When the power input voltage is higher than the under-voltage protection set point, the voltage at point A is higher than the Vref of U4, U4 is turned on, the voltage at point B is low, Q4 is turned on, and the Vcc power supply is normal; when the input voltage is lower than the protection voltage When the voltage at point A is lower than the Vref of U4, U4 is turned off, the voltage at point B is high, and Q4 is turned off, so Vcc has no voltage, and Vref is also low at this time. When the input voltage gradually increases, the voltage at point A It also gradually increases. When it is higher than the Vref of U4, the module works normally again. R4 can set the hysteresis of the undervoltage protection point.

Input undervoltage protection circuit

1. Input undervoltage protection circuit 1

1. Overview (circuit category, main function description):

This circuit is an input undervoltage circuit. When the input voltage is lower than the protection voltage, the power supply Vcc of the control chip is pulled down, thereby turning off the output.

2. Circuit composition (schematic diagram):

Detailed anatomy of various protection circuit examples of switching power supply

3. Analysis of working principle (main functions, performance indicators and implementation principle):

When the power input voltage is higher than the under-voltage protection set point, the voltage at point A is higher than the Vref of U4, U4 is turned on, the voltage at point B is low, Q4 is turned on, and the Vcc power supply is normal; when the input voltage is lower than the protection voltage When the voltage at point A is lower than the Vref of U4, U4 is turned off, the voltage at point B is high, and Q4 is turned off, so Vcc has no voltage, and Vref is also low at this time. When the input voltage gradually increases, the voltage at point A It also gradually increases. When it is higher than the Vref of U4, the module works normally again. R4 can set the hysteresis of the undervoltage protection point.

4. Advantages and disadvantages of the circuit:

The advantages of this circuit: simple circuit, accurate protection point
Disadvantage: higher cost.

5. Notes on application:

When using, pay attention to the value of R1 and R2. Sometimes two resistors need to be connected in parallel to get the required protection point. It is also necessary to pay attention to the temperature coefficients of R1 and R2, otherwise the undervoltage protection points will be quite different at high and low temperature.

2. Input under-voltage protection circuit 2

1. Overview (circuit category, main function description):

Input undervoltage protection circuit. When the input voltage is lower than the set undervoltage value, the output will be turned off; when the input voltage rises to the set recovery value, the output will automatically return to normal.

2. Circuit composition (schematic diagram):

Detailed anatomy of various protection circuit examples of switching power supply

3. Analysis of working principle (main functions, performance indicators and implementation principle):

When the input voltage is within the normal working range, Va is greater than the voltage regulation value of VD4, VT4 is turned on, Vb is 0 potential, and VT5 is turned off, and the protection circuit does not work at this time; when the input voltage is lower than the set undervoltage value, Va Less than the voltage regulation value of VD4, VT4 is off, Vb is high potential, VT5 is on, pulls COMP (pin 1 of the chip) to 0 potential, and the chip turns off the output, thus realizing the undervoltage protection function. R21, VT6, R23 form the hysteresis circuit during under-voltage shutdown and recovery. When the undervoltage is turned off, VT6 is turned on, and R21 is connected in parallel with R2,Detailed anatomy of various protection circuit examples of switching power supply;When recovering, VT6 is cut off,Detailed anatomy of various protection circuit examples of switching power supplythe hysteresis voltage is (Vin’-Vin).

4. Advantages and disadvantages of the circuit:

Advantages: The circuit form is simple and the cost is low.
Disadvantages: Due to the difference in voltage voltage between batches of voltage regulator tube VD4, the undervoltage protection point floats up and down, and relevant parameters need to be adjusted frequently during mass production.

5. Notes on application:

VD4 should choose a voltage regulator with better temperature coefficient, and components to be debugged, such as R2, should be considered in parallel to facilitate debugging.

Output overvoltage protection circuit

1. Output overvoltage protection circuit 1

1. Overview (circuit category, main function description):

Output overvoltage protection circuit. When an applied voltage higher than the normal output voltage range is applied to the output terminal or the circuit itself malfunctions (open loop or other) causing the output voltage to be higher than the regulated value, this circuit will clamp the output voltage at the set value.

2. Circuit composition (schematic diagram):

Detailed anatomy of various protection circuit examples of switching power supply

3. Analysis of working principle (main functions, performance indicators and implementation principle):

When the output is over-voltage, when the voltage applied to VD3 is greater than its voltage regulation value, VD3 is turned on, the output voltage is clamped, and is fed back to the primary side through IC4 at the same time.

