Using UC3842 single-ended flyback switching power supply design

It is a new type of control device developed by Unitrode . It is a current control type pulse width modulator that is widely used in China . The so-called current type pulse width modulator adjusts the pulse width according to the feedback current. The signal flowing through the output inductor coil is directly compared with the error amplifier output signal at the input end of the pulse width comparator, so as to adjust the duty cycle so that the output inductor peak current changes with the error voltage. Because there are voltage loop and current loop dual loop systems in the structure, the voltage regulation rate, load regulation rate and transient response characteristics of the switching power supply are improved, and it is a relatively ideal new type of controller.

Circuit Design and Principle

1.1 Working Principle of UC3842

UC3842 is an integrated chip with single power supply, current forward compensation, and single-channel modulation output. Its internal block diagram is shown in Figure 1. Among them, pin 1 is connected to an external resistor and capacitor to compensate for the frequency characteristics of the error amplifier. Pin 2 is the feedback voltage input terminal. The sampled voltage is added to the inverting input terminal of the error amplifier, and then compared with the reference voltage at the non-inverting input terminal to generate an error voltage. Pin 3 is the current detection input terminal, which cooperates with the resistor to form an overcurrent protection circuit. Pin 4 is connected to the external timing resistor of the sawtooth oscillator and the timing capacitor to determine the oscillation frequency. The reference voltage VREF is 0.5V. The output voltage will determine the transformer transformation ratio. As shown in Figure 1, it mainly includes functional circuits such as high-frequency oscillation, error comparison, undervoltage lockout, current sampling comparison, and pulse width modulation latch. UC3842 is mainly used for high-frequency small and medium-capacity switching power supplies. When the traditional offline flyback converter circuit composed of it drives the single-ended switch with isolated output, the signal obtained by the feedback winding through the resistor voltage division at the reverse input of the error comparator is usually compared with the internal 2.5V reference. The output of the error comparator and the reverse input are connected to form a PI compensation network. The output of the error comparator is compared with the current sampling voltage, thereby controlling the duty cycle of the PWM sequence to achieve the purpose of circuit stability.

1.2 System Principle

This paper uses UC3842 as the core control component to design a single-ended flyback switching power supply with AC 220V input and DC 24V output. The switching power supply control circuit is a voltage and current double closed-loop control system. The amplitude-frequency characteristic of the converter changes from a double pole to a single pole, so the gain-bandwidth product is improved, the stability amplitude is large, and it has good frequency response characteristics.

The main functional modules include: startup circuit, over-current, over-voltage and under-voltage protection circuit, feedback circuit and rectification circuit. The principles and functions of each module are analyzed below. The circuit schematic is shown in Figure 2.

1.2.1 Starting circuit

As shown in Figure 2, the AC power is low-pass filtered by C16, L1, C15, C14, and C13, where C16 and C15 form an anti-series mode interference circuit to suppress normal noise; C14, C13, and L1 form an anti-common mode interference circuit to suppress common mode noise interference. Their combined application has a strong attenuation bypass effect on electromagnetic interference. The filtered AC voltage is rectified by D1~D4 bridge and filtered by electrolytic capacitors C1 and C2 to become a pulsating DC voltage of 3lOV. This voltage is stepped down by R1 to charge C8 . When the voltage of C8 reaches the starting voltage threshold of UC3842, UC3842 starts to work and provides a driving pulse, which is output by pin 6 to drive the switch tube to work. With the startup of UC3842, the work of R1 is basically completed, and the remaining task is handed over to the feedback winding, which generates a voltage to power UC3842. Since the input voltage exceeds the working limit of UC3842, in order to avoid accidents, the D10 voltage regulator is used to limit the input voltage of UC3842, otherwise UC3842 will be damaged.

1.2.2 Short circuit overcurrent, overvoltage and undervoltage protection circuit

Due to the instability of input voltage or some other external factors, sometimes the circuit may have short circuit, overvoltage, undervoltage and other phenomena that are not conducive to the operation of the circuit. Therefore, the circuit must have certain protection functions. As shown in Figure 2, if the output end is short-circuited and overcurrent occurs due to some reason, the drain current of the switch tube will increase significantly, the voltage across R9 will increase, and the voltage on pin 3 of UC3842 will also increase. When the voltage of this pin exceeds the normal value of 0.3V to 1V (that is, the current exceeds 1.5A), the PWM comparator of UC3842 outputs a high level, resets the PWM latch, and turns off the output. At this time, pin 6 of UC3842 has no output, and MOS tube S1 is turned off, thereby protecting the circuit. If the power supply voltage is overvoltage (above 265V), UC3842 cannot adjust the duty cycle, the primary winding voltage of the transformer is greatly increased, the power supply voltage of pin 7 of UC3842 also rises sharply, and the voltage of its pin 2 also rises, turning off the output. If the voltage of the power grid is lower than 85V, the voltage of pin 1 of UC3842 will also drop. When it drops below 1V (normal value is 3.4V), the PWM comparator outputs a high level, resets the PWM latch, and turns off the output. If the output terminal is accidentally short-circuited, the output current will increase exponentially, causing the heat inside the automatic recovery switch RF to surge, and it will immediately disconnect the circuit to play an overvoltage protection role. Once the fault is eliminated, the automatic recovery switch RF quickly restores impedance within 5s. Therefore, this circuit has triple protection of short-circuit overcurrent, overvoltage, and undervoltage.

