Mosfets...Why so complicated?

Trying to move on from optoisolators and relays to mosfets or IBGTs due to power consumption. I want apply to projects running on battery.
I am getting so confused with SOA charts.
I am just looking for a logic level IC (5v) that would be able to switch (no pwm) a battery recharger on or off. 28v max current flow for the recharger is 4.5A.
Would prefer SMD small form factor without the need for a heatsink
Any suggestions?

Also if optoisolators are made with these components why are their specs so much more straight forward - rated by max volts and amps? no heatsinks / heat issues, just max ratings...simple! why are rating mosfets so much more complicated (SOA, ,heat dissipation depending on voltage/current ratios, etc.?


These are some of the ones I often use:

Make sure you design the cct. to prevent exceeding Vgs(max)

I don't have a recommendation for you...

The SOA curve isn't that important when using the MOSFET as a switch because it's either on (at minimum voltage) or off (at minimum current).

The important things are Rds-on (resistance when fully-on) and its wattage rating (and maybe its maximum current rating). Wattage can be calculated as current-squared x resistance. Usually the wattage and heat are the limiting factors so you may not get the full-rated current (without frying it).

The thing that gets tricky is the actual maximum wattage because that depends on the internal temperature which depends on the efficiency of the heat sink (if any) and the ambient temperature.

Opto-isolators and solid state relays usually don't rely on additional heat sinking and they designed to always be used "digitally" so there is usually just a current rating (and voltage rating) and a maximum ambient temperature rating.


N-channel (sink current to Gnd), or P-channel (source current to the load)?

Look at AOD4xxx parts at Digikey.
Several with Vds in the range, you want, able to handle the current, with very low Rds for minimal heating, and surface mount.

Here's an example

With 28V supply, you need 18V on the gate to turn it full on,
Rds will be < 0.044 ohm, power dissipated with 4.5A running thru the device will be P = I^2 x R = 4.5A x 4.5A x 0.044 ohm = 0.89W, so you'll want to have some heat sinking copper area under the part.

If you can us N-channel, there are parts with lower Rds, like this one

With 10V on the gate, P = 4.5A x 4.5A x 0.018 ohm = 0.36W.

This one has even lower Rds, but Vds is only 30V, not much margin above your 28V supply

With 10V on the gate, P = 4.5 x 4.5 x 0.004 = 0.08W
and just 0.12W with 5V driving the gate.

Great info. Thank you very much!
If I understand correctly all I need to check is

  1. supply limit rating
    and then
  2. resistance at given gate voltage -
    to make sure that current through that resistance will not dissipate more power (watts) than max rating.
    id that correct?. I can ignore the general SOA ?

What is SOA?

Safe operating area. Every mosfet datasheet has it.

For switching one generally does not need to be concerned about SOA. However if your circuit switches slowly (i.e. long rise and fall times) you can run into trouble.
For instance in your case of 28V and 4.5A . As the Mosfet Switches from on to off (or off to on) it is operating in the linear areas. So when the Mosfet is on enough to have 28/2V across it and 4.5/2 amps through it is momentarily at 14V and 2.25A Looking at your SOA curve, if you switch in 10µS you will be fine but at 100µs you would be on the edge for this device.

The difference between the opto and the mosfet is the opto doesn't handle any power.

Regarding why so complicated.... physics is complicated. Parts have limitations and if you ignore these limitations your design ends up having issues.

You will also find the datasheet featured specifications are like a marketing ad. They are true but not under usable conditions. One needs to refer to the charts for the real operating characteristics.
A note about Rds (on) this is usually measured during a 1 µs Pulse, hardly a usable number. Why is it even there you ask? Because it makes the Rds (on) measurement independent of the case or installation (heatsink). An engineer familiar with MosFet specifications will use this as a figure of merit.

I agree it makes understanding what a MosFet can or cannot due difficult on the novice.

SOA is normally irrelevant for using a MOSFET or IGBT as a switch - you are either in the
no current corner, or the (almost)no voltage corner...

high-side or low-side switching? For high-side you'll use p-channel and need some
ancilliary level-shifting components (and the pFET won't need to be logic-level).
With low side switching you break the ground connection which can be problematical
in some situations, but a single n-channel logic-level FET will suffice.

4.5A suggests an on-resistance of 10 milliohms or less for a nice low 200mW dissipation,
and Vds(max) or 40 to 60V would seem appropriate - parametric search on any of the
major vendor's websites will find possible devices - there are (many) thousands to
choose from and price/availability change every year so online search is the way to go.

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