Measuring MOSFET input capacitance?

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Am I calculating total input capacitance or gate capacitance or none of the above? and are my calculations even correct to begin with. The point of this is that I want to know what gate capacitance im dealing with to properly chose a gate resistor to give me the rise time I need, more or less.



So I have the following circuit using the old IRF540





schematic





simulate this circuit – Schematic created using CircuitLab



I am probing the gate directly and get the following readings:



enter image description here



I am loosing 500mV somewhere but anyways:



My voltage at 1 time constant is:
(This is where the dashed vertical line is)



1t



So measuring the time from 0V (solid vertical line) to the 1 time constant line (dashed) you can see is about 530nS. So my time constant is 530nS and the gate resistor is 100ohms then:



enter image description here



Is my logic correct that this is my input capacitance? And given will this vary if I have same Vds but a Vgs that is NOT equal to Vds?



Extra points: Where are my 500mV ?






mosfet capacitance charge







share|improve this question





















  • I have gone back to the bench and calculated a time constant of 100nS given this capacitance and found that I need a resistor of 18ohms or so, this will get me 7.2V in 100nS, and it works give or take a few nano seconds...I guess now I need to change Vds and see how this affects this capacitance value
    – Edwin Fairchild
    3 hours ago














up vote
3
down vote

favorite












Am I calculating total input capacitance or gate capacitance or none of the above? and are my calculations even correct to begin with. The point of this is that I want to know what gate capacitance im dealing with to properly chose a gate resistor to give me the rise time I need, more or less.



So I have the following circuit using the old IRF540





schematic





simulate this circuit – Schematic created using CircuitLab



I am probing the gate directly and get the following readings:



enter image description here



I am loosing 500mV somewhere but anyways:



My voltage at 1 time constant is:
(This is where the dashed vertical line is)



1t



So measuring the time from 0V (solid vertical line) to the 1 time constant line (dashed) you can see is about 530nS. So my time constant is 530nS and the gate resistor is 100ohms then:



enter image description here



Is my logic correct that this is my input capacitance? And given will this vary if I have same Vds but a Vgs that is NOT equal to Vds?



Extra points: Where are my 500mV ?






mosfet capacitance charge







share|improve this question





















  • I have gone back to the bench and calculated a time constant of 100nS given this capacitance and found that I need a resistor of 18ohms or so, this will get me 7.2V in 100nS, and it works give or take a few nano seconds...I guess now I need to change Vds and see how this affects this capacitance value
    – Edwin Fairchild
    3 hours ago












up vote
3
down vote

favorite









up vote
3
down vote

favorite











Am I calculating total input capacitance or gate capacitance or none of the above? and are my calculations even correct to begin with. The point of this is that I want to know what gate capacitance im dealing with to properly chose a gate resistor to give me the rise time I need, more or less.



So I have the following circuit using the old IRF540





schematic





simulate this circuit – Schematic created using CircuitLab



I am probing the gate directly and get the following readings:



enter image description here



I am loosing 500mV somewhere but anyways:



My voltage at 1 time constant is:
(This is where the dashed vertical line is)



1t



So measuring the time from 0V (solid vertical line) to the 1 time constant line (dashed) you can see is about 530nS. So my time constant is 530nS and the gate resistor is 100ohms then:



enter image description here



Is my logic correct that this is my input capacitance? And given will this vary if I have same Vds but a Vgs that is NOT equal to Vds?



Extra points: Where are my 500mV ?






mosfet capacitance charge







share|improve this question













Am I calculating total input capacitance or gate capacitance or none of the above? and are my calculations even correct to begin with. The point of this is that I want to know what gate capacitance im dealing with to properly chose a gate resistor to give me the rise time I need, more or less.



So I have the following circuit using the old IRF540





schematic





simulate this circuit – Schematic created using CircuitLab



I am probing the gate directly and get the following readings:



enter image description here



I am loosing 500mV somewhere but anyways:



My voltage at 1 time constant is:
(This is where the dashed vertical line is)



1t



So measuring the time from 0V (solid vertical line) to the 1 time constant line (dashed) you can see is about 530nS. So my time constant is 530nS and the gate resistor is 100ohms then:



enter image description here



Is my logic correct that this is my input capacitance? And given will this vary if I have same Vds but a Vgs that is NOT equal to Vds?



