Electronics > Beginners
High-side vs. low-side NPN transistor
spec:
--- Quote from: rstofer on January 12, 2019, 08:03:21 pm ---Logic Level MOSFETS might be better than BJTs but it sometimes takes 10V between the gate and source (equivalent to base and emitter) to turn a MOSFET fully on. You want to drive it to saturation to reduce heating.
There can be enormous gate capacitance and this slows the transition to whatever the final gate voltage is going to be. During the transition period, the MOSFET is heating. Add two transitions (rising and falling edge) and that is the reason that PWM is typically at a low frequency. The designers try to make the transition time a small percentage of the total period. Take 1 kHz = 1 mS. If the rise time and fall time are 250 ns, then half the time is spent in the transition region where heating occurs. I just grabbed those numbers, they aren't real, but it is important to understand that uCs can't always deal with conventional MOSFETs and may have a problem with Logic Level MOSTFETs as well. You just have to work the numbers!
Which leads us to why they invented MOSFET driver chips. These puppies can dump AMPS into the gate to get through the transition in a great big hurry.
MOSFETs are great devices but the user better read the datasheet and understand what is going on. Vgs is a big deal, so are all the capacitances.
--- End quote ---
Of course you have to look at the data sheets. And it is obvious that you have to take note of all the device characteristics. That is what design is all about.
spec:
--- Quote from: rstofer on January 12, 2019, 04:29:03 pm ---
--- Quote from: permal on January 12, 2019, 04:17:02 pm ---
I want to understand why I get such a low voltage reading in this particular scenario - 2.7 < 4.4. So yes, an explanation is what I'm asking for.
Re the low/high side switches, as long as I put the transistor on the right side, I should be good if I've understood things correctly.
--- End quote ---
In normal operation, an NPN transistor base will be about one diode junction drop higher than the emitter. It's 0.5V in the case of the 2N3055.
So, if the base is 3.1V, I wouldn't expect the emitter to be higher than 2.6V.
See Vbe(on) - second page here:
https://www.onsemi.com/pub/Collateral/2N3055-D.PDF
--- End quote ---
2N3055 :-//
permal:
I'm starting to like MOSFETs :)
Question re. them: In the SOA-diagrams a pulse length is mentioned, but it is assumed you know of *what*. I understand it as the pulse length of where Vgs is larger than its threshold voltage, i.e. Vgs > Vgs(th), is that correct?
What if you have a high duty cycle, would that effectively be considered as a longer pulse? Am I correct in that the the rise time of Vgs from 0 to lowest voltage where RDS(on) is specified [N-channel] matters, i.e. if it is short enough the MOSFET would pass through the linear mode very quickly, effectively reducing the power dissipation and thus allowing for a longer pulse?
@Spec: In your example schematics (thanks) you put a R120 in series with the gate - why is that needed if the gate of an NMOSFET already is so high (1014Ohms?) Is it purely as a protection for the MCP23017?
rstofer:
--- Quote from: spec on January 13, 2019, 08:07:55 am ---
--- Quote from: rstofer on January 12, 2019, 04:29:03 pm ---
--- Quote from: permal on January 12, 2019, 04:17:02 pm ---
I want to understand why I get such a low voltage reading in this particular scenario - 2.7 < 4.4. So yes, an explanation is what I'm asking for.
Re the low/high side switches, as long as I put the transistor on the right side, I should be good if I've understood things correctly.
--- End quote ---
In normal operation, an NPN transistor base will be about one diode junction drop higher than the emitter. It's 0.5V in the case of the 2N3055.
So, if the base is 3.1V, I wouldn't expect the emitter to be higher than 2.6V.
See Vbe(on) - second page here:
https://www.onsemi.com/pub/Collateral/2N3055-D.PDF
--- End quote ---
2N3055 :-//
--- End quote ---
Just a common pass transistor. Not the one in the OP, just an example I am familiar with. Nevertheless, I would expect the datasheets to have the same information. Probably not in the same order but essentially the same. The numbers will be slightly different, of course, but the ideas are identical.
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