4. Advantages and disadvantages of the circuit:

Advantages: The circuit form is simple and the cost is low.
Disadvantages: Due to the difference in voltage voltage between batches of voltage regulator tube VD3, the overvoltage clamp point floats up and down, and relevant parameters need to be adjusted frequently during mass production.

5. Notes on application:

VD3 should choose a voltage regulator tube with better temperature coefficient, and components to be debugged, such as R32, should be considered in parallel to facilitate debugging.
When the overvoltage protection circuit works, the circuit is in an abnormal working state. For circuits with output voltage up and down functions, the overvoltage protection point should be greater than the maximum output voltage increase.

2. Output overvoltage protection circuit 2

1. Overview (circuit category, main function description):

Output overvoltage protection circuit. When an applied voltage higher than the normal output voltage range is applied to the output terminal or the circuit itself is faulty (open loop or other) causing the output voltage to be higher than the normal value, this circuit will stabilize the output voltage at the set value.

2. Circuit composition (schematic diagram):

Detailed anatomy of various protection circuit examples of switching power supply

3. Analysis of working principle (main functions, performance indicators and implementation principle):

When the output is overvoltage, Va>Vref, IC3 is turned on, feedback to the primary side through IC4, the output voltage is stable at the set overvoltage protection value.

4. Advantages and disadvantages of the circuit:

Advantage: The output overvoltage protection value can be precisely set.
Disadvantages: The cost is slightly higher than the Zener tube clamping method.

5. Notes on application:

When the overvoltage protection circuit works, the circuit is in an abnormal working state. For circuits with output voltage up and down functions, the overvoltage protection point should be greater than the maximum output voltage increase.

Overvoltage protection self-locking control circuit

1. Overview (circuit category, main function description):

In the power supply system, when the feedback loop fails, the output voltage is uncontrolled, and the voltage rises beyond the specified range. At this time, the excessive output voltage may cause damage to the subsequent electrical equipment. To solve this problem, an overvoltage protection circuit is usually added to the power supply. There are generally three types of overvoltage protection.
A. Clamping type: When the feedback fails, the output voltage is clamped to a fixed value through the overvoltage clamping circuit.
B. Intermittent protection type: When the feedback fails, the output voltage is restarted back and forth through the protection circuit, and the highest point of the output voltage is the overvoltage protection point.
C. Self-locking type: When the output voltage reaches the overvoltage protection point, the circuit will act, and the PWM will be turned off so that the module has no output. After troubleshooting, restart the power supply output to supply power normally. The following circuit is a self-locking control circuit.

2. Circuit composition (schematic diagram):

Detailed anatomy of various protection circuit examples of switching power supply

3. Analysis of working principle (main functions, performance indicators and implementation principle):

The above figure shows the isolated self-locking control circuit. When the CONTROL end of the overvoltage protection signal gives a high level, the Transistor in U1 is turned on, and VCC is the power supply end of the entire circuit. Vcc gives Q2 a base current through R5, Q1 is turned on and enters a saturation state, the SHUT terminal is pulled to a low level by Q2, and the PWM turns off the power supply without output. Q2 also controls the conduction of Q1. When Q2 is turned on, the base current of Q1 goes to the ground through R2, Q1 is turned on, and a base current is supplied to Q2 through R3 to maintain the conduction of Q2. Q1 and R1, R2, R3 constitute the positive feedback circuit of Q2.

4. Advantages and disadvantages of the circuit:

Advantages: It can effectively carry out self-locking protection, and the whole circuit is equivalent to a thyristor.
Disadvantage: The entire circuit requires a fixed Vcc. When there is no power supply at the PWM power supply terminal, it is also necessary to ensure the existence of the VCC voltage in the above figure.

5. Notes on application:

1. This circuit must be self-locking with continuous power supply to be effective.
2. This circuit should not be used in an unattended power system.

Over temperature protection circuit

1. Overview (circuit category, main function description):

This circuit is an over-temperature protection circuit, but when the temperature is higher than the set protection point, the output of the module is turned off, and the module is automatically turned on when the temperature recovers.

2. Circuit composition (schematic diagram):

Detailed anatomy of various protection circuit examples of switching power supply

3. Analysis of working principle (main functions, performance indicators and implementation principle):

The voltage regulator tube provides 5V voltage to U103MAX6501. When the temperature is normal, the five pins of U103 output a high level. When the temperature exceeds the protection point, the five pins of U103 output a low level. When the temperature recovers, the five pins of U103 output a high level. .

4. Advantages and disadvantages of the circuit:

The advantages of this circuit: the circuit is simple and the accuracy is high.
Disadvantage: higher cost.

