Design considerations for a PWM controlled low voltage disconnect (LVDC)

Valve regulated lead acid (VRLA) batteries have been used in telecommunication power systems for over a decade and with them has followed demands on the handling of batteries at low voltages. Devices connected in series to the system are often incorporated to disconnect the batteries from the load in the event of a near deep discharge. This paper covers the design considerations for one such device that improves reliability through the use of pulse width modulation (PWM) techniques. By choosing a low voltage contactor and regulated PWM input, a voltage range covering both 24 V and 48 V systems can be achieved that greatly improves efficiency and reduces thermal stress, hence reducing the size of critical components. Detailed analysis of the total system reliability and EMC issues are also presented.