Home  Power Supplies

Jul 13 2019

Switching Power Supply Circuit Diagram with Explanation

Catalog

Article core

Switching Power Supply


1. Development History of Switching Power Supply



2. The Basic Principle of Switching Power Supply

2.1 The Basic Principle of PWM Switching Power Supply


2.2 Working Principle of Switching Power Supply


3. Circuit Composition of the Switching Power Supply



4. Principle of Input Circuit and Common Circuit

4.1 Principle of AC Input Rectification and Filtering Circuit


4.2 Principle of DC Input Filter Circuit


5. Power Conversion Circuit

5.1 Working Principle of MOS Transistor

5.1.1 Common Schematics

5.1.2 Working Principle

5.2 Push-pull Power Conversion Circuit


5.3 Power Conversion Circuit with Drive Transformer


6. Output Rectifier and Filter Circuit

6.1 Forward Rectifier Circuit


6.2 Flyback Rectifier Circuit


6.3 Synchronous Rectifier Circuit


7. Principle of Voltage Regulation Loop

7.1 Schematic of Feedback Circuit


7.2 Working Principle


8. Short Circuit Protection Circuit



9. Output Current Limiting Protection



10. Principle of Output Overvoltage Protection Circuit

10.1 Thyristor Trigger Protection Circuit


10.2 Photoelectric Coupling Protection Circuit


10.3 Output Voltage Limiting Protection Circuit


10.4 Output Overvoltage Lockout Circuit


11. Power Factor Correction Circuit (PFC)

11.1 Schematic Diagram


11.2 Working principle


12. Input Over/Under Voltage Protection

12.1 Intelligent heat dissipation


12.2 Working Principle


13. Battery Management

13.1 Schematic of Battery Management


13.2 Working Principle


13.3 The Voltage Regulated Principle of Battery Charging


13.4 The Principle of Battery Charging Current Limiting


13.5 The Principle of Battery Undervoltage Shutdown


14. Intelligent Fan Cooling

14.1 Intelligent Heat Dissipation


14.2 Working Principle


15. Current Sharing Technology

15.1 Current Sharing Technology


15.2 Working Principle



1. Development History of Switching Power Supply

The switching power supply has replaced the transistor linear power supply for more than 30 years. The first to appear is the series switching power supply. The main circuit topology is similar to that of the linear power supply. However, after the power transistor is in the switching state, the pulse width modulation (PWM) control technology has developed. It is used to control the switching converter to obtain PWM switching power supply. It is characterized by 20kHz pulse frequency or pulse width modulation. The efficiency of PWM switching power supply can reach 65%~70%, while the efficiency of linear power supply is only 30%~40%. In the era of the global energy crisis, it has aroused widespread concern. The linear power supply works at the power frequency, so it is replaced by a PWM switching power supply with a working frequency of 20 kHz, which can greatly save energy. It is known as the 20 kHz revolution in the history of power supply technology development. As ULSI chips continue to shrink in size, power supplies are much larger than microprocessors; aerospace, submarine, military switching power supplies, and battery-operated portable electronic devices (such as portable calculators, mobile phones, etc.) need the smaller and lighter power supply. Therefore, requirements of small size and lightweight are imposed on the switching power supply, including the volume and weight of magnetic components and capacitors. In addition, the switching power supply is required to have higher efficiency, better performance and higher reliability. 


2. The Basic Principle of Switching Power Supply

2.1 The Basic Principle of PWM Switching Power Supply

It is quite easy to understand the working process of the switching power supply. In a linear power supply, the power transistor is operated in a linear mode. Unlike a linear power supply, the PWM switching power supply allows the power transistor to operate in the on and off states. In both states, the product of volt-ampere applied to the power transistor is always small (the voltage is low and the current is large when turned on; the voltage is high and the current is small when turned off). The product of volt-ampere on a power device is the loss produced on a power semiconductor device.

Compared with linear power supplies, PWM switching power supplies work more efficiently by “chopper”, which is to convert the input DC voltage into a pulse voltage with an amplitude equal to the input voltage amplitude. The duty ratio of the pulse is regulated by the controller of the switching power supply. Once the input voltage is clamped into an AC square wave, its amplitude can be raised or lowered by the transformer. The number of voltage groups of the output can be increased by increasing the number of secondary windings of the transformer. Finally, a DC output voltage is obtained after these AC waveforms are rectified and filtered.

