UC3842 design tips for switching power supply circuits

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The typical circuit of the switching power supply made with UC3842 is shown in Figure 1.

Overload and short-circuit protection is generally achieved by sending a current signal to the 3rd pin of the 3842 by a resistor (R4) at the source of the switch. When the power supply is overloaded, the 3842 protection action reduces the duty cycle and reduces the output voltage. The supply voltage Vaux of the 3842 is also reduced. When the low voltage is not working, the entire circuit is turned off, and then the next start is started by R1 and R2. process. This is called "hiccup" protection.

In this protection state, the power supply only works for a few switching cycles, and then enters a long time (several hundred ms to several s) of the starting process, the average power is very low, even if the long-term output short circuit will not cause power supply damage. Due to leakage inductance and the like, some switching power supplies have large switching peaks in each switching cycle. Even when the duty ratio is small, the auxiliary voltage Vaux cannot be lowered enough, so it is generally on the rectifier diode of the auxiliary power supply. A resistor (R3) that forms an RC filter with C1 to filter out spikes at the turn-on instant. Careful adjustment of the value of this resistor can generally achieve satisfactory protection. To use this circuit, care must be taken to select a lower auxiliary voltage Vaux, which is typically 13 to 15V for the 3842, making the circuit easy to protect. Figures 2, 3, and 4 are common circuits.

Figure 2 Take the method of pulling down the first leg to turn off the power.

Figure 3 shows the method of disconnecting the oscillating circuit.

Figure 4 Take the method of raising the second leg and lowering the first leg.

In these three circuits, the R3 resistor is well protected even if it is not needed. Pay attention to the role of C4 in the circuit, the power supply starts normally, and the optocoupler is unreachable. Therefore, the protection circuit is delayed by C4 for a period of time. It also acts as a delay protection for overload or short circuit protection. When the starting current of the bulb, motor, etc. is large, the value of C4 is also larger.

Figure 1 is the most widely used circuit, however its protection circuit still has several problems:

1. In mass production, due to the difference of components, there will always be some power supply that cannot be well protected. In this case, the value of R3 needs to be adjusted individually to cause trouble to the production;

2. When the output voltage is low, such as 3.3V, 5V, because the output current is large, the output voltage drops little during overload, and it is difficult to adjust R3 to an ideal value;

3. In the forward application, the auxiliary voltage Vaux also follows the output change, but it has a greater relationship with the input voltage HV, and it is difficult to adjust R3 to an ideal value.

At this time, if the auxiliary circuit is used to achieve the protection shutdown, better results will be achieved. The implementation principle of the auxiliary shutdown circuit: in the case of overload or short circuit, the output voltage is reduced, the optocoupler of the voltage feedback is no longer turned on, and the auxiliary shutdown circuit detects that the optocoupler is no longer turned on, delays the action for a period of time, closes power supply

The above is part of the documentation, see the PDF document for more details!