| Electronics > Beginners |
| Picking a transistor. |
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| magic:
The KSD5401 posted by David Hess seems good too. For low threshold MOSFETs, simply look for SOT23 parts. I went to TME, punched in =20Vds, >5Ids, SOT23 and the first 3 parts it spat out were all rated <30mΩ at 2.5Vgs. |
| xlnx:
I have made an attempt to finding modern low-cost transistors that I can standardize on for all my designs. I have found a few which lets me to low-side and high-side switching at 1.8V, 2.5V, 3.3V and 5V. They are: Name Pkg Type US$ VDS(V) ID(A) RDS AO3400 SOT-23 N 0.01 30V 5.8A <52mΩ @ Vgs=2.5V AO3401 SOT-23 P 0.01 30V 4A <85mΩ @ Vgs=2.5V AO3415 SOT-23 P 0.03 20V 4A <65mΩ @ Vgs=1.8V AO3416 SOT-23 N 0.04 20V 6.5A <34mΩ @ Vgs=1.8V AO3407 SOT-23 P 0.02 30V 4.1A <87mΩ @ Vgs=4.5V These can be found on AliX for almost nothing, and they have absolutely fantastic specifications. Data sheets here: http://www.aosmd.com/ |
| Mr. Scram:
--- Quote from: MagicSmoker on August 11, 2019, 09:34:42 am ---Skipping the video and scrolling down the page to the schematic, it shows the expected tiny small-signal NPN BJTs being driven directly by the MCU, but that ain't gonna cut it if you need to switch 20W at 12V or so. The easiest solutions are either use an n-channel MOSFET driven by a MOSFET gate driver IC, or a "smart" low-side switch IC (which will also likely be protected against overvoltage and shorts, so even better). An example of a smart low-side switch is: https://uk.rs-online.com/web/p/mosfet-drivers/1630759/ A Zetex (nee Diodes, Inc.) BJT as suggested by @magic above might just work, but it would be marginal, in my opinion. You really don't want to demand more than about 5-10mA per output pin on an MCU. --- End quote --- These switch ICs can do the job by themselves? |
| MagicSmoker:
--- Quote from: Mr. Scram on August 11, 2019, 02:47:24 pm --- --- Quote from: MagicSmoker on August 11, 2019, 09:34:42 am ---... An example of a smart low-side switch is: https://uk.rs-online.com/web/p/mosfet-drivers/1630759/ ... --- End quote --- These switch ICs can do the job by themselves? --- End quote --- Not the specific one I suggested above, but, yes - these smart high and low side switches are specifically made for driving loads directly from an MCU output. However, the MOSFETs that @xlnx found above look like good choices, too. |
| David Hess:
--- Quote from: magic on August 11, 2019, 07:52:27 am ---Zetex (now Diodes Inc) makes good BJTs for saturated switching. ZTX851 might fit the bill with β>100 at 1A, 200 typical. --- End quote --- The Zetex parts should be ideal for this. Lots of companies make "Super E-Line" transistors now. The KSD5401 that I mentioned is one of them. --- Quote ---It also coincidentally has some of the lowest voltage noise available in any BJT, supposedly at 200pV/rtHz, but that's another story. --- End quote --- The way I remember it, Zetex bought old and depricated CMOS IC fabrication equipment and used it to make modernized RETs (Ring Emitter Transistors). You might be familiar with these in the form of the old D44/D45 TO-220 power transistors. Their construction includes multiple base connections and results is less Ft and hfe droop at high currents. They are very popular now for linear power amplifier but also work for fast linear regulators and switching regulators. The multiple base connections should also result in low base spreading resistance which otherwise limits low noise performance but this is not something I have investigated yet. Common small signal transistors are like 40 ohms but integrated transistors which use similar construction can be 2 ohms or less. |
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