Voltage-controlled Switching Power Supply Will switch the current out of control, not easy to over-current protection, and the slow response, poor stability. In comparison, current control type switching power supply is a voltage and current double closed loop control system, overcome the shortcomings of current out of control, and reliable, simple circuit. Accordingly, we UC3842 Chip Design a current control type switching power supply. In order to improve the accuracy of the output voltage, the system does not use off-line structure, and direct feedback structure. The system is designed to take full account of the electromagnetic compatibility and safety, can be widely used in industrial, household appliances, audio-visual and lighting equipment.

Current control block diagram of switching power supply

Current-mode control is a voltage-mode control for the shortcomings and developed, while retaining a voltage-controlled part of the output voltage feedback control, was added to a current feedback part, the principle of box shown in Figure 1.

Figure 1 Current Control of Switching Power Supply Block Diagram

Current control type switching power supply is a voltage and current double closed loop control system for the current control inner loop and outer loop for the voltage control. When UO UF changes cause changes in, or change lead to US changes I would make the circuit of the output pulse PWM duty cycle changes, thereby changing the UO, to achieve the purpose of output voltage stability.

Current-mode control chip UC3842 UC3842 is a fully functional, the more typical single-ended current-mode PWM control IC , Which contains the error amplifier, current sensing comparator, PWM latch, oscillator, power supply and the internal reference voltage lockout unit. It provides 8-port dual in-line plastic package and 14-port plastic surface mount package, the internal structure shown in Figure 2.

Figure 2 UC3842 internal circuit 8-port dual in-line plastic package features About UC3842 the control port.

port COMP is an internal error amplifier output.

Port feedback voltage VFB is input, and the internal error amplifier inverting input of +2.5 V reference voltage are compared, errors voltage control pulse width.

port ISENSE is the current sensing terminal. In the application circuit, the MOSFET's source and incorporates a small resistance of the sampling resistor, the Pulse transformer Converted into voltage and current into port, control pulse width.

port RT / CT is a regular client. Sawtooth oscillator, the oscillation frequency f = 1.8 / (RT? CT), current-mode frequency up to 500kHz.

port GND is grounded. OUTPUT is the output port, the port for the totem pole output drive current peaks up to l.0A.

port VCC is the power supply. When the supply voltage less than 16V when, UC3824 does not work, then 1mA power consumption in the following. Chip work, the input voltage can fluctuate between 10 ~ 30V, operating current is about 15mA.

VREF is the reference voltage output port can output +5 V precision reference voltage, current up to 50mA.

UC3842 constitute a current control type switching power supply

1 circuit UC3842 form of current control switching power supply circuit shown in Figure 3.

Figure 3 UC3842 current control type switching power supply form

2 works 220V AC pass filter network filter out all interference. Resistor R1 is mainly used to remove residual power transient voltage Thermistor RT1 can limit the inrush current protection circuit VDR varistor from the impact of lightning. Then, through the rectifier B1, C4 filter, a voltage of about 300V DC output by two routes: the way by switching transformer T added MOSFETQ1 the drain, add another pass R3 C17 positive terminal. When the C17 positive terminal when the potential to rise to R16, port was operating voltage, UC3842 circuit starts, port potential rise, Q1 starts to conduct, while port 5V voltage within the circuit through the establishment. C17 capacity of more than best lO0 F, otherwise the power will appear hiccup phenomenon. C12 filter Capacitance Elimination of the switch will generate peak pulse, C11 for the noise cancellation capacitor, R6, C13 determine the oscillation frequency sawtooth oscillator, R9, C15 is used to determine the error amplifier gain and frequency response. C14 from the slope compensation, can improve the reliability of the sampling voltage. After work, the high-frequency voltage coil N2 through D2, R17, C18, D3 to provide operating voltage for the UC3842.

When the switch turns on, the rectified voltage is added to the switch on the power transformer primary winding into magnetic energy stored in the switching transformer. After the closing switch, the energy released through the secondary winding to the load. D7, D8 is the pulse Rectifiers , C7, R5 instantaneous absorption occurs bypass boot pulse current, L3, C8, C9, C10 integral filter circuit. Output voltage can be described by the next.

UO = UI (TON / KTOFF) The formula, UO for the output voltage, UI for the rectifier voltage, K ratio for the transformer substation, TON for the Q1 on-time, TOFF deadline for the Q2.

From the above equation, the output voltage and switch on-time and the input voltage proportional to the transformer and the transformer and the switch than is inversely proportional to the deadline. C16, R12, D5 to limit the gate voltage and current, thereby improving the switching speed of Q1, will help to improve electromagnetic compatibility. R13 mainly to prevent the floating gate Q1, D1, R4, C5 and D6, R16, C20 constitute the two absorbing circuit for absorbing peak voltage to prevent damage Q1.

System voltage regulator are: current feedback circuit. Q1 source connected in series sampling resistor R15, the current signal into a voltage signal into the UC3842 current-sense comparator within the same phase side. When Q1 turns on, the current slope of the rise, increasing the voltage sampling resistor R15. Once R15 is equal to current detection voltage comparator inverting terminal voltage, internal trigger reset, Q1 ended, that is realized by current control port excitation pulse duty cycle to stabilize the output voltage. C19 is used to inhibit the sample current spike.

voltage feedback circuit. Mainly by the precision programmable linear regulator TL431 and optocoupler PC817 form. Output voltage by R21, R22 obtained by sampling voltage divider, and sent a programmable precision reference TL431 regulator port, change the R21, R22 the resistance, so that the regulator TL431 value changes, you can change the output voltage of switching power supply . C21, R19 TI431 programmable precision voltage regulator internal amplifier phase compensation. System by changing the optocoupler U2 to change the luminous intensity to achieve UC3842 feedback voltage regulator. When the output voltage rises, TL431

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