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| Selecting MOSFET |
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| permal:
Hi, During the weekend I got some great help re. transistors and MOSFETS. I hope I've now learned enough to dare select a MOSFET and use that in my design. My requirements are as follows * Controllable at 0-3.3V (via MCP2307 I2C IO Expander) * 500mA at VDS 5V continuous load, so 2.5W * VDS up to max 12V i.e. ~210mA. * Preferably no heat-sink needed. * SMD * Low-side switching. I'm currently looking at the IRLL110TRPBF, N-Channel for low-side switching. Alternatively the SQ1431EH for high-side switching (would of course need extra control circuitry as shown in this post by @Spec. Vgs(th) is 1V, so the 3.3V ought to be sufficient. If I'm reading the SOA graph correctly, I'm well within the safe area for both. Could I please get a second opinion on the choice of these MOSFET based on the above requirements? |
| aheid:
--- Quote from: permal on January 14, 2019, 08:59:53 pm ---My requirements are as follows * Controllable at 0-3.3V (via MCP2307 I2C IO Expander) * 500mA at VDS 5V continuous load, so 2.5W --- End quote --- --- Quote from: permal on January 14, 2019, 08:59:53 pm ---Vgs(th) is 1V, so the 3.3V ought to be sufficient. --- End quote --- If you look at figure 1, you'll see that the IRLL110TRPBF barely be able to do 500mA at 3.3V with that transistor. Looking at the V_GS(th), it lists 1V min, and 2V max... Compare this to say the AO3402, you can see that V_GS(th) is 1.5V max, and from figure it'll be more or less fully on at 3.3V. However your specification of "500mA at VDS 5V continuous load" is a bit ambiguous to me. Do you mean you want to turn on/off a load with an effective resistance of about 5V / 500mA = 10 Ohm, or that you want the transistor to have an effective resistance of 10 Ohm, so that it has 5V over it when 500mA is running through it? If the former, the IRLL110TRPBF won't really cut it. If the latter, then the SOA for the IRLL110TRPBF doesn't mention DC at all. While the SOA for the AO3402 does have a DC line, I wouldn't particularly trust it, given it's a trench construction. |
| Audioguru:
The request is very confusing. First the Mosfet has a current of 500mA and a Vds of 5V is to be used as a 2.5W heater. Then the Mosfet is a low side or high side switch when it barely gets warm. The tiny Mosfet shown will burn up if it must heat with 2.5W. The threshold voltage range of a Mosfet is the Vgs when it begins to conduct a very small current. |
| permal:
Yes, I can see now that the question is a bit confusing (very much, actually). Sorry about that, totally mixed things up. It is not Vds at 5 & /12V, but Vcc. So: 5V and 12 rails. 500 and 210mA continuous load at the respective voltages. 3.3 Vgs As @aheid points out, IRLL110TRPBF isn't suitable for those numbers. |
| spec:
--- Quote from: permal on January 14, 2019, 08:59:53 pm ---Hi, During the weekend I got some great help re. transistors and MOSFETS. I hope I've now learned enough to dare select a MOSFET and use that in my design. My requirements are as follows * Controllable at 0-3.3V (via MCP2307 I2C IO Expander) * 500mA at VDS 5V continuous load, so 2.5W * VDS up to max 12V i.e. ~210mA. * Preferably no heat-sink needed. * SMD * Low-side switching. I'm currently looking at the IRLL110TRPBF, N-Channel for low-side switching. Alternatively the SQ1431EH for high-side switching (would of course need extra control circuitry as shown in this post by @Spec. Vgs(th) is 1V, so the 3.3V ought to be sufficient. If I'm reading the SOA graph correctly, I'm well within the safe area for both. Could I please get a second opinion on the choice of these MOSFET based on the above requirements? --- End quote --- Hi permal Attached is a list of NMOSFETs that I put together for a similar application. The $ sign indicates merit. When switching current with a MOSFET it is important to make sure that the RDS (at the VGS that you are supplying) is as low as possible to minimize the voltage drop and also to limit the power dissipation in the MOSFET. You should be able to achieve around 50mR with a 2V5, or greater, G/S voltage. The threshold voltage, at its extremes needs to be low enough so that the 3V3 logic turns the MOSFET on and high enough so that the 3V3 logic can turn the MOSFET off. A good compromise is to choose a MOSFET with a maximum threshold voltage in the region of 1V to 1V5. Provided that you are turning the MOSFET on and off relatively infrequently, and/or are turning the MOSFET on and off relatively fast, you do not have to worry too much about the SOA. (your link above does not work) |
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