Been testing a few random power mosfets with the 7136 today and while it’s been interesting to see how each behave in the ramp up period I didn’t come across a really good candidate at 4.7khz.
Or indeed a much better idea of what specs are contributing most towards it working better or worse with the 7136 signal.
One particular standout comparison comes with two ON Semi power mosfets, the NTD60N02RG and the T70N03G
Both of these fets came on the old East092 DD drivers, the 70N first and then the 60N on the later models.
These two fets going by the datasheet appear to be very very similar, certainly I can’t really find any major differences in the usual suspects of mosfet specs. Nothing that’s jumping out as a fundamental performance difference between the two.
So how do they perform when hooked up to the 7136?
Well the 60N seems to be about the best performer I’ve tested so far with a pretty fast turn on while the 70N is by far the worst. It takes forever to fully turn on and as a result is completely useless in this application.
Here are the graphs of a 0-100% turn on capture.
NTD60N02RG
This fet actually switches on a fair bit faster than the standard small 30YLD fet I tested last time. It has about a 220uS delay from off to fully on compared with the 30YLD’s ~300uS
T70N03G
The 70N on the other hand is complete garbage, it takes so long to ramp up that the graph almost doesn’t fit in to the 100uS horizontal scaling. Just awful…
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So what’s causing this drastically different behaviour? No clue…
Total and individual gate charges are very close (and very low at only 9.5 and 13.2nC respectively) , they have identical threshold voltages (between 1v - 2v min-max on both) and capacitances are near as well.
The only major difference I found is that the 60N is apparently “Optimized for High Side Switching Requirements in High−Efficiency DC−DC Converters” and looking in the timing values, almost all the same again bar one; Tr or Rise Time.
Here there is a drastic difference between the 60N which has a Tr of 33ns and the 70N which has a staggeringly low Tr of 1.3ns! Haven’t come across a Tr this low up till now so I initially put it down to a datasheet error…but maybe this is really what makes it perform so poorly here?
I’m just speculating here but maybe having an exceptionally fast rise time is actually detrimental when combined with the 7136’s soft start signal output?
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Just for comparison here is the 30YLD fet again in the same test setup as above.
PSMN3R0-30YLD
And I also tested two other fets.
A philips PHD50N03LT
Looked promising initially but it’s a fairly slow one…strangely that fast initial start and soft ramp up plot actually seems to give a neater PWM trace on some levels. Non of the funky doubling seen on some modes with the “faster” looking fets like the 30YLD @4.7khz. But still nothing usable in terms of mode range.
DinTek DTU06N03
This looks like a variant of the DTU30N02 but with a higher Threshold voltage (1.2v-2.5v vs 0.6v-1.5v of the 30N02) and double the gate charge (total of 74nC vs ~30). Performance isn’t great unfortunately, had my fingers crossed it was all down to the DinTek name!
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Just putting it out there really, see if you guys can make any connections.
I’m thinking an exceptionally low gate threshold voltage is pretty important, seeing as the ramp up of the 7136 keeps below 1.5v for quite a while. A fet like the DTU30N02 that can start opening up at that stage (<0.6v) will have some advantage over these fets which seem to only start cooking in the 1-1.5v range.
Also not sure what exact effect the gate charges have, I suspect at these slow PWM speeds it’s not that critical. Especially as the two fets with the biggest difference in performance both have a very low charge spec.
Also hooking all of these fets up directly to the Attiny 13A pwm pin they seems to slam on and off almost instantly with the PWM signal, nothing at all like the leisurely pace things operate at under the supervision of the 7136.
Here is a trace showing what the “useless” T70N03 does when driven directly from the MCU. Everything else the same.
Nothing at all slow about that In comparison you can see what the open 7136 signal is doing in the comparison.
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My head is spinning…too many datasheets and still non the wiser… :Sp :weary: :arrow: Bed