The confirmation of the three pins of the MOS tube circuit

G pole: gate pole, the easiest to identify.

S pole: source pole, whether it is P-channel or N-channel, the intersection of two lines is the source pole.

D pole: drain pole, whether it is P-channel or N-channel, the one with a separate lead is the drain pole.

Orientation determination of channel and parasitic diode

The arrow pointing to the G pole is the N-channel, and the arrow pointing away from the G pole is the P-channel.

Whether it is an N-channel or P-channel MOS tube, the direction of the arrow on the substrate and the direction of the arrow of the parasitic diode are always the same: either both point from S to D or both point from D to S. N-channel, from S to D; P-channel, from D to S.

Main functions of MOS tube

Switching function: it can realize signal switching (high and low level switching) and voltage on/off. The direction of the parasitic diode in the MOS tube is crucial when used as a switch.

Conduction condition

Whether it is an N-channel or P-channel MOS tube, the G pole voltage is compared with the S pole voltage.

N-channel: conducts when UG>US, (simply put) cuts off when UG=US.

P-channel: conducts when UG<US, (simply put) cuts off when UG=US.

However, the voltage difference required for the MOS tube to saturate and conduct, whether UG is greater (or less) than US, depends on the specific MOS tube. Different MOS tubes require different voltage differences.

Isolation function: At this time, only a small part of the MOS tube, the diode, is working, which essentially achieves unidirectional conduction in the circuit. It is equivalent to a diode. However, in the circuit, we often use an isolation MOS tube because using a diode will result in a voltage drop during conduction, causing some voltage loss. By using an isolation MOS tube, when it is conducting in the forward direction, by applying the appropriate voltage to the control pole, the MOS tube can saturate and conduct, so that there is almost no voltage drop when passing current.

Summary of MOS tube functions:

If the MOS tube is used as a switch, (whether it is an N-channel or P-channel), the negative pole of the parasitic diode must be connected to the input side, and the positive pole must be connected to the output side or grounded. Otherwise, the switch function cannot be realized; if the MOS tube is used for isolation, (whether it is an N-channel or P-channel), the direction of the parasitic diode must be consistent with the unidirectional conduction direction required by the main circuit.

In some battery protection circuits, there are often two MOS tubes used together with a common drain, because in these protection circuits, both overcharge detection and overdischarge detection are required.

When we open the specifications of this type of MOS, we will see many parameters like the following:

How to test these parameters, please see the testing method below!

Note: The bold words in the following text are variables and need to be determined according to the MOS specifications.

VSSS withstand voltage

Connect the test circuit as shown in the figure, set VGS=0V, VSS1 gradually increases from 0V in steps of 0.1V until IS=1mA, and the MOS is not damaged, record the voltage VSS1.

Connect the test circuit as shown in the figure, set VGS=0V, VSS2 gradually increases from 0V in steps of 0.1V, instantaneously increase the voltage to IS=1mA, and the MOS is not damaged, record the voltage VSS2.

ISSS current

Connect the test circuit as shown in the figure, set VSS=12V, VGS=0V, and record ISSS.

VGS(th) turn-on voltage

Connect the test circuit as shown in the figure, set VSS=6V, increase VGS1 voltage in steps of 0.1V until ISS=1mA, record the voltage VGS1 at this time, and test VGS2 for the other FET in the same way.

IGSS gate leakage current

Connect the test circuit as shown in the figure, set VSS=0V, VGS1=±8V, and record IGSS1, test IGSS2 for the other FET in the same way.

RSS(on) internal resistance

Connect the test circuit as shown in the figure, set IS=6A, and test VS1S2 voltage when VGS is 4.5V, 4.0V, 3.8V, 3.1V, and 2.5V, RSS(on)=VS1S2/6A.

VFSS diode conduction voltage

Connect the test circuit as shown in the figure, set VGS2=4.5V, VGS1=0V, VS1S2=4.5V, IS1S2=6A, and record the tested VFSS1.

Similarly, set VGS1=4.5V, VGS2=0V, VS2S1=4.5V, IS2S1=6A, and record the tested VFSS2.