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Topic: Need help to control a high power LEDs matrix (Read 5493 times) previous topic - next topic


since the MOSFET input resistence is close to infinte, so no current flows into the gate (just a small leakeage one).

Yes but the gate of a FET looks like a capacitor, so when you first put a voltage on it that capacitor has to charge up. An uncharged capacitor looks like a short circuit so potentially an infinite amount of current can flow in the short time until that capacitor is charged up. Once it is charged up then no further current flows because, as you say, it has a high input impedance.


When both of the switches are at the bottom, then both should be N-channel. The point of a P-channel is that you can put it at top of the load, because you turn them on by pulling the gate negative. The draw-back of P-channels is that you have to get the gate all the way up to the drain voltage to turn them off -- not possible with 5V logic controlling a 12V load. The work-around for that is to use a pull-up resistor (1/2W, 100 ohm, say) to pull the gate high, and instead use a cheaper signal-level N-channel to pull the gate low when turning it on. The problem with that is that you are trading off power loss in the pull-up for fast enough turn-on time in the P-channel.

Now, why do I keep talking about turn-on time? It's because the MOSFET gate works like a capacitor. When it is fully charged, it lets current flow through gate and source. When it is depleted, it stops the current flow. When it is in an intermediate state, some current flows, but there's also a significant resistive loss. MOSFETs can have very low on resistance, and very high off resistance, so the loss in "on" or "off" states is very small, and little to no heat sinking is needed. However, in the "in-between" state, the resistive heating is significant, and if you stay in that state for any significant fraction of the time, they will heat up past their tolerance level, and burn out. Hence, why single on/off switches are safer, and PWM is safer with lower switching frequency. Also, why you want enough current into the gate of the device to build up (or deplete) that charge quickly.

What do the pros do? They use special MOSFET driver circuits that can deliver very large current "spikes" for short periods of time; enough to quickly push the device on/off, but then just deliver enough to compensate for the internal losses, which are very small. Additionally, these drivers often are able to "totem pole" the output voltage above that of the input voltage, so you can use a N-channel device (typically cheaper, and lower Rdson, and thus better and cooler) even as a high-end switch. There will be a dedicated "boost" capacitor attached to the driver, where it will build up charge that it can then dump into the gate of the switch it's driving. I haven't found any of these in a DIP package, though -- all the cool kids do surface mount these days. I'm going to have to get into toaster oven re-flow soldering soon :-(


Ok, now I got your point (; So the FET gate acts like a kind of low-pass filter more or less. Sorry if I ask basics concepts of the electronics but I didn't study the FETs at high school so now I feel a bit lost when I read very technical replyes. Anyway, everything's clear now... About the project, I thought about using some drivers which could help me a lot: CAT4101. I've also looked for some high speed MOSFETs and found this which could be useful. They look fast enough, since the propagation delay is ~30ns if 5V are applied to the input (anyway, as jwatte said, I don't think I'll have a lot of problems with the propagation times in any case since the frequency isn't that high and I can adjust in order to make it as fast as it needs to be). I'll buy some components as soon as I can and try these wirings, thank you all for your inputs! I will write something to make you know how the stuff goes on and to ak something if needed!   XD


It sounds like you pretty much got it now :-)

However, the "propagation delay" is not the right metric to look at. What you want is "gate charge" -- a higher gate charge needs more current, or the same current for a longer time, to reach saturation. The propagation delay means something slightly different, and is more important to worry about if you're doing very-high-frequency things like hundreds-of-megahertz clocks, or radio (RF) design, or physical interfaces for very-high-bitrate (gigabits, or at least dozens of megabits) serial busses etc.

CAT4101 seems like a decent enough device that combines voltage and current regulation with switching. One of those (plus the set resistor) per column, and a simple power MOSFET per row, would probably work fine. Note that you get "PWM" anyway because you have a 1/8 duty cycle if cycling the rows and driving the columns. If you want to drive all the LEDs, all the time, you need 64 of those drivers and resistors, and some way to get 64 separate outputs to each of the drivers. Perhaps a big enough shift register would do it :-)

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