Hi guys,
I've studied mosfets from an academic point but I've never really had to choose one for a circuit until now. I do now the basics of mosfets however I'm a bit confused right now about how could properly choose one for my situation.
I have 3 power supplies for my system and I want to switch between them with a priority rank. To do so I'll be using the LTC4417 which accepts three input voltages and selects the one with higher priority to drive the system.
The LTC4417 drives a P-channel mosfet gate. My circuit has a power consumption of 250W, and the input voltage can go from [12V to 33V] which gives [21 to 7.5] amps. The mosfet proposed in the datasheet of the LTC4417 cannot support that much of current so I chose to use the SUM110P04-04L.
Looking at the safe operating area curve of the mosfet , it should be able tolerate my operating range.
Where I get confused is :
- The specsheet of the LTC4417 says that when it wants to drive one of the mosfet it clamps its voltage by 6V below the voltage value of the input(I assume if the voltage is 33V it will clamp it to 27V).How do I know from the speecsheet of the mosfet that this value of clamping voltage will be sufficient to turn it on? Which parameters allow me to know this? I'm used to turn on P-channel mosfet by driving their gate to GND so I've never worried about this question before.
- The specsheet of the mosfet says that its Vds is 40V but that its Vgs is +/- 20V. How am I supposed to interprete this plus or minus value?
- One selecting a mosfet are they any others key parameters to consider except for power dissipation?
- The datasheet of this mosfet does not show the traditional VD/Id curve where we see the linear and saturation regions. Is there a reason why this curve wouldn’t be there? Is there another curve that can give me the same information?

Thank you very much for your replies I appreciate it!

 

Look to Vgs in order to figure out when your device will start to conduct. It starts to conduct at the threshold value, Vth. There are usually curves in the datasheet that show the typical response.

The Vgs value of +/-20V are absolute maximum values that the open-source connection can take. It tells you nothing about its current characteristics. Look to the Specification portion of the datasheet for more detailed information.

The curves that are available are typically Vgs vs Id and Vds vs Id. Vd vs Id does not provide enough useful information.

 

Vgs(th) for your part is between -1V and -3V, so that's where it starts to turn on. Rds(on) at Vgs=-4.5V and Id=-20A is 0.0062 ohms max at 25C, so it will definitely turn on with a -6V drive level. The top-left curve on datasheet p.3 confirms this, at Vgs=5V the >>typical<< FET is nearly in hard saturation. The top left curve on p.4 shows how that 0.0062 number scales up with high temp, as much as 75% above the 0.0062 value at 175C. So expect a 6V drive to give you an Rds(on) of maybe 0.011 Ohms max. over temp. The middle right graph on p.3 shows that for 0-40A the Rds(on) is flat, so you can ignore load current in figuring out true max Rds(on). At 0.011 Ohms and 21A you will have about 4.85W dissipation, so take that and your chosen max chip temp (I'd use 150C) minus your max ambient to figure out what kind of heatsink you will need.

 

The On-state resistance for your MOSFET is specified at a Vgs of -10V (data sheet excerpt below).
Using the on-resistance graphs, as was done in post #3 are for "typical" MOSFETs and will likely be different for the individual parts you buy

If you want the device to fully turn on at a Vgs of -6V, use a logic-level device, which fully turn on at a Vgs of -5V or lower.