Portable devices require a circuit that switches the power supply between battery and charger. This is usually done by using a diode on the battery line – when the charger is plugged in, the diode is reversely polarized and disconnects the battery from the rest of the circuit. This method however has drawbacks, namely power dissipation and voltage drop across the diode.
This app note from Maxim provides two examples which eliminate both of these drawbacks. They are considerably more complex, but the trade off is longer battery life
Thanks Rsdio!
I have heard about using a mosfet as an “ideal” diode but never bothered to look into it more, but I never thought it could be that simple!
Does anyone know why I see a diode more often in solar cell balancing instead of a mosfet? Or maybe I am asking the wrong question. What are the drawbacks of using a mosfet like this instead of a diode, other than another pin?
One of the downsides to using mosfets as low-loss diodes is that you *have* to know the voltages and current flows involved to be able to turn off the mosfet in the same timeframe that the diode would be off.
Use of the mosfet as a diode is seen often times in power electronics applications where currents are high. Since the power loss of a diode is the forward voltage drop * current, Pd = V_f*I_d, and the forward voltage is usually in the range of 0.6-2V, significant power savings can be achieved when using the mosfet with a low R_{ds(on)}. Pmosfet = I_d^2*R_{ds(on)}. Since most of the time R_{ds(on)}*Id <<<<< V_f. However, the major downside is that you have to know the current and voltages through and across the device to make smart decisions about when you turn the mosfet on and when you turn it off to make the mosfet act like a low loss diode.
Do a search for "synchronous rectification" if you want to learn more about it.
Also have a look at a new product from Linear: LTC4415.
Dual 4A Ideal Diodes with Adjustable Current Limit.
The data sheet shows it doing priority power source selection.
Thanks for the information and LTC4415 suggestion guys!