Let's take a look at the protection circuit we want to analyze. The picture above:
This is the power input protection circuit from DJI RM development board type A. I copied it here to AD for
analysis. We need to understand the power protection circuit. The protagonist of the MOS tube, the picture shows the name of each pin of an NMOS.
The left picture in the picture is NMOS, the right picture is PMOS
MOS transistors are divided into PMOS and NMOS, Q6 in the picture is NMOS, Q4 is PMOS, there is a difference between the two MOS before:
The characteristics of NMOS , Vgs is greater than a certain The value of will be turned on, which is suitable for the case when the source is grounded (low-side drive), as long as the gate voltage reaches 4V or 10V.
The characteristics of PMOS , Vgs is less than a certain value, it will be turned on, suitable for the situation when the source is connected to VCC (high-side drive). However, although PMOS can be conveniently used as a high-side driver, due to the large on-resistance, high price, and few replacement types, NMOS is usually used in high-side drivers.
Analysis of reverse connection protection:
The NMOS of Q6 in the figure plays a role of reverse connection protection. Its gate G is pin 4, pins 1, 2, and 3 are source S, and pin 5 is drain D. As we mentioned above, the turn-on characteristic of NMOS is that when VGS is greater than a certain voltage, it will turn on, and the default is off.
In the figure, when the power supply is normally turned on , the current flows through the resistance of R10 to pin 4, which is the G pole. We refer to the data sheet of PSMN1R4-40YLD, its VGS conduction voltage is 1.7V, and the voltage regulation of the D3 Zener diode is 9.1V, VGS (before pins 4 and 5) voltage is 9.1V , this voltage is enough to turn on Q6, and this zener diode can also prevent the MOS tube from being burned out due to excessive input voltage (the VGS limit voltage of this MOS Is ±20V).
In the figure, when the power supply is incorrectly connected , VGS (ie 4, 5 pins) is a negative voltage , and NMOS is turned off when VGS is a negative voltage , thereby cutting off, making the circuit open, protecting the subsequent circuit, and realizing reverse connection protection
Analysis of Overvoltage Protection
Before the analysis, let’s understand the protagonist of controlling overvoltage protection
-the conduction condition of PNP type triode, PNP type triode, Ve>Vb>Vc, that is, the emitter potential is greater than the base potential and the collector potential. .TPCA8122 is a PMOS type MOS tube. If PMOS is to be turned on, VGS must be a negative voltage (zero voltage is not acceptable ). The analysis is as follows:
When the power supply is normally connected : MMSZ5255 is a 28V voltage regulator tube. In, the voltage regulator tube is not conducting, R9 and R11 can be regarded as no current flowing, which is an open circuit. In this way, VB<VC, the transistor is not conducting, and the voltage of VGS is controlled by the voltage divider of R13 and R14, VS>VG When the PMOS is turned on, the current flows normally.
When the power supply is incorrectly connected : the power supply is incorrectly connected. At this time, Vin>Vbr, the voltage regulator tube is broken down, and the upper voltage is Vbr. The PNP transistor Q5 is turned on, VCE≈0, that is, Vgs≈0 of the PMOS tube, the PMOS tube is not turned on, and the circuit is open, that is, the overvoltage protection is realized.