Design of precise output voltage circuit using PWM

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Use PWM to get precise output voltage circuit design

in recent years, many single chip microcomputer manufacturers, such as ATMEL, analog divisions, Intel, Philips, Dallas, Maxim, etc., have launched new high-speed single chip microcomputer. Their instruction execution cycle is only 1/3~1/dozen of the original, and EEPROM, WDT, a/d converter and d/a converter are integrated in the single chip microcomputer, which greatly improves the performance of the single chip microcomputer and facilitates users. However, the output of d/a converter in many single chip computers adopts the form of pulse width modulation (PWM). PWM is very suitable for the control of switching power supply, thyristor and other devices. It can also be used in LCD brightness control, audio output and other occasions that do not need to output accurate voltage. Because PWM has no reference voltage, the amplitude of its output pulse is not very constant, which limits the application range of PWM. PWM cannot be used in occasions requiring precise control voltage output, such as precision adjustable voltage source, motor frequency converter, etc

however, only two cheap integrated circuits can be used to convert PWM output with variable amplitude into precise PWM output voltage

1 circuit principle

the circuit principle of obtaining voltage precision PWM pulse by using three terminal precision reference power supply and analog switch is shown in Figure 1. D1 is a three terminal reference voltage integrated circuit of oil tube TL431 that needs to be replaced with a higher strength, and U1 adopts single pole double throw analog switch max4544; Resistors R1, R2 and R3 are determined according to specific needs. Of course, other types of integrated circuits can also be used

when the PWM pulse is at high level (logic 1), the com terminal of U1 is thrown to the normally closed terminal (NC), the adjusting pin of TL431 is connected to the positive voltage pin, and the output voltage is 2.5 v. When the PWM pulse is low level (logic 0), the com terminal of U1 is thrown to the normal start (no), and the output voltage of TL431 is sent to the adjusting pin after R2 and R3 voltage division. At this time, the output voltage value is equal to [(r2+r3)/r3] 2.5 v. In this example, the output voltage is equal to 8 v. Thus, when a PWM signal is input to the in pin of U1, the circuit correspondingly outputs a PWM pulse with a high level of 8 V and a low level of 2.5 V, with an amplitude of 8 V - 2.5 V = 5.5 v. If you need to output a PWM signal with a low level of zero, it can be solved by adding a differential amplifier

simpler methods can be used in situations where precision is not required. Figure 2 is the circuit diagram of using precision zener diode to stabilize and limit the amplitude of PWM pulse. In Figure 2, the PWM signal is amplified into a 12 V output voltage by the high-speed operational amplifier U1. After the current limiting of R1 and the voltage stabilizing of D1, a 6.5 V PWM pulse output is obtained

2 error analysis

in Figure 1, as long as the reference power supply is properly selected, the error of the reference power supply itself can be completely ignored. In addition, the error sources mainly include the following aspects:

(1) the error caused by the on resistance of the analog switch

there is a certain on resistance when the analog switch is on. When the input current of TL431 adjusting pin passes through the analog switch, the voltage drop will be formed, resulting in error. The on resistance of max4544 is 35, while the input current of the adjusting pin of TL431 is less than 4 a. As a result, the reference voltage error is less than 140 V, which is 0.000056 of 2.5 V, equivalent to the binary 14 bit accuracy

(2) the error introduced by the switching delay time

the switching delay time will cause the change of pulse duty cycle, resulting in the error of PWM output pulse. The max4544 has an on time of 30 ns and an off time of 25 ns. The calculation shows that when the PWM frequency is 10 kHz, the maximum error is 0.0003, which is equivalent to the accuracy of 12 bits. If the PWM frequency is selected low, the influence of the switching delay time will be reduced accordingly. For example, when 1 kHz is selected, the introduced error is 0.00003, which is equivalent to the accuracy of 15 bits

among the above two items, what really affects the accuracy of output power supply is the drift of these parameters with temperature and time. Since the absolute values of these two parameters are very small, it can be inferred that their drift is smaller

from the above analysis, it can be seen that the error introduced by the additional circuit can fully meet the accuracy requirements of PWM

in the circuit shown in Figure 2, the error is mainly caused by three aspects:

(1) the error introduced by the dynamic resistance of the zener diode

the dynamic resistance of the zener diode is relatively large, generally about dozens (when the working current is 5~10 MA); The driving ability of the operational amplifier is relatively small, which can only make the zener diode work at a small working current. In addition, the dynamic resistance of the zener diode at low current operation is larger, which is more likely to cause voltage changes

(2) error caused by zener diode temperature drift

2dw7 (2dw230~236) internal structure can be considered as two zener diodes connected in series. The positive voltage drop (with negative temperature coefficient) of one diode compensates the temperature drift (with positive temperature coefficient) of the other zener diode, and a very low temperature coefficient is obtained for 1million intraocular lenses per year. However, when 2dw7 is applied in reverse, its temperature drift can not be properly compensated, resulting in a higher temperature coefficient in the negative pulse part

(3) error introduced by operational amplifier

the drift of input offset voltage of operational amplifier can directly lead to the error of pulse amplitude; If the conversion rate (SR) is too low, the pulse square waveform will be distorted, and then the voltage error will be caused. Common fault 2 of operational amplifier with low offset voltage temperature drift and high conversion rate: the price of will be very high

however, for 8-10 bit PWM, the circuit can meet the requirements. For occasions with lower requirements, two cheap zener diodes can be used to replace 2dw7

3 application example

using PWM output of single chip microcomputer, RC filter circuit and level 1 operational amplifier are added on the basis of Figure 1 to obtain 0~10 V DC output voltage, which is used as the control signal of frequency converter and has achieved good results. Figure 3 shows an example of using PWM output to control a frequency converter


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