Bias supply provides short-circuit protection

Dave Kim, Linear Technology Corp, Milpitas, CA -- EDN, 9/16/1999

Most power-supply designs require protection from a short-circuit fault condition. One of the methods of short-circuit protection is cycling, or the "hiccup-mode" method. This method is an effective way of controlling a short-circuit fault condition while minimizing power loss in the circuit. Figure 1 shows a trickle-charged bias supply that uses an LT1431 programmable reference, IC₁, to start an LT1680 high-power dc/dc step-up controller, IC₂, which operates here in a forward-converter topology. This circuit is useful for applications with wide input-voltage ranges, such as telecommunication applications with input ranges of 18 to 72V.

IC₂'s undervoltage-lockout (UVLO) hysteresis, which you can adjust using the R₁/R₂ voltage divider at the RUN/SHDN pin, sets the short-circuit cycling. An overwinding, L₁, on L₂'s output inductor provides a V_CCof 12V. When a short-circuit fault at the output drags V_CCdown to IC₂'s UVLO threshold, IC₂ shuts down, and a constant bias current starts to charge the bias supply capacitor. IC₁ controls the scaled current mirror that generates a constant current and continually adjusts itself to keep the input of the current mirror at 3 mA. When this constant current charges the bias capacitor to the level of IC₂'s turn-on threshold (approximately 12V), IC₂ wakes up. You can calculate the turn-on delay using the following equation:

With a constant charging current of 3 mA, the turn-on delay for the circuit is approximately 400 msec. You can produce shorter turn-on delays by reducing the value of the bias capacitor or increasing the bias current. If you increase the bias current, you must scale the transistors according to their power-dissipation ratings.

Figure 2a shows the bias-capacitor voltage and the output of the forward-converter voltage during turn-on. When the bias capacitor charges to the turn-on threshold voltage of 12V set by IC₂, the output turns on and the bias capacitor's voltage dips to 11.5V, which is the level that the overwinding sets. Figure 2b shows the bias capacitor and output voltage in cycling mode during a short-circuit fault. IC₁ sets the constant current, which charges the bias capacitor to the turn-on threshold, and IC₂ starts up. However, the overwinding generates no voltage because of the short at the output. And because IC₂'s quiescent current is greater than the charge current, the bias capacitor discharges. Discharging the bias capacitor to less than the UVLO threshold shuts down IC₂. This charging and discharging cycle repeats until you remove the fault.

You can calculate the restart time using the following equation, where V_RUN equals the turn-on threshold set by RUN/SHDN:

When implementing a cycling type of short-circuit protection, a constraint is on the maximum capacitive load for which the power supply starts. The capacitive load depends on the short-circuit current and the threshold voltage at which the overwinding back-feeds the bias supply. The output rise time of the LT1680 must be less than the holdup time that the hysteresis and the bias capacitor provide.

You can calculate the start-up time using: