Telecom hot-swap reference design effectively solves the problem of instantaneous input overvoltage and power outage

Overview

Typical telecommunications systems must be able to operate at full load from two voltage sources of -36V to -72V. In addition, they must provide protection against lightning induction, high-voltage pulses, and input voltages that drop to 0V within a few milliseconds. Pluggable line cards typically contain a high-voltage to low-voltage DC-DC converter with hot-swap protection circuitry. This circuit limits the inrush current to slightly above full load current when the line card is plugged into a backplane while powered. After the on-board filter capacitor is fully charged, the hot-swap circuit must also provide a power-ready signal (/PGOOD) to turn on the subsequent DC-DC converter. This article details the requirements for hot-swap circuits . Most of the requirements are the same as the AdvancedTCA® (ATCA®) level one hot-swap circuit.

Typical telecommunications system requirements

1. Input voltage range: -43V to -72V

2. Input uses dual diode OR logic

3. -32V to -36V off (power supply before diode)

4. 80W (max) input power, C _LOAD = 680µF

5. Withstand voltage -150V, 1ms overvoltage pulse¹

6. Power-down support, 0V, 16ms instantaneous power-down starting from -43V input voltage; /PGOOD must remain low during this period

7. When starting at 43V, surge current ≤ 1.5 times full load current

Reference design features and considerations

1. The undervoltage lockout (UVLO) rising threshold can be set to ~43V, and the overvoltage lockout (OVLO) rising threshold can be set to ≥ 72V.

2. Two 100V Schottky diodes installed at the input implement the "OR" logic of two independent power supplies.

3. The UVLO drop threshold can be set to ~32V, which must meet item 6 of the telecommunications system specification. When the voltage drops below the UVLO drop threshold, /PGOOD remains valid and the system maintains normal operation.

4. When V _{IN = 43V, P IN} = 80W, the input current is 1.86A.

5. For U1, C3, C4, and Q2, the 70V TVS diodes clamp the input voltage to a safe level. This diode can be connected directly across the input, or to the V _DD – V _{EE pins of the} MAX5921 hot-swap controller, and isolated from V _IN with a 5Ω resistor in series .

6. Within 16ms of the drop from 43V to 32V, the energy storage capacitor (C3) stores enough energy to support the power supply of the 80W load. The capacitor charges through a resistor to limit the charging current to a fraction of the input surge current specified in Item 7 of the Telecommunications System Specification. During discharge, the Schottky diode bypasses this charge current limiting resistor.

7. The minimum triggering threshold of the circuit breaker is optional, allowing it to be set to 2.8A at 43V, 150% startup surge current, and maintain 2.5A full load operation when the voltage is as low as 32V.