Power Transistor / Mosfet Suggestion?

[exelanoz]

Hi all.

I am using a PIC16F767 to control some 12V light strips of LEDS. It will use the HPWM at the lowest available frequency at 4Mhz which is around 250Hz to control light dimming

The FULL 100% Duty Cycle load will be close to 15amps at 12V DC....

Can anyone suggest or guide me in the right direction for the right type of switching device?
I have quite a few Motorola TIP Series Power transistors around (TIP120,121,122)
I linked up only 1/4 of the lights so load would have been around 4amps. But I have successfully blown them up by overheating them, even on a massive fan cooled heatsink.

The TIP 121 datasheet says that it is a low speed medium power general purpose amplifier/switching transistor. Is 250Hz considered low speed for this type of transistor?

What are the forum members using to PWM high amperage loads?

Obviously I am doing something incorrectly or wrong part choice...

[rmteo]

BJT's (particularly Darlingtons like the TIP120/121/122) are not the ideal choice for switching applications such as yours.

Looking at the datasheet, the CE saturation voltage of a TIP12x is about 3V at a collector current of 4A. So 1/4 of your lights will give 12W (3x4) dissipation in the TIP. Max collector current of the TIP is just 5A. You won't be able to get the 15A you need out of them.

Using a MOSFET with a RDS on of 0.010 Ohm (such as an FDD10AN06A0) the power dissipated will 0.16W (4x4*0.1) at 4A current. So the TIP (12/0.16) 75 times power as the MOSFET - big difference. Also, the FDD has a max. current rating of 50A.

You can switch the FDD at several hundred KHz.

[skimask]

Can anyone suggest or guide me in the right direction for the right type of switching device?

I'm using IRF9Z24N P-channel MOSFETs to drive LEDs at 3Khz PWM. I'm driving the P-channel gate with a 2N7000 N- type under PIC control because that seems to work the best. As a test awhile back, I took 6 of these, paralleled them (because I've got 3 on each board, RGB setup, see www.srt.com/~jdgrotte/kromatoobz.htm), and used them as a starter motor relay. The traces on the PCB let loose before the MOSFETs did...but I was just screwing around, trying to blow something up!
I'm sure if you could find the N-channel equivalent, you'd be in good shape.

I could go on, but I'd just be repeating what rmteo said...
Low Rds(on) at a low Vgs with a high Id is what you want.
In general...
The lower the Rds, the less heat the package dissipates at a given current across the drain-source.
The lower the Vgs, the lower the voltage you can put on the gate and still have the drain-source conduct a decent amount of current.
The higher the Id, the more it will conduct at a given Vgs.

Read up on your MOSFETs in wikipedia or something and you'll easily see why MOSFETs are almost always better than standard ol' transistors in this sort of application.

[sayzer]

One small note from me:

Whatever the choice is, mine IRFZ44 and IRFZ46 most of the time, make sure you do not use resistor between PWM pin and the gate (or base) of the power mosfet or transitor.

Directly connect the pin to the gate (or base).

One other thing to do: if using mosfet, like IRFZ44, have a 10K or 15K res from gate to gnd.


Just test this, load it up and post the result!


------------

[mister_e]

RFP40N10, RFP50N06, IRF3205 are way overkill... but are the first that spring to mind.

For HIgh Current drive i would use a Mosfet Driver in between... maybe a opto-coupler as well.

[exelanoz]

So after listening (or is it reading?) the suggestions offered by the helpful members here, I have settled to use the IRF3205 mosfet. Seems to work well, as sayzer mentioned, you must use a resistor in the gate to pull it down, otherwise it is a bit eratic on the scope. In driving the LED strips, i have changed the strips to 5V type instead of the 12V.
I tested it with a small strip, seems ok.

The TOTAL powersupply requirement I need is whopping 18Amps @ 5VDC.
I am ready to connect ALL the strips and before I blow anything up (again), I have a question.

I have 2 switchmode powersupplies, <a href="http://jaycar.com.au/productView.asp?ID=MP3123&CATID=27&keywords=&SPECI AL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pag eNumber=&priceMin=&priceMax=&SUBCATID=647">Powersu pply</a>

On the casing they are rated 10Amps for the 5V. Can also do 24V @4.5amps (but not needed)
Is it possible (and safe) to connect the outputs in parallel to achieve a 20Amp capacity?
My other idea was to use a computer PSU where normally the +5V DC rail can deliver up to 30Amps.
Not quite sure how else I can achieve a high output PSU.

[BrianT]

Your mileage may vary but I have come to grief several times when trying to switch high currents in LED star cloths (1000+ white LEDs for a theatre backdrop), heaters, and high brightness LED strings. Screw terminals get hot if not perfectly terminated, board currents give ground bounce, a single short takes out the entire display, etc, etc.

What I found is much more fault tolerant is to divide the load into several parallel strings each about 2 -3 amps. The Philips (NXP) PMV31XN will switch a whopping 5.9 amps in a tiny SOT-23 package so several handling smaller currrents means you don't end up with hot spots in connectors handling 15 amps. The inputs can be driven in parallel with a small series R to each gate and the source pins can be paralled into a very large all copper fill area on the PCB. Each output goes via its own fuse and screw terminal block. Multiple strings can be individually fused so most of the show goes on even if some clown has sliced one of your display lines.

HTH
Brian

[exelanoz]

Hello Brian,

Many thanks for your suggestion, I now realise that I should have split the load.
This won't be for any commercial setup. It is going on my verendah outside.
Sort of a party light RGB led type effect.

So if any clown is going to slice the wire, it will be me(and probably will)....and if the verendah burns down......., - well... that will also be me and if that is the case, I will be running for the hills so that my other half doesn't kill me. (She can be threatening at times)

At this stage I have already etched the PCBs and the best that I can do is split the 18A load in 2 parts which would suit the PSU I have (9Amps to each).

I am using screw terminals, (18Amps spread over 30 screw terminals) all decent size and I made the board with heavy duty tracks.
And your fuse is not a bad idea either. I think I can include that on all screw terminals.
Alu box is also fan assisted (120mm) cooling with lots of heatsinking.

I guess my primary concern was whether it is an acceptable practice to parallel independant power supplies to get more amperage.

Many thanks. Hopefully I can post some success pictures of the project, - and not the verendah burning.

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