Electronic Design

Simple Circuit Provides Latching Fault Protection

Most switching power-supply controllers provide non-latching fault protection. Satisfying a requirement for a latchoff response to a fault often requires the addition of excessive and redundant circuitry. For power-supply controllers with an external soft-start pin, however, a simple circuit can be added to convert its non-latching fault protection into one with latched protection.

For typical controllers, the startup sequence begins by charging the VDD bias capacitor C4 from the input voltage. Once sufficient bias voltage is present, the controller (U1) begins charging external soft-start capacitor C8 from an internal current source. This allows the power-supply output voltage to rise in a controlled manner to its nominal regulated voltage.

Whenever the controller detects a fault, it attempts to shut down the power supply by quickly discharging the soft-start capacitor C8. This quick soft-start discharge can be used as a trigger for a latch circuit (Fig. 1).

At initial turn-on, the clamp capacitors C5 and C6 are completely discharged. As the bias voltage VDD begins to rise, these clamp capacitors and the bleeder resistors, R3 and R4, prevent false triggering of the latch. Capacitor C7 ac-couples the soft-start pin to the latch circuit.

During the startup sequence, the soft-start capacitor slowly charges and the latch remains off. In response to a fault, though, the controller will quickly discharge the soft-start capacitor, consequently creating a voltage change across C7 that initiates base current in Q1.

The latch transistors Q1 and Q2 feed base current to each other and discharge the VDD bias through R13. The line input voltage keeps a small amount of current flowing into the latch through the pull-up resistor R2 to keep the transistors active. The latch can thus only be reset by removing the input voltage and waiting for the clamp capacitors to completely discharge.

The value of R13 is important for proper operation of the latch. If this resistance is too high, the latch can’t keep the bias voltage discharged and the controller will attempt a restart. If this resistance is too small, a large instantaneous current spike that occurs when the latch is initially triggered may damage transistors Q1 and Q2.

The current-mode controller in this example, the UCC28600, can provide input as well as output overvoltage protection by sensing the voltage on the transformer bias winding through the resistor divider of R5 and R12. Measured waveforms from the latch circuit during an output overvoltage event demonstrate how the circuit operates (Fig. 2).

The power-supply output voltage is configured for a nominal 35 V. When a fault occurs that causes the supply to lose regulation, the output voltage rises to 45 V. The controller detects the overvoltage condition and pulls the soft-start pin to ground. The discharge of the soft-start capacitor triggers the latch, which discharges the bias voltage and holds it at 2 V until the input voltage source is removed.

This circuit can be applied to most isolated, switching powersupply controllers. With only eight inexpensive components, any fault protection provided by the controller can be leveraged to create a latched protection scheme.

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