Fast battery charger. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network, uninterruptible power supplies
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The circuit of a key-type charger is considered.
The advantages of step-down key battery chargers over analog devices are simplicity and high efficiency. However, sometimes the power source does not have enough voltage to charge the batteries, for example, when you need to charge a battery with more cells than a car battery from a car battery.
This problem can be solved by using a DC boost converter. The device (see figure) contains a MAX770 boost key regulator and a fast charge controller. This device allows you to charge NiCd or NiMH batteries consisting of five or more cells from a DC source with a nominal voltage of 5 V.
Since the battery voltage varies greatly during charging, the use of a linear regulator to increase (decrease) the voltage at the terminals of the battery being charged will lead to a decrease in the efficiency (COP) of the process and excessive heat generation.
This scheme is free from this shortcoming. In the above circuit, the sensor resistor R2 is included in the load circuit, but its effect on the efficiency of the circuit is negligible. The feedback input FB of the DA1 chip is used to turn on the boost converter. This happens when the current reaches its maximum value.
The power output (15) of the DA2 chip is connected to the input source through the resistor R1. The resistance of this resistor is selected so that the voltage at pin 15 does not exceed 5 V. The voltage at the input of the VIN circuit can reach 16 V.
For the type of transistor VT1 indicated in the diagram, the load power should not exceed 20 watts.
The charge current can be calculated by the formula
I(A) = 0,25 / R2 (Ohm).
The operating voltage of the capacitor C1 (in volts) must be at least U = 2N, where N is the number of battery cells.
References:
- Quick-charge IC routes step-up switcher output to battery. MAXIM. Engineering Journal. Volume Nineteen.
Author: A.A.Kovpak
See other articles Section Protection of equipment from emergency operation of the network, uninterruptible power supplies.
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