|
Basic Mosfet usage:
Nmos.... see schematic on the left, it simulates how a real standard N-Mosfet would normally operate.
1) Always keep the Source arrow immediately connected to Earth (no resistance at all between arrow and Earth).
2) View the settings for the Nmos in the left schematic here and always put your Nmos settings exactly like this until you are skilled enough to know if they should be altered.
3) Attach the drain (non arrow) side of the Nmos to the other components in your circuit as you wish.
4) Ensure that the gate terminal of the Nmos swings between a V of less than 3v to turn it off (no bar seen), and more than 4v to turn it on (bar seen fully)..... a switch or pulse can do that....such as the 5v seen here.
5) Use a 10k resistor as shown here between gate and earth to actively pull the Nmos off properly for when the gate V is below 3v
6) The supply voltage (12v shown) can be whatever you wish within reason....it will not significantly alter the Nmos switching operation here.
Pmos....
1) Always best to keep the Source arrow of a Pmos immediately connected to the supply positive .... (no resistance at all between the arrow and the supply V)
2) View the settings for the Pmos in the right schematic here and always put your Pmos settings exactly like this until you are skilled enough to know if they should be altered.....apart from the VTO.
Alter the VTO to be - 3v normally. eg1... If your Supply V is 12v then set the VTO as -3v ....ignore the minus sign which will be seen there. eg2...If your supply is 9v then set VTO to be -3v too etc:
A standard Pmos VTO (ie Vth) is approx 3v below the supply v , unlike the Nmos Vth which is 3v above zero.
3) Attach the drain (non arrow) side of the Pmos to the other components in your circuit as you wish.
4) Ensure that the gate terminal of the Pmos swings between a V above zero of more than [Supply V - VTO] to turn it off (bar not seen),
and less than [Supply V - VTO] to turn it on (bar seen)..... a divider switch or pulse can do that.
In the Pmos schematic on the right here, 12-9=3v ie supply V minus the VTO setting v drop amount.
so a gate pulse of 5v above zero being more than that 3v will turn the Pmos off, the bar will be not be seen.
Note carefully here though.... I have reversed the normal VTO setting of a mosfet -3v to now be -9v here in order to show the bar operating by a 3v above zero pulse.....EC settings can be altered like this as reqd. but remember to keep that Pmos VTO setting as -3v if you are looking to simulate a real standard Pmos.
When the gate pulse drops below 3v in the top right circuit here, the gate will be on with the bar seen. P-Mosfets are normally in the On position and need to be actively switched Off by putting a high enough v at the gate....
Extra note: The high pulse (above our gate 3v more than zero earth point) switches the Pmos off, a drop in pulse allows the Pmos to stay in its default on state.
See how the open switch allows more than the 3v to the gate...which thus turns the Pmos off.
Closing the switch drops the gate below the 3v ie below [Supply V - VTO]....which thus turns the Pmos on.
Replace the gate switch with the pulse if you wish, and toggle its pulse High V between 2.99v and 3.01v to see how [Supply V - your VTO setting] being 12v- 9v here, has determined the on/off state of the Pmos.
I chose 3v above zero gate for the Mosfet threshold cut-off point because that is approx. the same as a real LL Mosfet. ..... but do bear in mind that the non LL, standard Pmos VTO needs to be set at -3v and you will then need to increase your gate volts to go above [Supply V - VTO]. ie above [supply-3] for the bar to shut off.
Summary:
For a Nmos the threshold cut off point above zero earth is easy to understand....being the VTO setting.
For a Pmos the threshold cut off point above zero earth is [Supply V - VTO] . Remember though that if you see Vth mentioned for Pmos.....it refers to the VTO drop amount below the supply v , hence the minus sign in the VTO settings.
Be careful to use the Pmos VTO correctly and not use a double negative.....that minus sign is just a reminder that setting values are a drop below the supply.
|
