Voltage control is not desirable, current control UC3842 is shortlisted as an ideal power supply

The switching power supply controlled by voltage will lose control of the current, which is not conducive to overcurrent protection. The shortcomings of slow response and poor stability are obvious. In comparison, using current to control the switching power supply is a voltage and current dual closed-loop control system, which overcomes the disadvantages of current loss of control, has reliable performance and simple circuit. Therefore, we use the UC3842 chip to design a current-controlled switching power supply to improve the output voltage accuracy. The system does not adopt an offline structure, but a direct feedback structure. This system fully considers electromagnetic compatibility and safety factors in its design and is widely used in industry, home appliances, audio-visual and lighting equipment.

Schematic diagram of current-controlled switching power supply

Current-mode control is developed to address the shortcomings of voltage-mode control. In addition to retaining the output voltage feedback control part of the voltage-mode control, a current feedback link is added. Its principle block diagram is shown in Figure 1.

Figure 1 Schematic diagram of a current-controlled switching power supply

The current-controlled switching power supply is a voltage and current double closed-loop control system, with the inner loop being the current control loop and the outer loop being the voltage control loop. When changes in UO cause changes in UF, or changes in I cause changes in US, the output pulse duty cycle of the PWM circuit will change, thereby changing UO and achieving the purpose of output voltage stability.

Current mode control chip UC3842

UC3842 is a typical single-ended current-mode PWM control integrated circuit with complete functions, including error amplifier, current detection comparator, PWM latch, oscillator, internal reference power supply and undervoltage lockout. It provides 8-port dual in-line plastic package and 14-port plastic surface mount package, and its internal structure is shown in Figure 2.

The current-controlled switching power supply is a voltage and current double closed-loop control system, with the inner loop being the current control loop and the outer loop being the voltage control loop. When changes in UO cause changes in UF, or changes in I cause changes in US, the output pulse duty cycle of the PWM circuit will change, thereby changing UO and achieving the purpose of output voltage stability.

Current mode control chip UC3842

UC3842 is a typical single-ended current-mode PWM control integrated circuit with complete functions, including error amplifier, current detection comparator, PWM latch, oscillator, internal reference power supply and undervoltage lockout. It provides 8-port dual in-line plastic package and 14-port plastic surface mount package, and its internal structure is shown in Figure 2.

Figure 2 UC3842 internal circuit UC3842 8-port dual in-line plastic package UC3842 port function of each tube port brief introduction.

① Port COMP is the output of the internal error amplifier.

②Port VFB is the feedback voltage input terminal, which is compared with the +2.5V reference voltage at the non-inverting input terminal of the internal error amplifier to generate an error voltage to control the width of the pulse.

Port ③ ISENSE is the current sensing terminal. In the application circuit, a small-resistance sampling resistor is connected in series to the source of the MOSFET to convert the current of the pulse transformer into a voltage and send it to port ③ to control the width of the pulse.

④ Port RT/CT is the timing terminal. The oscillation frequency of the sawtooth oscillator is f=1.8/(RT·CT), and the current mode operating frequency can reach 500kHz.

⑤Port GND is grounded.

⑥ Port OUTPUT is the output port. This port is a totem pole output with a peak driving current of up to 1.0A.

⑦ Port VCC is the power supply. When the supply voltage is lower than 16V, UC3824 does not work, and the power consumption is less than 1mA. After the chip works, the input voltage can fluctuate between 10 and 30V, and the working current is about 15mA.

⑧Port VREF is the reference voltage output, which can output accurate +5V reference voltage and the current can reach 50mA.

UC3842 constitutes a current-controlled switching power supply

1 Circuit composition

The current-controlled switching power supply circuit composed of UC3842 is shown in Figure 3.

Figure 3 UC3842 constitutes a current-controlled switching power supply

2 Working Principle

The 220V AC power first passes through the filter network to filter out various interferences. The resistor R1 is mainly used to eliminate the residual voltage at the moment of power failure, the thermistor RT1 can limit the surge current, and the varistor VDR protects the circuit from the impact of lightning. Then, after rectification by B1 and filtering by C4, a DC voltage of about 300V is obtained and then output in two ways: one is added to the drain of MOSFET Q1 through the switching transformer T, and the other is added to the positive end of C17 through R3. When the positive terminal potential of C17 rises to ≥R16, the working voltage of port ⑦ is obtained, the UC3842 circuit is started, the potential of port ⑥ rises, Q1 starts to conduct, and the 5V voltage of port ⑧ is established through the internal circuit. The capacity of C17 is preferably above 100μF, otherwise the power supply will hiccup. The C12 filter capacitor eliminates the spike pulse generated during switching, C11 is a noise elimination capacitor, R6 and C13 determine the oscillation frequency of the sawtooth oscillator, and R9 and C15 are used to determine the gain and frequency response of the error amplifier. C14 acts as a slope compensation to improve the reliability of the sampling voltage. After normal operation, the high-frequency voltage on coil N2 provides the working voltage for UC3842 through D2, R17, C18, and D3.

When the switch is turned on, the electric energy of the rectified voltage added to the primary winding of the switch transformer is converted into magnetic energy and stored in the switch transformer. After the switch 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 appears at the moment of bypass startup, and 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 ratio, TON is the on-time of Q1, and TOFF is the off-time of Q2.