1.2.3 Feedback Circuit

The feedback circuit uses the precision voltage regulator TL431 and the linear optocoupler PC817. The TL431 adjustable precision voltage regulator is used to form an error voltage amplifier, and the output is precisely adjusted through the linear optocoupler. As shown in Figure 2, R4 and R5 are external control resistors of the precision voltage regulator. They determine the output voltage and form an external error amplifier together with TL431. When the output voltage increases, the sampling voltage VR7 also increases. The set voltage is greater than the reference voltage (the reference voltage of TL431 is 2.5V), which increases the output voltage of the error amplifier in TL431, causing the output voltage of the on-chip driving transistor to decrease, and also causing the output voltage Vo to decrease, and finally Vo tends to stabilize; conversely, the output voltage decreases, causing the set voltage to decrease. When the output voltage is lower than the set voltage, the output voltage of the error amplifier decreases, and the output voltage of the on-chip driving transistor increases, which eventually causes the compensation input current of pin 1 of UC3842 to change, prompting the on-chip PWM comparator to adjust and change the duty cycle to achieve the purpose of voltage regulation. The resistance values ​​of R7 and R8 are calculated as follows: first fix the resistance value of R7, then calculate the resistance value of R8, that is,

1.2.4 Rectification and filtering circuit

The output rectifier filter circuit directly affects the size of the voltage ripple and the performance of the output voltage. The influence of the ripple amplitude at the output end of the switching power supply mainly includes the following aspects.

(1) Input power noise refers to the AC component contained in the input power. The solution is to add capacitor C5 at the power input to filter out this noise interference.

(2) High-frequency signal noise. In the switching power supply, the DC input is chopped at high frequency and then transmitted through a high-frequency transformer. In this process, high-frequency noise interference is inevitable. There is also high-frequency noise caused by the power tube device during the switching process. The solution to this type of high-frequency noise is to use π-type filtering at the output end. The filter inductor uses a 150μH inductor to filter out high-frequency noise.

(3) Adopt fast recovery diodes D6 and D7 for rectification. Based on the characteristics of low voltage, low power consumption and high current, it is beneficial to improve the efficiency of the power supply. Its short reverse recovery time is beneficial to reduce high-frequency noise.

Parallel rectifier diodes reduce peak voltage

In high-power rectifier circuits, the secondary rectifier bridge circuit has large stray inductance. When the output rectifier tube is commutating, due to the parasitic oscillation in the circuit, the rectifier tube will be subjected to a large peak voltage. The existence of the peak voltage increases the withstand voltage requirements for the rectifier diode and will also bring additional circuit losses. The parasitic oscillation of the rectifier bridge is generated between the leakage inductance of the transformer (or additional resonant inductance) and the winding capacitance of the transformer and the junction capacitance of the rectifier tube.

When the secondary voltage is zero, all four diodes in the full-bridge rectifier are turned on, and the output filter inductor current is in a natural freewheeling state. When the secondary voltage changes to a high voltage Vin/K (K is the transformer ratio), two diodes in the rectifier bridge are turned off, and two diodes continue to be turned on. At this time, the leakage inductance of the transformer (or the additional resonant inductance) begins to resonate with the capacitance of the turned-off rectifier diode. Even if a fast recovery diode is used, the diode will still withstand at least twice the peak voltage. Therefore, an effective snubber circuit must be used. Many literatures have studied this, and there are 5 ways in summary: RC snubber circuit, RCD snubber circuit, active clamping snubber circuit, the third winding plus a diode clamping snubber circuit, and the primary side plus a diode clamping snubber circuit. Here is another effective method to reduce the diode peak voltage: that is, the rectifier diodes are connected in parallel, and its specific circuit diagram is shown in Figure 3.

This method has been applied in the project of high-power full-bridge phase-shifted DC/DC power converter. The experimental waveform verifies the method. The experimental results are shown in Figure 4, where Figure 4(a) is the rectifier bridge voltage waveform. It can be seen that due to the high-frequency oscillation between the leakage inductance of the transformer, the junction capacitance of the diode and the winding capacitance of the transformer, the diode has a very high peak voltage; Figure 4(b) is the rectifier bridge voltage waveform after using parallel rectifier diodes. Obviously, the peak voltage is much reduced, which verifies the effectiveness of this method.

Experimental results and analysis

The designed circuit was experimented, and the experimental waveforms are shown in Figure 5. The upper waveform in Figure 5 (a) is the triangle wave oscillation waveform of pin 4 of UC3842, and the lower waveform is the PWM wave of pin 6 of UC3842 driving the switch tube; the upper waveform in Figure 5 (b) is the DC component Vdc of the output voltage at full load, and the lower waveform is the AC ripple Vripp.

UC3842 is a high-performance fixed-frequency current-mode controller with single-ended output. It can directly drive transistors and MOSFETs. It has the advantages of small number of pins, simple peripheral circuits, easy installation and debugging, excellent performance, and low price. It has good application prospects in switching power supplies below 100W.