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Topic: Fast switching of Power MOSFETs (Read 1 time) previous topic - next topic

VFXCode


First you have to figure out what the problem is.


The problem is that the MOSFETs take more time than they should to turn on and off thus leds that should not light up do so.


What are the capacitors for?

for exactly that
It looks to me like they might be keeping the LEDs lit up for a short time after your MOSFET switches off.


Why do you write to the output ports multiple times?


Ignore it. It has no impact on my problem whatsoever

VFXCode


Quote
It would take about 8.2ms just to put the data to the column shift registers plus the time i need to get that data from the other arduino
(an other 8.2ms at best) which kinda limits my refresh rate.


I am not sure how your have structured your hardware but it takes about 0.1ms to shift a byte into a HC595, on a 1MIPS AVR, and much faster if you use hardware spi.

To drive 64 leds, you need to shift at most two bytes and that should be no more than 0.2ms, and likely much lower than that if you are running your chip at 16MIPS.

As fungus has pointed out, your hardware doesn't make a whole lot of sense to me and you may want to rethink how you approach the whole thing.



its 64x64=4096 leds

dhenry

Quote
its 64x64=4096 leds


To multiplex that many LEDs, you have to either run them at incredibly high levels of current, or they are very dim.

Think about a different approach, like a static driver (4096 / 8 = 500 HC595), or multiplex sections of it.

fungus



What are the capacitors for? It looks to me like they might be keeping the LEDs lit up for a short time after your MOSFET switches off.

for exactly that


I still think they're the problem, not the MOSFETs. The cathodes all seem to be joined together when they arrive at a MOSFET, Is it possible that charge from the capacitor is leaking into other LEDs via. that junction? If so you need to put more diodes in there to prevent it.

If you want smaller capacitances in your MOSFETS you probably have to use smaller ones. T220 MOSFETS have a big chunk of silicon inside them so capacitance is always an issue. You can try something like a VN1206L which comes in a T0226.

An oscilloscope would solve this mystery far, far faster than debating in forums...
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

VFXCode


I still think they're the problem, not the MOSFETs. The cathodes all seem to be joined together when they arrive at a MOSFET, Is it possible that charge from the capacitor is leaking into other LEDs via. that junction? If so you need to put more diodes in there to prevent it.

If you want smaller capacitances in your MOSFETS you probably have to use smaller ones. T220 MOSFETS have a big chunk of silicon inside them so capacitance is always an issue. You can try something like a VN1206L which comes in a T0226.

An oscilloscope would solve this mystery far, far faster than debating in forums...



You are correct. I need a diode in the cothode as well to completly isolate each led. This seems to solve the problem.

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