Practical information | Detailed analysis of various protection circuit examples of switching power supplies

1. Input undervoltage protection circuit 1

1. Overview (circuit category, description of main functions):

This circuit belongs to the 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):

3. Working principle analysis (main functions, performance indicators and implementation principles):

When the power input voltage is higher than the undervoltage protection setting 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 Vcc is powered normally; when the input voltage is lower than the protection voltage, the voltage at point A is lower than the Vref of U4, U4 is turned off, the voltage at point B is high, Q4 is turned off, and Vcc has no voltage. At this time, Vref is also low. When the input voltage gradually increases, the voltage at point A 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

Advantages of this circuit: simple circuit, precise protection point

Disadvantages: higher cost.

5. Application precautions:

When using, pay attention to the values ​​of R1 and R2. Sometimes two resistors need to be connected in parallel to get the required protection point. Also pay attention to the temperature coefficients of R1 and R2, otherwise the undervoltage protection point will differ greatly at high and low temperatures.

2. Input undervoltage protection circuit 2

1. Overview (circuit category, description of main functions):

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. Working principle analysis (main functions, performance indicators and implementation principles):

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, 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 is less than the voltage regulation value of VD4, VT4 is turned off, Vb is high potential, VT5 is turned on, 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 a hysteresis circuit when the undervoltage is turned off and restored. When the undervoltage is turned off, VT6 is turned on, and R21 is connected in parallel with R2; when it is restored, VT6 is turned off, and 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 voltage regulation values ​​between batches of voltage regulator tube VD4, the undervoltage protection point fluctuates up and down, and related parameters need to be frequently debugged during mass production.

5. Application precautions:

VD4 should be a voltage regulator with a good temperature coefficient. Components that need to be debugged, such as R2, should be connected in parallel to facilitate debugging.

1. Output overvoltage protection circuit 1

1. Overview (circuit category, description of main functions):

Output overvoltage protection circuit. When an external 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 the regulated voltage value, this circuit will clamp the output voltage at the set value.

2. Circuit composition (schematic diagram):

3. Working principle analysis (main functions, performance indicators and implementation principles):

When the output is overvoltage, when the voltage applied to VD3 is greater than its regulated value, VD3 is turned on, the output voltage is clamped, and at the same time, it is fed back 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 voltage regulation values ​​between batches of voltage regulator tube VD3, the overvoltage clamping point fluctuates up and down, and related parameters need to be frequently debugged during mass production.

5. Application precautions:

VD3 should choose a voltage regulator with a good temperature coefficient. Components that need to be debugged, such as R32, should be connected in parallel to facilitate debugging.

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

2. Output overvoltage protection circuit 2

1. Overview (circuit category, description of main functions):

Output overvoltage protection circuit. When an external 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 the normal value, this circuit will stabilize the output voltage at the set value.

2. Circuit composition (schematic diagram):

3. Working principle analysis (main functions, performance indicators and implementation principles):

When the output is overvoltage, Va>Vref, IC3 is turned on, and feedback is given to the primary side through IC4, and the output voltage is stabilized 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 the Zener diode clamping method.

5. Application precautions:

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

1. Overview (circuit category, description of main functions):

In a power supply system, when the feedback loop fails, the output voltage is not controlled and the voltage rises beyond the specified range. At this time, the excessively high 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 ways 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 is activated, and the PWM is turned off so that the module has no output. After eliminating the fault and restarting the power output, the power supply will be normal. The following circuit is a self-locking control circuit.

2. Circuit composition (schematic diagram):

3. Working principle analysis (main functions, performance indicators and implementation principles):

The figure above shows an isolated self-locking control circuit. When the overvoltage protection signal CONTROL terminal gives a high level, the transistor in U1 is turned on, and VCC is the power supply terminal of the entire circuit. Vcc gives Q2 a base current through R5, Q1 is turned on and enters the saturation state, the SHUT terminal is pulled to a low level by Q2, and the PWM power supply is turned off 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 another base current is provided to Q2 through R3 to maintain the conduction of Q2. Q1 and R1, R2, and R3 constitute the positive feedback circuit of Q2.

