Defects of 2 UC3842 protection circuit

2. 1 overload protection defect

When the power supply is overloaded or the output is short-circuited, the protection circuit of UC3842 will reduce the duty cycle of the output pulse, the output voltage and the power supply voltage of UC3842. When it is too low for UC3842 to work, the whole circuit is closed, and then the next startup process is started through R6. This protection 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) start-up process, so its average power is very low. However, due to the leakage inductance of the transformer, the switching peak voltage of some switching power supplies is very high in each switching cycle, and even in the case of small duty cycle, the auxiliary power supply voltage cannot be lowered enough to achieve the ideal protection function.

2.2 Defects of overcurrent protection

The overcurrent protection function of UC3842 is realized through pin 3. When the voltage detected by pin 3 is higher than 1V, the comparator inside UC3842 will flip, the PWM latch will be set to zero, and the pulse modulator will be turned off, thus realizing the overcurrent protection of the circuit. Because the sensing resistor can sense the peak inductance current, a pulse-by-pulse current limiting circuit is naturally formed. As soon as the level on the detection resistor reaches 1V, the pulse width modulator will be turned off immediately. Therefore, this peak inductance current detection technology can accurately limit the maximum output current, so that the magnetic components and power devices in the switching power supply can ensure the reliable operation of the regulated power supply without designing a large margin. But the sampling resistor we usually use is metal film or oxide film resistor, which is inductive. When the current flows through the sampling resistor, a certain induced voltage will be induced. This inductance element will present great impedance at high frequency, so it will consume a lot of power. With the increase of frequency, the current flowing through the sampling resistor may not be fully discharged before the next oscillation period comes, and the current borne by the sampling resistor will become larger and larger, resulting in the misoperation of UC3842 and even the explosion of the machine. Therefore, this overcurrent protection function of UC3842 is sometimes difficult to play a good role in protection, and there are certain defects.

2.3 Defects of circuit stability

In the circuit shown in figure 1, when the duty cycle of the power supply is more than 50%, or the transformer works under the condition of continuous current, the whole circuit will produce subharmonic oscillation, resulting in instability of the power supply output. Fig. 2 shows the change process of inductance current in transformer. At time t0, the switch starts to conduct, so that the inductor current rises with a slope of m 1, which is a function of the input voltage divided by the inductor. At t 1, the current sampling input reaches the threshold established by the control voltage, which causes the switch to open and the current decays with the slope m2 until the next oscillation period. If a disturbance is added to the control voltage at this time, a δ I will be generated, so we will find that the circuit is unstable, that is, in a fixed oscillator period, the current decay time will be reduced, the minimum current conduction time t2 will be increased by δ I+δ IM2/M 1, and the minimum current will be reduced to (δ I+δ IM2/M 1) in the next period t3. In each subsequent cycle, the disturbance m2/m 1 doubles, and when the switch is turned on, the inductor current alternately increases and decreases. It may take several oscillator cycles to make the inductor current zero and restart the process. If m2/m 1 is greater than 1, the converter is unstable. Therefore, the circuit shown in figure 1 has certain instability hidden trouble in some states.

Fig. 2 Inductor current waveform diagram

3. Improvement of protection circuit

According to the above analysis, the improved circuit is shown in Figure 3 and has the following characteristics.

1) By connecting an emitter follower at the sampling voltage of UC3842 and adding an artificial ramp synchronized with the pulse width modulation clock to the control voltage, the δ I disturbance can be reduced to zero in the subsequent cycle. Therefore, the system will not be unstable even if the duty cycle is greater than 50% or the inductor current is continuous. But the slope of compensation slope must be equal to or slightly larger than m2/2, so that the system can have real stability.

2) The sampling resistance is changed to non-inductive resistance. Non-inductive resistance is a kind of winding resistance with two wires wound in parallel, which has high precision and is easy to realize high power. After using non-inductive resistor, its impedance will not increase with the increase of frequency. In this way, even at high frequency, the power consumed by the sampling resistor will not exceed its nominal power, so there will be no explosion.

3) The feedback circuit is controlled by TL43 1 and optocoupler. We all know that amplifier needs transmission time when it is used for signal transmission, and the output and input are not established at the same time. If the feedback signal is connected to the voltage feedback terminal of UC3842, the feedback signal needs to pass through two high-gain error amplifiers continuously, and the transmission time will increase. Since TL43 1 is a high gain error amplifier, in Figure 3, the pin 1 is directly used as feedback, and a resistor is pulled between pin 8 (reference voltage pin) and pin 1 of UC3842, and the pin 2 is grounded through R 18. The advantage of this method is to skip the internal amplifier of UC3842, thus shortening the transmission time of feedback signal by half and making the dynamic response of power supply faster. In addition, directly controlling the 1 pin of UC3842 can also simplify the frequency compensation and low output power of the system.

Fig. 3 Improved UC3842 application circuit

4 experimental results

Fig. 4 shows the voltage waveform and sampling signal waveform of the UC3842 detection resistor. As can be seen from Figure 4, the sampling signal waveform of the improved circuit closely follows the voltage waveform of the detection resistor, and there is no great peak voltage. Therefore, the circuit can effectively avoid the misoperation of power supply caused by abnormal interference such as transformer leakage inductance, and can also effectively avoid the system instability caused by excessive duty cycle of power supply.

Fig. 4 Detecting resistance voltage and sampling signal waveform

5 conclusion

UC3842 is a current-controlled pulse width modulator with excellent performance, but its protection circuit has some defects in practical application. Therefore, the protection circuit must be improved in the design of power supply. Experiments show that the improved protection circuit makes the system performance more stable and reliable.

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