Extra points: Where are my 500mV ?






mosfet capacitance charge




mosfet capacitance charge






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 3 hours ago









Edwin Fairchild

30418




30418











  • I have gone back to the bench and calculated a time constant of 100nS given this capacitance and found that I need a resistor of 18ohms or so, this will get me 7.2V in 100nS, and it works give or take a few nano seconds...I guess now I need to change Vds and see how this affects this capacitance value
    – Edwin Fairchild
    3 hours ago
















  • I have gone back to the bench and calculated a time constant of 100nS given this capacitance and found that I need a resistor of 18ohms or so, this will get me 7.2V in 100nS, and it works give or take a few nano seconds...I guess now I need to change Vds and see how this affects this capacitance value
    – Edwin Fairchild
    3 hours ago















I have gone back to the bench and calculated a time constant of 100nS given this capacitance and found that I need a resistor of 18ohms or so, this will get me 7.2V in 100nS, and it works give or take a few nano seconds...I guess now I need to change Vds and see how this affects this capacitance value
– Edwin Fairchild
3 hours ago




I have gone back to the bench and calculated a time constant of 100nS given this capacitance and found that I need a resistor of 18ohms or so, this will get me 7.2V in 100nS, and it works give or take a few nano seconds...I guess now I need to change Vds and see how this affects this capacitance value
– Edwin Fairchild
3 hours ago










1 Answer
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up vote
4
down vote













You are measuring the lump capacitance in accord with this standard model,



enter image description here



You even have observed the Miller Plateau.



The capacitances do depend on applied voltages however, see this ROHM article. As VDS increases the capacitance decreases.






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  • After reading this answer, OP will ask "So... how do I measure $C_gs$?"
    – Harry Svensson
    14 mins ago










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1 Answer
1






active

oldest

votes








1 Answer
1






active

oldest

votes









active

oldest

votes






active

oldest

votes








up vote
4
down vote













You are measuring the lump capacitance in accord with this standard model,



enter image description here



You even have observed the Miller Plateau.



The capacitances do depend on applied voltages however, see this ROHM article. As VDS increases the capacitance decreases.






share|improve this answer




















  • After reading this answer, OP will ask "So... how do I measure $C_gs$?"
    – Harry Svensson
    14 mins ago














up vote
4
down vote













You are measuring the lump capacitance in accord with this standard model,



enter image description here



You even have observed the Miller Plateau.



The capacitances do depend on applied voltages however, see this ROHM article. As VDS increases the capacitance decreases.






share|improve this answer




















  • After reading this answer, OP will ask "So... how do I measure $C_gs$?"
    – Harry Svensson
    14 mins ago












up vote
4
down vote










up vote
4
down vote









You are measuring the lump capacitance in accord with this standard model,



enter image description here



You even have observed the Miller Plateau.



The capacitances do depend on applied voltages however, see this ROHM article. As VDS increases the capacitance decreases.






share|improve this answer












You are measuring the lump capacitance in accord with this standard model,



enter image description here



You even have observed the Miller Plateau.



The capacitances do depend on applied voltages however, see this ROHM article. As VDS increases the capacitance decreases.







share|improve this answer












share|improve this answer



share|improve this answer










answered 2 hours ago









Ale..chenski

25.1k11858




25.1k11858











  • After reading this answer, OP will ask "So... how do I measure $C_gs$?"
    – Harry Svensson
    14 mins ago
















  • After reading this answer, OP will ask "So... how do I measure $C_gs$?"
    – Harry Svensson
    14 mins ago















After reading this answer, OP will ask "So... how do I measure $C_gs$?"
– Harry Svensson
14 mins ago




After reading this answer, OP will ask "So... how do I measure $C_gs$?"
– Harry Svensson
14 mins ago

















 

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