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A high-precision self-oscillating switching power supply multi-output regulator design

In Electronic Infomation Category: A | on March 01,2011

Abstract: A self-excited high-precision multi-output switching power supply regulator, more multi-output switching power supply in the past, the use of minimal elements, including some self-excited control elements commonly used to achieve only 11, but The output power accuracy is very high. And only slightly modified, can be converted circuit 9 V to 12 V, 15V, which is also slightly modified the main circuit can be changed to 3. 3 V / 4 A precise output. The power circuit is simple, widely.

0 Introduction

Switching power supply is a switching power device and AD9200JST datasheet and through the use of power conversion technology into the DC power supply. It has small size, light weight, high efficiency, the grid voltage and AD9200JST price and frequency changes in the characteristics of adaptability. Switching power supply is also known as energy-efficient power supplies, high frequency switching circuit in the internal state, its energy consumption is very low, the power efficiency up to 80% more than double the normal linear power supply.

The main circuit topology switching power supply there are many, from the DC / DC conversion without input and AD9200JST suppliers and output isolation transformers, switching power supply into a transformer isolation and without transformer isolation, there are several of each type of topology, which Buck ( Buck), Boost (boost), Buck-Boost (boost - buck), Cuk (tandem) and Sepic (parallel), etc.; by incentives points, a self-excited, and it excited ; according to the control types, including PWF (Frequency Modulation), PWM (transfer-width), PAM (amplitude modulation) and RSM (resonant) 4; energy transfer by way of a continuous mode and discontinuous mode. The most widely used is the width-modulated converter. Width modulated converter are the following: Forward (Forward), the flyback (Feedback), half-bridge (Half Bridge Mode), Full-Bridge (Full Bridge Mode) and push-pull (Push Draw Mode) and so on. If the switch by switch can be divided into hard-switching conditions (Hardswitching) and soft switch (Softswitching) two. According to the switching power supply topology and control the various technical requirements, ease of implementation of engineering, electrical and other indicators of performance and cost summary, this paper used a self-excited type transformer isolated flyback topology to achieve this multi- High output switching power supply.

1 the principle of switching power supply

Precision multi-output switching power supply shown in Figure 1.

Figure 1, switching power supply schematic

General switching power supply shown in Figure 1, composed of four parts. Input circuit mainly by lightning, filtering, surge current suppression, rectification circuit composition.

Role is to input into AC power grid to meet the requirements of DC power input. Conversion circuit including the switching circuit, transformer and RCD snubber circuit, are the main energy conversion switching power supply channels. Control circuit with sampling circuit, this paper uses a high-value resistor divider samples, including baseline power generation used here TL431 2. 5V reference supply. There is also an error amplifier and pulse drive circuit, sampling the error signal amplified by the linear optocoupler feedback error signal, the driving while the rectangular pulse switch to achieve the purpose of regulating the output voltage. Output circuit includes rectifier, filter, the output voltage into a pulsating DC rectifier and smoothing into a low ripple DC voltage. In this thesis, the power to achieve, with simple design, small size, high efficiency, ripple, and apply a wide range of advantages.

2 switching power supply design and implementation

Multi-output switching power supply high-precision key design requirements are as follows:

Input voltage: AC132 V ~ 264 V

Input Frequency: 50/60 Hz

Quad Voltage Output: U1 24 V/0.5 A, U2-9 V/0.8 A,

U3 9 V/0.8 A, U4 5 V / 3 A

Output power: 40 W

Ripple voltage: 100 mV

Load regulation: 3%.

2.1 Design Principles

Figure 2 is a self-oscillating switching power supply multi-output regulator schematic.