4. Advantages and disadvantages of the circuit

Advantages: It can effectively perform self-locking protection, and the entire circuit is equivalent to a thyristor.

Disadvantages: The entire circuit requires a fixed Vcc. When the PWM power supply is not powered, the VCC voltage in the figure above must also be guaranteed.

5. Application precautions:

1. This circuit needs continuous power supply to be self-locking.

2. This circuit should not be used in unattended power supply systems.

1. Overview (circuit category, description of main functions):

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

2. Circuit composition (schematic diagram):

3. Working principle analysis (main functions, performance indicators and implementation principles):

The voltage regulator 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

Advantages of this circuit: simple circuit and high accuracy.

Disadvantages: higher cost.

5. Application precautions:

5.1 When pin 3 and pin 1 of MAX6501 are connected, the hysteresis temperature is 10℃. When pin 3 and ground are connected, 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, description of main functions):

This circuit uses a thermistor to detect the substrate temperature. The resistance of the thermistor changes with the substrate temperature. The change in the resistance of the thermistor causes the op amp input voltage to change, thereby flipping the op amp to control the output of the PWM chip and shutting down the module.

2. Circuit composition (schematic diagram):

3. Working principle analysis (main functions, performance indicators and implementation principles, key parameter calculation and analysis):

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

As the substrate temperature increases, the resistance of R99 decreases. When it decreases to a certain value, the negative input of the op amp is greater than the positive input. The op amp outputs a low level, pulling down the SS of LM5025, thereby shutting down the module output. The temperature protection point can be adjusted accordingly by appropriately adjusting the resistance values ​​of R94, R23, and R97.

After the module turns off the output (over-temperature protection), the substrate temperature will decrease, the resistance of R99 will increase, and the negative input of the op amp will decrease. In order to make the op amp flip normally, the resistor R98 is introduced. The principle is that after the op amp output is low, R98 is equivalent to being connected in parallel with R97, lowering the op amp reference and widening the voltage gap between the positive and negative inputs of the op amp, thereby achieving temperature hysteresis. For example, the protection is on when the substrate temperature is 90°C and it is on when it is 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 The resistance of thermistor during temperature protection is obtained: R99 (t) = (Vref * R24 / (Vref * R97 / (R23 + R97) - 0.007)) - R94

4.4 Calculations taking tolerance into account are shown in the following table:

4.5 R99 value during over-temperature protection

4.6 R99-SDNT2012X104J4250HT(F) is a thermistor with a negative temperature coefficient. The resistance is 100k at 25°C and about 10k during 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: room temperature 25°C, in the above formula T2=273.15+25=298.15; B: 4250±3%; R: resistance value at 25°C, 100k, the calculated T1 value is also the value after adding 273.15, so t1=T1-273.15 in the table below, in degrees Celsius. Rt: resistance value after temperature change, 10k, 9.704k, 10.304k, see the table above

4.7 Hysteresis

After the op amp output is low, resistor R98 (51k) is connected to R97 to pull the reference down. The new reference voltage Vref1 = Vref* (R98//R97) / (R23+R98//R97) = 2.28V. When it reaches 2.44V, the resistance of R99 R99 = Vref*R94/Vref1-R94 = 11.9k. When R99 reaches 10.49k, the temperature is calculated as shown in the following table.

Temperature difference = 82.6-77.3 = 5.3℃

5. Advantages and disadvantages of the circuit

Advantages: Temperature protection point and temperature hysteresis are easy to adjust

Disadvantages: Low temperature accuracy

The circuit is slightly more complicated than using a temperature switch

The temperature protection reflects the substrate temperature near the thermistor, which cannot reflect the temperature of the highest device in the module. However, this can be solved during design. For example, if the substrate temperature is protected at 90°C, but the actual highest device temperature on the board has reached 130°C, the temperature protection point can be adjusted appropriately to achieve a protective effect.

6. Application precautions

Try to place the thermistor near the heat generating device.

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