Electrical – mosfet turn off / turn on


My appplication require mosfets to switch between two power sources from adapter to batter source and battery to adapter source. There is a confusion in my mind relating to turn on / turn off time of mosfets. Turn on time of mosfets is significantly smaller then turn off time due to its physics. My questions is if a lower mosfer is not switched off and upper mosfet gets switched on then current might flow from first power source (adapter) to second source (battery) through lower mosfet.In this case circuit can malfunction, am i right. :)


Best Answer

Normally the on/off switching times of a MOSFET are largely determined by how fast you can charge /discharge the gate. So you need a strong driver circuit. Then the "physical reasons" why switching off takes longer become insignificant. I mean, this is in the order of nano seconds and only relevant in certain applications !

Depending on your circuit indeed an NMOS and PMOS in series (like in an inverter) do "short" the supply very briefly. I write short but actually it is not a short, just a low resistance for a very short time. It happens in all digital ICs all the time. That is why supply decoupling is needed to provide that short burst of current.

This effect gets more troublesome at very high switching frequencies. Switching some supply lines is usually not done at high frequencies but at extremely low frequencies, so low that you can almost neglect it.

To prevent the effect altogether you need a "break before make" circuit, the second transistor can only switch on after the first has had time to switch off. Some switching MOSFETs in switched mode power supply ICs use this principle.