Input undervoltage protection circuit

1、 Input undervoltage protection circuit 1

1. Overview (circuit category, main function description)

When the input voltage is lower than the protection voltage, the power supply VCC of the control chip is pulled down, so as to turn off the output.

2. Circuit composition (schematic diagram) 3. Working principle analysis (main function, performance index and realization principle)

When the input voltage of power supply is higher than the set point of undervoltage protection, the voltage of point a is higher than VREF of U4, U4 is on, the voltage of point B is low level, Q4 is on, and the power supply of VCC is normal; when the input voltage is lower than the protection voltage, the voltage of point a is lower than VREF of U4, U4 is off, the voltage of point B is high level, and Q4 is off, so there is no voltage in VCC, and VREF is also low level. When the input voltage increases gradually, a When VREF is higher than U4, the module works normally. R4 can set the return difference of undervoltage protection point.

The advantages of this circuit are simple circuit and precise protection point

5. Precautions for 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 coefficient of R1 and R2, otherwise the difference of undervoltage protection point is large 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 will be closed; when the input voltage rises to the set recovery value, the output will automatically return to normal.

2. Circuit composition (schematic diagram) 3. Working principle analysis (main function, performance index and realization principle)

When the input voltage is within the normal working range, VA is greater than the regulated value of VD4, vt4 is on, VB is 0 potential, vT5 is off, and the protection circuit does not work; when the input voltage is lower than the set undervoltage value, VA is less than the regulated value of VD4, vt4 is off, VB is high potential, vT5 is on, comp (one pin 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 constitute the return differential circuit for under voltage shutdown and recovery. When the undervoltage is turned off, VT6 is turned on and R21 and R2 are connected in parallel, VT6 is cut off during recovery, The return voltage is (VIN ‘- VIN).

Advantages: simple circuit and low cost.

Disadvantages: due to the difference of voltage stabilizing value between VD4 batches, the undervoltage protection point floats up and down, so the relevant parameters need to be adjusted frequently in mass production.

5. Precautions for application:

VD4 should choose the regulator with better temperature coefficient, and the components to be debugged, such as R2, should consider multiple parallel connection 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 there is an applied voltage higher than the normal output voltage range added to the output terminal or the circuit fault (open-loop or other) causes the output voltage to be higher than the regulated value, the circuit will clamp the output voltage at the set value.

2. Circuit composition (schematic diagram) 3. Working principle analysis (main function, performance index and realization principle)

When the output voltage is over-voltage, when the voltage applied to VD3 is greater than its regulated value, VD3 turns on, the output voltage is clamped and fed back to the primary side through iC4.

Advantages: simple circuit and low cost.

Disadvantages: due to the difference of voltage stabilizing value between VD3 batches of voltage stabilizing tube, the position of overpressure clamp fluctuates up and down, and the relevant parameters need to be adjusted frequently in mass production.

5. Precautions for application:

VD3 should choose the regulator with better temperature coefficient, and the components to be debugged, such as R32, should consider multiple parallel connection to facilitate debugging.

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

2、 Output overvoltage protection circuit 2

1. Overview (circuit category, main function description)

Output overvoltage protection circuit. When the applied voltage higher than the normal output voltage range is added to the output terminal or the circuit itself fault (open-loop or other) causes the output voltage higher than the normal value, the circuit will stabilize the output voltage at the set value.

2. Circuit composition (schematic diagram) 3. Working principle analysis (main function, performance index and realization principle)

When the output voltage is over-voltage, VA > VREF, IC3 are on, and feedback to the primary side through iC4, the output voltage is stable at the set over-voltage protection value.

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

Disadvantages: the cost is slightly higher than that of the clamping mode of the pressure stabilizing tube.

