Current flow in CHG & DSG Fet in Bq20z95

Deepak Bansal

In Bq20z95, there are two fets on high side -> CHG & DSG. As it can see in below image:

During discharging, (when both CHG & DSG fet are on), current flows in direction as I have indicate by red arrow

My question is, when current flows it will pass through diode of CHG FET & Channel of DSG FET. But why there is no drop of voltage caused by diode in CHG FET (0.7V typically). When i/p voltage is 14V, I get 13.8V at o/p.

over 14 years ago

0 Cui Mao over 14 years ago

TI__Prodigy 350 points

Hi Deepak,

For normal charging and discharging process, both Q1 and Q2 are on.

Best Regards,

Cui

0 Deepak Bansal over 14 years ago in reply to Cui Mao

Expert 2395 points

Hi Cui Mao,

Yes, you are right both fets are on. But my question is when both fets are on, then during discharging current will flow through diode of CHG fet & channel of DSG fet as I indicated in above ckt. (same in charging case). Then why don't there is any drop across diode.

I don't have indepth knowledge of mosfets. But what I read, mosfet conduct through its channel in only one direction, but an internal diode make in bidirectional switch.Now why don't any drop across this diode.

0 Jason Blackman over 14 years ago in reply to Deepak Bansal

TI__Genius 14015 points

Deepak,

Mosfet's are symmetrical devices - current can flow in either direction. Basically, as long as the device is ON, current will flow from the terminal with higher potential to the terminal with lower potential. So the drop in voltage you are seeing is actually just the drop across both transistors, which is very small.

Regards,

Jason

0 Deepak Bansal over 14 years ago in reply to Jason Blackman

Expert 2395 points

Thanks Jason

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Current flow in CHG & DSG Fet in Bq20z95