Reading the SOA graph in a MOSFET datasheet - Page 1
Forum section: electronics / beginners

author: logancane
author_rank: Regular Contributor
timestamp: August 04, 2021, 11:49:58 am
content: 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.

author: logancane
author_rank: Regular Contributor
timestamp: August 04, 2021, 11:58:12 am
content: 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.

author: magic
author_rank: Super Contributor
timestamp: August 04, 2021, 12:16:31 pm
content: Quote from: logancane on August 04, 2021, 11:49:58 amOf 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 amMost 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.editIt 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.

author: logancane
author_rank: Regular Contributor
timestamp: August 04, 2021, 12:58:21 pm
content: Quote from: magic on August 04, 2021, 12:16:31 pmOk, 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 = 3mId = 20ARth(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)?

author: magic
author_rank: Super Contributor
timestamp: August 04, 2021, 01:31:59 pm
content: 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").

author: RoV
author_rank: Regular Contributor
timestamp: August 04, 2021, 02:37:44 pm
content: apart from what has already been said, I recommend to check for the presence of a capacitive load, that could cause a very high current spike on switch closing, possibly violating the SOA curve limits. If the power source is very low impedance (e.g. lithium batteries and/or a large capacitor) and the load includes a large shunt capacitor on the input, you may need to take countermeasures, e.g.:- slow the Vgs to avoid exceeding the maximum current (but also check staying within the SOA for the closing time)- make a double switch with a precharge resistor. But I don't think this is the case, more typical of high voltages like 200 V or more.

author: logancane
author_rank: Regular Contributor
timestamp: August 05, 2021, 12:13:31 am
content: Quote from: magic on August 04, 2021, 01:31:59 pmAh, yes, that makes sense. I'm so used to thinking of MOSFETs as switches that I forgot people use them in their linear region as amplifiers.Quote from: RoV on August 04, 2021, 02:37:44 pmThat is a good point which I hadn't considered. One of the downstream loads is a radio that has a massive bulk cap on it so that if it loses power, it can quickly send a power failure message. So how does one slow the Vgs? Do you use some kind of RC circuit on the gate?

author: logancane
author_rank: Regular Contributor
timestamp: August 05, 2021, 01:26:19 am
content: Quote from: logancane on August 05, 2021, 12:13:31 amNevermind, I found it in the gate driver datasheet. It appears to be a parallel RC circuit between the gate driver output and the gate

author: logancane
author_rank: Regular Contributor
timestamp: August 05, 2021, 05:19:13 am
content: Sorry to keep spamming this, but one more question just to make sure I fully understand these graphs.In a switch operation such as this, if the switching action took say 1ms, would I use the 1ms line on the graph to determine the maximum current it can handle at a given Vds?For example in the attached simulation, the greatest dissipation of power seems to occur when the current (blue) peaks at 1.1A while Vds (difference between green and red) is still pretty much at 19V. So since the switching operation takes 1ms, to be safe, possibly even overly conservative, I just need to make sure that 1.1A at 19V is below the 1ms line, even if it is above the DC line?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 05, 2021, 06:58:37 am
content: yes the 1 ms curve would be relevant for slow switching over some 1 ms. The curves are for rectangular power pulses, so only a approximation for a real world more rounded pulse and with droping voltage.The radio part may want some extra measures to reduce the peak current from the capacitor (e.g. series resistor for the cap).

author: logancane
author_rank: Regular Contributor
timestamp: August 05, 2021, 07:04:27 am
content: Quote from: Kleinstein on August 05, 2021, 06:58:37 amAwesome, thank you for confirming that. I think I understand what I'm doing now. I talked to the guy working on radios and he's taken measures to limit charging current.