How to judge the direction of MOS tube parasitic diode?

1. MOS tube switching circuit learned analog circuits, people know that the transistor is a flow control device, which is the base current control collector and emitter current; and MOS tube is a voltage control device, which is The voltage applied to the gate controls the current between the drain and the source. The MOSFET is a kind of FET and can be manufactured as an enhancement type or a depletion type. There are four types of P-channel or N-channel, but only the enhanced N-channel MOS transistor and the enhanced P-channel are actually used. Road MOS tube. In practical applications, NMOS is mostly.

On the left side of Figure 1 is an N-channel MOS transistor. On the right is the direction of the P-channel MOS transistor parasitic diode. ** Its rule of thumb is that for the N channel, the S pole points to the D pole; for the P channel, the D pole points to the S pole.

How to distinguish the three poles? The D pole is located on one side alone, and the G pole is the 4th PIN. The remaining three feet are S poles. Their position is relatively fixed, and it is useful to remember this. Please note: Regardless of NMOS tube or PMOS tube, the method for determining the above PIN pin is the same.

The turn-on nature of the MOS tube means that it acts as a switch and is equivalent to a switch being closed. Characteristics of NMOS: Vgs is greater than a certain value and the tube will conduct. It is suitable for use when the source is grounded (low-side drive), as long as the gate voltage reaches 4V. PMOS characteristics: Vgs is less than a certain value, the tube will be turned on, and it is suitable for the case when the source is connected to VCC (high-end drive).

The following figure shows the MOSFET switching circuit. The input voltage is Ui and the output voltage is Uo. When Ui is small, the MOS transistor is turned off and Uo=Uoh=Vdd; when Ui is large, the MOS transistor is turned on, and Uo=Ron/(Ron+Rd)*Vdd due to Ron<

Application example: The following is a schematic circuit diagram analysis of a notebook motherboard, in which the mos tube is a switching function: the PQ27 control pin is low, PQ27 is off, and the right mos tube is on, so the output is pulled low;

Circuit principle analysis: PQ27 control pin is high, PQ27 conduction, so the drain is low, the right mos tube is in the off state, so the output is high.

As a whole, the collocation function of the two tubes is high and low level switching. This circuit comes from the circuit of the notebook motherboard, but this circuit module is also more common in the power-up sequence control module of the complex circuit, the operation module of the GPIO, etc. in.

2. The MOS transistor isolation function MOS tube to achieve the role of voltage isolation is another very important and common function, the importance of isolation is: worry about the current leakage of the previous pole to the back of the circuit, the power system timing of the circuit system The work of the processor or logic device caused a misjudgment, which eventually caused the system to fail to work. Therefore, in the actual circuit system, the role of isolation is very important.

For example, the upper and lower two graphs control the on-off of the MOS transistor through the source's high and low levels to achieve isolation of the signal level, because the MOS tube has a body diode and is reversed, so no signal will pass through. MOS pipe leaked. This is a very classic circuit, and many practical circuits can be derived from the collocation.

For example, the level conversion circuit in the following IIC bus actually has a great similarity with the circuit above.

Circuit analysis: When SDA1 is high (3V3), TR1 is off and SDA2 output is high (5V); when SDA1 is low (0V), TR1 is on and SDA2 is output low.

Summary: NMOS used in notebook motherboards can be divided into two categories: signal switching MOS tube: Ug than Us 3V --- 5V can be large, in fact, as long as the conduction can not be saturated conduction. For example, the common ones are: 2N7002, 2N7002E, 2N7002K, 2N7002D, and FDV301N. Voltage on and off MOS tube: Ug than Us should be greater than 10V, and must be working in the saturated conduction state. Common are: AOL1448, AOL1428A, AON7406, AON7702, MDV1660, AON6428L, AON6718L

The above circuit is a very good circuit, where? In fact, it is not necessary to use R73 and C566, but it will generally be added. What are the advantages? It is worth thinking about (welcome analysis)!