Using a Switching Power Supply for Battery Charging

Constant current charging is a way to charge common batteries. This is a charging method where batteries are charged with a constant current from beginning to end.

A standard switching power supply is a constant voltage power supply, so it monitors fluctuations in output voltages, inputs the results in the control circuit, and executes constant voltage controlling also known as feedback controlling.

The output voltage is maintained constant, but the output current is left fluctuating depending on load conditions, which means it cannot be used as a battery charger until you achieve constant current.

The power supplies overcurrent protection function will be used. The overcurrent protection function is designed to suppress current values and prevent deterioration and damage to the power supply when it overloads and a current exceeding the rating flows. The suppressing of the current values is diverted, and the suppressed current is used as a constant charging current for the battery.

Keep in mind that there are some characteristics of overcurrent protection that make it inappropriate for charging batteries.
In Figure 1, these are the three-main power supply overcurrent protection characteristics. The most appropriate method for charging batteries among them is with a power supply that has constant current voltage drooping type characteristics (Far Left) where a constant current range is used for charging batteries with a constant current. The other two characteristics should not be used to charge batteries.

The constant current range is the state where overcurrent protection for avoiding deterioration and damage in the power supply is operating as explained above, and its use is the exceeding of the rated current stipulated in the specifications of the power supply (rated current < overcurrent protection setting current).

In this case, a substandard for changing the overcurrent protection setting current value of the power supply is needed so that the overcurrent protection setting current is less than the rated current.

Another method of controlling constant currents is by connecting the external circuitry to the power supply in addition to the method explained previously where the overcurrent protection function is diverted.

The example below is using TDKs HWS1000 and will explain the process.

Shunt resistance (1) is applied to the load line of the power supply, a voltage conversion for the output current is made with the shunt resistance, and the output current is detected (2). A conditioning circuit for constant current drooping points is then designed and a set standard voltage for over currents is established (3). The voltage of (2) and (3) is compared in the operational amplifier (4) and the output voltage of the power supply is controlled in the photo coupler (5).

This method is an example of a constant current circuit configuration of power supplies with external controls of output voltages and should be used at the rated power or below.

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Content and images provided courtesy of TDK’s FAQ.