Introduction The UC3842 is a high-performance current-controlled pulse width modulation chip manufactured by Untmde Systems, Inc. It is widely used due to its small pin count and simple peripheral circuitry. However, with the increase in the switching frequency of the UC3842, many problems have arisen in the protection circuits of switching power supplies constructed using it. This article analyzes the defects of the UC3842 protection circuit and proposes improvement methods. 1 Typical Application of UC3842 The typical application circuit of the UC3842 is shown in Figure 1. This circuit mainly consists of a bridge rectifier circuit, a high-frequency transformer, a MOSFET power transistor, and the current-mode pulse width modulation chip UC3842. Its working principle is as follows: 220V AC power passes through the bridge rectifier and filter circuit to obtain approximately +300V DC high voltage. This DC voltage is chopped by the MOSFET power transistor and stepped down by the high-frequency transformer, becoming a rectangular wave voltage with a frequency of tens of kHz. After output rectification and filtering, a stable DC output voltage is obtained. The voltage induced in the self-feeding coil N2 of the high-frequency transformer, after rectification by D2, is fed back to the error amplifier inside the UC3842 and compared with the reference voltage to obtain the error voltage Vr. Simultaneously, the DC voltage established across the sampling resistor R11 is also fed back to the same input terminal of the UC3842 current measurement comparator. This detected voltage is compared with the error voltage Vt to generate a pulse-width-adjustable drive signal, used to control the on and off times of the switching power transistor, thus determining the on/off state of the high-frequency transformer and achieving output voltage regulation. In Figure 1, R5 is used to limit the peak charging current generated by C8. Considering that the noise voltages on Vi and Vref will also affect the output pulse width, noise-suppressing capacitors C4 and C2 are connected to pins 7 and 8 of the UC3842, respectively. R7 is the gate current-limiting resistor of the MOS power transistor. Additionally, there is a 34V Zener diode between the input terminal and ground of the UC3842. If a high voltage is applied to the input terminal, this Zener diode will break down in reverse, clamping Vi to 34V and protecting the chip from damage. 2. Defects of the UC3842 Protection Circuit 2.1 Defects of Overload Protection When the power supply is overloaded or the output is short-circuited, the UC3842's protection circuit activates, reducing the duty cycle of the output pulse and lowering the output voltage. The UC3842's supply voltage also decreases accordingly. When it drops to a level where the UC3842 cannot operate, the entire circuit shuts down and then starts the next startup process through R6. This type of protection is called "hiccup" protection. In this protection state, the power supply only operates for a few switching cycles and then enters a long startup process (hundreds of milliseconds to seconds), therefore, its average power is very low. However, due to leakage inductance in the transformer, some switching power supplies have very high switching spike voltages in each switching cycle. Even with a very small duty cycle, the auxiliary supply voltage cannot drop low enough, thus failing to achieve the ideal protection function. 2.2 Circuit Stability Defects In the circuit shown in Figure 1, when the power supply duty cycle is greater than 50%, or the transformer operates under continuous current conditions, the entire circuit will generate subharmonic oscillations, causing instability in the power supply output. Figure 2 illustrates the change process of the inductor current in the transformer. At time t0, the switch begins to conduct, causing the inductor current to rise with a slope m1, which is a function of the input voltage divided by the inductance. At time t1, the current sampling input reaches the threshold established by the control voltage, which causes the switch to open, and the current decays with a slope m2 until the next oscillation cycle. If a disturbance is applied to the control voltage at this time, it will generate a ΔI. This will cause instability in the circuit. Specifically, within a fixed oscillator cycle, the gate current decreases as the current decays. The minimum current increases by ΔI + ΔIm2/m1 at the switch-on time t2, and decreases to (ΔI + ΔIm2/m4)(m2/m1) in the next cycle t3. In each subsequent cycle, this disturbance m2/m1 is multiplied, alternating between increasing and decreasing the inductor current when the switch is turned on. It may take several oscillator cycles to bring the inductor current to zero and restart the process. If m2/m1 is greater than 1, the converter will be unstable. Therefore, the circuit shown in Figure 1 has a certain potential for instability under certain conditions. 3. Improvement of the Protection Circuit Based on the above analysis, the improved circuit is shown in Figure 3. This circuit has the following characteristics. 1) By adding an emitter follower to the sampling voltage of the UC3842, an artificial ramp synchronized with the pulse width modulation clock is added to the control voltage, which can reduce the ΔI disturbance to zero in subsequent cycles. Therefore, even if the system operates under a duty cycle greater than 50% or continuous inductor current conditions, the system will not become unstable. However, the slope of this compensation ramp must be equal to or slightly greater than m²/2 for the system to have true stability. 2) The sampling resistor is replaced with a non-inductive resistor. A non-inductive resistor is a two-wire wound resistor, which has high precision and is easy to make into high-power resistors. With a non-inductive resistor, its impedance will not increase with the frequency. Thus, even at high frequencies, the power consumed by the sampling resistor will not exceed its nominal power, and therefore, the failure phenomenon will not occur. 3) The feedback circuit is replaced with a TL431 with an optocoupler for control. We all know that amplifiers used for signal transmission require transmission time; the output and input are not established simultaneously. If the feedback signal is connected to the voltage feedback terminal of the UC3842, the feedback signal needs to pass through two high-gain error amplifiers consecutively, increasing the transmission time. Since the TL431 itself is a high-gain error amplifier, pin 1 is directly used for feedback in Figure 3. A resistor is pulled from pin 8 (reference voltage pin) of the UC3842 to pin 1, and pin 2 is grounded through R18. The advantage of this is that it bypasses the internal amplifier of the UC3842, thereby halving the transmission time of the feedback signal and making the dynamic response of the power supply faster. In addition, directly controlling pin 1 of the UC3842 can also simplify the frequency compensation of the system and address issues such as low output power. 4 Experimental Results Figure 4 shows the voltage waveform of the sensing resistor and the sampling signal waveform of the UC3842. As can be seen from Figure 4, after the improvement, the waveform of the sampling signal closely follows the voltage waveform of the sensing resistor, without any very large voltage spikes. Therefore, this circuit can effectively avoid power supply malfunctions caused by abnormal interference such as transformer leakage inductance, and can also effectively avoid system instability caused by excessive power supply duty cycle. 5. Conclusion The UC3842 is a high-performance current-controlled pulse width modulator, but its protection circuit has certain shortcomings in practical applications. Therefore, its protection circuit must be improved during power supply design. Experiments show that the improved protection circuit makes the system performance more stable and reliable.