Improvement of a switching power supply protection circuit

introduction

UC3842 is a high-performance current-controlled pulse width modulation chip produced by Unltmde, USA. It has the characteristics of small number of pins and simple peripheral circuits, so it has been widely used. However, with the increase of UC3842 switching frequency, the protection circuit of the switching power supply composed of it has also encountered many problems. This article analyzes the defects of UC3842 protection circuit and the improvement method.

1 Typical Applications of UC3842

The typical application circuit of UC3842 is shown in Figure 1. The circuit is mainly composed of a bridge rectifier circuit, a high-frequency transformer, a MOS power tube and a current-type pulse width modulation chip UC3842. Its working principle is: after the 220V AC passes through the bridge rectifier filter circuit, a DC high voltage of about +300V is obtained. This DC voltage is chopped by the MOS power tube and stepped down by the high-frequency transformer to become 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 is fed back to the error amplifier inside UC3842 after rectification by D2, and the DC voltage obtained is compared with the reference voltage to obtain the error voltage Vr. At the same time, the DC voltage established on the sampling resistor R11 is also fed back to the same input terminal of the current measurement comparator of UC3842. This detection voltage and the error voltage

Compared with Vt, a driving signal with adjustable pulse width is generated to control the on and off time of the switching power tube to determine the on and off state of the high-frequency transformer, thereby achieving the purpose of output voltage regulation. In Figure 1, R5 is used to limit the charging peak current generated by C8. Considering that the noise voltage on Vi and Vref will also affect the output pulse width, noise elimination capacitors C4 and C2 are connected to pins 7 and 8 of UC3842 respectively. R7 is the gate current limiting resistor of the MOS power tube. In addition, there is a 34V voltage regulator between the input end and the ground of UC3842. Once high voltage appears at the input end, the voltage regulator will be reversely broken down, clamping Vi at 34V to protect the chip from damage.

2 Defects of UC3842 protection circuit

2.1 Defects of overload protection

When the power supply is overloaded or the output is short-circuited, the protection circuit of UC3842 is activated, reducing the duty cycle of the output pulse, lowering the output voltage, and the supply voltage of UC3842. When it is so low that UC3842 cannot work, the entire circuit is closed, and then the next startup process is initiated 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 very long (hundreds of ms to a few seconds) startup process, so its average power is very low. However, due to the leakage inductance of the transformer and other reasons, some switching power supplies have a high switching spike voltage in each switching cycle. Even when the duty cycle is very small, the auxiliary supply voltage cannot be reduced to a low enough level, so the ideal protection function cannot be achieved.

2.2 Defects of overcurrent protection

The overcurrent protection function of UC3842 is realized through pin 3. When the voltage detected on pin 3 is higher than lV, the comparator inside UC3842 will flip, the PWM latch will be set to zero, and the pulse modulator will be in the off state, thus realizing the overcurrent protection of the circuit. Since the detection resistor can sense the peak inductor current, a pulse-by-pulse current limiting circuit is naturally formed. As long as the level on the detection resistor reaches lV, the pulse width modulator will be immediately turned off. Therefore, this peak inductor current detection technology can accurately limit the maximum output current, so that the magnetic components and power devices in the switching power supply do not need to be designed with a large margin, and the operation of the voltage-stabilized power supply can be guaranteed to be reliable. However, the sampling resistors we usually use are metal film or oxide film resistors. This type of resistor is inductive. When current flows through the sampling resistor, a certain inductive voltage will be induced. The impedance presented by this inductor component will be very large at high frequencies, so it will consume a lot of power. As the frequency increases, the current flowing through the sampling resistor may not be fully discharged before the next oscillation cycle arrives, and the current borne by the sampling resistor will become larger and larger, which will cause UC3842 to malfunction or even cause the machine to explode. Therefore, the overcurrent protection function of UC3842 sometimes cannot play a good protective role and has certain defects.

2.3 Circuit stability defects

In the circuit shown in Figure 1, when the duty cycle of the power supply 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 shows the change process of the inductor current in the transformer. At t0, the switch begins to conduct, causing the inductor current to rise at a slope m1, which is a function of the input voltage divided by the inductance. At t1, the current sampling input reaches the threshold established by the control voltage, which causes the switch to turn off and the current to decay at a slope m2 until the next oscillation cycle. If a disturbance is added to the control voltage at this time, it will generate a △I, so we will find that the circuit is unstable, that is, in a fixed oscillator cycle, the gate decreases when the current decays, and the minimum current switch is turned on at t2, which rises by △I+△Im2/m1. The minimum current decreases to (△I+△Im2/m4) (m2/m1) in the next cycle t3. In each subsequent cycle, the disturbance m2/m1 is multiplied, and the inductor current is alternately increased and decreased when the switch is turned on. It may take several oscillator cycles to make the inductor current zero and restart the process. If m2/m1 is greater than l, it will be unstable. Therefore, the circuit shown in Figure 1 has a certain risk of instability in a certain state.

3 Improvement of protection circuit

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

1) By connecting an emitter follower at the sampling voltage of UC3842, an artificial slope synchronized with the pulse width modulation clock is added to the control voltage, which can reduce the △I disturbance to zero in the subsequent cycle. Therefore, even if the system operates under the condition of duty cycle greater than 50% or continuous inductor current, the system will not be unstable. However, the slope of the compensation slope must be equal to or slightly greater than m2/2 for the system to have real stability.

2) The sampling resistor is replaced by a non-inductive resistor. A non-inductive resistor is a winding resistor with two wires wound in parallel. It has high precision and is easy to achieve high power. After using a non-inductive resistor, its impedance will not increase with the increase of frequency. In this way, even in high-frequency conditions, the power consumed by the sampling resistor will not exceed its nominal power, so there will be no explosion phenomenon.

3) The feedback circuit is controlled by TL431 plus an optocoupler. We all know that when an amplifier is used for signal transmission, it needs transmission time, 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 increases. Since TL431 itself is a high-gain error amplifier, in Figure 3, pin 1 is directly used for feedback, and a resistor is pulled from pin 8 (reference voltage pin) of UC3842 to pin l, and pin 2 is grounded through R18. The advantage of this is that the internal amplifier of UC3842 is skipped, thereby shortening the transmission time of the feedback signal by half and making the dynamic response of the power supply faster. In addition, directly controlling pin l of UC3842 can also simplify the system's frequency compensation and low output power.

4 Experimental Results

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

5 Conclusion

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