The main purpose of the controller is to ensure that the output voltage is stable, and its working process is very similar to that of a linear controller. This means that the controller's functional block voltage reference and error amplifier can be designed to be identical to a linear regulator. They differ in that the output of the error amplifier (error voltage) passes through a voltage pulse conversion unit before driving the power transistor.

Switching power supplies have two main modes of operation: forward conversion and boost conversion. Although the arrangement of the various parts differs little, the working process varies greatly and they have different advantages in specific situations.

The advantage of the forward converter is that the output voltage has a lower ripple peak than the boost converter, and can output relatively high power. The forward converter can provide several kilowatts of power.

The boost converter has a high peak current and is therefore only suitable for applications with a power of no more than 150 W. In all topologies, these converters use the smallest components and are therefore popular in applications with small to medium power. 

2.2 Working Principle of Switching Power Supply

(1) AC power input is rectified and filtered into DC.

(2) Control the switching tube by high-frequency PWM (pulse width modulation) signal, and apply DC to the primary of switching transformer.

(3) The secondary of the switching transformer induces a high-frequency voltage, which supplies to the load through rectification and filtering.

(4) The output part feeds back to the control circuit through a certain circuit to control the PWM duty ratio to achieve stable output.

 

3. Circuit Composition of the Switching Power Supply

The main circuit of the switching power supply is composed of an input electromagnetic interference filter (EMI), a rectification and filtering circuit, a power conversion circuit, a PWM controller circuit, and an output rectification and filtering circuit. The auxiliary circuit has an input over-voltage protection circuit, an output over-voltage protection circuit, an output over-current protection circuit, and an output short-circuit protection circuit.

The circuit block diagram of the switching power supply is as follows:

Figure 1. Block Diagram of Switching Power Supply Circuit

Figure 1. Block Diagram of Switching Power Supply Circuit

 

4. Principle of Input Circuit and Common Circuit

4.1 Principle of AC Input Rectification and Filtering Circuit

Figure 2. Schematic of Input Filter, Rectifier Circuit

Figure 2. Schematic of Input Filter, Rectifier Circuit

① Lightning protection circuit: When there is lightning strike and high voltage is generated through the power grid, the circuit consists of MOV1, MOV2, MOV3, F1, F2, F3 and FDG1. When the voltage applied across the varistor exceeds its operating voltage, its resistance decreases, so that the high-voltage energy is consumed on the varistor. If the current is too large, F1, F2, and F3 will burn and protect the subsequent circuit.

② Input filter circuit: The double π-type filter network composed of C1, L1, C2 and C3 mainly suppresses the electromagnetic noise and clutter signals of the input power source to prevent interference to the power supply and it also prevents high-frequency clutter generated by the power supply itself from interfering with the power grid. When the power is turned on, the C5 should be charged. Because the instantaneous current is large, adding RT1 (thermistor) can effectively prevent the surge current. Since the instantaneous energy is completely consumed on the resistor RT1, the resistance of RT1 decreases after the temperature rises after a certain time (RT1 is the negative temperature coefficient component). At this time, the energy consumption is very small, and the subsequent circuit can work normally.

③ Rectifier filter circuit: After the AC voltage is rectified by BRG1, it is filtered by C5 to obtain a relatively pure DC voltage. If the capacity of C5 becomes smaller, the output AC ripple will increase.

4.2 Principle of DC Input Filter Circuit

Figure 3.

Figure 3.

① Input filter circuit: The double π-type filter network composed of C1, L1, C2 and C3 mainly suppresses the electromagnetic noise and clutter signals of the input power source to prevent interference to the power supply and it also prevents high-frequency clutter generated by the power supply itself from interfering with the power grid. C3 and C4 are safety capacitors, and L2 and L3 are differential mode inductors.

