I find I learn a lot by writing tutorials, because when you write a tutorial you want to explain exactly why you did something, rather than something else. Also it helps you clarify concepts in your mind (my mind, that is).
I am working on a page about interfacing with a high-side driver (ie. to source current rather than sink it) using a P-channel MOSFET.
There are some great YouTube videos about this subject, but I find at times that I want to just “glance at a circuit” rather than sit there watching a video for 10 minutes, to try to find the part where they explain about the relationship between current, RDSOn and heat.
Below is the proposed schematic:
You are welcome to make suggestions about the values of the components (eg. R1 and R2).
Also D1, I am using a 1N4148 for a 300 mA motor. Would this be adequate? Would a Schottky be better, or is that overkill? I believe the 1N4148 is a fast (4nS) recovery, 200 mA, 100V diode.
I have tested with a 2 ohm load, driving the whole thing from a power supply limited to 1 amp. The input pin is a 1 kHz square wave (at 5V).
Overall results are:
The blue trace is the Gate of Q2 and the yellow trace is the Drain of Q2.
As expected, the FET turns on when the gate becomes more negative, and turns off when the gate becomes less negative. In particular:
The delta there is around 2V which agrees with the datasheet which quotes VGS(th) as -2V mininum.
Once concern I have is the fairly lengthy turn-off time:
That is around 58 µS which seems a lot. It also accounts for, I think, that the FET is getting hot (around 30 °C) because it is running unsaturated for quite a while.
Would a lower value for R2 help here? Any suggested values? If R2 was lower, then R1 might need to be lower too to provide adequate current to sink the gate to 0V when required. Would that be right?
Turn-on time seems to be somewhat faster (around 1 µS) which seems OK to me:
Power consumption and heat
I calculate the power through the load as being I2R where I is 1 and RDS(on) is 0.021, thus:
power = 1 * 1 * 0.021 = 0.021 watts
The “Thermal Resistance Junction to Ambient” (RØJA) is 62.5 °C/W, thus I expect a rise of:
0.021 * 62.5 = 1.3125 °C (over ambient)
Does that sound right? However I am measuring more like 10 degrees rise.
Any other comments or suggestions?