Extension for LiPo Charger

Extension for LiPo Charger Project Image

The ‘Simple LiPo Charger’ published in Elektor Electronics April 2005 is a small and handy circuit that allows you to quickly charge two or three LiPo cells. Especially in the model construction world are LiPo batteries used a lot these days, particularly model aeroplanes. It is usual to use a series connection of three cells with these models. Since working with these model aeroplanes usually happens in the field, it would be nice if the batteries could be charged from a car battery. We therefore designed a voltage converter for the LiPo charger concerned, which makes it possible to charge three cells in series. The voltage per cell increases while charging to a value of about 4.2 V, which gives a total voltage of 12.6 V. The converter, therefore, raises the 12-V voltage from the car battery to 16.5 V, from which the LiPo charger can be powered.

Extension for LiPo Charger Circuit Diagram

A step-up controller type MAX1771 in combination with an external FET carries out the voltage conversion. The IC operates at a moderately high switching frequency of up to 300 kHz, which means that quite a small coil can be used.

Because the IC uses pulse frequency modulation (PFM) it combines the advantages of pulse width modulation (high efficiency at high load) with very low internal current consumption (110µA).

The IC is configured here in the so-called non-bootstrapped mode, which means that it is powered from the input voltage (12 V). The output voltage is adjusted with voltage divider R2/R3. This can be set to any required value, provided that the output voltage is greater than the input voltage.

Extension for LiPo Charger PCB Layout

Finally, sense resistor R1 determines the maximum output current that the circuit can deliver. With the 25 mΩvalue as indicated, this is 2.5 A.

COMPONENTS LIST:

Resistors:
R1 = 25mΩ(e.g., Digikey # 2FR025-ND)
R2 = 100kΩ
R3 = 10kΩ

Capacitors:
C1,C4,C8 = 100nF
C2,C3 = 47µF 25V radial
C5,C7 = 100µF 25V radial
C6 = 100pF

Semiconductors:
D1 = 31DQ05 (e.g., Digikey #31DQ05-ND)
IC1 = MAX1771-CPA (e.g., Digikey #MAX1771EPA-ND)
T1 = IRFU3708 (e.g., Digikey #IRFU3708-ND)

Miscellaneous:
K1,K2 = 2-way PCB terminal block,lead pitch 5mm
L1 = 47µH high current suppressor
coil, (e.g, Digikey # M9889-ND)
PCB.,ref. 054012-1 from The PCBShop

Author : Unknown - Copyright : Elektor