Support the utilization of small capacitors for telecommunication power supply during the period of insufficient voltage
Time：2021.11.01
View：
This design example shows how to keep telecommunications equipment working during a short period of undervoltage. First of all, we must understand a few details of the dedicated power supply for telecommunications equipment. The common mode voltage of the power supply to the telecommunications equipment is -48V, although the actual voltage range may be -42.5V "-56V, -40V" -60V, or even beyond these ranges. Public power supply-"brick" DC/DC converters work in the -36V "-75V range. When the -48V source drops to 0V and lasts for 10ms, there will be insufficient voltage.
Using a capacitive storage device connected to the input of the "brick" is a way to overcome this problem, but when people understand the real situation of the -48V power supply, a shortcoming becomes obvious. For example, the energy in a capacitor charged to a predetermined voltage is (C×V2)/2, where C is the capacitance value and V is the voltage. When the capacitor discharges to 36V, the "brick" stops working. Generally speaking, the energy that can be used to support the work of "bricks" is:
Among them, V1 and V2 are the initial and final voltages respectively, and U is the energy. In addition, U=P×t, where P is power and t is time. Using these equations, you can find the time the device keeps working normally:
Or define the value of the capacitor:
Suppose there is a voltage shortage when the input voltage of the "brick" is -39V. This happens when -48V drops to -40V, but the "brick" is lost due to the protective OR diode in the hot-swappable structure At least 1V. In addition, assume that the storage capacitor is charged to -39V. The device works normally until the storage capacitor is discharged to -36V. Assume that the power consumption of the device is 100W. In order to store enough energy for 5ms, the value of the capacitor must be approximately 4500mF. The capacitor's rating must be adapted to the maximum possible input voltage, which may exceed 75V, so the minimum rating must be 100V. The 4500mF100V capacitor is a fairly large component. If the design requires twice the working time at 300W power consumption, the value of the capacitor must be 27000mF and 100V.
This design example still needs a capacitor, but the value of the capacitor should be low, 200mF instead of 4500mF, and maintain 100W during the 5ms undervoltage period. This method improves reliability and reduces cost and size. A hidden feature is that the power brick can keep working in the -36V"-75V input range, even under surges greater than -80V. Figure 1 depicts how to use this feature. The figure depicts the positive input voltage. The "brick" power supply is isolated, so the polarity does not matter, but it is easier to describe the positive interpretation.
It should be remembered that the energy stored by the capacitor increases exponentially, while the voltage of the capacitor increases linearly. The multiplier charges C1 to twice the input voltage or at least 80V. Even assuming that there is a 5ms undervoltage every 10s, the current for charging 200mF is still only about 3mA. The comparator observes the input voltage, and once it drops below 37V, switch S1 closes and the energy from C1 discharges to the "brick" power supply.
