| Electronics > Beginners |
| Selecting MOSFET |
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| permal:
Ok, so taking inspiration from @Spec's schematic in an earlier thread I've ended up with this. Just added a pull down to the left mosfet and the 5/12V switch so what acould possibly go wrong? :) |
| spec:
The switching circuit was designed for 3V3 and 5V systems (max 7V), so it will not be suitable for 12V switching because the VGSmax of the PMOSFET of 8V will be exceeded. For the same reason the input control voltage must not exceed +-7V. The circuit can be modified to handle 12V or another PMOSFET can be used with a VGSmax of 13V or greater (20V most likely). The same goes for the NMOSFET if the input voltage will exceed +-7V. Taking about preferred component lists, a 6V8 zener diode type CZRQR52C6V8-HF is always handy to have for gate protection (in general, get a range of zener diodes). Another consideration is speed. The circuit, as is, will be reasonably fast but for higher speeds the 120R gate stopper resistors could be reduced to 10R and the 4k7 pull-up resistor value could be reduced. If you post a specification for a switching circuit to suit all your requirements, we can have a look at that, or do you want to do the design yourself? We can talk you through it if necessary. http://www.vishay.com/docs/72024/72024.pdf http://www.vishay.com/docs/65900/SI2312CD.pdf http://www.comchiptech.com/admin/files/product/CZRQR52C2-HF%20THRU%20CZRQR52C39-HF-RevB.pdf |
| spec:
Here is one way to do it (R3/D1 and R7/D2 must be mounted on their respective MOSFET terminals, or as close as possible, using as short leads as possible): |
| permal:
Wow, do I feel the fool now. :palm: Thank you for your patience, @Spec. The circuit as I drew it is what I need - 5 and 12V output so your latest schematic will do the trick. Questions: I suppose D2 strictly needed since I'll be controlling that only via the MCP23107 at 3.3V? --- Quote --- ...as close as possible, using as short leads as possible --- End quote --- Can please you elaborate on the reason behind that? |
| spec:
--- Quote from: permal on January 17, 2019, 08:01:12 am ---Wow, do I feel the fool now. :palm: --- End quote --- Not at all. I wont tell you about the mistakes I have made. :) --- Quote from: permal on January 17, 2019, 08:01:12 am ---Thank you for you patience, @Spec. --- End quote --- No sweat --- Quote from: permal on January 17, 2019, 08:01:12 am ---The circuit as I drew it is what I need - 5 and 12V output so your latest schematic will do the trick. --- End quote --- That's good. By the way you can always trade speed for lower current consumption. --- Quote from: permal on January 17, 2019, 08:01:12 am ---I suppose D2 strictly needed since I'll be controlling that only via the MCP23107 at 3.3V?. --- End quote --- Yes, and you can drop the input resistor value back to 120R. --- Quote from: permal on January 17, 2019, 08:01:12 am --- --- Quote --- ...as close as possible, using as short leads as possible --- End quote --- Can please you elaborate on the reason behind that? --- End quote --- Sure :) MOSFETs are odd. They are as fast as hell, but they have huge parasitic capacitances from drain to source, drain to gate, and gate to source. These capacitances are not in the tens of pF like for BJTs (normal transistors). Instead they are in the nF range- take a look at the data sheets. The consequence of this high frequency response and high parasitic capacitances is that MOSFETs tend to oscillate, typically from 500kHz to 20mHz, if their physical layout in not compact and you do not include a gate stopper resistor. They may not oscillate continuously, but only at certain voltages, temperatures, or currents. Because traces/wires have inductance and capacitance this also adds to the chance of parasitic oscillations, so you must make traces/wires as short as possible. I have said this many times before, but in general, the nice circuit that you see depicted in schematics is nothing like the actual physical circuit that you build, because there are parasitic resistances, capacitances, and inductances all over the place. This is especially the case for prototype breadboards with sockets and jumper leads. There is a further complication: skin effect. At DC, current flows through the whole cross-section of a trace/wire, but as the frequency increases, say from 1kHz upward, skin effect causes the current to bunch towards the outer surfaces of the traces/wires, and the skin effect increases with frequency. The net result is that the nice conductor you see on the schematic is an even less nice resistor at high frequencies. The skin effect is the reason why Litz (insulated multi-strand wire) is used to wind high-frequency transformers. https://en.wikipedia.org/wiki/Litz_wire |
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