70t03h replacment

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vectraboyv6

Member
hi

I'm am copying a simple surface mount circuit which has 3x 70t03h 's on it. could anyone recommend a similar device that is non surface mount?

I believe these are Mosfets. I am requiring a 5v feed in 12v output of about 4 - 5 amps.

many thanks stu
 
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inglewoodpete

Senior Member
Yours are probably SSM70T03H from Silicon Standard Corp.

You may not need a specific MOSFET. If the function is for switching, there are probably a range of substitutes.

Edit: Was sort of switching are you doing? PWM?
What sort of load are you switching? Resistive? Reactive?
What are you driving the MOSFET with? PICAXE? MOSFET Driver?
 
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vectraboyv6

Member
hi

its PWM for RGB led strings there commond (+) so will switching the (-) side and will be powered by the picaxe directly via a resistor (10K???)

ta
 
premelec

premelec

Senior Member
That's great - I didn't see any brand marks in picture - too bad no ship to USA... 1-2v Vgth is good to work with.
 
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Goeytex

Senior Member
I don't particularly like the Fairchild RFP12N10L. It has a relatively high on resistance (200 milliohms). This means it will not be very efficient compared to many others.

For most logic level applications where it must be through hole, I generally use an STP36NF06L. It has much lower on resistance (35 milliohms) and comparable gate threshold specs at 1V - 2.5V. Currently in stock and available from Farnell.
 
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nekomatic

Member
Switching 5 amps with a RFP12N10L you're going to lose a volt across the on resistance - can you live with that? Consequently each one will need to dissipate 5 watts, so it'll need a heatsink, and don't forget the metal tab is connected to the drain terminal so it'll need to be isolated from the heatsink.

From a quick search on Farnell it looks like the IRLIZ44NPBF would do the job, which has the advantage of an insulated tab package, but I haven't used this one myself so there may be something I've overlooked. What's the PWM frequency going to be?
 
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vectraboyv6

Member
I wanted a 0 to 255 scales on each RGB lines to correspond with a computer scale... therefore im running a 14m2 at 16mhz and PWM at 64000hz (pwmout b.4, 62, b1 b1 ranges from 0 to 255)
 
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nekomatic

Member
I'm not expert in driving MOSFETS except as simple DC on/off switches so someone else is probably better placed to advise here, but one complete cycle of 64 kHz takes 15.6 microseconds and the gate capacitance of the MOSFETs under discussion here is on the order of 1000 pF so if you're driving them through a 10K resistor this will have a time constant of 10 microseconds, and it seems to me your MOSFET will never have time to completely turn on (or off) - it'll always be in partial conduction, getting very hot, and not switching your load as you want it to.

I'd have thought you'll need to reduce your PWM frequency and/or reduce the size of the resistor from PICAXE output to MOSFET gate - is there any reason it needs to be as high as 10K?
 
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inglewoodpete

Senior Member
I'd recommend you run the PWM at around 100Hz. That will keep the MOSFETs much cooler and more efficient. A 10K resistor is also way too high. If you are not using MOSFET driver chips, I suggest you try 470 ohms and see how hot everything gets.
 
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vectraboyv6

Member
hi

will have a play around to see if i can get it down to 100hz the circuit board im copying has a 10k to ground (i read the circuit wrong) and a 1k to a unknow pic chip. i presume the 10k is a pull down to help the mosfet switch corectly?

thanks
 
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Goeytex

Senior Member
The most efficient way to drive a MOSFET ( Even a Logic Level type) is via a dedicated MOSFET driver. Microchip has quite a few different models that will do the job quite well. However this advise is many times ignored, especially by those inexperienced with driving FETs. Microchip MOSFET Drivers

Why a driver? Because a Picaxe can only source about 20 ma of current. While MOSFETs are voltage controlled devices, it takes time for the current to overcome the gate capacitance and the gate voltage to rise. The faster the gate capacitance can be overcome, the less time the FET stays in the linear region and the less heat is generated. This is particularly true at higher switching speeds. If you must drive the FET directly from a Picaxe I/O pin, actual testing has shown that peak current is limited to ~20ma with a 270 ohm series resistor and a 5V supply. This is ok for switching at speeds below ~1000Hz but CERTAINLY NOT for 64Khz.

LEDs do not need to be driven at high frequencies. Typical PWM frequencies are from around 100Hz to 1Khz. With a Picaxe the frequency should be selected so that the duty cycle has good granularity. For example, with a Picaxe 20M2 operating at 16Mhz, a PWM Frequency of 100Hz is not possible. The lowest possible frequency is 245Hz.
A frequency of 250Hz will give a resolution of .1 percent duty. (1000 steps). This may be a good choice.
 
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vectraboyv6

Member
Goeytex said:
The most efficient way to drive a MOSFET ( Even a Logic Level type) is via a dedicated MOSFET driver. Microchip has quite a few different models that will do the job quite well. However this advise is many times ignored, especially by those inexperienced with driving FETs. Microchip MOSFET Drivers

Why a driver? Because a Picaxe can only source about 20 ma of current. While MOSFETs are voltage controlled devices, it takes time for the current to overcome the gate capacitance and the gate voltage to rise. The faster the gate capacitance can be overcome, the less time the FET stays in the linear region and the less heat is generated. This is particularly true at higher switching speeds. If you must drive the FET directly from a Picaxe I/O pin, actual testing has shown that peak current is limited to ~20ma with a 270 ohm series resistor and a 5V supply. This is ok for switching at speeds below ~1000Hz but CERTAINLY NOT for 64Khz.

LEDs do not need to be driven at high frequencies. Typical PWM frequencies are from around 100Hz to 1Khz. With a Picaxe the frequency should be selected so that the duty cycle has good granularity. For example, with a Picaxe 20M2 operating at 16Mhz, a PWM Frequency of 100Hz is not possible. The lowest possible frequency is 245Hz.
A frequency of 250Hz will give a resolution of .1 percent duty. (1000 steps). This may be a good choice.
Click to expand...
thanks for your reply

im using a 14m2 chip at the moment and having to run 16mhz to get the 9600 baud rate to comunicate back to my house automation server. the reason why im using a range of 0 - 255 is that its the same range as the computer and makes the colour displayed on screen the same as the led strip. also its easyer to program as html and css etc are not my strong point.

will have a look at the fet drivers as well

thanks stuart
 
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