Electronics > Projects, Designs, and Technical Stuff
How slow are zener diodes?
theleakydiode:
Hi,
I'm using a zener diode to clip the negative going part of a power transistors base drive waveform so it doesn't damage my transistor, but it seems to take a while to kick in. This is for a little self oscillating 20khz flyback converter I put together.
Is this normal zener diode behavior? I'm using a 1N5343 7.5v 5w type, and it sees about 500mA peak zener current. Could the 5w type just be too slow to respond in time?
The transistor I'm using seems to perform better with a negative turn off voltage hence why I'm using this set up, it can take up to 9v emitter-base according to the datasheet.
If it is too slow then I'm thinking a TVS might be faster but they also don't clamp as tight as zeners so I'm not sure which one to pick, maybe one of the 1.3w or 3w zeners might be faster.
Thanks.
exe:
How slow is slow?
EDIT: I believe "real" zeners are fast (those that are below ~6.2V). Avalanche "zeners" can be slower (but please check that). So, it might be beneficial to substitute a 7.5 zener with two devices. Or may be even with a 5.6/6.2V zener.
One thing that limits performance of a zener is long leads (when really fast clamping is needed, such esd protection), but I don't think this is the case here.
theleakydiode:
Slow enough that there is a significant delay before the clamping kicks on, switching frequency is about 20khz.
I've included a drawing of the waveform in the image.
So I should try a lower voltage zener then? Could their wattage ratings affect their speed in clamping/clipping applications too?
exe:
--- Quote from: theleakydiode on April 30, 2020, 06:48:25 am ---I've included a drawing of the waveform in the image.
--- End quote ---
It doesn't contain timebase :(. Anyway, I'd try to change zener. May be a diode too if changing zener doesn't help.
trobbins:
Let's say it was junction charge related delay - then use the lowest power zener (ie. not 5W) and a signal diode (not the UF4004 which may well have a turn-on recovery voltage characteristic).
If you are probing a very fast waveform, are you using a probe with the springy clip ground tip and the probe and scope have sufficiently high bandwidth?
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