Electronics > Projects, Designs, and Technical Stuff
Confusing MOSFET transition times
mbless:
I’m evaluating negative high-voltage pulser designs (see this thread), part of which I’m testing different mosfets. For the current topology I tested 4 mosfets, but I ended up with confusing results regarding the turn on/off times.
Here are the mosfets and a summary of what I believe are their relevant numbers. Data sheets are linked.
PartInput capacitance @ 150V Total gate charge End of Miller plateau @ 400VST STF12N50M2560pF15nC @ 400V12nCInfineon IPA60R280P7S 760pF18nC @ 400V10nCON Semi FQPF8N80C1580pF35nC @ 640V22nCON Semi FQA19N602800pF70nC @ 480V49nC
Based on the parameters, I would expect the rise/fall times to increase as you move down the list. I tested the parts by pulsing -150V into a 50ohm non-inductive resistor for a commanded 100ns. I used the same PCB for all parts so everything would be the same except for the mosfet. All mosfets are also TO-220 package. What I found, though, is that the 3rd part (FQPF8N80C) was the fastest and closest to the commanded on time. Here is a table of rise, fall and on times extracted from the attached plot of output voltage. Note that I didn't measure current, so I'm treating the on time as conduction time.
PartRise time -10V to -140V Fall time -140V to -10V On time -10V to -10VST STF12N50M27.6ns20.4ns130.8nsInfineon IPA60R280P7S 5.6ns22.0ns132.0nsON Semi FQPF8N80C5.6ns12.8ns109.6nsON Semi FQA19N608.8ns26.8ns130.8ns
I'm trying to figure out why the FQPF8N80C performed so well when its parameters are 2-3x worse than the STF12N50M2. Normally I wouldn't even have selected FQPF8N80C for testing based on its parameters; I only chose it because it was in an Art of Electronics example circuit for nanosecond pulsers. So far the only two distinguishing characteristics I can find are
* different MOSFET technology from different manufacturers
* STF12N50M2 and IPA60R280P7S have built-in gate zener diodes where as the FQPF8N80C and FQA19N60 do not. Not sure why the fall time would be so adversely affected unlike the rise time.
What am I missing?
EDIT: Imgur album https://imgur.com/a/AYe0QeD
TimNJ:
I don't have enough experience to give a good answer, but what does your test PCB look like? Sometimes a poor gate drive layout with high inductance can cause a "high performance MOSFET" to underperform, maybe even worse than a MOSFET that's worse on paper. Reasons may be include parasitic oscillation during switching transitions.
jmelson:
Input capacitance is not the whole story. A major component is the "Miller" capacitance, ie. the capacitance from the drain to the gate. While much less than the gate-source capacitance, the drain swings, perhaps, 50 X the g-s voltage swing.
During turn-on, the gate is brought up to, perhaps 12 V, with respect the the source. As the transistor turns on, the drain voltage falls toward the source voltage. But, the falling drain voltage pulls charge away from the gate, causing the gate voltage to plateau until the drain has come most of the way down to the source voltage.
To turn it off, the reverse happens. You want to bring the gate down to zero, with respect to the source. But, then the rising drain voltage puts charge back into the gate.
Jon
temperance:
Rising edge: You can actually see what's happening from the measurements you've made. With a 50R load, the FQPF8N80C will be the fasted MOSFET because it has the smallest output capacitance.
Falling edge: artefacts in timing are caused by differences in the miller plateau voltage and the output resistance of the driver circuit.
magic:
--- Quote from: temperance on July 13, 2020, 08:16:02 pm ---Falling edge: artefacts in timing are caused by differences in the miller plateau voltage and the output resistance of the driver circuit.
--- End quote ---
Also my thought. The FQPF8N80C has higher threshold voltage, therefore more volts is applied across internal and external gate resistance during turn-off. Internal gate resistance is unspecified, perhaps it's smaller than the 7Ω specified for the other two parts. At an amp or two of drive current it may make a difference.
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