Voltage-controlled switching power supplies suffer from current runaway, making overcurrent protection difficult, and exhibit slow response and poor stability. In contrast, current-controlled switching power supplies are dual closed-loop control systems (voltage and current), overcoming the current runaway problem while offering reliable performance and a simpler circuit. Based on this, we designed a current-controlled switching power supply using the UC3842 chip. To improve output voltage accuracy, the system does not employ an offline structure but rather a direct feedback structure. This system design fully considers electromagnetic compatibility and safety, and can be widely used in industrial, home appliance, audio-visual, and lighting equipment. [Figure 1 : Block Diagram of Current-Controlled Switching Power Supply] Current-controlled switching power supplies were developed to address the shortcomings of voltage-controlled power supplies. In addition to retaining the output voltage feedback control section of voltage-controlled power supplies, a current feedback loop is added. The block diagram is shown in Figure 1. [align=center] Figure 1: Block Diagram of Current-Controlled Switching Power Supply[/align] The current-controlled switching power supply is a dual closed-loop control system (voltage and current), with an inner current control loop and an outer voltage control loop. When a change in UO leads to a change in UF, or a change in I leads to a change in US, the duty cycle of the PWM circuit's output pulse will change, thereby altering UO and achieving output voltage stability. The UC3842 is a fully functional and typical single-ended current-type PWM control integrated circuit, containing an error amplifier, current sensing comparator, PWM latch, oscillator, internal reference power supply, and undervoltage lockout. It is available in an 8-port dual in-line plastic package and a 14-port plastic surface mount package. Its internal structure is shown in Figure 2. [align=center] Figure 2 UC3842 Internal Circuit[/align] Brief description of the port functions of the UC3842 in the 8-port dual in-line plastic package: ① Port COMP is the output of the internal error amplifier. ② Port VFB is the feedback voltage input, compared with the +2.5V reference voltage at the non-inverting input of the internal error amplifier to generate an error voltage, controlling the pulse width. ③ Port ISENSE is the current sensing terminal. In the application circuit, a small sampling resistor is connected in series with the source of the MOSFET to convert the current of the pulse transformer into voltage and send it to port ③ to control the pulse width. Port ④ RT/CT is the timing terminal. The oscillation frequency of the sawtooth wave oscillator is f=1.8/(RT·CT), and the current mode operating frequency can reach 500kHz. Port ⑤ GND is ground. Port ⑥ OUTPUT is the output terminal. This port is a totem-pole output, and the peak drive current is as high as 1.0A. Port ⑦ VCC is the power supply. When the supply voltage is lower than 16V, the UC3824 does not work, and the power consumption is less than 1mA. After the chip is working, the input voltage can fluctuate between 10 and 30V, and the operating current is about 15mA. Port ⑧ VREF is the reference voltage output, which can output a precise +5V reference voltage, and the current can reach 50mA. The circuit composition of the current-controlled switching power supply constructed by UC3842 is shown in Figure 3. [align=center]Figure 3 UC3842 forming a current-controlled switching power supply[/align] 2. Working Principle The 220V AC power first passes through a filter network to remove various interferences. Resistor R1 is mainly used to eliminate the residual voltage at the moment of power failure, thermistor RT1 can limit surge current, and varistor VDR protects the circuit from lightning strikes. Then, after rectification by B1 and filtering by C4, approximately 300V DC voltage is obtained and output in two paths: one path is applied to the drain of MOSFET Q1 through switching transformer T, and the other path is applied to the positive terminal of C17 through R3. When the potential of the positive terminal of C17 rises to ≥R16, the working voltage is obtained at port ⑦, the UC3842 circuit starts, the potential at port ⑥ rises, Q1 starts to conduct, and at the same time, the 5V voltage at port ⑧ is established through the internal circuit. The capacitance of C17 should preferably be above 100μF, otherwise the power supply will hiccup. C12 is a filter capacitor to eliminate spike pulses generated during switching. C11 is a noise-suppressing capacitor. R6 and C13 determine the oscillation frequency of the sawtooth wave oscillator. R9 and C15 are used to determine the gain and frequency response of the error amplifier. C14 provides slope compensation, which improves the reliability of the sampling voltage. After normal operation, the high-frequency voltage on coil N2 provides the operating voltage for UC3842 through D2, R17, C18, and D3. When the switching transistor is turned on, the electrical energy applied to the primary winding of the switching transformer is converted into magnetic energy and stored in the switching transformer. After the switching transistor is turned off, the energy is released to the load through the secondary winding. D7 and D8 are pulse rectifier diodes. C7 and R5 absorb the pulse current that occurs at the moment of bypass power-on. L3, C8, C9, and C10 form a filter circuit. The output voltage can be described by the following formula. UO = UI(TON/KTOFF) Where UO is the output voltage, UI is the rectified voltage, K is the transformer's turns ratio, TON is the on-time of Q1, and TOFF is the off-time of Q2. From the above formula, it can be seen that the output voltage is directly proportional to the on-time of the switching transistor and the input voltage, and inversely proportional to the transformer's turns ratio and the off-time of the switching transistor. C16, R12, and D5 are used to limit