NexGen Badboy overdrive voltage?

[PeterB]

I'm surprised by the high allowed Vin overdrive voltage of the nexgen (0.7V). I thought this is a synchronous boost converter, which replaces the schottky diode with a pmos switch to improve the efficiency. And in this case the allowed overdrive should be reduced, since the voltage drop is reduced (for example the madmax has also much lower overdrive tolerance).
This is a very interesting topic for all LiIon users.

 

[dat2zip]

Good observation!

Yes, you would think with the internal FET replacing the external schottky diode the overdrive would be zero.

What happens is that when Vin = Vout, the regulator shuts down or turns off. As with most MOSFETS, there is a body substrate diode that is inherit in the way the constuct most MOSFETS including this one. The body MOSFET is the conduction path when Vin > Vout. The body diode is a generic silicon diode.

What I don't know, is how beefy this diode is. So, be careful.

Wayne

 

[UncleFester]

It makes sense. If it's PMOS, the cathode of the parasitic diode will be pointing toward the load. If the converter isn't running, you've got one diode drop in the forward direction (toward the load). Hadn't thought of that.

 

[PeterB]

Thanks for the explanation!
The source or drain to body diode of a mosfet is typically capable to carry at least the same current as the transistor it belongs to, therefore this should work quite well.

One more question: Does this mean that the Nexgen works in a kind of PWM mode for Vout<Vin<Vout+0.7. I would assume so, since it should alternate between shut down (if the current is to high) and on.

 

[dat2zip]

You guys worry me. What? I said something that you understood? /ubbthreads/images/graemlins/smile.gif

PeterB,

Yes, near Vin=Vout I can see a case where the battery ripple will have the converter turning on, then off. I've seen it happen at the low end as well. Not so much at the high end.

 

[PeterB]

Wayne,

I have just tested a NexGen500 with a QWOH star and a bench supply. I'm really impressed by the efficiency.
I have only observed a steep increase of current, as soon as Vin>Vout. At LiIon voltages, I would have something like 1A or more.
Is it possible, that the overvoltage of 0.7V is only possible for high current versions (due to battery voltage sag and other losses)?