient temperature (t_amb) 30°C – maximum current 5 A,
– ambient temperature (t_amb) 40°C – maximum current 3.5 A.

High quality power supply usually feature high operating temperature and high maximum rated current. Depending on its efficiency, some energy supplied to the device is converted into heat. If a low-efficiency power supply is installed in a closed enclosure, the components that become heated may increase the temperature inside the enclosure. Which in turn may significantly reduce the current efficiency of the power supply.

A temperature test (Fig. 5) using a thermal imaging camera of a cheap 12V/10A modular power supply at different load currents at 25°C is shown below.

Some power supply components may become very hot, i.e. the output filter reactor. The power supply has a low efficiency and a great amount of energy is lost due to heating.

For power supplies with the output voltage control, its current efficiency, i.e. the maximum current supplied to the load is specified by the manufacturer for the rated output voltage. If the output voltage is increased, the output current must be reduced.

Example 3.

Power supply with 12 V output voltage and 500 W power with output voltage control (11.6 to 16.2 V). The current efficiency is calculated for 12 V rated voltage.

If the output voltage is increased to 16.2 V, the current efficiency is:

The maximum current (current efficiency of the power supply) is reduced compared to its rated current by over 10 A.