Experience sharing use of MOSfet as power switches

Experience sharing  use of  MOSfet as power switches

In general power switching circuits, the purpose of controlling power is to save power and control quiescent current. However, the following circuit has several disadvantages:

1. The pressure drop of theMOSfet is large

We know that the saturation voltage drop of the PNP MOSfet as the switch tube is 0-0.3V, which is unacceptable in low-voltage circuits. The maximum error of the 3.3V control power supply becomes 3V, and some 1.5V power supply becomes 1.2V, which will cause damage to the chip powered by it.

The MOSfet pressure drop of PMOS is Vdrop=Id×Rdson, Rdson can be selected, and the actual value is within 1 ohm.

2. Control current

We know that Ib and Ic are related, and the saturation amplification factor is generally designed to be 30, so when we pass a current of 200mA, Ib=200/30=7mA, which leads to a large power consumption of the control circuit.

3. Switch diode power consumption

We know that the power consumption calculation formula of the triode is Pd=Veb×Ib+Vec×Ic. When Vec is saturated, under the condition of 0-0.3V, when the passing current is large, the power consumption of the switch diode will be very large.

In comparison, the on-resistance Rdson of PMOS is small (also optional), P=Rdson×Id^2.

PMOS high voltage circuit design (12V) circuit

Comparing PNP circuit design

Low voltage switch (NMOS) [5V, 3.3V, 1.5V]

NMOS turn-off conditions:

Here, the PNP MOSfet is used to directly conduct the G and Vin of the NMOS, so that the NMOS can be completely turned on. If Vin cannot be completely connected to the G of the NMOS, use the top PMOS solution. If it is connected as follows Condition:

result:

When Photo Control is set to high level, 9014 is turned on and 640 is turned off. When Photo Control is at low level, 9014 is disconnected and 640 is turned on, but at this time, the voltage drop between ds on 640 is 3~4V, the original intention is DC=VBAT(8V), and now the DC is only 3 volts;

reason:

Only NPN+PMOS is used for power control, this is NPN+NMOS, R21 can be directly replaced with 0 ohms, but nothing is controlled.

In addition, the substrate of NMOS is generally connected to the source when it is made. The NMOS needs to control the voltage (relative to the substrate) to be greater than the turn-on voltage when it is turned on. In this way, the substrate is completely suspended. No matter how the current circuit is controlled, it can reach If the requirements are not met, the design of the circuit itself is wrong. NMOS is generally used for low output, and PMOS is for high output circuit. This property is similar to that of NPN and PNP of triode. Transistors are used in TTL, TTL can drive MOS, and MOS cannot drive TTL.