Can back-to-back series MOS achieve bidirectional current flow? What about a single MOS tube?

When two MOS tubes are connected in series back to back, they form a bidirectional switch.

 Two-way switch

An active device built from MOSFET or IGBT that allows current to flow in both directions when powered on, and blocks voltage flow in both directions when powered off. It can conduct in both directions.

For example, if a positive voltage is applied to the switch at point A and a negative voltage is applied to point B, the current will flow from A to B. But when the voltage polarity is changed, B can flow to A.

 A bidirectional switch is constructed by connecting MOSFETs or IGBTs back to back in series.

 So how does it achieve bidirectional flow of current?

When the USB charges the battery, Q3 is turned on, and the USB power forms a path through the body diode of Q1 and Q3. Then Q1 and Q2 are turned on to charge the battery.

 When the battery supplies power to the USB external load, Q3 is turned on, the battery power forms a path through the body diode of Q2 and Q3, Q1 and Q2 are turned on, and the USB interface circuit is powered.

When Q3 is controlled to be turned off, the USB cannot charge the battery and the battery cannot power the load. At this time, the reverse current resistance or current backflow prevention function can be realized.

Why do we need to use back-to-back series connection? Can't we achieve this with just one MOS tube?

If only one MOS tube is used, when the control MOS tube is turned off, the battery can be discharged through the body diode of the MOS tube, and the shutdown effect will not be achieved.

However, if two MOS tubes are used, when the two MOS tubes are controlled to be turned off, the body diodes of the MOS tubes are connected in reverse series, which can block the discharge of the battery.

That’s all for today.

Some of the above pictures are from the Internet