The majority of communications equipment is designed to operate from a DC power system with a nominal voltage of -48 V. DC-DC converters are used on each card in the equipment to reduce the voltage to a suitable level for the ICs and other components, and to maintain isolation for safety. In some applications, mainly cell sites, a nominal system voltage of +24 V is used rather than -48 V, and there is therefore a need for equipment that can operate from either power system voltage. This article examines the range of voltage needed and considers what options can be used to minimize the impact of the wide voltage range on the DC-DC converters.
Nominal 48 V input range
Although the power system voltage is referred to as -48 V, the actual voltage varies over a range from -42 to -56 V depending on the state of charge of the batteries. The DC-DC converters themselves must operate down to a significantly lower input voltage, to allow for voltage drops within the equipment. Typically the minimum is about -38 V, derived as illustrated in Figure 1.
Figure 1 " Minimum input voltage at DC-DC converter
Most systems include a low input shutdown (LISD) function, to shut down the DC-DC converters if the input voltage falls below the minimum. Assuming the LISD is set to 37±1 V, the minimum voltage at the DC-DC converter is 36 V.
At the other extreme, the maximum voltage for a -48 V power system is normally specified to be -60 V continuous, with transients up to -100 V (refer to ANSI T1.315). However, some central offices use a nominal -60 V power system rather than the normal -48 V. Even though this type of power system is only used in a limited number of areas, mainly in Europe, most large equipment manufacturers prefer to make a single product that can be used in all applications. Consequently, it has become commonplace to design equipment that can operate on either -48 V or -60 V, and the maximum continuous input voltage to the DC-DC converter is increased to -72 V. Most commonly available DC-DC converters are therefore designed for a 2:1 input voltage of 36 to 72 V, or sometimes 36 to 75 V.