Comments requested on MOSFET high-side driver

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introduction

i find learn lot writing tutorials, because when write tutorial want explain why did something, rather else. helps clarify concepts in mind (my mind, is).

i working on page interfacing high-side driver (ie. source current rather sink it) using p-channel mosfet.

there great youtube videos subject, find @ times want "glance @ circuit" rather sit there watching video 10 minutes, try find part explain relationship between current, rdson , heat.

schematic

below proposed schematic:



for testing, mosfet fqp47p06 , transistor 2n3904.

you welcome make suggestions values of components (eg. r1 , r2).

also d1, using 1n4148 300 ma motor. adequate? schottky better, or overkill? believe 1n4148 fast (4ns) recovery, 200 ma, 100v diode.


i have tested 2 ohm load, driving whole thing power supply limited 1 amp. input pin 1 khz square wave (at 5v).

overall results are:




the blue trace gate of q2 , yellow trace drain of q2.

as expected, fet turns on when gate becomes more negative, , turns off when gate becomes less negative. in particular:



the delta there around 2v agrees datasheet quotes vgs(th) -2v mininum.



turn-off time

once concern have lengthy turn-off time:



that around 58 µs seems lot. accounts for, think, fet getting hot (around 30 °c) because running unsaturated quite while.

would lower value r2 here? suggested values? if r2 lower, r1 might need lower provide adequate current sink gate 0v when required. right?



turn-on time

turn-on time seems faster (around 1 µs) seems ok me:





power consumption , heat

i calculate power through load being i2r 1 , rds(on) 0.021, thus:

code: [select]

power = 1 * 1 * 0.021 = 0.021 watts


the "thermal resistance junction ambient" (rØja) 62.5 °c/w, expect rise of:

code: [select]

0.021 * 62.5 = 1.3125  °c  (over ambient)


does sound right? measuring more 10 degrees rise.



any other comments or suggestions?

yes, lower r2. 10k pretty high resistance discharge gate charge.

unless on tight current budget arduino pins, think i'd start 1k resistor r1, , 470 ohms r2.

using 200ma rated diode on 300ma motor seems unwise. i'd use @ least 1a diode. has been discussed, 1n400x diode fast enough turn-on time, turn-off time slow.


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