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I am trying to look for a solution that can help me in turning ON/OFF my 24VDC, 2A LED load. So as per the application needs I may have to turn ON/OFF this load 30 times max per day. So it is a DC operation.

Below sketch shows a quick pictorial representation of what I am trying to do. Not a final layout yet.

enter image description here

I am looking if I can use a MOSFET rather electro mechanical relay. I am referring to the ROHM RQ7L050AT datasheet. This is a 60V, 5A Mosfet.

But by looking at the SOA curve as below, I am not sure if this MOSFET approach is advisable for continuous DC loads.

As per the graph, I guess at the desired 24VDC VDS the maximum allowed Id current is less than 0.07A before I hit the thermal runaway situation? (whereas my actual requirement is for 2A Id).

Let me know if I am getting if right. Can anyone help me understanding the MOSFETs for DC load operations?

Will it help if I choose higher VDS and Id rated MOSFETs?

enter image description here

Thanks in advance!

-Yugandhar

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2 Answers 2

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VDS depends on the current and the resistance of the MOSFET.

Your RQ7L050AT has a maximum RDSON of 39 milliohms. At 2 amperes of current, that's a VDS of less than 0.1 volts.

0.1V and 2A is well within the safe operating area for continuous DC.

You've got a Zener diode in there protecting the gate from the 24V. The NZX84C10L should keep the gate voltage below 11V, so the gate is safe.

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  • \$\begingroup\$ Oh I got it now! I was assuming the other way round, that VDS is the OFF state voltage of the mosfet drain-source. Thanks for clarifying. \$\endgroup\$
    – Yugandhar
    Commented Apr 20, 2021 at 14:40
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When fully ON your Vds will be around 80mV (39mOhm*2A), which is inside your safe operating area. However you must consider this is with the MOSFET mounted on a ceramic plate for dissipation. This MOSFET mounted on a PCB with no dissipation will not handle the current of your application.

enter image description here

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  • \$\begingroup\$ Thanks! So in the datasheet, the Rthj-a was mentioned as 113degC/W (for FR4 PCB base of 25x25mm). However I could not find the Rthj-c in the datasheet. So in the worst case, I can assume 113degC/W as my thermal resistance? \$\endgroup\$
    – Yugandhar
    Commented Apr 20, 2021 at 14:52

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