Detailed anatomy of various protection circuit examples of switching power supply!

Although the protection function of the switching power supply is an additional function required by the electrical performance of the power supply device, it is very important to the safety and reliability of the power supply device whether the protection circuit is complete and works according to the predetermined settings under the harsh environment and accident conditions.

Input Undervoltage protection circuit

Input Undervoltage protection circuit one

1 Overview (circuit category, realization of main function description):

This circuit is an input under-voltage circuit. When the input voltage is lower than the protection voltage, the power supply Vcc of the control chip is pulled down to turn off the output.

2 Circuit composition (schematic diagram):

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

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 When 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 cut off, so that Vcc has no voltage, and Vref is also low at this time. When the input voltage gradually rises, the voltage at point A also gradually increases. When the Vref of U4 is higher than that 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: simple circuit, accurate protection point
 
Disadvantages: Higher cost.

5 Application notes:

Pay attention to the values of R1 and R2 when using them. Sometimes it is necessary to connect two resistors in parallel to get the required protection point. Also need to pay attention to the temperature coefficients of R1 and R2, otherwise, the Undervoltage protection points will be quite different at high and low temperatures.

Input Undervoltage protection circuit two

1 Overview (circuit category, realization of 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 Working principle analysis (main functions, performance indicators, and implementation principles):

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 turned off. At this time, the protection circuit does not work; when the input voltage is lower than the set Undervoltage value, Va Less than the voltage value of VD4, VT4 is off, Vb is high, VT5 is on, COMP (pin 1 of the chip) is pulled to 0 potential, and the chip turns off the output, thus realizing the Undervoltage protection function. R21, VT6, and R23 form a hysteresis circuit for Undervoltage shutdown and recovery. When the Undervoltage is turned off, VT6 is turned on, and R21 is connected in parallel with R2, Va=Vin'*R2/R21/(R1+R2/R21); when it is restored, VT6 is turned off, Va=Vin*R2/(R1+R2 ), 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 it is necessary to frequently adjust the relevant parameters during mass production.

5 Application notes:

VD4 should select a voltage regulator tube with a good temperature coefficient, and components that need to be debugged, such as R2, should consider multiple parallel connections to facilitate debugging.

Output overvoltage protection circuit

Output overvoltage protection circuit one

1 Overview (circuit category, realization of main function description):

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 malfunctions (open loop or other) causing the output voltage to be higher than the regulated value, this circuit will clamp the output voltage to 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 feedback to the primary side through IC4 at the same time.

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 clamping point fluctuates up and down, and the relevant parameters need to be adjusted frequently during mass production.

5 Application notes:

VD3 should choose a voltage regulator tube with a good temperature coefficient, and components that need to be debugged, such as R32, should consider multiple parallel connections to facilitate debugging.
 
When the overvoltage protection circuit works, the circuit is in an abnormal working state. For circuits with output voltage up and downregulation, the overvoltage protection point should be greater than the maximum output voltage up-regulation.

Output overvoltage protection circuit two

1 Overview (circuit category, realization of main function description):

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 malfunctions (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 to the primary side through IC4, 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 accurately set.
 
Disadvantages: The cost is slightly higher than the voltage regulator clamp method.

5 Application notes:

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

Overvoltage protection self-locking control circuit

1 Overview (circuit category, realization of main function description):

In the power 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 to a fixed value through an overvoltage clamp 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 turn off the PWM to make the module 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 Working principle analysis (main functions, performance indicators, and implementation principles):

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

4 advantages and disadvantages of the circuit

Advantages: self-locking protection can be effectively carried out, and the entire circuit is equivalent to a silicon-controlled rectifier.
  
Disadvantages: 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 Application notes:

1. This circuit needs a continuous power supply and self-locking to be effective.
 
2. This circuit should not be used in unattended power supply systems.

Over-temperature protection circuit

Over-temperature protection circuit

1 Overview (circuit category, realization of main function description):

This circuit is an over-temperature protection circuit, but 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 is restored.

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 a high level. When the temperature exceeds the protection point, the five pins of U103 output a low level. When the temperature is restored, the five pins of U103 output a high level.

4 advantages and disadvantages of the circuit

Advantages: simple circuit and high accuracy.
 
Disadvantages: Higher cost.

5 Application notes:

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

Over-temperature protection circuit-thermistor

1 Overview (circuit category, realization of 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 in the resistance of the thermistor causes the input voltage of the op-amp to change, to realize the flip of the op-amp and control the output of the PWM chip. The module is closed.

2 Circuit composition (schematic diagram):

3 Working principle analysis (main functions, performance indicators, and realization principles, calculation and analysis of key parameters):

The R99 thermistor is a negative temperature coefficient thermistor. At room temperature, R99=100k, and the partial voltage of R99 and R94 is 0.45V for 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 points Pressure), so the output of the op-amp is high, which does not affect the SS end of the LM5025, and the module works normally.
 
As the substrate temperature increases, the resistance of R99 resistance 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, which pulls the SS of LM5025 low, thereby turning off the module Output; the temperature protection point can be adjusted appropriately by adjusting the resistance of R94, R23, R97 accordingly.
 
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. To make the op-amp normal flip, 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 to lower the reference of the op-amp and open up the voltage gap between the positive and negative inputs of the op-amp to achieve 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 Get the resistance value of the thermistor for 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 following table:

4.5 When over-temperature protection, the value of R99

4.6 R99-SDNT2012X104J4250HT(F) is a thermistor with a negative temperature coefficient, 100k at 25°C, the resistance value of about 10k for over-temperature protection (see the above table), 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, 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, 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 above table

4.7 Backlash
 
After the op-amp output is low, the resistor R98 (51k) is connected to R97 to pull the reference low. The new reference voltage Vref1=Vref*(R98//R97)/(R23+R98//R97)=2.28V to reach 2.44 At 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 difference=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 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 component of the module, but this can be solved in the design. For example, the substrate temperature is protected at 90℃, and the actual maximum temperature of the device on the board has reached 130℃. The temperature protection point can be adjusted appropriately to play a protective role.

6 Application notes

Try to place the thermistor near the heating device.