Detailed analysis of the switching power supply circuit diagram
Switching power supply circuit diagram analysis
The so-called switching power supply, hence the name, is that there is a door here, a door power supply will pass, a power supply will stop passing, then what is the door, some of the switching power supply uses thyristor, and some use the switch tube, this The performance of the two components is similar. It is connected to the base (switching tube) control electrode (thyristor) with a pulse signal to complete the turn-on and turn-off. The positive half-cycle of the pulse signal comes, and the voltage on the control electrode rises. The switch tube or the thyristor is turned on, and the 300V voltage outputted by the 220V rectification and filtering is turned on, passed to the secondary through the switching transformer, and then the voltage is raised or lowered by the transformation ratio for each circuit to work. When the oscillation pulse is negative for half a week, the base voltage of the power supply regulating tube or the control electrode voltage of the thyristor is lower than the original set voltage, the power supply regulating tube is cut off, the 300V power supply is turned off, and the switching transformer secondary has no voltage. At this time, each circuit The required operating voltage is maintained by the secondary capacitor rectified filter capacitor discharge. The previous process is repeated until the signal of the positive half cycle of the next pulse arrives. This switching transformer is called a high frequency transformer because its operating frequency is higher than the 50Hz low frequency. Then how to get the pulse of the switch tube or thyristor, this requires an oscillation circuit. We know that the transistor has a characteristic that the base-to-emitter voltage is 0.65-0.7V, which is an amplification state, 0.7V. The above is the saturation conduction state, -0.1V- -0.3V works in the oscillation state, then after the working point is adjusted, the deep negative feedback is used to generate the negative pressure, so that the oscillation tube starts to oscillate, the frequency of the oscillation tube It is determined by the length of time that the capacitor on the base is charged and discharged. The oscillation frequency is high and the output pulse amplitude is large, and vice versa. This determines the output voltage of the power supply regulating tube. Then how to regulate the working voltage of the secondary output of the transformer, generally on the switching transformer, winding a group of coils, the voltage obtained at the upper end is rectified and filtered, as a reference voltage, and then the reference is passed through the optocoupler. The voltage returns to the base of the oscillating tube to adjust the oscillation frequency. If the secondary voltage of the transformer rises, the voltage output from the sampling coil also rises, and the positive feedback voltage obtained by the photocoupler also rises. This voltage is added. On the base of the oscillating tube, the oscillation frequency is reduced, which stabilizes the stability of the secondary output voltage. The too thin working condition does not need to be elaborated, and it is not necessary to understand the fineness, so that the high-power voltage is controlled by the switching transformer. Passed and separated from the latter stage, the returned sampling voltage is transmitted by the optocoupler and also separated from the latter stage, so the mains voltage of the former stage is separated from the latter stage. This is called cold plate, it is safe, the transformer The front power supply is independent, which is called a switching power supply.
Switching power supply circuit diagram
First, the main circuit
The entire process from AC grid input and DC output, including:
1. Input filter: Its function is to filter the clutter existing in the power grid, and also hinder the clutter generated by the machine from being fed back to the public power grid.
2. Rectification and filtering: The AC power supply of the power grid is directly rectified into a smoother DC power for the next level of transformation.
3. Inverter: The rectified DC power is changed into high frequency AC power, which is the core part of the high frequency switching power supply. The higher the frequency, the smaller the ratio of volume, weight and output power.
4. Output rectification and filtering: Provide stable and reliable DC power supply according to load requirements.
Second, the control circuit
On the one hand, sampling from the output, comparing with the set standard, then controlling the inverter, changing its frequency or pulse width to achieve output stability, on the other hand, according to the data provided by the test circuit, identified by the protection circuit, provided The control circuit performs various protection measures on the whole machine.
Third, the detection circuit
In addition to providing various parameters in the protection circuit, various display instrument data are also provided.
Fourth, auxiliary power supply
Provides different power requirements for all single circuits.
Switch control voltage regulation principle
The switch K is repeatedly turned on and off at a certain time interval. When the switch K is turned on, the input power source E is supplied to the load RL through the switch K and the filter circuit, and the power source E supplies energy to the load during the entire switch-on period; When the switch K is turned off, the input power source E interrupts the supply of energy. It can be seen that the input power supply provides energy to the load intermittently. In order to provide continuous energy supply to the load, the switching power supply must have a set of energy storage devices to store a part of the energy when the switch is turned on. Released to the load when disconnected. In the figure, a circuit composed of an inductor L, a capacitor C2, and a diode D has such a function. The inductor L is used to store energy. When the switch is turned off, the energy stored in the inductor L is released to the load through the diode D, so that the load obtains continuous and stable energy. Since the diode D makes the load current continuous, it is called freewheeling. diode. The voltage average EAB between AB can be expressed by the following formula:
EAB=TON/T*E where TON is the time each time the switch is turned on, and T is the duty cycle of the switch on and off (ie, the sum of the switch-on time TON and the off-time TOFF).
