P-Channel FETS vs N-Channel FETS for LED Matrix usage ??

T&E Engineer

I have never really used FETs before but finding out slowly how to use them. At least P-Ch FETs to allow more current for some LED Matrix modules.

I currently understand that P Channel FETs allow a positive voltage (e.g. 5vdc or 0VDC) into it's Gate (G) from an SX-28·output. The Source is connected to 5VDC and the Drain is connected to a current limiting resistor and then to an LED Anode or series of LED Anodes. If I send 0VDC (GND) from the·SX-28 output to the Gate of the P-CH FET then the LED turns on. Likewise if I send 5VDC from the·SX-28 output to the Gate of the P-CH FET, then the LED or LEDs turn off. This works fine.

However, when using an N-CH FET, I would assume just the opposite is true but it is not because I am missing something due to never using them before.

Is it not possible to control the LED Cathode using an N-CH FET in the same or similar fashion?

The reason I ask is that I currently·am doing this with·an SX-28 microcontroller. On the 8 RB outputs I have it going to 8 P-CH FET Gates and then it's drains going to 8 LED Anodes of an 8x8 LED Matrix module. The 8 sources are all tied to 5VDC. The Cathodes I currently have running back to the 8 outputs on RC.

I wrote a program (modified from JonnyMac's LIFE program) that the P-FET Anode rows are scanning and the data is being pushed out into RC (Cathode Columns).· All is working well with this setup.

If I swap RB with RC in the code, the P-FETs anodes are no longer scanning the rows so that data is moving across the columns AND the message scrolling the other way is not as bright because it is not using the P-FETs effectively. I would like to scroll data from top to bottom as well as from side to side. This tells me that I will need some FET's of some sort between RC and the Columns.
I thought I could simply use N-FETs like I did P-FETs but I get nothing. However, after reading a little bit more I think·N-FETs are only used in combination with P-FETs for using higher voltages like 12V (TTL or something greater than 5V).

Can anyone confirm this. ?

I suppose I can use UDN2803's between RC and the cathode columns but I would like the option of having full current draw like I have with the P-FETs.
Is this possible or how can it be done?

Thanks for your help.

PS: See attached SX-28 working code··

Post Edited (T&E Engineer) : 9/8/2007 11:20:54 PM GMT

Phil Pilgrim (PhiPi)

There's no reason you can't use N-CH FETS for the cathodes. Just ground the sources and connect the drains, through current-limiting resistors, to the cathodes. But remember that your output polarity is reversed now: to pull a cathode down, you have to output a "high" to the gate of its associated FET.

-Phil

T&E Engineer

Phil,

Thanks so much! I beleive that did it for me. I did a test and had to change some 0's into 1's but beleive it is working as I would·expect.·Also I would have never have thought to connect·the Source to·GND but it makes sense all the same.

You saved me alot of time and guess work. Problem now is that·all my·FETS are all in SO-8 size (surface mount) so I have to make·more mini PCB's (hand drawn) and soak them in Etchant solution. What a pain. Aren't there any higher current FETs in standard sizes (non surface mount)?

BTW: I·will be using (8) 6A P-FETs and (8) 4A N-FETs on my 8x8 LED Matrix for full brightness. This must be what they·use on the big·outdoor signs as I can't imagine them using P-FETs (high current) and UDN2803's (only 500mA) outputs. Anyway, when finished I should be able to scroll them up/down/left and right possibly using both P-FETs and N-FETs.

Does this sound like the right method to do this? I think it does but I'm new to FETs.

Thanks again!

T&E Engineer

Phil or anyone,

From what I can tell from the Fairchild Semiconductor website, all they have as far as N-FETs and P-FETs are all surface mount types either being single or combination types (2 max per package). Does anything exist in a DIP format or larger package than just 2 (like 8 or 16 would be nice)? I am working with anything between 3-6 A or more would be great. I haven't found anything like this yet.

Phil Pilgrim (PhiPi)

I'm a bit confused about your current requirements. On the one hand, you've been able to drive the cathodes directly from an SX. On the other hand you claim to need between three and six amps of drive. Is that a real requirement? What kind of LEDs are they?

-Phil

T&E Engineer

Phil,

I am using 8 P-FETs (6A) to drive the rows (Anodes) of (5) 8x8 Bi-Colored LED Matrix modules.

If I only use (10) ULN2803's (or Allegro Microsystems A6821's - Serial version of ULN2803's basically)·across the 40 Bi-Colored·= 80 total Cathode Collumns then I can only scroll (or scan) the display left·or right. However, if I add N-FETs to replace the ULN2803's or Allegra A6821, then I can scroll (or scan)·up·or down with full current brightness on the LEDs. Currently if I don't use the N-FET's on the collumns and change RB with RC in the code (attached) then many of the LEDs look have lit so I am assuming it is because it is looking to draw more current from RC (which does not have any FETs for the moment). Keep in mind that this SX program is for a single 8x8 LED module.

I have calculated the larger scale current to be about 3.2A but have some FDS6825 (6A) P-FETs (see attachment).

The calculations were done assuming I was using 5mA LEDs. However, I found out later they were 20mA so the 6A P-FETs should be a good start with some higher current N-FETs to suppliment.

Am I on track here or am I missing something?


Post Edited (T&E Engineer) : 9/10/2007 3:32:15 PM GMT

Phil Pilgrim (PhiPi)

Getting eight or sixteen 3-6A drivers in one package is asking a lot. Nonetheless, I would check out Allegro and STMicro — maybe TI. If you want through-hole, some sort of tabbed SIP would be preferrable to a DIP, since you can heat-sink it. Pay particular attention to the "on" resistance at the gate voltage you'll be using. For your current requirements, it should be in the tens of milliohms range, or less, to avoid excess power dissipation. Also check the gate capacitance and make sure the circuitry that drives the gate is capable of switching that much capacitance at the multiplexing and/or PWM dimming rate you plan to use. Otherwise, the MOSFET will spend too much time in the linear region and overheat, and you will need to add some MOSFET drivers (probably from Micrel).

-Phil