② R1, R2, R3, Z1, C6, Q1, Z2, R4, R5, Q2, RT1 and C7 form an anti-surge circuit. At the moment of starting, due to the presence of C6, Q2 does not conduct, and the current forms a loop through RT1. Q2 turns on when the voltage on C6 is charged to the regulated value of Z1. If the C8 leakage or the subsequent circuit is short circuited, the voltage drop generated by the current on RT1 increases at the moment of starting, and Q1 is turned on so that Q2 is not turned on without the gate voltage, and RT1 will burn out in a short time to protect the subsequent circuit.

 

5. Power Conversion Circuit

5.1 Working Principle of MOS Transistor

At present, the most widely used insulated gate field effect transistor is a MOSFET (MOS transistor), which works by utilizing the electroacoustic effect of the semiconductor surface and is also known as surface field effect devices. Since its gate is in a non-conducting state, the input resistance can be greatly improved up to 105 ohms. The MOS transistor uses the magnitude of the gate-source voltage to change the amount of induced charge on the semiconductor surface, thereby controlling the drain current.

 5.1.1 Common Schematics 

Figure 4.

Figure 4.

 5.1.2 Working Principle 

R4, C3, R5, R6, C4, D1 and D2 form a buffer, and is connected with the switch MOS transistor in parallel, so that the voltage stress of the switch tube is reduced, EMI is reduced, and secondary breakdown does not occur. When the switch tube Q1 is turned off, it is easy for the primary winding of the transformer to produce spike voltage and spike current. These components together can absorb the spike voltage and current well. The current peak signal measured from R3 participates in the duty ratio control of the current working cycle and is therefore the current limit of the current working cycle. When the voltage on R5 reaches 1V, UC3842 stops working and switch tube Q1 turns off immediately. The junction capacitances CGS and CGD in R1 and Q1 together form an RC network, and the charge and discharge of the capacitor directly affects the switching speed of the switching transistor. If R1 is too small, it will cause oscillation and electromagnetic interference will be very large; if R1 is too large, it will reduce the switching speed of the switching tube. Z1 usually limits the GS voltage of the MOS transistor to 18V or less, thus protecting the MOS transistor. The gate controlled voltage of Q1 is saw-shaped wave. When the duty ratio is larger, the longer the Q1 conduction time is, the more energy the transformer stores. When Q1 is cut off, the transformer releases energy through D1, D2, R5, R4 and C3. At the same time, it achieves the purpose of magnetic field reset, which is ready for the next storage and transmission of energy of the transformer. The IC adjusts the duty ratio of the saw-shaped wave of pin 6 according to the output voltage and current, thus stabilizing the output current and voltage of the whole machine. C4 and R6 are spike voltage absorption loops.

5.2 Push-pull Power Conversion Circuit

Figure 5.

Figure 5.

Q1 and Q2 will turn on in turn.

5.3 Power Conversion Circuit with Drive Transformer

Figure 6.

Figure 6.

T2 is the drive transformer, T1 is the switching transformer, and TR1 is the current loop.

 

6. Output Rectifier and Filter Circuit

6.1 Forward Rectifier Circuit

Figure 7.

Figure 7.

T1 is a switching transformer whose phase of the primary and secondary poles are in phase. D1 is a rectifier diode, D2 is a freewheeling diode, and R1, C1, R2, and C2 are despiking circuits. L1 is a freewheeling inductor, and C4, L2, and C5 form a π-type filter.

6.2 Flyback Rectifier Circuit

Figure 8.

Figure 8.

T1 is a switching transformer with opposite phases of the primary and secondary poles. D1 is a rectifier diode, and R1 and C1 are Despiking circuits. L1 is a freewheeling inductor, R2 is a dummy load, and C4, L2, and C5 form a π-type filter.

6.3 Synchronous Rectifier Circuit

Figure 9.

Figure 9.

Working principle: When the upper end of the secondary of the transformer is positive, the current makes Q2 turn on through C2, R5, R6 and R7, the circuit forms the loop, and Q2 is the rectifier. The gate Q1 is turned off due to the reverse bias. When the lower end of the secondary of the transformer is positive, the current makes Q1 turn on through C3, R4, and R2, and Q1 is a freewheeling tube. The gate Q2 is turned off due to the reverse bias. L2 is a freewheeling inductor, and C6, L1, and C7 form a π-type filter. R1, C1, R9, and C4 are despiking circuits.