5. Notes on application:

5.1 When the 3-pin of MAX6501 is connected to the 1-pin, the hysteresis temperature is 10℃, and when its 3-pin is connected to the ground, the hysteresis temperature is 2℃.
5.2 The supply voltage of MAX6501 cannot exceed 7V, otherwise it will be damaged.
5.3 The MAX6501 must be placed near the hottest part.

Over temperature protection circuit – thermistor

1. Overview (circuit category, main function description):

This circuit uses the thermistor to detect the temperature of the substrate. The resistance of the thermistor changes with the temperature of the substrate. The change of the resistance of the thermistor causes the input voltage of the op amp to change, so as to realize the inversion of the op amp to control the output of the PWM chip, and then convert the output of the PWM chip. Module is closed.

2. Circuit composition (schematic diagram):

Detailed anatomy of various protection circuit examples of switching power supply

3. Analysis of working principle (main functions, performance indicators and implementation principles, calculation and analysis of key parameters):

The R99 thermistor is a negative temperature coefficient thermistor. At normal temperature, R99=100k, and the divided voltage of R99 and R94 is 0.45V, which is the negative input of the U2 op amp, which is much lower than the positive input of the op amp 2.5V (R23 and R97 divide voltage), so the output of the op amp is a high level, which has no effect on the SS terminal of the LM5025, and the module works normally.

As the temperature of the substrate increases, the resistance value of the R99 resistor decreases. When it decreases to a certain value, when the negative input of the op amp is greater than the positive input, the op amp outputs a low level and pulls the SS of the LM5025 low, thereby turning off the module Output; the temperature protection point can be adjusted accordingly by adjusting the resistance values ​​of R94, R23 and R97.

After the module turns off the output (over-temperature protection), the temperature of the substrate will decrease, the resistance value of R99 will increase, and the negative input of the op amp will decrease. R98 is equivalent to being connected in parallel with R97, lowering the reference of the op amp and opening the voltage distance between the positive and negative inputs of the op amp, so as to realize the temperature hysteresis. For example, when the substrate temperature is 90°C, it is protected, and it is turned on at 80°C.

4. Calculation and analysis of key parameters:

4.1 Op amp positive input voltage:

VR97=Vref2=5/(1+R23/R97)=5/(1+10/10)=2.5V
4.2 Op amp negative input voltage:
VR94+0.007=VR97=5*R94/(R99+R94)+0.007,
4.3 Obtain the resistance value of the thermistor during temperature protection:
R99

4.4 The calculation when considering the tolerance is shown in the table below:

Detailed anatomy of various protection circuit examples of switching power supply

4.5 When over temperature protection, the value of R99

Detailed anatomy of various protection circuit examples of switching power supply

4.6 R99-SDNT2012X104J4250HT(F) is a thermistor with negative temperature coefficient, 100k at 25°C and about 10k in over-temperature protection (see the table above). The calculated temperature is:

Rt=R*e(B(1/T1-1/T2)) T1=1/(ln(Rt/R)/B+1/T2))
T2: normal temperature 25°C, T2=273.15+25=298.15 in the above formula; B: 4250±3%; R: resistance value at 25°C, 100k, the calculated T1 value is also the value after adding 273.15, Therefore, in the table below, t1=T1-273.15, which is Celsius. Rt: resistance value after temperature change, 10k, 9.704k, 10.304k, see the table above

Detailed anatomy of various protection circuit examples of switching power supply

4.7 Hysteresis

After the output of the op amp is low, the resistor R98 (51k) is connected to R97 to pull the reference down, and the new reference voltage Vref1=Vref*(R98//R97)/(R23+R98//R97)=2.28V to reach 2.44 When V, the resistance value of R99 R99=Vref*R94/Vref1-R94=11.9k When R99 reaches 10.49k, the temperature is calculated according to the table below

Detailed anatomy of various protection circuit examples of switching power supply

Temperature hysteresis=82.6-77.3=5.3℃

5. Advantages and disadvantages of the circuit:

Advantages: The temperature protection point and temperature hysteresis can be easily adjusted
Cons: Low temperature accuracy
The circuit is slightly more complicated than using a temperature switch
The temperature protection reflects the temperature of the substrate near the thermistor, and cannot reflect the temperature of the highest device of the module, but this can be solved during design. The temperature protection point can be adjusted appropriately to play a protective role.

6. Notes on application:

Place the thermistor as close to the heat-generating device as possible.

The Links:   CM300DXD-24A CM600HU-12F