Electronics > Beginners

Reading the SOA graph in a MOSFET datasheet

(1/3) > >>

logancane:
So in my project, I want to implement a high-side switch. It is fairly high power (up to approximately 175W at DC) and most of that load contains sensitive electronics that must have a good ground reference (hence the high-side switch). The supply voltage can be anywhere between 9.6V and 19V and thus the supply current (Id) could be between 9.2A and 18.3A, so that covers the y-axis of the graph, but what I am struggling with is the Vds on the x-axis.

In a low-side configuration I can easily see how Vds is equal to the supply voltage, but is this the same for a high side configuration? Should I be looking for a transistor that can handle 9.2A at 19V?

I've attached an example of what I mean. In this example it would seem that if Vds is referring to supply voltage, 9.6V/18.3A is within limits but 19V/9.2A is not therefore I shouldn't use this MOSFET.

logancane:
I should probably add that I am looking for a n-type MOSFET and will use a high-side gate driver to obtain a sufficient Vgs.

magic:

--- Quote from: logancane on August 04, 2021, 11:49:58 am ---In a low-side configuration I can easily see how Vds is equal to the supply voltage, but is this the same for a high side configuration?
--- End quote ---
Of course. When the switch is off, source is grounded by the load and drain sees the input voltage.


--- Quote from: logancane on August 04, 2021, 11:49:58 am ---Should I be looking for a transistor that can handle 9.2A at 19V?

--- End quote ---
Most likely not. Switching means that the MOSFET is either turned on and conducts high current at minimum Vds (source voltage is almost equal input voltage, the difference is load current times Rds(on)) or it is turned off and conducts no current at all. If that sounds like what you are doing than this part is good enough.

You just need to ensure that the transition between the two states is a matter of microseconds rather than milliseconds (any gate driver will do that) because at that time both current and voltage can be high.

edit
It seems that Vds may be somewhat high at 20A due to Rds(on) so maybe a lower Rds(on) FET would be better. Or a heatsink may be necessary.

logancane:

--- Quote from: magic on August 04, 2021, 12:16:31 pm ---
Most likely not. Switching means that the MOSFET is either turned on and conducts high current at minimum Vds (source voltage is almost equal input voltage, the difference is load current times Rds(on)) or it is turned off and conducts no current at all. If that sounds like what you are doing than this part is good enough.

You just need to ensure that the transition between the two states is a matter of microseconds rather than milliseconds (any gate driver will do that) because at that time both current and voltage can be high.

edit
It seems that Vds may be somewhat high at 20A due to Rds(on) so maybe a lower Rds(on) FET would be better. Or a heatsink may be necessary.

--- End quote ---

Ok, thanks for that. So in what situation would there be a high Vds and Id for an extended period of time (i.e. in what situation would the DC curve on the graph be relevent)? Because even in a low side switch there is hardly a voltage drop across it during operation?

Also, have I got this right?

Rds_on = 3m
Id = 20A
Rth(j-mb) = 0.77K/W

=>P = I^2R = 400 * 3m = 1.2W
=>deltaT = 0.77 * 1.2 = 0.924 K above ambient? (Doesn't seem like much, is this just a really good MOSFET or have I made an incorrect assumption/done something wrong)?

magic:
In linear applications, like a DIY voltage regulator. If you apply 14V to the gate to get roughly 10V at the source and the load pulls 20A then Vds may be up to 9V and power dissipation up to 180W - not good.

Rth(j-mb) is most likely thermal resistance from junction to the metal mounting bracket of TO220 or similar package. It matters insofar as you are able to keep the metal tab at a reasonable temperature. Power transfer from TO220 to air is not great and it depends on air temperature and rate of flow, but at 1W they get quite hot usually. The spec for that is Rth(j-a) or something like that ("junction to ambient").

Navigation

[0] Message Index

[#] Next page