 

7. Principle of Voltage Regulation Loop

7.1 Schematic of Feedback Circuit

Figure 10. Schematic of Voltage Feedback Loop Circuit

Figure 10. Schematic of Voltage Feedback Loop Circuit

7.2 Working Principle

When the output U0 rises, after the voltage is divided by the sampling resistors R7, R8, R10, VR1, the voltage of pin 3 of U1 rises. When it exceeds the reference voltage of pin 2 of U1, pin 1 of U1 outputs a high level, so that Q1 is turned on, and the optocoupler OT1 LED lights, the phototransistor is turned on, and the potential of pin 1 of the UC3842 is correspondingly low, thereby changing the output duty ratio of pin 6 of U1 to decrease, and U0 is lowered. When the output U0 decreases, the voltage of pin 3 of U1 decreases. When it is lower than the reference voltage of pin 2 of U1, pin 1 of U1 outputs a low level, Q1 does not conduct, the optocoupler OT1 LED does not emit light and the phototransistor does not conduct. The potential of pin 1 of the UC3842 rises high, thus changing the output duty cycle of pin 6 of U1 to increases and U0 decreases. Repeatedly , the output voltage is kept stable. Adjusting VR1 can change the output voltage value.

The feedback loop is an important circuit that affects the stability of the switching power supply. Feedback resistor capacitance error, leakage, virtual soldering and so on will produce self-oscillation. The fault phenomenon is: waveform abnormality, empty or full load oscillation, output voltage instability and so on.

 

8. Short Circuit Protection Circuit

— In the case of a short circuit at the output, the PWM control circuit can limit the output current to a safe range. It can implement the current limiting circuit in a variety of ways. When the power limiting current does not work when it is short-circuited, only another part of the circuit will be added.

— There are usually two types of short-circuit protection circuits. The following figure shows a low-power short circuit protection circuit.

Figure 11.

Figure 11.

The principle is as follows:

When the output circuit is short circuited, the output voltage disappears, the optocoupler OT1 is not turned on, the voltage of pin 1 of UC3842 rises to about 5V, the voltage division of R1 and R2 exceeds the reference of TL431 and makes it turn on. The VCC potential of pin 7 of UC3842 is pulled low, and the IC stops working. After UC3842 stops working, the potential of pin 1 disappears and TL431 does not turn on. The potential of pin 7 of UC3842 rises, and UC3842 restarts, and it starts again and again. When the short circuit disappears, the circuit can automatically return to normal operation.

 

— The figure below is a medium power short circuit protection circuit.

Figure 12.

Figure 12.

The principle is as follows:

When the output is short circuited, the voltage of pin 1 of UC3842 rises, and the potential of pin 3 of U1 is higher than that of pin 2. The pin 1 of comparator outputs high potential to charge C1. When the voltage across C1 exceeds the reference voltage of pin5, the pin 7 of U1 outputs a low potential. When the voltage of pin 1 of UC3842 is lower than 1V, UCC3842 stops working and the output voltage is 0V, the circuit starts again. When the short circuit disappears, the circuit works normally. R2 and C1 are charge and discharge time constants, and the short circuit protection does not work when the resistance value is incorrect.

— The figure below is a common current limiting and short circuit protection circuit.

Figure 13.png

Figure 13.

Its working principle is briefly described as follows:

The output duty ratio of pin 6 of UC3842 is gradually increased. When the voltage of pin 3 exceeds 1V, the UC3842 is turned off and has no output.

— The following figure is a protection circuit for sampling current with a current transformer. It has low power consumption, but high cost and complicated circuit.

Figure 14.

Figure 14.

The working principle is as follows:

If the output circuit is short circuited or the current is too large, The voltage induced by the TR1 secondary coil will be higher. When the pin 3 of UC3842 exceeds 1 volt, the UC3842 stops working and repeats. When the short circuit or overload disappears, the circuit recovers itself.

 

9. Output Current Limiting Protection

Figure 15.

Figure 15.