Figure X1_C1, X1_C2, Y_C1, Y_C1 and L components EMI circuits used to filter out the common-mode power differential mode signal, while avoiding the pollution caused by switching power on the grid. L for the common mode choke, common mode choke is the switching power supply, inverter, UPS power supplies and other equipment is an important part. When the operating current flows through two coils around the opposite, the resulting magnetic field of the two cancel each other out, if there are common mode interference signals flowing through the coil, the coil of the common mode signal that is showing a high impedance, resulting in a strong damping effect to achieve the attenuation effect of interfering signals. X1_C1, X1_C2, Y_C1 and Y_C2 for the safety capacitor, which is X X1_C1 and X1_C2 safety capacitor, Y_C1 and Y_C2 Y-safety capacitor, where they are used in the power supply filter, and L play a role in power supply filtering, respectively were mode and differential mode interference from the filtering effect of work. On the capacitance of the feature is, after failure of the capacitor will not lead to electrical breakdown, without compromising personal safety. RV for the varistor with a lightning effect, can also be used TVS (transient voltage suppressor), the varistor has good nonlinear properties, through current, low levels of residual pressure, the action fast and free-wheeling, etc. characteristics, are widely used in electronic equipment mine.

Varistor switching power supply system is equivalent to D-level mine, for 220V line, selected for the varistor, 220 1 .4 1 .4 = 430 V, so the selection for the 470 V varistor . RT is the thermistor, boot, 220 V AC rectified through the large capacitor, which capacitor is characterized by the maximum instantaneous charge current to the fuse on the front of the bridge and an impact when the power is likely to cause damage to the device, In order to improve the safety factor of power supply design, often in the insurance after limiting resistor, the greater the resistance, although limiting effect, but the resistance is also a great energy consumption, switching power supply starts, the current limiting resistor has no effect , but a waste of electricity. In order to achieve better results but limit power, switching power supply is now often used as a negative temperature limiting thermistor used to absorb the surge current. Negative temperature characteristic thermistor, the higher the temperature, the resistance is smaller. In order to reduce the size and complexity of power, here to avoid the use of relays and other components of the anti-surge circuit, after testing, to achieve very good results. Transformer T, RCD (R2, C1, D1) absorption circuit and switch form of energy for this power converter circuit, the circuit uses a single-ended flyback, is a relatively mature power conversion circuits, transformers, both as isolated devices, but also as the energy storage device. This transformer uses EI framework, the primary coil inductance in the 1 .2 ~ 1 .3 mH, between the transformer winding, the outer ring to rely on self-excited winding, primary winding divided into two superimposed, a group including a group of outside, to prevent flux saturation power efficiency. This transformer is conventional transformer, not to repeat, the selection switch should pay attention to the drain-source voltage level, the maximum operating current, resistance, power dissipation and some switch time. RCD snubber circuits function is to absorb due to the transformer primary winding inductance generated in the work potential, the collector cut-off switch to avoid excessively high peak inverse moment of high voltage damage to the switch of the word. Switch has been at work, turn-on and off, cycle work, so there is a current absorbing circuit has been adopted, the current size of the power switching power supply with different size, making the absorption of circuit components values are not the same, RCD R-value of the absorption circuit, such as too small, it will reduce the switching power supply efficiency. However, if the R value is too large, MOS tube to the existence of the breakdown risk. This circuit use 10 k / 2 W, capacitors use 0.01 F / 1 kV, D1 use voltage 1 kV of HER107. Output rectification filter circuit mainly consists of the three terminal regulator and false resistance and so on. Appropriate choice of filter capacitor and resistor can reduce the power off output ripple and improve power efficiency.

Figure 2 self-oscillating switching power supply multi-output regulator schematic

