mosfet
I was looking at this P-channel MOSFET datasheet and I am a bit confused.
(Source: Vishay Siliconix Si5435BDC Datasheet)
How it is possible to allow for a different current for the source and the drain? The datasheet states more A for the drain than the source. As I get it all the current that enters drain must exit the source. Where the extra current for the drain comes from?
Note that this is not a power converter but effectively a chip that uses a PFET to implement active rectification. As such, the current is intended to flow from the battery to the load when the switch is on. The FET is being used like a diode, making use of the inherent body diode of the part. The chip then turns on the FET to make the diode look more ideal when it should already be on. The chip will turn off the FET quickly when is sees SENSE go higher than Vin. If it didn't, the "diode" would conduct backwards.
The advantage of such a design is that the effective diode has very low forward drop, which is useful when it's in series with a battery since less of the battery power will be wasted. The drawback is that the overall diode has slow reverse recovery time, since the chip has to sense the reverse voltage and then actively shut off the FET. In this case the diode is used for power ORing, so a few µs reverse on time won't matter much.
Yes it does conduct in either direction.
Due to the body diode, most discrete MOSFETs cannot block in the reverse direction, but the channel will conduct in either direction when the gate is biased "on".
If you want to conduct and block in both directions you need two MOSFETs in series.
MOSFETs used as near-perfect rectifiers are usually used in the reverse direction for conduction (so they can block in the other direction).
Edit: Your schematic here:
Illustrates one example of switching AC with two MOSFETs (one of which will be conducting in reverse at any given time when the switches are on).
Another example is here, from the LT4351 datasheet:
Best Answer
With source current they mean the current over the body diode. This current flows, when the drain potential is higher than the source potential. Check for example the specification of the diode forward voltage, here Is is mentioned whereas for Rdson Id is mentioned.