The figure above is a common output current limiting protection circuit. Its working principle is as shown in the above figure: When the output current is too large, the voltage across RS (manganese copper wire) rises, the voltage of pin 3 of U1 is higher than the reference voltage of pin 2. Pin 1 of U1 outputs high voltage, Q1 is turned on and the optocoupler has photoelectric effect. The voltage of pin 1 of UC3842 is lowered, and the output voltage is lowered, thereby achieving the purpose of output overload current limiting.

 

10. Principle of Output Overvoltage Protection Circuit

The function of the output overvoltage protection circuit is to limit the output voltage to a safe value when the output voltage exceeds the designed value. When the internal voltage regulation loop of the switching power supply fails or the output overvoltage phenomenon is caused by improper operation of the user, the overvoltage protection circuit protects to prevent damage to the power equipment of the subsequent circuit. The most common overvoltage protection circuits are as follows:

10.1 Thyristor Trigger Protection Circuit

When the output circuit is short circuited or over-current, the primary current of the transformer increases, the voltage drop across R3 increases, and the voltage of pin 3 rises.

Figure 16.

Figure 16.

As shown in the figure above, when the Uo1 output rises and the Zener diode (Z3) breaks through, the control terminal of the SCR1 (SCR1) gets the trigger voltage, so the SCR is turned on. The voltage of Uo2 is short circuited to ground, and the overcurrent protection circuit or the short circuit protection circuit will work to stop the operation of the entire power supply circuit. When the output overvoltage phenomenon is eliminated, the control terminal trigger voltage of the thyristor is discharged to the ground through R, and the thyristor is restored to the off state.

10.2 Photoelectric Coupling Protection Circuit

Figure 17.

Figure 17.

As shown above, when Uo has an overvoltage phenomenon, the Zener diode breaks through and conducts current through the optocoupler (OT2) R6 to the ground, and the LED of the photocoupler lights, thereby making the phototransistor of the photocoupler on. The base of Q1 is electrically turned on, and the pin 3 of 3842 is reduced, so that the IC is turned off and the operation of the entire power supply is stopped. Uo is zero, and the cycle is repeated.

10.3 Output Voltage Limiting Protection Circuit

The output voltage limiting protection circuit is as shown in the figure below. When the output voltage rises, the Zener diode and the optocoupler turn on, and the Q1 base turns on with a driving voltage. The voltage of UC38423 rises, the output decreases, and the Zener tube does not conduct. The voltage of UC38423 is lowered and the output voltage is raised. Repeatedly, the output voltage will stabilize within a range depending on the voltage of regulator.

Figure 18.

Figure 18.

10.4 Output Overvoltage Lockout Circuit

Figure 19(a)、(b)

Figure 19(a)、(b)

The working principle of Figure 19(a) is that when the output voltage Uo rises, the Zener diode and the optocoupler are turned on, and the base of Q2 is electrically turned on. Because Q2 is turned on, the base voltage of Q1 is lowered and also turned on, and the Vcc voltage makes Q2 always on through R1. Q1 and R2. Pin 3 of UC3842 is always high and stops working. In Figure 19(b), UO raises and the voltage of pin 3 of U1 rises. Pin 1 outputs high level and pin 1 of U1 always outputs high level due to the presence of D1 and R1. Q1 is always on, and pin 1 of UC3842 is always low and stops working.

 

11. Power Factor Correction Circuit (PFC)

11.1 Schematic Diagram

Figure 20.

Figure 20.

11.2 Working Principle

In one way, the input voltage sends PFC inductor through EMI filter composed of L1, L2, L3, etc. and BRG1 rectification. In another way, it is divided by R1 and R2 and then sent to PFC controller as sampling of input voltage for adjusting the duty ratio of controlled signal, that is, changing the on and off time of Q1 and stabilizing the PFC output voltage. L4 is a PFC inductor that stores energy when Q1 is on and releases energy when Q1 is off. D1 is the start diode, D2 is a PFC rectifier diode, and C6, C7 are filters. The PFC voltage is sent to the subsequent circuit, and it is divided by R3 and R4 and then sent to PFC controller as sampling of input voltage for adjusting the duty ratio of controlled signal and stabilizing the PFC output voltage in another way.

