Detailed anatomy of various protection circuit examples of switching power supply

Input undervoltage protection circuit

1. Input undervoltage protection circuit 1

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

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

2. Circuit composition (schematic diagram):

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

When the power supply input voltage is higher than the undervoltage 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 At this time, the voltage at point A is lower than the Vref of U4, U4 is cut off, the voltage at point B is high, and Q4 is 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 rises gradually. 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

Advantages of this circuit: simple circuit, precise protection point

Disadvantages: higher cost.

5. Precautions for application:

Pay attention to the value of R1 and R2 when using, 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 coefficient of R1 and R2, otherwise, the undervoltage protection point will be quite different at high and low temperature.

2. Input undervoltage 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 is turned off; when the input voltage rises to the set recovery value, the output automatically returns to normal.

2. Circuit composition (schematic diagram):

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

When the input voltage is within the normal operating range, Va is greater than the regulated value of VD4, VT4 is turned on, Vb is at 0 potential, and VT5 is cut off. At this time, the protection circuit does not work; when the input voltage is lower than the set undervoltage value, Va If it is less than the regulated voltage value of VD4, VT4 is cut off, Vb is high potential, VT5 is turned on, and COMP (pin 1 of the chip) is pulled to 0 potential, and the chip turns off the output, thereby realizing the undervoltage protection function. R21, VT6, and R23 form the hysteresis circuit for undervoltage shutdown and recovery. When the undervoltage is turned off, VT6 is turned on, and R21 and R2 are connected in parallel,

; When recovering, VT6 cuts off,

, the hysteresis voltage is (Vin’-Vin).

4. Advantages and disadvantages of the circuit

Advantages: simple circuit form and low cost.

Disadvantages: Due to the difference in the voltage regulation value of the voltage regulator tube VD4 between batches, the undervoltage protection point fluctuates up and down, and the relevant parameters need to be frequently adjusted during mass production.

5. Precautions for application:

VD4 should choose a voltage regulator tube with a better temperature coefficient. For components that need to be debugged, such as R2, multiple parallel connections should be considered to facilitate debugging.

Output Overvoltage Protection Circuit

1. Output overvoltage protection circuit 1

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

Output overvoltage protection circuit. This circuit will clamp the output voltage at the set value when an applied voltage higher than the normal output voltage range is applied to the output terminal or the circuit itself fails (open loop or other) to cause the output voltage to be higher than the regulated value.

2. Circuit composition (schematic diagram):

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

When the output is overvoltage, when the voltage added to VD3 is greater than its regulated value, VD3 will be turned on, the output voltage will be clamped, and feedback will be sent to the primary side through IC4.

4. Advantages and disadvantages of the circuit

Advantages: simple circuit form and low cost.

Disadvantages: Due to the difference in the voltage regulation value of the voltage regulator tube VD3 between batches, the overvoltage clamp point fluctuates up and down, and the relevant parameters need to be frequently adjusted during mass production.

5. Precautions for application:

VD3 should choose a voltage regulator tube with a better temperature coefficient. For components that need to be debugged, such as R32, multiple parallel connections should be considered to facilitate debugging.

When the overvoltage protection circuit works, the circuit is in an abnormal working state. For a circuit with the function of adjusting the output voltage up and down, the overvoltage protection point should be greater than the maximum value of the output voltage.

2. Output overvoltage protection circuit 2

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

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

2. Circuit composition (schematic diagram):

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

When the output is overvoltage, Va>Vref, IC3 is turned on, and 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

Advantages: The output overvoltage protection value can be set accurately.

Disadvantages: The cost is slightly higher than that of the Zener tube clamping method.

5. Precautions for application:

When the overvoltage protection circuit works, the circuit is in an abnormal working state. For a circuit with the function of adjusting the output voltage up and down, the overvoltage protection point should be greater than the maximum value of the output voltage.

Overvoltage protection self-locking control circuit

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

In the power 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 subsequent electrical equipment. To solve this problem, an overvoltage protection circuit is usually added to the power supply. There are generally three methods of overvoltage protection.

A. Clamping type: When the feedback fails, the output voltage is clamped at 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 operates, and the PWM is turned off so that the module has no output. After troubleshooting, restart the power output to supply power normally. The following circuit is a self-locking control circuit.

2. Circuit composition (schematic diagram):

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

The figure above is an isolated self-locking control circuit. When the overvoltage protection signal CONTROL end gives a high level, the triode in U1 is turned on, and VCC is the power supply end of the entire circuit. Vcc supplies a base current to Q2 through R5, Q1 is turned on and enters a saturated 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 provided 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: Effective self-locking protection, the entire circuit is equivalent to a silicon controlled rectifier.

Cons: The whole circuit needs 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. Precautions for application:

1. This circuit must have a continuous power supply self-locking 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 belongs to the 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):

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

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

4. Advantages and disadvantages of the circuit

The advantage of this circuit: the circuit is simple and the precision is high.

Disadvantages: higher cost.

5. Precautions for application:

5.1 When pin 3 of MAX6501 is connected to pin 1, the hysteresis temperature is 10°C; when pin 3 is connected to ground, the hysteresis temperature is 2°C.

5.2 The power 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 a 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 (thus realizing the flipping of the op amp to control the output of the PWM chip, and then Turn the module off.

2. Circuit composition (schematic diagram):

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

The R99 thermistor is a negative temperature coefficient thermistor. At room temperature, R99=100k, 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 high level, which has no effect on the SS end of the LM5025, and the module works normally.

As the temperature of the substrate increases, the resistance value of R99 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. SS is pulled low to turn off the module output; the temperature protection point can be adjusted by appropriately 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 drop, the resistance of R99 will increase, and the negative input of the op amp will decrease. In order to make the op amp turn over normally, a resistor R98 is introduced. R98 is equivalent to being connected in parallel with R97, lowering the reference of the op amp and widening the voltage gap between the positive and negative inputs of the op amp, thereby realizing the temperature hysteresis. For example, when the substrate temperature is 90°C, it is protected, and when it is 80°C, it is turned on.

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 Get the resistance value of the thermistor in temperature protection: R99(t)=(Vref*R24/(Vref*R97/(R23+R97)-0.007))-R94

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

4.5 The value of R99 at the time of over-temperature protection

4.6 R99-SDNT2012X104J4250HT(F) is a thermistor with negative temperature coefficient, 100k at 25°C, and the resistance value is about 10k when 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, t1=T1-273.15 in the table below is Celsius. Rt: resistance value after temperature change, 10k, 9.704k, 10.304k, see the table above

4.7 Hysteresis

After the output of the operational amplifier 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 reaches 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 following table

Temperature hysteresis=82.6-77.3=5.3℃

5. Advantages and disadvantages of the circuit

Advantages: temperature protection point and temperature return difference are easy to adjust

Disadvantages: low temperature accuracy

The circuit is slightly more complex 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 component of the module, but this can be resolved during design. For example, the substrate temperature is protected at 90°C, and the maximum temperature of the components on the actual board has reached 130°C. The temperature protection point can be adjusted appropriately to play a protective role.

6. Precautions for application

Try to place the thermistor near the heating device.