MOS tube-MOS tube short circuit protection circuit and short circuit protection circuit schematic summary

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MOS tube-MOS tube short circuit protection circuit and short circuit protection circuit schematic summary [Copy link]

The simplest short circuit protection circuit diagram (I)

Simple AC power short circuit protection circuit

When the AC power supply voltage is normal, the relay is closed and the load (Rfz) circuit is connected. When the load is short-circuited, the voltage across KA drops rapidly, KA is released, and the load circuit is cut off. At the same time, the light-emitting diode VL lights up, indicating that the circuit is short-circuited.

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The simplest short circuit protection diagram (II)

This is a self-locking protection circuit. When short-circuited: Q3 is pulled low, Q2 is turned on, forming a self-locking, forcing Q3 to be cut off. After Q3 is cut off, there is no voltage on the load. At this time, it doesn't matter whether there is a load or not, so there will be no output even if the load is removed. To restore the output after removing the load, you can connect a resistor of about 1K to the CE junction of Q3.

C2 and c3 are very important. After self-locking, the restart circuit depends on these two capacitors, otherwise the startup will fail. The principle is that when power is turned on, the voltage across the capacitor cannot change suddenly. C2 keeps the base of Q2 at a high level at the moment of power-on, so that Q2 is not turned on. C3 keeps the base of Q3 at a low level at the moment of power-on, so that Q3 is turned on and Vout has voltage. In this way, R5 is at a high level, locking the conduction.

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The simplest short circuit protection diagram (III)

Phase loss protection circuit

Due to the power grid's own reasons or unreliable power input wiring, the switching power supply sometimes runs in a phase-loss situation, and the phase-loss operation is not easy to be discovered in time. When the power supply is in phase-loss operation, one arm of the rectifier bridge has no current, while the other arms will be seriously overcurrent and cause damage, and at the same time, the inverter will work abnormally, so the phase loss must be protected. Current transformers or electronic phase loss detection circuits are usually used to detect grid phase loss. Due to the high cost and large size of current transformer detection, electronic phase loss protection circuits are generally used in switching power supplies. Figure 5 is a simple electronic phase loss protection circuit. When the three phases are balanced, the H potential of the R1~R3 node is very low, and the optical coupling output is approximately zero level. When the phase is lost, the H point potential is raised, the optical coupler outputs a high level, and after comparison by the comparator, it outputs a low level and blocks the drive signal. The reference of the comparator is adjustable to adjust the phase loss action threshold. This phase loss protection is suitable for three-phase four-wire system, but not for three-phase three-wire system. With a slight change in the circuit, the PWM signal can also be blocked with a high level.

The figure is a phase loss protection circuit for a three-phase three-wire power supply. If any phase A, B, or C is missing, the output level of the optocoupler is lower than the reference voltage of the inverting input of the comparator. The comparator outputs a low level, blocks the PWM drive signal, and turns off the power supply. If the polarity of the comparator input is slightly changed, the PWM signal can also be blocked with a high level. This phase loss protection circuit uses optocoupler isolation of strong electricity, which is safe and reliable. RP1 and RP2 are used to adjust the phase loss protection action threshold.

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The simplest short circuit protection diagram (IV)

In some DC/DC converters, on-chip cycle-by-cycle current limiting may not be sufficient to prevent failure during a short circuit. A nonsynchronous boost converter provides a direct path from the input to the short circuit through the inductor and clamping diode. When the load is shorted, the large current flowing through the load path can damage the clamping diode, inductor, and IC, regardless of the current limiting protection in the IC. In a SEPIC (single-ended primary inductor converter) circuit, coupling capacitors interrupt this path. Therefore, when the load is shorted, there is no direct path for current to flow from the input to the output. However, if the required minimum on-time is shorter than the specified duty cycle, the inductor current and switch current can increase rapidly, causing IC failure, input overload, or both. Even in some buck regulators, duty cycle limitations sometimes keep the switch on time too long to maintain control during an output short circuit, especially at very high input voltages for very high frequency ICs. Using a single transistor approach, the VC pin (output of the error amplifier) voltage can be pulled down when the inductor current starts to run away due to load overload or short circuit, thus protecting the SEPIC circuit from short-circuit failure.

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The simplest short circuit protection (V)

The implementation circuit of the high-reliability short-circuit protection circuit is shown in Figure 1, where VMP is the power MOS tube of the linear regulator, R1 and R2 are the feedback resistors of the regulator; VMO and VMP tubes are current mirror circuits, and the VMOS tube copies the current of the power tube at a certain ratio and converts it into a detection voltage through the resistor R4; the transistor VM1 completes the level shift function, and finally connects to the positive input terminal (Vinp) of the comparator composed of MOS tubes such as VM8~VM12, and the negative input terminal (Vinm) of the comparator is connected to the output terminal (0UT); VM13 and VM14 form a common source amplifier circuit with a diode connection form as a load; VM14 and VMp1 form a current mirror circuit; the transistor VMp1 completes the switch control of the power tube VMP. When working normally, the gate potential (Vcon) of VMp1 is high, which will not affect the normal operation of the system. When a short circuit occurs, Vcon will be low, turning off the power tube.

Qualitative analysis of working principle

When a short circuit occurs, the potential of the negative input terminal of the comparator (Vinm) is 0 V; at the same time, the VM1 tube will be turned on, so the potential of the positive input terminal of the comparator is greater than 0 V, and finally the output node potential (Vcom) of the comparator is high. Under the action of MOS tubes VM13 and VM14, the control signal Vcon will be low, and finally the gate voltage of the VMP tube will increase, thereby turning off the P power tube to achieve short circuit protection. After achieving short circuit protection, the VM1 tube will be turned off; VM3 and VM4 form a current mirror, and the role of transistor VM2 is to ensure that the voltage of the positive input terminal of the comparator is always higher than the voltage of the negative input terminal of the comparator during the short circuit (VM1 tube is turned off) (even if there is ground plane noise in the system), so that the Vcon voltage is always low, ensuring that the circuit can always turn off the P power tube during the short circuit, and achieve high reliability of the protection circuit. At the same time, when a short circuit occurs (i.e. the Vcon signal is at a low level), the VM7 tube works normally, the VM5 tube will be turned on, and a certain current will flow to the 0UT end; therefore, once the short circuit is eliminated (i.e. a load resistor is connected to the 0UT end), the VM5 tube will charge the parallel RC network composed of the load capacitor and the load resistor, the 0UT end voltage will increase, the Vcon signal will become a high level, and the circuit will automatically return to normal.