the gate voltage and current, thereby improving the switching speed of Q1 and improving electromagnetic compatibility. R13 mainly prevents the Q1 gate from being floating. D1, R4, C5 and D6, R16, C20 form a two-stage absorption circuit to absorb voltage spikes and prevent damage to Q1. The voltage regulation circuit in the system includes: ● Current feedback circuit. A sampling resistor R15 is connected in series with the source of Q1 to convert the current signal into a voltage signal, which is sent to the non-inverting input of the current detection comparator inside the UC3842. When Q1 is turned on and the current slope increases, the voltage across the sampling resistor R15 increases. Once the voltage across R15 equals the voltage at the inverting input of the current sensing comparator, the internal trigger is reset, Q1 is cut off, thus stabilizing the output voltage by controlling the duty cycle of the excitation pulse at port ⑥. C19 is used to suppress spikes in the sampling current. ● Voltage Feedback Circuit. This mainly consists of a programmable precision voltage regulator TL431 and a linear optocoupler PC817. The output voltage is divided by R21 and R22 to obtain the sampling voltage, which is sent to the reference port of the programmable precision voltage regulator TL431. Changing the resistance values ​​of R21 and R22 changes the voltage regulation value of TL431, thereby changing the output voltage of the switching power supply. C21 and R19 provide phase compensation for the internal amplifier of the programmable precision voltage regulator TL431. The system achieves voltage regulation by changing the luminous intensity of optocoupler U2 to change the voltage at the feedback terminal of UC3842. When the output voltage increases, the voltage UKA across TL431 remains constant, the current at the optocoupler control terminal increases, and the voltage at port ② (feedback terminal) increases accordingly. The voltage at the inverting input of the current detection comparator inside UC3842 decreases, the duty cycle of the pulse signal at output port ⑥ decreases, the on-time of the switching transistor decreases, and the output voltage decreases. Conversely, if the output voltage decreases, the duty cycle of the UC3842's output pulse increases, and the output voltage increases, achieving voltage stabilization. On the other hand, the power supply voltage at port ⑦, rectified by D2 and filtered by C18, reflects changes in the output voltage, providing feedback and stabilizing the output voltage. ● The circuit has a feedforward adjustment function. When the load remains constant, a sudden increase in input voltage causes the induced current in the switching transformer to rise rapidly due to the increased input voltage. Because the feedback and error signals have not yet changed, the current limiting effect occurs relatively quickly, resulting in a narrower pulse width. Therefore, changes in the mains power are compensated for before affecting the output, thus improving the response speed to the input voltage. [align=center]Figure 4 Slope Compensation[/align] When the system operates with a duty cycle greater than 50% or under continuous inductor current conditions, harmonic oscillations will occur. This is caused by the simultaneous operation of a fixed frequency and peak current sampling, as shown in Figure 4A. At time t0, Q1 is turned on, and the inductor current rises with a slope m1. At time t1, the current sampling input reaches the threshold established by the control voltage. This causes Q1 to turn off, and the current decreases with a slope m2 until the next oscillation cycle. If a disturbance is applied to the control voltage, generating a small ΔI (dashed line in the figure), the system will be unstable. To ensure reliable operation of the system under duty cycle greater than 50% or continuous inductor current conditions, the sawtooth wave voltage at port ④ is fed into port ③ through emitter follower Q2, thus adding an artificial slope synchronized with the pulse width modulation clock at the current sampling terminal. This can reduce the ΔI disturbance to zero in subsequent cycles, as shown in Figure 4B. The slope of this compensation slope must be equal to or slightly greater than m2/2 for the system to be stable. The system's protection circuits include: ● Output overvoltage protection circuit I. When the output voltage is high, the voltage feedback circuit causes the voltage at port ② to exceed 2.5V, resetting the internal trigger and cutting off external Q1, thus achieving output overvoltage protection. ● Output overvoltage protection circuit II. When the output voltage rises above the breakdown voltage of D9, the Zener diode D9 breaks down, triggering the SCR to conduct, causing the voltage at the negative terminal of the optocoupler diode to drop to 0V, saturating the optocoupler, and reaching its maximum value at port ②. Q1 remains cut off, achieving output overvoltage protection. ● Output overcurrent and overload protection circuit. In case of overcurrent or overload, the output voltage decreases. Q3, D4, and R8 form the secondary overcurrent and overload protection circuit. When the secondary is not overloaded, Q3 and D4 are cut off; when the secondary is overloaded, Q3 and D4 conduct, the potential at port ④ drops, and the sawtooth oscillator stops oscillating, achieving overcurrent and overload protection. ● Q1 overcurrent protection circuit. When the power supply voltage is abnormal, the current in the switching circuit increases. When the voltage across the sampling resistor R15 exceeds 1V, the internal trigger resets, and the external Q1 is cut off, effectively protecting Q1. Conclusion This system uses a current-controlled switching power supply designed with UC3842, overcoming the shortcomings of poor voltage and load regulation in voltage-controlled switching power supplies. It also boasts reliable performance and a simple circuit. This power supply is ideal for low-power switching power supplies ranging from 20 to 80W.