Electronic Design

Eliminate Latching Current limit In Power Supplies

Most modern power-supply control ICs offer built-in overcurrent or short-circuit protection. While some overcurrent-protection schemes are latching, others are nonlatching. When the overcurrent trip level is reached in a latching controller, the supply shuts off and stays off until either the input power or the enable signals are cycled off and on. In a nonlatching scheme, the power supply shuts down, then automatically restarts. Often, the best IC for the power conversion has a latching overcurrent-protection scheme, but the system re-quires a nonlatching plot.

The circuit of Figure 1 converts the latching overcurrent protection of the TPS5120 power-supply controller into nonlatching overcurrent protection. This circuit uses the built-in power-good (PG) signal of the TPS5120. At startup, PG is pulled high, allowing VREF5 (5 V) to be applied to the enable pin (STBY1). This action enables the power supply.

In normal operation, the voltage on both the PG and STBY1 pins is 5 V, so the voltage across C1 is 0 V. During an overcurrent condition, the TPS5120 shuts down and pulls the PG pin to ground. Because the voltage across C1 can't change instantaneously, the voltage on STBY1 also gets pulled to ground. That resets the enable signal to the supply.

A requirement to restart the controller after a fault, resetting the enable pin, forces the designer to modify the circuit so that it provides a nonlatching overcurrent protection. At this point, the power supply is off because STBY1 is low. As C1 is charged up through R1, the voltage on the STBY1 pin slowly rises. The supply stays off until the voltage on STBY1 crosses the turn-on threshold.

The scope image in Figure 2 illustrates the sequence of events that occur during a fault condition. Trace 1 shows the output voltage and Trace 2 depicts the soft-start voltage. Trace 3 and Trace 4 show the STBY1 voltage and the supply output current, respectively.

Trace 4, the initial overcurrent fault, is the point where the output voltage drops out of regulation and the PG pin goes low. When PG goes low, C1 pulls STBY1 low. The supply stays off as STBY1 charges up to its turn-on voltage (approximately 1 V) at a time constant determined by R1 and C1.

After STBY1 charges up to its turn-on voltage, the supply enters soft start. When this happens, PG enters a high-impedance state and STBY1 is pulled high again. The output attempts to supply load current into the short during soft start. If the short circuit remains when the soft-start period ends, the PG pin goes low again and the process starts over. But if the short circuit isn't present, the supply restarts normally.

The on and off times of the power supply during the fault condition are determined by the RC time constant on STBY1 and the soft-start time. With the time constants shown in Figure 1, the power supply can operate indefinitely into a short-circuit fault while dissipating only 1 W.

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