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MOSFET parasitic capacitance and charge

Bjtpower_magic

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Hi,

I have started working on Mosfet analysis, i have to replace IRFZ44VPbF (https://www.infineon.com/dgdl/irfz44vpbf.pdf?fileId=5546d462533600a40153563b51fa2214) with alternate mosfet which is IRF60B217 (https://www.infineon.com/dgdl/Infin...N.pdf?fileId=5546d46256fb43b301576e3c7f77665e)

i have done pre analysis of both the Mosfet and have found below parameters.
Column 2 is for IRFZ44VPbF and Column 3 for IRF60B217
Typical Gate to Drain Charge (nC)2514
Typical Input Capacitance @ Vds (pF)1812@25V2230@25V
Typical Turn-On Delay Time (ns)138.3
Typical Turn-Off Delay Time (ns)4024
Typical Fall Time (ns)5720
Typical Rise Time (ns)9737

I tried to read Mosfet capacitance effect and gate to drain charge but i did not find any relation well explained and confused after reading lot of datasheet.

Can you please help relationship with other important parameters, some equations?
I have Flyback converter (Already designed by ex colleague, who left organization :( )
Input: 20-32VDC
Ouput: 5V at 8A
Switching Frequency: 100 khz
Gate driver : UCC3809 https://www.ti.com/lit/ds/symlink/u...ttps%3A%2F%2Fwww.ti.com%2Fproduct%2FUCC3809-2
 
Your questions (?) are not very well put - what exactly are you attempting to ask about ?
Hi,
I would like to replace Mosfet, i have selected close one but still there are few important parameters are different.
I would like to Understand/Learn the relation of other parameters like gate charge, Capacitance? How it will impact on design? Any co relation of this parameters?
 
Hi,
I have read some documents and understood that UCC3809 only source 0.4A and Sink 0.8A.
1689357858828.png


FOR IRFZ44VPbF
From Datasheet:
Total Gate Charge: 67nC
Total Rise Time: 97nS


I_Peak=dQ/dT=67nC/97nS=0.69A

FOR IRF60B217

From Datasheet:
Total Gate Charge: 66nC
Total Rise Time: 37nS

I_Peak=dQ/dT=66nC/37nS=1.78A
now if i select IRF60B217 then i need to update Gate Driver as well.
is my understanding correct?
 
One thing to look closer at, is how those "drive currents" are defined
(test conditions). Sometimes you will see "peak current" meaning the
output is at the rail it's leaving. Sometimes, Iout@Vdrop. These two
will differ by maybe a factor of 5-10 as headroom is removed as a
factor in the "peak current" test.

And two FETs from two mfrs might well disagree on basis / conditions.
 
You need compare rds(on).
Also, can it handle peak drain voltage.
Do i=Cdv/dt to get rise time of gate.
 
a 22 ohm gate resistor will limit the peak gate currents, limit the di/dt at turn on, and the dv/dt at turn off - reducing RFI noise from your circuit - try it ...!
 
You need compare rds(on).
Also, can it handle peak drain voltage.
Do i=Cdv/dt to get rise time of gate.
What of we have Tr which is allready provided in Datasheet.
T=C*(dV/dI)
C=Capacitance (CGS+CGD)
dV= ?
dI=?
--- Updated ---

a 22 ohm gate resistor will limit the peak gate currents, limit the di/dt at turn on, and the dv/dt at turn off - reducing RFI noise from your circuit - try it ...!
I did not get it, UCC3809 Recommended to use 3.9 ohm resistor.
--- Updated ---

Can someone explain.. Gate drive calculations are correct?

I have impression that, I=Total Charge/Rise Time,
1) If Rise time is equal to great, would be good for lower drive current assuming charge is constant.
2) Total charge shall be less, assuming Rise time constant.

So its trade off between Gate charge and Rise time to use same Mosfet drive.

Please add your comments.
 
Last edited:
There is not a magic resistor value. You have received a couple of suggestions.
Choosing is (part of) what engineers do.

If you can find credible models for your FET candidates and key passives /
magnetics then I would recommend ditching calculation, make a schematic
with your driver represented reasonably and then you can play with the
gate drive network and gate driver attributes to figure out what you want.
 
There's only so much you can get by reading datasheets and app notes, OP. IMO the IRF60B217 looks like a fine candidate. It's specs overall are better than the IRFZ44VPbF (only exception I noticed was the Rjc, which is a little higher).

The faster switching speed and lower capacitances of the IRF60B217 might actually be problematic though, in particular regarding EMC. Also you should keep an eye on the peak Vds. For a 30V in flyback, I personally would go for a FET rated for 80V or more (though with proper snubber design it's not necessary). A faster FET will likely produce a higher peak Vds though.
 