 

12. Input Over/Under Voltage Protection

12.1 Schematic

Figure 21.

Figure 21.

12.2 Working Principle

The principles of input over or under voltage protection of the AC input and DC input switching power supplies are approximately the same. The sampling voltage of the protection circuit is derived from the input filtered voltage. The sampling voltage is divided into two ways, one is divided by R1, R2, R3, R4 and then input to pin 3 of comparator. If the sampling voltage is higher than the reference voltage of pin 2, pin 1 of the comparator outputs a high level to control the main controller to shutdown and power supply has no output. The other way is divided by R6, R8, R9, R10 and then input to the pin 6 of comparator. If the sampling voltage is lower than the reference voltage of pin 5, pin 7 of the comparator outputs a high level to control the main controller to turn off, and the power supply has no output.

 

13. Battery Management

13.1 Schematic of Battery Management

Figure 22. Schematic of  Battery Management

Figure 22. Schematic of  Battery Management

The parts in the dotted box A constitute the battery starting and shutdown circuit; the dotted box B is the battery charging linear voltage regulated circuit; the dotted box C is the electronic switching circuit; and the dotted box D is the battery charging current limiting circuit.

13.2 Start Principle of Battery

The input voltage is input from the INPUT and AGND terminals and is divided into three paths. The first way is directly sent to the subsequent circuit and the battery starting and shutdown circuit via D7. The voltage after division of R28, R27 and R26 turns U3 on and the optocoupler OT1 is turned on. R25 provides the operating voltage for U3, and R23 and R24 are the current limiting and protection resistors of the optocoupler.

After the optocoupler is turned on, the power supply provides a base bias voltage to Q4 via R22, OT1, and D9 and Q4 is turned on. R21 is the lower bias resistor of Q4. A current flows through the coil of the relay RLY1-A, and the relay contact RLY1-B pulls in, and the battery BAT is connected to the circuit. D4 is to prevent the electromotive force generated by the relay coil from affecting the subsequent circuit when Q4 is turned off, and D5 releases the energy generated by the relay coil which is to prevent the electromotive force generated by the relay coil from damaging Q4 when Q4 is turned off.

13.3 The Voltage Regulated Principle of Battery Charging

At the beginning of electrification, since Q3 is not biased and does not conduct, there is no voltage at the positive terminal of D3. The power supply provides voltages to U1 and U2 via voltage drop of R1 and regulation of Z1. R2 and U1 form the reference voltage, R13, R4, R5, R6 and VR1 form the battery voltage detection circuit. When the detection voltage of pin 2 of U2 is lower than the voltage of pin 3 , pin 1 outputs a high level, and the bias voltage is supplied to Q2 via R14. Q2 is turned on and Q3 is also turned on. The power supply charges the battery BAT via Q3, D3, and relay contacts RLY1-B and F1.

When the detection voltage of pin 2 of U2 is higher than the voltage of pin 3, its pin 1 outputs a low level, Q2 loses the bias voltage and is turned off. Q3 is turned off, the positive terminal of D3 has no voltage and its negative voltage drops. The detection voltage of pin 2 of U2 also decreases. When the detection voltage of pin 2 of U2 is lower than the voltage of pin 3, pin 1 outputs a high level, and Q2 and Q3 are turned on to continue charging. So that the negative terminal voltage of D3 is maintained at a certain set value. Adjusting VR1 can change the charging voltage value.

13.4 The Principle of Battery Charging Current Limiting

Figure 23.

Figure 23.

During charging, the current returns to ground (AGND) via Q3, RLY1-B, F1, BAT, and R20. At the beginning of battery charging, because the battery voltage is relatively low, the current flowing through Q3, RLY1-B, F1, BAT, and R20 will increase, and the voltage drop generated on R20 will increase (R20 is the current sampling resistor). The upper terminal S of the resistor R20 is connected to the non-inverting input terminal 5 of U2B via R11, and the inverting input terminal 6 of U2B has a fixed reference voltage. When the voltage drop on R20 exceeds the reference voltage, pin 7 of U2 outputs a high level and provides a bias voltage to Q1 through D2 and R15. And Q1 is thus turned on. After Q1 is turned on, Q2 is turned off due to the loss of the base voltage, which will turn off the output of the linear regulator. No current flows through the loops of Q3, RLY1-B, F1, BAT, and R20, and the voltage drop on R20 disappears. Pin 7 of U2 outputs low level and Q1 is cut off. Q2 and Q3 are turned on to continue charging, so the charging current is limited to a certain set value range. Adjusting R10 and R11 can change the current limit point.

