Q.How should the standard switching power supply be used for battery charging applications?
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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 (hereunder “power supply”), on the other hand, 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 (feedback controlling).
The output voltage is therefore maintained constant but the output current fluctuates depending on load conditions so it cannot be used as is for battery charging that requires a constant current (constant current controlling).
The overcurrent protection function of the power supply is to therefore be used. The overcurrent protection function was originally designed to suppress current values and avoid 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.
It is important to keep in mind here that there are some characteristics of overcurrent protection that make it inappropriate for charging batteries.
As shown in Figure 1, there are 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 where a constant current range is used for charging batteries with a constant current.
Caution must be exercised when selecting power supplies because other power supplies with overcurrent protection characteristics are not appropriate for charging batteries.
Furthermore, 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 < the rated current.
There is also a method of controlling constant currents by connecting the external circuitry to the power supply in addition to the method explained previously where the overcurrent protection function is diverted.
This is a concrete explanation using our HWS1000 as an example (Figure 2).
Figure 2 Constant current circuitry for charging batteries when using the HWS1000
Figure 3 Block diagram
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 (PV variable functions) and always requires use at the rated power or below.