There's only so much you can get by reading datasheets and app notes, OP. IMO the IRF60B217 looks like a fine candidate. It's specs overall are better than the IRFZ44VPbF (only exception I noticed was the Rjc, which is a little higher).

The faster switching speed and lower capacitances of the IRF60B217 might actually be problematic though, in particular regarding EMC. Also you should keep an eye on the peak Vds. For a 30V in flyback, I personally would go for a FET rated for 80V or more (though with proper snubber design it's not necessary). A faster FET will likely produce a higher peak Vds though.
Hi,
Thanks for your reply.
But if you look into Mosfet Driver UCC3809, capable of provide 0.4A source current and i feel it cant drive IRF60B217.

FOR IRFZ44VPbF
From Datasheet:
Total Gate Charge: 67nC
Total Rise Time: 97nS


I_Peak=dQ/dT=67nC/97nS=0.69A

FOR IRF60B217

From Datasheet:
Total Gate Charge: 66nC
Total Rise Time: 37nS

I_Peak=dQ/dT=66nC/37nS=1.78A

What is your opinion?
 
A few points:
1. The rise time parameter tr refers to the measured transition time of the drain voltage with some arbitrary gate voltage and resistance, and drain resistance. It is not the gate rise time, but it is related to it.
2. The source/sink current of the driver is, like the tr of the FET, characterized under specific arbitrary conditions (though in the case of the UCC3809 it doesn't seem to explain this). Those numbers generally don't indicate the max current the driver is capable or providing, or a limit which you should not exceed.
3. The gate voltage/current waveform is somewhat complex, and determined by a number of factors, including the driver circuit. If you use a weaker drive circuit, then you will get slower transitions on the gate and drain.

I strongly recommend taking a step back and getting familiar with what these different parameters mean. Just because the datasheet says the tr=97ns, does not mean that it will actually switch in 97ns (could be more or less). It also does not mean you have to drive it hard enough to actually switch in 97ns.

These parameters are still useful though because usually different vendors and devices will be characterized with similar arbitrary setups. They're useful for comparing devices against each other, but not very useful for designing your actual circuit.
 
I strongly recommend taking a step back and getting familiar with what these different parameters mean. Just because the datasheet says the tr=97ns, does not mean that it will actually switch in 97ns (could be more or less). It also does not mean you have to drive it hard enough to actually switch in 97ns.

Can you please help with exact calculation required? or can you please guide where i can get these hidden parameters?
 
I am out of by league here but Vishay has a good ap note on measured versus calculated,
and I would be of the inclination to also do a sim, and a test on bench, all in addition to confirm.
If reliability and reproducability are of concern. Careful on the sim, look at the spice model
for the part you want to use. That it has key parameters. Also make sure time step is in nS
area not uS.

Vishay note AN608A key one.

Supporting ones in zip.

As already stated here by supporters load and actual circuit very importasnt, and Vishay
Ap Note discusses that.

Doesnt LTC have a series of sim setups for DC/DC you could piggyback off ? I am sure
there must be other simulators that do.



Regards, Dana.
 

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Last edited:
What "hidden parameters" are you referring to?
For Gate drive calculations for Particular mosfet.

I see that,
IRF60B217 is good candidate over IRFZ44VPbF
RDS ON is better, Gate charge is lower
but i have concern on Rise time.

I want to keep UCC3809 which has source current of 0.4A

Can you please help me with Gate drive current required for IRF60B217 and IRFZ44VPbf?
 
For Gate drive calculations for Particular mosfet.
You can't derive the gate drive current from the MOSFET parameters alone. It's a function also of how fast you want/need the MOSFET to switch in your application.

The most basic relation between switching speed (of the drain voltage) and gate current is the gate-to-drain charge Qgd:
Qgd/tr=Ig for turn off
Qgd/tf=Ig for turn on

Keep in mind here that the tr/tf in these equations is not from the datasheet, but from your actual circuit. Do you know what sort of tr/tf are typical of the IRFZ44VPbF in your circuit?
I see that,
IRF60B217 is good candidate over IRFZ44VPbF
RDS ON is better, Gate charge is lower
but i have concern on Rise time.
If the UCC3809 was sufficient for driving the IRFZ44VPbF in your application, then it should also be sufficient for driving the IRF60B217 at the same speed or faster. However it may not switch as fast as the tr/tf values given in the datasheet. And that may be desirable if you want to minimize change in the overall design and performance. That's one reason why engineers often add additional gate resistance.
 

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