13.5 The Principle of Battery Undervoltage Shutdown

When the input voltage is not available, the battery voltage is sent to the subsequent circuit and the battery starting and shutdown circuit via D6. When the battery voltage drops, the voltage of pin 1 of U3 also drops. When the battery voltage drops to the designed shutdown point (that is, when the voltage of pin 1 of U3 is lower than 2.5V), U3 does not conduct, OT1 has no photoelectric coupling and Q4 is unbiased and cut off. There is no current flows through the coil of the relay RLY1-A, and the relay contact RLY1-B is disconnected, and the battery BAT is disconnected from the circuit to prevent the battery from being over-discharged and damaged. Changing the resistance of R26 and R27 can change the voltage value when the battery is shutdown because of undervoltage.


14. Intelligent Fan Cooling

14.1 Intelligent Heat Dissipation

In the switching power supply, there are many ways to dissipate heat from the power supply. Intelligent heat dissipation is one of them. It adjusts the operating voltage of the cooling fan to change the wind pressure according to the temperature of the power supply to achieve the best heat dissipation effect and  the purpose of energy saving. The schematic diagram is as follows:

Figure 24.

Figure 24.

14.2 Working Principle

The input voltage is input from the INPUT terminal (12~13V), R6 provides the working voltage for U2, and the resistance values of R7 and R8 are the same. After the voltage division, the trigger voltage is provided for TL431, so that the reference voltage of point A is +5V; RT1 is a negative temperature coefficient thermistor, which is applied to the inverting input terminal 6 of U1 via voltage division of R1 and R2. R5 is the output voltage sampling resistor, which is added to the non-inverting input terminal 5 of U1 after being divided by R4; Q1 is the electronic switch tube; the fan voltage is output from the FANOUT terminal.

At the time of power-on, since Q1 is not turned on, there is no voltage at point C, and the voltage of pin 6 of U1 is higher than pin 5. Therefore, the pin 7 of U1 outputs a low level, Z1 is turned on, Q1 is turned on, and there is voltage at point C; the emitter of Q1 is connected to the input voltage terminal, so the voltage at point C is approximately equal to the input voltage. After being divided by R5 and R4, it is applied to pin 5 of the non-inverting input terminal of U1, so that the voltage of pin 5 is higher than the voltage of pin 6, pin 7 of U1 outputs high level. Z1 is not conducting, Q1 is not conducting, and there is no voltage output at point C; the voltage of pin 5 is lower than that of pin 6. Pin 7 of U1 outputs low level again, so repeatedly it makes voltage of C stable at some value (because the voltage of pin 6 does not change); that is, the voltage at point C changes with the voltage at point B.

At the beginning of the switching power supply operation (or light load operation), the internal temperature is low, the internal resistance of the thermistor RT1 is large, and the voltage at point B is relatively low, so the output voltage at point C is also low, and the speed and wind power of the fan slow down due to the low operating voltage. When the temperature inside the switching power supply is gradually increased (or full load operation), the internal resistance of the thermistor RT1 gradually decreases, and the voltage at point B rises, so the output voltage at point C rises, and the fan speeds up and the wind power increases because the operating voltage rises. When the temperature inside the machine drops, the internal resistance of the thermistor gradually increases, the voltage at point B decreases, and the output voltage at point C also decreases. The fan has a slower rotation speed and a lower wind power due to a lower operating voltage. When the voltage (temperature) at point B rises to a certain level, the voltage of pin 3 of U1 is higher than the reference voltage of pin 2, and pin 1 of U1 outputs a high level and it goes back to point B via D1 and R13, so that pin 1 of U1 always outputs a high level,that is, self-locking. The other way will be output to the over-temperature protection circuit via D2 to realize over-temperature protection.