It can be known from the formula that changing the ratio of the switch-on time to the duty cycle, the average value of the voltage between the ABs also changes. Therefore, the output voltage V0 can be automatically adjusted by the ratio of the load and the input power supply voltage to automatically adjust the ratio of TON and T. stay the same. Changing the on-time TON and the duty cycle ratio also changes the duty cycle of the pulse. This method is called Time Ratio Control (TRC).
According to the TRC control principle, there are three ways:
1. Pulse Width Modulation (PWM) The mode in which the switching period is constant and the duty ratio is changed by changing the pulse width.
Second, Pulse Frequency Modulation (PFM) The conduction pulse width is constant, and the duty ratio is changed by changing the switching operating frequency.
Third, the mixed modulation on-pulse width and the switching operating frequency are not fixed, and each can change the way, it is a mixture of the above two ways.
Self-excited switching power supply circuit diagram
Self-excited switching power supply circuit diagram, STR41090 power supply is a self-excited parallel switching power supply, adapting to the grid voltage capability of 150-280V.
In the oscillation process C808, about 300V DC voltage is applied to the B pole of the internal switch of the (2) leg of the N801 through R811, and is added to the internal switch of the (3) pin of the N801 through the (1) and (3) windings of the T802. The pole starts to conduct, the current flows through the (1) and (3) windings of T802, and the induced voltage is generated in the (1) and (3) windings. The polarity is (3) positive (1) negative, coupled, The (6) and (7) windings also generate induced voltages with a polarity of (7) positive (6) negative. This positive feedback voltage is sent back to the (2) pin of N801 via C819, R817, and R816 to make the switch current Further increasing, the avalanche process quickly saturates the switching tube. During the saturation of the switch tube, the current of the T802(1) and (3) windings increases linearly, VD821 and VD822 are cut off, and T802 stores the magnetic field energy. As the C819 is continuously charged, the voltage of the (2) pin of the N801 is continuously decreased. At a certain moment, the voltage on the N802(2) pin cannot maintain the saturation of the internal switch, the switch tube is out of saturation, and the C-pole current is reduced. The induced voltage polarity of each winding of T802 is reversed. The voltage polarity of the feedback windings (6) and (7) is (6) positive (7) negative, and sent to the (2) pin of N801 via C819, R817 and R816. The voltage of the N801(2) pin is further reduced, and another avalanche process causes the switching tube to be quickly turned off. During the off period of the switch, VD821 is turned on, forming 112V voltage on the C822 capacitor; VD822 is also turned on, forming 18V voltage on the C824 capacitor, and the magnetic field energy stored by T802 is released. On the other hand, the voltage on the C819 is discharged through R817, R816, VD812, VD813, and the 300V voltage is reversely charged to the C819 via the R811. These two factors cause the voltage at the left end of the C819 to rise, that is, the voltage at the N801(2) pin rises. When the voltage of the (2) pin rises by 0.6 V or more, the switching transistor is turned on again, and the oscillation of the next cycle is started. The voltage regulator circuit is internally completed by STR41090. The (5) and (6) pins of T802 are sampling windings. After VD814 rectification and C817 filtering, the sampling voltage is formed on C817. Under normal conditions, the voltage on C817 is about 84V, if the output voltage is 112V, the sampling voltage must also rise. The sampling voltage is sent to the (1) pin of N801 via R815, and the internal voltage is adjusted to finally stabilize the output voltage at 112V. Protection circuit R814, V801 is the switch tube over-current protection circuit, R814 string is between the E-pole of the switch tube and the ground. The voltage drop on R814 reflects the current of the switch tube. Under normal conditions, the voltage on R814 cannot make V801. Turn on, once the switch tube is overcurrent, the voltage drop on R814 increases, making V801 turn on, and the (2) pin of N801 is shorted to ground by V801, which prevents the switch tube from overcurrent. R812 and C812 are soft start circuits. The characteristics of the voltage across C812 can not be abrupt. At each instant of starting, the (5) pin of N801 is grounded instantaneously through R812, so that the internal switch tube is instantaneously cut off to avoid the switch tube saturation time at the start of the switch. It is too long and damaged.
