True, Rdson has to be figured at the operating temperature. Hopefully that isn't 160*C. Figure 4 is a little confusing; it's 'normalized'. At 160*C Rdson is (2.6 * 44mΩ) or 114mΩ. That's the figure to use if you actually allow 160*C.

I agree the cheapest way to fx this with these old transistors is to put a few in parallel. Every transistor decreases the Rdson and at the same time increases the number of devices which share the dissipated power.

Three in parallel gives Rdson of (44mΩ / 3) at 25*C, or if you get it to 160*C it's 38mΩ total. The power shared by the 3 transistors is 9.7 watts, 3.2 each. Too much to run bare, but with forced air it might go.

If you were running PWM, the extra gate capacitance would kill the switching speed, but that's not a problem here..

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de KI6RWX

 

Hi Andrew,

here is a suggestion with two parallel IRF540 (not the best choice).

It uses an outdated Pentium IV heatsink with fan. If necessary you might place three transistors on that heatsink.

3 resistors are added: R19 is a gate resistor for the second transistor and R20 and R21 are zero Ohm resistors instead of wire jumps.

Boncuk

Attached Thumbnails

 

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Proper Planning Prevents Piss Poor Performance

 

Hi guys,

this is all great info thanks.

Boncuk,

a) you said this is not a good choice, I assume you mean the choice of Mosfets rather than the fact that they are paralleled? I happen to have a P4 heatsink and fan combo waiting for a job. Any suggestions for the Mosfets?

b) I don't mind whacking 4 Mosfets in, there is lots of space where this is going. If I used 4 would probably take them offboard and arrange around the heatsink in a North, E, W and South arrangement with a ringmain connection

c) what drawing program are you using?

d) on the gate resistors, based on your comment one obviously needs one per Mosfet? I have been trying to teach myself electronics with LTSpice, playing around with values etc. The one 390R resistor feeding 4 Mosfets gives the same output trace, what am I missing here?

e) Would the Mosfets in the Spice model be better?

f) What are the connections for the P4 fan, G, B, Y


Cheers
Andrew

Attached Thumbnails

 

 

Deleted posted in the wrong place.

Cheers
Andrew

 

Hi,

Okay I arbitarily selected IRFP240 Mosfets rated at Id = 20A. Applying the formulae from the spreadsheet was interesting. I calculated the "free air" parameters i.e. what the limit of the devices would be without cooling.

Max Operating Temp 25 to 125°C
Ambient 40°C (yes this is real)
Rds ON = .0.14Ω
Rθ(ja) = 30°C/W

Maximum allowable power dissipated from the device
P = ((125-40)/30)
P = 2.8W

Maximum switching current
P = I²R
√(2.8/.14) = I
I = 4.5A

So 4 of these devices in parallel should be OK. Would anyone care to check if I am correct please.

Cheers
Andrew

 

Hi Andrew,

I didn't say one gate resistor is necessary for each transistor, but the resistor is a good method to get across the annoying traces between the pot and the gate pin.

On the other hand individual gate resistors won't hurt.

Here is a transistor I would recommend for high current. Using two of them is already an overkill for your application. A single transistor can dissipate 306W.

The transistor is PSMN1R6-30PL (NXP-Semiconductors). Check out the attached datasheet.

The colours at the connector outside the board indicate: black=ground, green=+12V, yellow=not used for the fan.

I use that heatsink and fan with an SLA-battery charger and the fan is on whenever the charger is on.

The software I'm using is supposedly the same as you are using. I use Eagle (version 3.55)

Regards

Boncuk

Attached Files

PSMN1R6-30PL.pdf (225.3 KB, 10 views)

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Proper Planning Prevents Piss Poor Performance

 

Hi Boncuk,

thanks a lot. I will check if these are available locally.

I am slowly getting a hang of this. It would appear as if the important criteria of any Mosfet is the RdsON, the lower that is the more current can pass for the same given heat generated?

I have also noticed that 2 to 2.5W appears to be the limit for a T220 package in air with no cooling.

Would I be correct in stating the two of these device could be used with no heatsink? Just trying to confirm the answers to the maths I am learning to calc these numbers.

Thanks for the help
Andrew

"The software I'm using is supposedly the same as you are using. I use Eagle (version 3.55)"
Did not recognize the fancy resistor shape thats what confused me, it looks nice.

 

According to my calculation a PSMN1R6-30PL has to dissipate 410mW at the minimum RDS(ON), V(GS)=10V of 17mΩ with a load of 25W.

So you might get away without heatsink. I like electronics "cool" and always use overdimensioned heatsinks for my applications, never to exceed +35deg/C at 25deg/C ambient temperature. I use to fix fried eggs using a pan.

Have you ever seen a resistor looking like a rectangle from the top view? I haven't till now. That's why I modified the package to look like the "real thing".

Regards

Boncuk

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Proper Planning Prevents Piss Poor Performance