The hottest current controlled PWM switch based on

2022-07-26
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Current controlled PWM switching power supply based on UC3842

Abstract: the characteristics and principle of UC3842 are introduced, and its current controlled PWM switching power supply is analyzed and tested. The experimental results show that the switching regulated power supply has the characteristics of fast frequency response, high voltage regulation rate and load regulation rate. It is a switching regulated power supply with good performance

key words: UC3842; Switching regulated power supply; Voltage regulation rate; Load regulation rate

1 introduction

power supply device is an important field in the application of power electronics technology. Among them, high-frequency switching DC regulated power supply has been widely used because of its outstanding advantages such as high efficiency, small size and light weight. The control circuit of switching power supply can be divided into voltage control type and current control type. The former is a single closed-loop voltage control system. What are the functional characteristics of the system in response to the 100t microcomputer controlled electro-hydraulic servo hydraulic universal experimental machine? Slow, it is difficult to achieve high accuracy of linear adjustment rate; The latter is a voltage and current double closed-loop control system, and the current control type has incomparable advantages over the voltage control type

UC3842 introduced in this paper is a high-performance single terminal output current controlled pulse width modulator chip produced by United States. Compared with the voltage controlled pulse width modulation switching regulated power supply, the switching regulated power supply composed of this integrated circuit has the following characteristics:

1) the number of pins is small, the peripheral circuit is simple, and the price is low

2) good voltage regulation rate

3) the load adjustment rate was significantly improved

4) good frequency response characteristics and large stable range

5) it has the functions of overcurrent limitation, overvoltage protection and undervoltage locking. Therefore, it is an ideal new type of pulse width modulator at present. Its internal principle block diagram is shown in Figure 1

2 brief introduction to the working state of UC3842 spring change experimental machine together with the installed standard torque meter

2.1 main features

working voltage 8 ~ 40 v

current sensing and voltage feedback input - 0.3 ~ + 5.5 v

error amplification output suction current 10 ma

undervoltage latch function

adjustable duty cycle

maximum switching frequency 500 kHz, stability 0.2%, High power efficiency

there is a high stability reference voltage source inside. The 5.0 v

has good stability, and the voltage adjustment rate can easily reach 0.01%. The operating principle of

2.2

uc3842 is an 8-pin dual in-line package, as shown in Figure 1. It is mainly composed of a 5.0 V reference voltage source, an oscillator used to accurately control the duty cycle setting, a step-down device, a current measurement comparator, a PWM latch High gain E/a error amplifier and high current push-pull output circuit suitable for driving power MOSFET

end 1 is comp end

end 2 is the feedback end

terminal 3 is current measurement

terminal 4 is connected to RT and CT to determine the sawtooth frequency; Terminal 5 is grounded

terminal 6 is a push-pull output terminal with the ability to pull and inject current

terminal 7 is the working power supply voltage of the integrated block, which can be between 8 and 40 V

terminal 8 is a reference voltage of 5 V for internal and external use, with a load capacity of 50 ma

the current controlled pulse width modulator can generate a driving signal with a fixed frequency and adjustable pulse width, control the on-off state of the high-power transistor to adjust the output voltage, and achieve the purpose of voltage stabilization. The sawtooth wave generator provides a constant clock frequency signal, uses the error amplifier and the current measurement comparator to form a voltage closed loop, and uses the current measurement and current measurement comparator to form a current closed loop, At the input end of the pulse width comparator, the signal flowing through the output inductance current is directly compared with the output signal of the error amplifier, so as to adjust the duty cycle of the driving signal so that the output inductance peak current changes with the change of the error voltage. If the output voltage rises for some reason, the pulse width modulator will change the pulse width of the driving signal, that is, the duty cycle D, so that the average voltage after chopping will decrease, So as to achieve the purpose of voltage stabilization, and vice versa

3 application circuit

Figure 2 shows the current controlled PWM switching regulated power supply composed of chip UC3842, bridge rectifier circuit, high-frequency transformer and high-power transistor. This new control mode makes the switching power supply have high voltage regulation rate and load regulation rate at the same time

its working principle is as follows: 220 V AC gets a DC voltage through the bridge rectifier circuit and filter circuit, and then the rectangular wave voltage with a frequency of tens of kHz is obtained through the continuous on and off of the high-power transistor and the step-down effect of the high-frequency transformer T2, and then the DC output voltage is obtained after rectification and filtering, The DC voltage induced in the self feeding coil NS2 of the high frequency transformer T2 is fed back to the error amplifier and compared with the reference voltage to obtain the error voltage VR. The DC voltage established by the current flowing through the primary coil on the sampling resistance 0.25 is fed back to the in-phase input terminal of the current measurement comparator and compared with the error voltage VR to generate a drive signal with adjustable pulse width, The output inductance peak current changes with the change of error voltage, and the output voltage is stabilized at a fixed value

for example, when the output voltage decreases due to power supply voltage change or load change, the pulse width modulator will correspondingly increase the duty cycle of the output PWM waveform, making the conduction time of the high-power transistor longer; On the contrary, when the output voltage rises due to the change of power supply voltage or load, the pulse width modulator will correspondingly reduce the duty cycle of PWM waveform to shorten the on time of high-power transistor, so as to maintain the output voltage at a constant value

4 experimental results

in order to test the performance of the current controlled PWM switching regulated power supply, this paper has conducted tests according to the above application circuits. The waveform, load adjustment rate and voltage adjustment rate of the switching power supply are shown in Figure 3, table 1 and table 2

4.1 voltage regulation rate (anti voltage fluctuation ability) test

adjust RP1 to reduce vs (DC input voltage). Use multimeter 42 to measure output voltage Vo1 and VO2 before and after vs change. Measure 5 data to form 4 groups. The voltage regulation rate is vo1-vo2/Vo1 100%

from the data obtained in Table 1, it can be calculated that the voltage regulation rate of the regulated power supply is:

(0.009%+0.018%+0+0.018%)/4=0.011%v

4.2 load regulation rate (anti load fluctuation ability) test

rotate RP3 to the left, measure the output voltage Vo1 with a multimeter, then rotate RP3 to the right, measure the output voltage VO2, measure 5 data, and form 4 groups, then the load regulation rate is vo1-vo2/Vo1 100%

from the data obtained in Table 2, it can be calculated that the load adjustment rate of the regulated power supply is:

accuracy problem

(2.17%+2.59%+1.61%+2.03%)/4 = 2.10%/v

4.3 switching power supply waveform test

observe the waveform of output voltage Vo with an oscilloscope, as shown in Figure 3

5 conclusion

this paper uses the integrated chip UC3842 to design the current mode PWM switching regulated power supply, which overcomes the shortcomings of the voltage control type PWM switching regulated power supply with slow frequency response, low voltage regulation rate and load regulation rate in order to detect the maximum service life of these small parts. The circuit structure is simple, low cost, small volume and easy to realize. This stabilized voltage power supply is a practical and ideal stabilized voltage power supply at present, and has a great development prospect

references

1 〕 Wang Zhaoan, Huang Jun. power electronics technology [ m ]. Beijing: Machinery Industry Press, 2002.

2 [ us ] Gottlieb im. regulated power supply [ m ]. Beijing: Science Press, 1993

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