Self-excited switching power supply circuit diagram 2:
1 Introduction Switching power supply is a DC regulated power supply that uses switching power devices and is powered by power conversion technology. It has small size, light weight, high efficiency, adaptability to changes in grid voltage and frequency, and output voltage retention. Long time, good for computer information protection, etc., it is widely used in various terminal devices and communication devices led by electronic computers. It is an indispensable power source for the rapid development of electronic information industry. Switching power supply is also called high efficiency. Energy-saving power supply, internal circuit works in high-frequency switching state, the energy consumed by itself is very low, the general power supply efficiency can reach about 80%, which is double that of ordinary linear regulated power supply. Currently, there is no power frequency transformer type, switch The power supply still uses the principle of pulse width modulator PWM or pulse frequency modulator PFM. According to the PWM principle, the switch tube BU508A is used to introduce the design of a flyback isolated switching regulator power supply without power frequency transformer.
2 main technical parameters input voltage: AC220V
Input frequency: 50Hz
Input voltage range: AC165V-265V
Output voltage: DC24V, 2A
Output power: 48W
3 Working principle The working principle of the switching power supply is shown in Figure 1. The input voltage is AC220v, 50Hz AC power, after filtering, and then converted to DC by the rectifier bridge, and the high frequency is turned on and off by the switching tube in the control circuit. The low-voltage high-frequency voltage is generated by the first measurement of the transformer, and is coupled to the secondary measurement through the low-power high-frequency transformer, and then rectified and filtered to obtain the DC voltage output. In order to stabilize the output voltage, the TL431 sampling is used, and the error is amplified by optical coupling. The PWM is used to control the on and off time (ie, duty cycle) of the switching transistor, so that the output voltage remains stable.
Figure 1 Working principle of the switching power supply
4 switching power supply design
The circuit diagram of the switching power supply is shown in Figure 2. In this power conversion circuit, a single-ended flyback converter is used, and the single-ended one is because the core of the high-frequency transformer only works in the first quadrant. The wiring of the diode is different. The single-ended converter can be divided into two types: forward and flyback. The primary side main power switch tube and the secondary side rectifier tube have opposite switching states (when the switch tube is turned on, the secondary side is rectified). Diode cutoff is called single-ended flyback. When the primary side is applied to the high-level excitation pulse to make Q1 turn on, the DC input is applied to the primary side of the high-frequency transformer. At this time, the secondary side is up and down, making the rectification The diode is turned off; when the driving pulse is low level, Q1 is turned off, the polarity of both ends of the primary side is reversed, so that the two ends of the secondary winding become upper and lower negative, the rectifier diode is forwardly turned, and then the magnetic energy of the secondary side of the transformer It is released to the load. Therefore, the single-ended flyback converter only stores energy when the primary side Q1 is turned on, and releases it to the load when it is turned off. Therefore, the high-frequency transformer functions as both voltage-separating and inductive during the switching process. Energy storage component.
The electromagnetic interference filter connected to the input end of the AC power source is composed of common mode chokes L1, C2 and C3. The midpoints of C2 and C3 should be grounded to suppress common mode interference. C1 is used for filtering and filtering. Series mode interference, large capacitance. In view of the moment when the switch BU508A is turned off, the leakage inductance of the high-frequency transformer will produce a spike voltage. C8, R3 and D1 form a clamp circuit, and the function of C9 is to filter the switch tube set. The peak voltage of the electrode determines the automatic restart frequency. C9 and R4 together compensate the control loop. At the same time, C9 and R4 also function as the primary side quick reset, which can effectively protect the switch tube from being damaged.
4.1 Switching Control Section of Switching Power Supply
The core of the switching power supply is the switch control part. The main working process is to control the time when the main power switch Q1 is turned on and off (that is, the duty ratio) through the high and low voltages of point B and point C in Figure 2. When Q1 is cut off When point A is high, C5 discharges to Q1, so that the potential of point B is rapidly increased, so that the base potential of the switching transistor Q1 is higher than the emitter, so Q1 is saturated and the C5 is charged. The current at this time is The current between the primary current of the transformer and the current when Q1 is turned on, so the current flowing through R5 is very large, the potential at point C rises, the saturation conducts the potential at point A, and Q1 is cut off.
D2 and D3 function is to make the potential of point C not high when Q1 is turned on, otherwise the discharge time of C5 is too long, so that the turn-off time toff of Q1 is too large, and the on-time of ton remains constant, so the frequency becomes low. If Q1 is boosted too high when Q1 is turned on, Q1 will be turned off. At this time, D2 and D3 are forwarded, and the potential of point C is lowered, so that the discharge time of C5 is short, Vb>Vc, so that Vb>Vc The toff is also small, so the frequency can be made very high.
