The UCC3580 (www.ti.com), IC₁ in Figure 1, is an active-clamp/reset-PWM IC that has all the requisites of a power-supply IC except for current limit. You can use the shutdown pin (16) for this purpose (see the UCC3580 data sheet). But when the shutdown pin activates, the soft-start capacitor normally connected to Pin 15 discharges, and the converter starts again, resulting in hiccup mode. Poor dynamic response ensues because of this circuit behavior. The problem becomes especially serious when you hot-plug a load card, with typically 100-µF decoupling across the converter's output, into the motherboard. You can avoid this problem by not using the shutdown pin for current limiting. Instead, use the error amplifier's output pin, Pin 12. Q₁ is the main switch, T₁ is the main transformer, and C₁ is the bulk capacitor. Q₂, D₃, and D₄ provide power (+V) for IC₁ during start-up and current limit. D₁ and C₂ rectify and filter the voltage from the auxiliary winding of T₂ to power the control circuit during normal operation. R₂ senses the current in the main switch, Q₁.

The LM358 op amp provides current limit. Diode D₆ isolates the op amp and Pin 12 of IC₁ during normal operation. According to the data sheet of UCC3580, the error amplifier's output should pull down to 0.3V or lower for 0% duty cycle. This condition forces you to use a negative supply (–V) for the op amp to obtain rated performance. As Figure 1 shows, the same supply voltage (+V) serves for the PWM IC and the op amp. You can obtain the negative supply voltage (–V) for the op amp from another winding on T₂. Even though the voltages that T₂ generates decrease in foldback condition, the op amp continues to obtain +V from Q₂. A negative voltage (–V) from T₂ is adequate for proper operation of the op amp. The Schottky diode, D₇, from Pin 12 to ground protects the error amplifier's output from large negative voltages.

The inverting input of the op amp senses the peak pulse voltage across R₂. Diode D₅ helps C₅ to charge to peak voltage (peak detection), and R₃ helps to discharge C₅ when the peak voltage across R₂ decreases. Use a slow diode for D₅. The reference voltage at the noninverting input has two components—first, from the fixed 5V reference (Pin 14) of IC₁; second, from the voltage generated by the T₂ winding. This voltage decreases with output voltage, and voltage foldback occurs at the noninverting input. The 50-mV component from the fixed 5V reference is necessary for start-up. As the current limit starts, the output voltage starts to decrease, and the noninverting-input voltage also decreases, causing foldback. With this circuit, if the current limit starts at an output current of 10A, the output short-circuit current is 2A. The output recovers even with minimal current hysteresis. The positive voltage that the T₂ winding generates decreases from 13V just before current limit to 3V at short circuit. This circuit does not influence the transient response of the converter.