Several techniques and problems in designing switching power supply using UC3842

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

Overload and short circuit protection is generally achieved by connecting a resistor (R4) in series with the source of the switch tube to send the current signal to the third pin of 3842. When the power supply is overloaded, the 3842 protection is activated, the duty cycle is reduced, the output voltage is reduced, and the supply voltage Vaux of 3842 is also reduced. When it is so low that 3842 cannot work, the whole circuit is closed, and then the next startup process starts with R1 and R2. This is called "hiccup" protection.

In this protection state, the power supply only works for a few switching cycles, and then enters a very long startup process (hundreds of ms to a few seconds). The average power is very low, and even if the output is short-circuited for a long time, it will not cause damage to the power supply. Due to leakage inductance and other reasons, some switching power supplies have large switching spikes in each switching cycle. Even when the duty cycle is very small, the auxiliary voltage Vaux cannot be reduced to a low enough level. Therefore, a resistor (R3) is usually connected in series with the rectifier diode of the auxiliary power supply. It forms an RC filter with C1 to filter out the spike at the moment of opening. Carefully adjusting the value of this resistor can generally achieve satisfactory protection. When using this circuit, it is necessary to pay attention to selecting a relatively low auxiliary voltage Vaux, which is generally 13~15V for 3842, so that the circuit is easy to protect.

Figures 2, 3, and 4 are common circuits.

Figure 2 uses the method of pulling down pin 1 to turn off the power.

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FIG. 3 adopts the method of disconnecting the oscillation loop.

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

In these three circuits, even if the R3 resistor is not needed, it can still provide good protection. Note the role of C4 in the circuit. When the power supply starts normally, the optocoupler is not connected, so C4 is used to delay the action of the protection circuit for a period of time. In overload or short circuit protection, it also plays a role in delayed protection. In situations where the starting current is large, such as light bulbs and motors, the value of C4 should also be larger.

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

1. In mass production, due to differences in components, there will always be some power supplies that cannot be well protected. At this time, the value of R3 needs to be adjusted individually, causing trouble for production;

2. When the output voltage is low, such as 3.3V and 5V, due to the large output current, the output voltage does not drop much when overloaded, and it is difficult to adjust R3 to an ideal value;

3. In forward applications, although the auxiliary voltage Vaux also changes with the output, it is more related to the input voltage HV, and it is difficult to adjust R3 to an ideal value.

At this time, if an auxiliary circuit is used to implement protection shutdown, better results will be achieved. The implementation principle of the auxiliary shutdown circuit: when overloaded or short-circuited, the output voltage decreases, and the optocoupler of voltage feedback is no longer conducting. When the auxiliary shutdown circuit detects that the optocoupler is no longer conducting, it will act after a delay and shut down the power supply.