4.2 PWM adjustment section
When Q1 is turned on, the winding N2 is positive and negative, C10 absorbs the peak voltage just after discharge, preventing the diode D10 from being damaged in the forward direction, and D10 is conducting the conduction, so that the potential of point B rises, so that Q1 is saturated and turned on faster. At the same time, Q2 is turned on, and then Q3 is also turned on. The voltage at point B drops, and the current of the primary coil decreases to cutoff. At this time, the N2 side is positive and negative, D4 ​​and D5 are turned on, and the base of Q4 becomes high. When Q4 is turned on, the potential at point C decreases, the cut-off time becomes shorter, and the feedback current of TL431 causes the current flowing into the base of Q4 to decrease, the potential at point C decreases slowly, and the cut-off time becomes longer. When Q1 is turned on, TL431 feedback The current determines the speed at which the potential at point C rises to achieve the purpose of voltage regulation. C12 is used to protect Q3. When the cut-off, the reverse peak voltage is too high, and the damage is Q3. The feedback control is to compare the sampled voltage with the reference voltage and convert it into The current is then amplified by current to adjust ton and toff to control the duty cycle to achieve the purpose of voltage regulation.
R12 is the minimum load of the output voltage, preventing the voltage from being too high when the load is unloaded. It is used to improve the voltage regulation rate at light load. C17 can appropriately reduce the high frequency gain of the error amplifier. The reference voltage of TL431 is compared with the output voltage Vo. The error voltage is formed at R14, so that the diode of IC1 generates different current. R14 is the current limiting resistor of IC1 diode. The frequency of error amplification should be determined by R13, R16, VR and C17. The RC absorption network composed of C14 and R10, Can eliminate high frequency self-oscillation and reduce radio frequency interference.
4.3 High frequency converter part
Since the power supplied by the primary side of the high-frequency transformer in unit time is proportional to the square sum frequency of ton and proportional to the square of the DC voltage of the input primary side, it is inversely proportional to the number of turns of the primary winding, if the consumption of the transformer is not considered, The conservation of energy can obtain the power of the secondary side of the transformer, that is, the output power is independent of the number of turns on the secondary side of the transformer, and the load is only determined by the power supplied by the primary side. Therefore, to obtain different output power, it is only by changing the primary side of the high-frequency transformer. The power of the change ton has the greatest influence on the output power, but it should not be changed greatly due to the limitation of the magnetic flux reset condition. To change the DC voltage of the input primary side, only the parameters of the filter inductance and filter capacitor of the previous circuit can be changed. You can add a potentiometer in front, you can also change the DC voltage, and the frequency is limited by the condition of the power switch tube. Therefore, it is a good method to change the number of turns of the primary winding. The width of the primary winding is not too long. Divide it into multiple layers, each layer is controlled by a switch, which requires different winding turns to access different switches to control the power on the primary side. Different output power is obtained. However, in the toff time, the primary magnetic flux of the high-frequency transformer should be reset, and the secondary magnetic flux should be reset in the ton time. If the magnetic flux is not returned to the cycle at the end of the switching duty cycle. At the beginning of the starting point, the magnetic flux in the transformer core will gradually increase, causing the core to saturate and damage the power switch tube. To meet the flux reset condition of the single-ended converter, it is necessary to make the time of Ton and Toff appropriate. Too long, otherwise the frequency of the switching tube will be low, and it is related to the number of turns of the primary side and the secondary side of the high frequency transformer.
4.4 TL431
The TL431 is a three-terminal adjustable regulator that uses two external resistors to set any reference voltage in the range of 2.50 to 36V. The voltage temperature coefficient of the TL431 is small. The dynamic impedance is low, typically 0.2 ohms, and the output noise is low. It has the thermal stability specified in the temperature range of the automotive industry, and the effective output circuit has very steep conduction characteristics, which makes these devices very suitable for applications such as on-board voltage regulation, adjustable power supply and switching power supply. A good alternative to Zener diodes.
5 Conclusion
According to the above principle, the prototype is designed and manufactured, and the performance is stable after debugging. The characteristic of this circuit is that the duty ratio is proportional to the input voltage (the frequency is inversely proportional), and it is not affected by the load, so it is easy to control in a large range. The frequency is limited, and can reach 50KHz-100KHz. The power efficiency is slightly lower than that of the integrated switch. In order to improve the power efficiency of this circuit, it is better to use a switch with a higher frequency. The higher the frequency, the better the energy saving effect.
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