From the above formula, we can know that the output voltage is proportional to the on-time of the switch tube and the input voltage, and inversely proportional to the transformer ratio and the cut-off time of the switch tube. C16, R12, and D5 are used to limit the gate voltage and current, thereby improving the switching speed of Q1, which is beneficial to improving electromagnetic compatibility. R13 is mainly used to prevent the gate of Q1 from being suspended. D1, R4, C5 and D6, R16, and C20 form a two-stage absorption circuit to absorb peak voltage and prevent Q1 from being damaged. The voltage stabilization circuits in the system are:

● Current feedback circuit. The source of Q1 is connected in series with the sampling resistor R15, which converts the current signal into a voltage signal and sends it to the non-inverting terminal of the current detection comparator inside UC3842. When Q1 is turned on and the current slope rises, the voltage of the sampling resistor R15 increases. Once the voltage of R15 is equal to the voltage of the inverting terminal of the current detection comparator, the internal trigger is reset and Q1 is turned off, which means that the duty cycle of the excitation pulse of port ⑥ is controlled by current to stabilize the output voltage. C19 is used to suppress the sharp pulse of the sampling current.

● Voltage feedback circuit. It is mainly composed of programmable precision voltage regulator TL431 and linear photocoupler PC817. The output voltage is divided by R21 and R22 to obtain the sampling voltage, which is sent to the reference port of programmable precision voltage regulator TL431. By changing the resistance values ​​of R21 and R22, the voltage regulation value of TL431 can be changed, and the output voltage of the switching power supply can be changed. C21 and R19 perform phase compensation on the internal amplifier of programmable precision voltage regulator TI431. The system changes the feedback voltage of UC3842 by changing the luminous intensity of photocoupler U2 to achieve voltage regulation. When the output voltage increases, the voltage UKA across TL431 remains unchanged, the current at the control end of the optocoupler increases, and the voltage value at the feedback end of port ② increases accordingly. The voltage at the inverting end of the current detection comparator inside UC3842 becomes lower, and the duty cycle of the pulse signal at the output port ⑥ becomes lower, the conduction time of the switch tube decreases, and the output voltage decreases; on the contrary, if the output voltage decreases, the output pulse duty cycle of UC3842 increases, and the output voltage increases, achieving the purpose of voltage stabilization. On the other hand, the power supply voltage at port ⑦ is generated by D2 rectification and C18 filtering, reflecting the change of the output voltage, playing a feedback role, and stabilizing the output voltage.

● The circuit has a feedforward adjustment function. When the load remains unchanged, the input voltage suddenly increases, and the inductive current of the switching transformer rises rapidly due to the increase in input voltage. Since the feedback signal and error signal have not changed, the current limiting effect occurs relatively quickly, so the pulse width becomes relatively narrow. Therefore, the change of the mains power has been compensated before affecting the output, which improves the response speed to the input voltage.

Figure 4 Slope compensation

When the system operates under duty cycle greater than 50% or continuous inductor current conditions, harmonic oscillation will occur, which is caused by the simultaneous operation of fixed frequency and peak current sampling. Figure 4A shows this phenomenon. At t0, Q1 is turned on and the inductor current rises with a slope of m1. At t1, the current sampling input reaches the threshold established by the control voltage. This causes Q1 to be turned off and the current to decrease with a slope of m2 until the next oscillation cycle. If the system has a disturbance added to the control voltage, a small △I (dashed line in the figure) is generated, and the system will be unstable.

In order to make the system work reliably under the condition of duty cycle greater than 50% or continuous inductor current, the sawtooth voltage of port ④ is sent to port ③ through emitter follower Q2, thereby adding an artificial slope synchronized with the pulse width modulation clock to the current sampling end, which can reduce the △I disturbance to zero in the subsequent cycle, as shown in Figure 4B. The slope of the compensation slope must be equal to or slightly greater than m2/2 for the system to be stable.

The protection circuits designed in the system are:

● Output overvoltage protection circuit I. When the output voltage is high, the voltage at port ② exceeds 2.5V through the voltage feedback circuit, the internal trigger is reset and the external Q1 is cut off, thus achieving the purpose of output overvoltage protection.

● Output overvoltage protection circuit II. When the output voltage rises and is higher than the breakdown voltage of D9, the voltage stabilizing diode D9 breaks down, and the thyristor SCR is triggered to conduct, causing the negative terminal voltage of the photocoupler diode to drop to 0V, the photocoupler is saturated, the voltage at port ② is at the maximum value, and Q1 is always cut off, achieving the purpose of output overvoltage protection.

● Output overcurrent and overload protection circuit. When the circuit is overcurrent or overloaded, the output voltage decreases, and Q3, D4, and R8 form a secondary overcurrent and overload protection circuit. When the secondary is not overloaded, Q3 and D4 are turned off; when the secondary is overloaded, Q3 and D4 are turned on, the potential of port ④ decreases, and the sawtooth oscillator stops oscillating, achieving the purpose of overcurrent and overload protection.

● Q1 overcurrent protection circuit: When the power supply voltage is abnormal, the current in the switch circuit increases, and when the voltage on the sampling resistor R15 exceeds 1V, the internal trigger is reset and the external Q1 is cut off, effectively protecting Q1.

Summarize

This system uses the current-controlled switching power supply designed by UC3842 , which overcomes the shortcomings of the voltage-controlled power supply in terms of voltage regulation and load regulation, and has reliable performance and simple circuit. This power supply is an ideal power supply for 20-80W low-power switching power supplies.