Figure 2 optocoupler PC817, TL431, and self-excited circuit box, this power supply control circuit. R16, R17 is a sampling resistor, TL431 adjustable precision shunt regulator for the device, by changing the resistors R16 and R17 of the partial pressure value, small-scale change in output voltage. TL431 R15 and C20 for the frequency compensation circuit to improve TL43l transient frequency response. The design on the feedback loop, in fact, is to determine the R13, R14 the resistance and selected pieces of suitable optical coupler, the first optical coupler to the selected pieces of good linearity, because it can output a linear response to the self-excited circuit, the circuit can be generated by a linear self-excited pulse, inversely proportional to the linear cut-off control switch and turn. Commonly used in China at present non-linear optocoupler 4N25 optocoupler series, should not be used. Second, we must pay attention to parts optocoupler CTR (current transfer ratio) values. Optocoupler is mainly used to provide isolation, while changes in the output response in the Self-excited linear control circuit, the easy control of the linear switch duty cycle. Optocoupler CTR of the allowed range is 50% to 200%, because when the CTR 200%, in the starting circuit or when the load is mutated, may affect the normal output. PC817s CTR linear range of 80% to 160%, can meet the design requirements of the feedback loop. Determine a good optocoupler, it has to determine the R13, R14, should be noted that, the choice of resistance TL431 must ensure the necessary conditions of work, that is, the current through the cathode is greater than 1 mA. R13 for the PC817 external limiting resistor. In fact in addition to limiting protection role, it is also the gain control circuit has an important influence. When R13 is changed, will affect the optocoupler input current, then the output current of optocoupler, thereby affecting the duty cycle switch, it changes the control loop is equivalent to the current magnification. Take this circuit R13 100 , the optocoupler input and R13 R14 in parallel to the optocoupler input current close to 0, in order to ensure the TL431 cathode is not less than 1 mA operating current and setting.

2 .2 power supply feedback circuit

Block diagram in Figure 2 is a self-excited circuit, is the core of this thesis is the power of the pulse generator and the control part. Transistor working in the off state, the opening and closing time of the output current by the PC817 and the impact of the sampling resistor R7. Can be observed with an oscilloscope block diagram in Figure 2, the regulator, transistors, resistors are working in the sample under the rectangular pulse. Below is a brief analyze feedback loop to achieve regulation of the working process. When the output voltage Uo fluctuation, through the sampling resistor R16, R17 partial pressure, you make TL431 If the optocoupler input current produces a corresponding change, and thus make PC817 change in the output current Ic, Ic changes in the transistors base current Ib to change, then change the base voltage Ub, changing the transistors on-time, thus changing the switch on-time, that the duty cycle D, to produce the opposite change in Uo, Uo to maintain stability. The regulation process can be summarized as: Uo If Ic Ib Ub D Uo . Uo down after such a cycle, so that power supply to stabilize.

Switch voltage waveform shown in Figure 3.

Figure 3 switch waveforms

Figure 3 is the input AC220 V, output load, half load and full load, the switch drain and source-level output waveform points the oscilloscope is 10 times the attenuation, the vertical axis for the graphical display 5 V / div , the abscissa is 2 s / div.

Switching frequency can be seen with the change in inverse proportion to the size of the load.

3 Conclusion

According to this principle, the power supply design and production of a prototype, debugging and stable performance. Figure 3 is the power input AC220 V, drain-source-level switch in the power load, half load and full load waveform. Table 1 is the no-load, half load and full load output voltage of the four measured values.

four outputs measured results in Table 1

This power circuit is simple, self-excited circuit design, the use of less components, more than 82% efficiency, ripple voltage 90 mV, thus fully meet the design requirements. In addition switch duty cycle is proportional to the input AC voltage, and inversely proportional to frequency. When the voltage from the low-key to AC220 V, the switch from the oscilloscope to see the waveform will find the drain to source from less than 100 kHz switching frequency rose to power in a no-load as shown in Figure 3 the power load frequency 300 kHz. When the power supply voltage in the design, the load does not vary with input voltage changes, and then see from the oscilloscope waveform switch drain to source will find the same when the input voltage, switch the operating frequency is inversely proportional to the load size. Thus the power to adapt to a wide range of input and output changing work environment. Since this power supply a few hundred kHz in frequency, more efficient, so they requested switching high frequency switch tubes. This power can also be slightly modified, the TL431 to TLV431, then R16, R17 a change in resistance, the other can not change the output 5 V / 3 A to 3. 3 V / 4 A output, 9 V can also be increased by 2 laps of transformer winding or 4 laps, respectively, three-terminal regulator to 7812 and 7912, 7815 and 7915, which can output 12 V, 15V voltage , when the current does not exceed 0. 6 A. This wide range of power applications, the output voltage accuracy is not low.

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