 

15. Current Sharing Technology

15.1 Current Sharing Technology

In communication equipment or other electrical equipment, in order to make the system work uninterruptedly, the requirements for the power supply system are very high. In addition to requiring the performance of the power supply itself to be stable, another method is to use the 1+1 backup method, that is, one device is powered in parallel with two power supplies. When one of them is damaged, the other one can continue to supply power to the system. In normal operation, each power supply provides the same energy, that is, the voltage and current they output are basically the same. In order to make the voltage and current output of each power supply basically the same, the current sharing technology is used. The principle is as shown below:

Figure 25.

Figure 25.

15.2 Working Principle

U1A, R1 to R7, C1 to C5, VR1 form a current sampling voltage amplifier; U1B and D1 form a voltage follower; R10 is a current sharing voltage output resistor; R11 to R14, U2A, C6 to C10 form a balanced voltage comparator; R15 to R17 and Q1 are electronic switches; R30 to R33, C17, C18, and U2B form an overcurrent protection circuit; R19 to 28, D2, D3, D4, C12 to C14, and Q2 are output voltage regulation loops of the power supply, of which D2 D3 and R19 to R21 are output voltage sampling circuits. D6 is an output isolation diode.

When the power supply is working, the current sampling voltage detected by the current loop or manganese copper wire is amplified by the voltage amplifier composed of +IS and -IS added to U1A. After being divided by R5, R6, R7 and VR1, the output is divided into two ways. One way is sent to the voltage follower U1B, D1 acts as an isolation to prevent the voltage change on the current sharing bus from affecting the previous stage circuit, and the other is sent to the overcurrent protection circuit. The current sampling voltage is divided into two ways after getting through the voltage follower. One gets through the R10 and is output as the current sharing signal voltage JL+, and the other is sent to the balanced voltage comparator composed of U2A through R11 and compared with the reference voltage of pin 2 of U2. When the voltage of pin 3 of U2 is higher than that of pin 2, its pin 1 outputs a high level. Base Q1 is electrically conducted, and R17 and R18 are incorporated into the output voltage sampling circuit, so that the output voltage rises and the output current decrease. The detected current sampling voltage is also reduced, and the current sharing signal voltage JL+ is lowered. The voltage of pin 3 of U2 is lower than the voltage of pin 2, and its pin 1 outputs a low level. Q1 is cut off, R17 and R18 are exited from the output voltage sampling circuit, and the output voltage is lowered.This cycle finally stabilizes the output voltage and current. As shown in the following figure, when the two power supplies work in parallel, the output terminals are connected together, and the current sharing signal lines are also connected. Now suppose that the output current Io1 of the power supply A is larger than the output current Io2 of the power supply B, then the current sampling voltage of A inside the two power supplies will be higher than B, that is, JL1+ is higher than JL2+, and JL1+ and JL2+ are connected on the same line (current flow bus). Therefore, JL2+ rises, the output voltage rises by the control of the internal current sharing circuit of power supply B, Io2 increases and Io1 decreases (load current does not change); when Io2 is higher than Io1, its control process is on the contrary. This cycle will eventually make the output voltage and current of the two power supplies consistent.

Figure 26. Parallel Current Sharing Diagram

Figure 26. Parallel Current Sharing Diagram

The function of the circuit composed of Q3, C19, R34 to R36 is that Q3 is turned on when the output voltage is low or the output is under voltage at the initial stage of power supply, so that pin 3 of U2A is at a low level. Pin 1 of U2A outputs a low level, and Q1 is cut off, that is, the current sharing circuit does not work. VR1 can adjust the voltage value of the current sharing signal, and can also adjust the output current limit point.

 

You May Also Like

Circuit Design of Linear DC Regulated Power Supply

Design of Single-Phase Sine Wave SPWM Inverter Power Supply Based on SG3525

Circuit Design Schematic of Adjustable Voltage Regulated Power Supply

Principle and Application of DC Stabilizing Power Supply

The Working Principle of High-Power Adjustable Switching Power Suppl


0 comment

Leave a Reply

Your email address will not be published.

 
 
   
Rating: