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MOSFET output current limited by Inductor Saturation current? 
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#1
Apr1411, 06:59 PM

P: 12

So the MOSFET's output current in a simple buck converter with a PWm a driver and a MOSFET and LC circuit. The Inductor's saturation or Irms current limits the MOSFET's current output, correct? I heard that if it exceeds saturation current that the MOSFETs could be damaged.
is this true? Thanks 


#2
Apr1411, 08:51 PM

Sci Advisor
P: 2,751

Yes it is true. Once the inductor saturates the mosfet current will increase very rapidly which could damage it.



#3
Apr1411, 09:00 PM

P: 12

it will damage the MOSFET, correct?
Will there be any damage to the inductor or the bulk output capacitors? 


#4
Apr1511, 12:55 AM

Mentor
P: 41,258

MOSFET output current limited by Inductor Saturation current?
How do you model an real inductor; with what circuit components? (What series components? We don't need to consider parallel winding capacitance for your present question) And given that model, how does the model's impedance change when the current exceeds the saturation current? And is it usually a sharp knee or a soft knee? And why would the MOSFET or Bipolar highside switch be damaged? What sizing considerations go into each component in a DCDC converter's design? And what kind of current limiting schemes should you consider in your design? Nothing should get damaged if the output of your DCDC gets shorted out... 


#5
Apr1511, 10:03 AM

P: 12

Each phase, or driver with one pari of highside and pair of lowside FET outputs to an inductor. The MOSFETs can output 40A, has Rdson of 3.4mOhm and output capacitance of 7.7nF. Come in a canPAK package. THere are 4 MOSFETs and one driver per phase along with a single inductor for both pairs of MOSFETs, i know its an odd design. Totally overkill considering the current rating of the Inductor. but the inductor has a saturation current of 28A and Irms of 28A as well. Metal alloy. DRC is 0.0012ohm typical and 0.0014 with 0.68uH inductance. the PWM probably has current limiting but I have no idea what it is set too. I just want to know if there is was way that the inductor limits the current output, in this case would a good engineer design the circuit so that current is limited at 28A. I want to know, or better understand how it is going to work if lets say there is NO current limiting and the MOSFETs output all together 3040A through the inductor. 


#6
Apr1511, 12:41 PM

Mentor
P: 41,258

Wow, monster stuff!
That's a tiny resistance for the inductor. You would calculate the resistive heating for the wire in the inductor, to figure out if the inductor would actually fry with that current. 


#7
Apr1511, 12:59 PM

P: 12

Wouldn't the MOSFET fry first? or what? which equation?



#8
Apr1511, 01:04 PM

Mentor
P: 41,258

And where is all this current going? If the inductor saturates, it's not likely that the DCDC can stay in regulation. Other things may fry too... 


#9
Apr1511, 03:10 PM

P: 12

The current will go to the bulk output capacitor bank with something like 5000uF9000uF, then to high frequency ceramic caps and then to the CPU(processor).
Some of the Intel processors, all of the i7 900 series have an 100A load step from idle to load. We overclock which means we take 2.8ghz processors to 4 or 5 or 6 or 7ghz depending on the cooling. The VRM (voltage regulator module) in question have 16 pairs of these caps, but only 8 of these inductors, which is weird, usually pair of high and low side MOSFETs have their own inductor, but this VRM has a weakness, and I want people to be informed. In our community we had very high end $600 dollar GPUs very new ones that would have their 35A DriverMOSFETs or 30A low Rdson MOSFETs blow up because of their inductors, that was the theory of why. Our processors have a 130watt TDP, which is their thermal spec, so they pull about 105watts and then loss a bunch to efficiency. but the GPUs have 200300 watt TDPs. Motherboards now have very sophisticated SMPS. Some have 24 phase designs, some 8 some even as little at 6, and everywhere in between. The top end GPUs I have seen one with 12 phases and the ones blowing up have 5. 


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