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Classic application of overcurrent protection circuit

Date: 2018-03-27
Viewed: 26
案例名称: Classic application of overcurrent protection circuit

Overcurrent protection circuit works

 When the circuit is in a normal state, the current through the overcurrent protection PTC thermistor is less than the rated current, and the overcurrent protection PTC thermistor is in a normal state, the resistance value is small, and does not affect the normal operation of the protected circuit. When the circuit fails and the current greatly exceeds the rated current, the PTC thermistor for overcurrent protection suddenly heats up and is in a high-impedance state, so that the circuit is in a relatively 'off' state, thereby protecting the circuit from damage. When the fault is removed, the PTC thermistor for overcurrent protection also automatically returns to the low resistance state, and the circuit resumes normal operation.

      Application of overcurrent protection circuit on motor

When the motor starts, press the lock button SBi once, the start is finished (after the motor speed is stabilized), and then press SBi, then the protection circuit is put into operation. For motors with a short start-up time (such as a few seconds), the SBi can also use a normal button, as long as the SBi is pressed during the start-up process.

When the motor is running normally, the secondary inductive potential of the current transformers TAi~TA3 is small and is not enough to trigger the thyristor V. As shown below.


Comparison of several overcurrent protection methods in switching power supplies

Power supply as the power supply equipment for all electronic products, in addition to the performance to meet the requirements of power supply products, its own protection measures are also very important, such as overvoltage, overcurrent, overheat protection. Once the electronic product fails, such as when the input side of the electronic product is short-circuited or the output side is open, the power supply must be turned off its output voltage to protect the power MOSFET and the output side equipment from being burnt, otherwise it may cause further damage to the electronic product, or even It causes electric shock and fire of the operator. Therefore, the overcurrent protection function of the switching power supply must be perfected.

1 Overcurrent protection method commonly used in switching power supplies

There are many forms of overcurrent protection, as shown in Figure 1, which can be divided into rated current drooping type, that is, font type; constant current type; constant power type, most of which are current drooping type. The set value of the overcurrent is usually 110% to 130% of the rated current. Generally it is automatic recovery type.


In Fig. 1, 1 indicates a current droop type, 2 indicates a constant current type, and 3 indicates a constant power type.

1.1 Current limiting circuit used in transformer primary direct drive circuit

In the design of a circuit directly driven by a transformer (such as a single-ended forward converter or a flyback converter), it is relatively easy to achieve current limiting. Figure 2 is two ways to achieve current limiting in such a circuit.

The circuit of Figure 2 can be used for single-ended forward converters and flyback converters. In Figure 2(a) and Figure 2(b), a current limiting resistor Rsc is connected in series with the source of the MOSFET. In Figure 2(a), Rsc provides a voltage drop to drive transistor S2 on, in Figure 2(b). The current-limit voltage comparator connected across the Rsc can short-circuit the drive current pulse to protect it when an overcurrent is generated.


Figure 2(a) compares with Figure 2(b), Figure 2(b) protects the circuit faster and more accurately. First, it presets the threshold voltage of the current-sense drive of the comparison amplifier to a more precise range than the threshold voltage Vbe of the transistor. Second, it takes the preset threshold voltage sufficiently small, and its typical value is only 100mV~ 200mV, therefore, the value of the current-limiting sampling resistor Rsc can be made smaller, thus reducing power consumption and improving the efficiency of the power supply. When the AC input voltage is varied within the range of 90-264V and the output is equal, the peak currents of the transformers are greatly different, resulting in severe drift of the high- and low-side overcurrent protection points, which is not conducive to the consistency of the overcurrent points. A pull-up resistor R1 from +VH is added to the circuit for the purpose of making the base of S2 or the non-inverting comparator of the current-limit comparator have a pre-value to achieve the highest possible consistency of the high- and low-side overcurrent protection points.

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