Equip switchers with overcurrent protection

Robert N Buono, Buono Consulting, Ringwood, NJ -- EDN, 6/24/1999

Overcurrent protection is usually a necessary design feature of a switch-mode power supply to safeguard both the switcher and the load. Most PWM control ICs have internal overcurrent-protection circuits, and you can typically add auxiliary circuits around the IC to enhance this protection. For example, simple circuits enhance the operation of the common family of UC3842/3/4/5 (Unitrode Corp, www.unitrode.com) PWM-control ICs (Figure 1). These circuits allow the switcher to respond to an overcurrent fault condition by latching off if the overcurrent condition persists for more than some defined time interval or by cycling off and then on again at a low duty cycle until the fault clears. When the fault clears, the switcher then resumes normal operation on the next restart cycle. You can add these circuits without affecting other desirable features that the PWM IC may already include, such as soft start and maximum current-limit clamping.

In most switcher designs, operation in current limit imposes the greatest stress on the power devices. Therefore, decreasing the time that the switcher must operate in current-limit mode can enhance the reliability of the switcher. In some cases, decreasing this time may even result in reduced heat-sinking requirements for the power devices. A design that doesn't permit sustained operation in current-limit mode can have a lower average power dissipation than a design that permits a longer time in current-limit mode.

The circuits function solely by manipulating Pin 1 of the PWM IC; the schematics omit all other details of the PWM IC because the remainder of the IC's operation stays the same. In the UC384x family of control ICs, Pin 1 is the output of the internal error amplifier. The voltage at Pin 1 controls and is directly proportional to the peak current level in the main power-switching transistor. Therefore, Pin 1 is a logical place to exert control of the switching current.

The circuit in Figure 1a is a commonly used network that adds a slow-start feature as well as maximum current-limit clamping to the PWM- control circuit. The IC's internal error amplifier sources only a limited amount of current, typically 0.8 mA. Therefore, pnp transistor Q₁ easily clamps the voltage at Pin 1 to 0.6V higher than the voltage at its base by diverting the current from Pin 1 through its collector to ground. Thus, the voltage divider of R₁ and R₂ determines the clamping voltage at Pin 1. This voltage sets the maximum current limit of the main power-switching transistor.

A logical conclusion is that adding a capacitor, C₁, from the base of Q₁ to ground will allow a ramp-up characteristic for the clamped voltage at Pin 1 and, therefore, will allow an analogous ramp-up characteristic for the power-switch current. This ramp-up characteristic is usually a desirable feature for initial switcher start-up because it allows the switcher's output voltage to ramp up in a controlled manner, reducing output-voltage overshoot. The 5V REF supply in Figure 1 is a precision voltage reference that IC₁ develops internally. This 5V REF is commonly used to power auxiliary circuits; this output delivers as much as 20 mA. The circuits in Figure 1 are very low-power, and draw only a small fraction of that available current.

The design in Figure 1b adds a low-cost, dual-comparator circuit to the network at Pin 1. R₂ and C₁ remain the same, but this circuit splits R₁ into R_1A and R_1B. R₃ is an additional component in the collector leg of Q₁. Whenever Q₁ clamps the output of IC₁'s Pin 1, a voltage develops across R₃. This voltage indicates the onset of current limiting. R₃ does not interfere with the soft-start and maximum-current-limit clamp performance.

Cycling occurs when you install R₄ and remove R₅. When the switcher is in current-limit mode, a voltage of approximately 400 mV develops across R₃. When the voltage across R₃ exceeds the voltage at IC_2A's Pin 2, which is approximately 150 mV, the output at IC_2A's Pin 1 becomes an open-collector output and allows C₂ to charge through R₄. The time it takes for C₂ to charge from approximately 0V to the threshold established at IC_2B's Pin 5 is the "fault-delay time." During fault-delay time, the switcher remains on and in current-limit mode. IC_2B functions as a low-frequency oscillator with low duty cycle. When the voltage on C₂ exceeds the voltage on IC_2B's Pin 5, the open-collector output of IC_2B switches low. This low level pulls the base of Q₁ to ground through R_1B, and the voltage at Pin 1 of IC₁ clamps to approximately 0.6V. When Pin 1 of IC₁ is less than approximately 1.1V, current through the power-switching transistor is 0A, and the switcher is off.

The resistor ratios of R₆, R₇, and R₈ define the duty cycle of the low-frequency oscillator. For the values shown, the duty cycle is approximately 12%; the switcher is off for 1.4 sec and on for 200 msec. Each time the switcher restarts, it does so with soft start because the output of IC_2B fully discharges C₁ through R_1B. When the circuit pulls the base of Q₁ low, collector current continues to flow; the circuit maintains the voltage across R₃ even when the switcher is off. IC_2B continues to cycle until the overcurrent fault clears. When the fault clears, the circuit reestablishes the nominal output voltage of the switcher, and the voltage at IC₁ Pin 1 drops below the clamp level. The voltage across R₃ drops to 0V and ensures an open-collector output at IC_2B's oscillator output.

Removing R₄ and adding R₅ causes the switcher to latch off and not restart, in response to current limit. You can adjust the ratios of R₆, R₇, and R₈ to optimize the fault-delay time. Increasing the threshold voltage at IC_2B's Pin 5 increases the fault-delay time before latch-off.

Oscilloscope waveforms (Figure 2) show the power-switch current cycling on and off when the switcher is in current-limit mode (Figure 2a). A time expansion of the ramp-characteristic of the power-switch current for each restart event shows that each restart benefits from the soft-start characteristic (Figure 2b). These waveforms are the result of measuring the voltage across a current-sensing resistor; the waveforms represent the current through the main power-switching transistor switching at 50 kHz.(DI #2369)