Understanding Two MOSFET with sources connected

Question

I am having problem understanding how this MOSFET configuration (used in battery charger) works?

In this slide,

what does it mean "blocking voltages in both directions while allowing bidirectional current"?

Answer 1

The below assumes NMOS, just for simplicity. Also note that I'm describing which directions current can flow, not which way it will flow. Which way it will flow depends on the voltages the circuit is connected to.

A single MOSFET has two states: ON and OFF.

ON: current can flow both directions

OFF: current can flow one direction (source-drain, due to the body diode) and not the other direction (drain-source, because the FET is off)

A single MOSFET in series with a diode:

also has two states, ON and OFF.

ON: current flows one direction (drain-source, through the FET and the second diode), but not the other direction (source-drain, because the second diode is pointing the wrong way)

OFF: current won't flow either way, because whichever way you look there's a diode opposing the flow of current.

Two MOSFETs in series, pointing opposite directions:

have four possible states. ON-ON, ON-OFF, OFF-ON, and OFF-OFF. For this example, I'll describe current flow with directions as seen in the picture above.

ON-ON: current can flow both directions through the circuit.

ON-OFF: current can flow from left to right (through the left FET and the right diode) but not from right to left (because of the right diode)

OFF-ON: current can flow from right to left (through the right FET and the left diode) but not from left to right (because of the left diode)

OFF-OFF: current can not flow in either direction, because with both FETs off all you have is two diodes blocking current flow in either direction.

So the third topology has the option of either blocking currents in both directions, or allowing current flow in either direction, depending on how it's gated.

Answer 2

I agree that the wording in the second slide is confusing, mainly because it doesn't distinguish between the switch being on or off. I think they mean:

• When the switch is turned off (using the gate), no current can flow, so if one of the supplies is off, it will not have a voltage forced backwards onto it as can happen with a simple single MOSFET with no extra diode. This is the "blocking voltages in both directions" part.

• When the switch is turned on (using the gate), current can flow in either direction from the side of higher voltage to lower voltage.

Answer 3

In the first slide, the body diodes allow current to flow to Vcc from either Vin or Vbatt.

When the gate is low, the P-MOSFETs short out the body diode, presumably with the purpose of a higher voltage at Vin to charge the battery Vbatt.

In other words: - when Vin is connected, the current flows from there - otherwise, from the battery - when the gate is low, battery is charged from Vin

In the second slide, the default state blocks current in either direction.