5. Precautions for application:

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

Self locking control circuit for overvoltage protection

1. Overview (circuit category, main function description)

In the power supply system, when the feedback circuit fails, the output voltage is not controlled, and the voltage rise exceeds the specified range. At this time, the excessive output voltage may cause damage to the subsequent electrical equipment. In order to solve this problem, the overvoltage protection circuit is usually added in the power supply. There are 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 acts and turns off PWM to make the module have 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 function, performance index and realization principle)

The figure above shows the isolated self-locking control circuit. When the over-voltage protection signal control terminal gives a high level, the triode in U1 is on, and VCC is the power supply terminal of the whole circuit. VCC gives a base current to Q2 through R5, Q1 turns on and enters the saturation state, the shut end is pulled to the low level by Q2, PWM turns off the power supply and has no output. Q2 controls the conduction of Q1 at the same time. When Q2 is on, the base current of Q1 goes to the ground through R2, Q1 is on, and another base current is provided to Q2 through R3 to keep Q2 on. Q1 and R1, R2, R3 constitute the positive feedback circuit of Q2.

Advantages: can effectively carry out self-locking protection, the whole circuit is equivalent to a thyristor.

Disadvantages: the whole circuit needs a fixed VCC. When there is no power supply at the PWM power supply end, the VCC voltage in the figure above should also be ensured.

5. Precautions for application:

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

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

Over temperature protection circuit

1. Overview (circuit category, main function description)

The circuit belongs to the 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 function, performance index and realization principle)

The voltage regulator 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 is restored, the five pins of u103 output high level.

The advantages of this circuit are simple circuit and high accuracy.

5. Precautions for application:

5.1max6501’s return differential temperature is 10 ℃ when its three pins are connected to one pin, and 2 ℃ when its three pins are connected to the ground.

The power supply voltage of 5.2max6501 should not exceed 7V, otherwise it will be damaged.

5.3max6501 must be placed near the hottest part.

Over temperature protection circuit – Thermistor

1. Overview (circuit category, main function description)

In this circuit, thermistor is used to detect the temperature of the substrate. The thermistor value changes with the temperature of the substrate. The change of the thermistor value leads to the change of the input voltage of the op amp, so as to realize the flip of the op amp, control the output of the PWM chip, and then turn off the module.

2. Circuit composition (schematic diagram) 3. Working principle analysis (main function, performance index and realization principle, calculation and analysis of key parameters)

R99 thermistor is a negative temperature coefficient thermistor. At room temperature, R99 = 100k. The partial voltage of R99 and r94 is 0.45V, which is the negative input of U2 operational amplifier, much lower than the positive input of 2.5V (R23 and r97 partial voltage). Therefore, the output of operational amplifier is high level, which has no effect on the SS terminal of lm5025, and the module works normally.

With the increase of substrate temperature, R99 resistance decreases. When it decreases to a certain value, so that the negative input of the op amp is greater than the positive input, the op amp outputs low level, which lowers the SS of lm5025, thus closing the module output; the temperature protection point can adjust the resistance of r94, R23, r97 appropriately and adjust accordingly.

After the module turns off the output (over temperature protection), the substrate temperature will decrease, the R99 resistance will increase, and the negative input of the op amp will decrease. In order to make the op amp flip normally, the resistance r98 is introduced. The principle is that after the op amp output is low, r98 is equivalent to parallel with r97, which reduces the reference value of the op amp, and enlarges the voltage gap between the positive and negative input of the op amp, so as to realize the temperature difference. For example, when the substrate temperature is 90 ℃, it is protected and turned on at 80 ℃.

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 thermistor during temperature protection

R99（t）=（Vref*R24/（Vref*R97/（R23+R97）-0.007））-R94

4.4 the calculation considering tolerance is shown in the table below:  4.5 the value of R99 in over temperature protection 4.6r99-sdnt2012x104j4250ht (f) is a thermistor with negative temperature coefficient, 100k at 25 ° C and 10K at over temperature protection (see the table above)

Rt=R*e（B（1/T1-1/T2））T1=1/（ln（Rt/R）/B+1/T2））

T2: normal temperature 25 ° C, where 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 is centigrade. RT: resistance after temperature change, 10K, 9.704k, 10.304k, see the table above 4.7 return difference

When the output of the op amp is low, the resistor r98 (51K) is connected to r97 and the reference is pulled down. When the new reference voltage vref1 = VREF * (r98 / / r97) / (R23 + r98 / / r97) = 2.28v reaches 2.44v, the resistance of R99 is R99 = VREF * r94 / vref1-r94 = 11.9k, and the temperature is calculated as follows when R99 reaches 10.49k Temperature return difference = 82.6-77.3 = 5.3 ℃