works with 3205 for switching with high amper

[baby_1]

i want to ontrol a element that needs 100 amper 24volt
i want to use two of IRF3205 to control it.
could you tell me how can i do that?
with equation and datasheet determine the vgs and input current to switch faster?

i need use PWM control with micro controler(1Khz)

 

[IanP]

To switch a 100A load you need something like 10 of the 3205s (TO220 package), and then you face the issue of combined gate capacitance ..
I’d suggest that you go for a proper power MOSFET packaged in a, say, ISOTOP, something like the STE180NE10 ..

:wink:
IanP

 

[baby_1]

Thanks ianp
but could you tell me how did you know that i should use 10 mosfet instead of 1?(becuase according datasheet it has a mili ohm RDS and with 100 amper it has max 2 wat in switching mode)

 

[IanP]

TO220's leads are about 0.6mm x 1mm ..
Wire of that size would probably be rated for no more than about 5---10 amps ..
10x10 ..

:wink:
IanP

 

[baby_1]

it means that 3205 can withstand up to 110 amper but it depends on the pins physical?

 

[baby_1]

so its better think that this mosfet pins can withstand up to 100 amper , could you help me to create a sample circuit ?

 

[IanP]

Try the following test-circuit:
**broken link removed**

:wink:
IanP

 

[baby_1]

i know the pwm circuit because i want to genreate this wave with micro or 555 timer, i want to know the calculation of mosfet for designin? not somethnig wrong that is on some electronics book.

the GS resistor?
mosfet saturated with special current?
a simple driving a mosfet?
drive mosfet with a mosfet driver?

 

[jpanhalt]

I have been reluctant to respond, because my design for a high-current control using IRF3205's is very old and crude. I built it to control a 12V-winch that was made from a 6V Ford, long-shaft started motor (used in trucks, tractors, and old cars). Those motors were beasts, and at 12V draw well in excess of 100A, probably in excess of 200A when loaded. We loaded them almost to stall.

What you have been told is absolutely correct. An industrial mosfet will probably work better. But they cost a lot more. Second, read footnote #5 in the datasheet. It explains that the junction limitation for the 3205 may be 100 A, but the package limitation is only 75A. I think that is optimistic, for a continuous load. Heat is the issue. You have to keep the solder from melting.

Anyway, here are some pictures of what I did. I used the LT1158 to drive the top and bottom mosfets in a half bridge. You would need another for a full bridge. But since, the starter motor is not reversible, that was a non-issue for me. I paralleled 5 IRF3205's for each leg of the half-H controller. In the picture, you will see the typical gate resistors and a larger value resistor from the gates to ground. That was necessary as a "fix" so the gates would shut down. It may not be necessary in your application with the gate driver you chose. The conductors were made from 0.025 " copper plate and mosfets were soldered directly to it to aid in heat dissipation.

While I am sure this design will handle well over 100A for short periods, I am much less sure how it would work for continuous use at that current. At a minimum, you may need better heat sinks. Note, the connector shown on the schematic is only for control of the mosfets. Power to the source and drains was by fat cables.

John

 

[baby_1]

thanks john,but i want to know
how can we saturate a Mosfet in special current? with data sheet and calculation find Resistor for vgs and the amper and voltage of gate in special current?

 

[jpanhalt]

That mosfet requires approximately 10V to turn on completely. The Rds(on) is specified at a Vgs of 10V. Figure 3 in its datasheet shows drain current vs. Vgs. If that is not the answer you need, I am not sure what your question is about Vgs. Perhaps, try rephrasing it.

As for the gate resistor and gate drives, there is a wealth of information:

http://www.irf.com/technical-info/appnotes/an-937.pdf
http://www.irf.com/technical-info/appnotes/an-978.pdf
**broken link removed**
**broken link removed**
**broken link removed**

I have in my mind an additional application note from Fairchild, I believe, that studied gate resistors specifically. Unfortunately, I couldn't find it. As I recall, its conclusion was that about 2 to 3 ohms was all that was needed. That struck me as a bit lower than is shown in many designs. In any event, you need a gate resistor for two reasons, one is to control ringing, and second, to balance gate currents when using multiple mosfets in parallel. In other words, the individual resistors help keep the mosfets turning on at the same time. By analogy, think of how resistors are used to balance current in LED's. Circuit layout and construction contribute inductance and capacitance that contribute to ringing and cannot easily be calculated, at least by me. So, in the design I showed, the values were basically determined empirically. Note that 5, 15Ω resistor in parallel = 3Ω seen by the driver.

If you will draw your circuit and present it here, it would be easier to address your specific questions.

John

 

[baby_1]

john could you tell me why most of the time we should 10 voltage greater than source to gate? because it most of the datasheet we read that vgs=2 to 5 volt?

 

[jpanhalt]

Vgs of 10 V is extremely common for completely turning on the gate. Logic-level mosfets have lower voltages of 5 volts or less. With respect to the IRF3205, which is not logic level, you may be confusing the threshold gate voltage -- the voltage at which it just begins to conduct -- with the full turn-on voltage.

Operating in the threshold region at 100A will create a lot of heat from I^2R. That is why when used as switching devices, the gate drivers are designed to deliver several amperes for a short period, which minimizes the time between reaching threshold and fully conducting.

John

 

[baby_1]

how can we exaclty define the vgs?

 

[jpanhalt]

This Wikipedia link has a definition: MOSFET - Wikipedia, the free encyclopedia

Search on Vgs.

Here is the relevant section from the IR3205 datasheet. Note the Rds(on) is at 10V. The Vgs(th) is at 250 uA. TH = threshold.



John

 

[alexan_e]

I think you will find this tutorial very useful http://www.irf.com/technical-info/appnotes/mosfet.pdf
it describes nicely the most important parameters of a mosfet.

The gate threshold voltage is the minimum voltage you should use to turn on the mosfet but when you want the lowest possible Rds-on (mosfet on resistance between drain and source) so that the losses and heat on the mosfet are minimized you should apply a much higher Vgs.
The datasheet specifies a Vgs for the given Rds-on , for example in your case it says 8m ohm @10v , usually the manufacturer wants to show a low Rds-on so the Vgs he uses is a good level to drive the mosfet (10v or higher but below the specified max of 20v).

Alex

 

[baby_1]

Thanks jan & alex , now please wait me to read more about mosfet and then ask you the question,but Thanks anyway for about useful information

 

[baby_1]

hello again and thanks
i attach the IRFP250 datasheet
according data sheet it can withstand approxmately 33A
but in the data sheet we can see that it can withstand up to 80A,what is different?
and if we get it VGS=10 it goes to be burn,so how can we get it 10volt to saturate?
im sorry if my english is poor

 

[jpanhalt]

I think you should focus on the working values, not a test value that leads to destruction. 100A across 0.073Ω generates 730W of heat. The junction to case differential is 0.66°C/W = 482°C. That's hot. In contrast, 30A produces 66W @ 0.66°C/W = 43°C. That is much more realistic.

John

 

[baby_1]

and could you tell me about driving... if i get 10 volt to Gate what can we see?