Measuring MOSFET input capacitance?
up vote
5
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
simulate this circuit – Schematic created using CircuitLab
I am probing the gate directly and get the following readings:
I am loosing 500 mV somewhere but anyways:
My voltage at 1 time constant is:
(This is where the dashed vertical line is)
So measuring the time from 0 V (solid vertical line) to the 1 time constant line (dashed) you can see is about 530 ns. So my time constant is 530 ns and the gate resistor is 100 ohm then:
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 500 mV?
mosfet capacitance charge
add a comment |
up vote
5
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
simulate this circuit – Schematic created using CircuitLab
I am probing the gate directly and get the following readings:
I am loosing 500 mV somewhere but anyways:
My voltage at 1 time constant is:
(This is where the dashed vertical line is)
So measuring the time from 0 V (solid vertical line) to the 1 time constant line (dashed) you can see is about 530 ns. So my time constant is 530 ns and the gate resistor is 100 ohm then:
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 500 mV?
mosfet capacitance charge
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
Nov 7 at 1:42
2
S for Siemens. s for second.
– winny
Nov 7 at 7:06
Your calculations are only valid for a fast enough switch
– PlasmaHH
Nov 7 at 7:38
add a comment |
up vote
5
down vote
favorite
up vote
5
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
simulate this circuit – Schematic created using CircuitLab
I am probing the gate directly and get the following readings:
I am loosing 500 mV somewhere but anyways:
My voltage at 1 time constant is:
(This is where the dashed vertical line is)
So measuring the time from 0 V (solid vertical line) to the 1 time constant line (dashed) you can see is about 530 ns. So my time constant is 530 ns and the gate resistor is 100 ohm then:
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 500 mV?
mosfet capacitance charge
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
simulate this circuit – Schematic created using CircuitLab
I am probing the gate directly and get the following readings:
I am loosing 500 mV somewhere but anyways:
My voltage at 1 time constant is:
(This is where the dashed vertical line is)
So measuring the time from 0 V (solid vertical line) to the 1 time constant line (dashed) you can see is about 530 ns. So my time constant is 530 ns and the gate resistor is 100 ohm then:
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 500 mV?
mosfet capacitance charge
mosfet capacitance charge
edited Nov 7 at 7:06
winny
4,39231727
4,39231727
asked Nov 7 at 1:33
Edwin Fairchild
34318
34318
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
Nov 7 at 1:42
2
S for Siemens. s for second.
– winny
Nov 7 at 7:06
Your calculations are only valid for a fast enough switch
– PlasmaHH
Nov 7 at 7:38
add a comment |
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
Nov 7 at 1:42
2
S for Siemens. s for second.
– winny
Nov 7 at 7:06
Your calculations are only valid for a fast enough switch
– PlasmaHH
Nov 7 at 7:38
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
Nov 7 at 1:42
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
Nov 7 at 1:42
2
2
S for Siemens. s for second.
– winny
Nov 7 at 7:06
S for Siemens. s for second.
– winny
Nov 7 at 7:06
Your calculations are only valid for a fast enough switch
– PlasmaHH
Nov 7 at 7:38
Your calculations are only valid for a fast enough switch
– PlasmaHH
Nov 7 at 7:38
add a comment |
1 Answer
1
active
oldest
votes
up vote
8
down vote
accepted
You are measuring the lump capacitance in accord with this standard model,
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.
Brief description of methods used to characterize individual capacitances can be found in this Vishay Siliconix appnote AN-957, see Fig.17, 18, and 19. Three configurations of a capacitance bridge are used, and then individual caps are algebraically determined.
that is a great App note you linked. Thanks A lot!
– Edwin Fairchild
Nov 8 at 4:37
add a comment |
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
8
down vote
accepted
You are measuring the lump capacitance in accord with this standard model,
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.
Brief description of methods used to characterize individual capacitances can be found in this Vishay Siliconix appnote AN-957, see Fig.17, 18, and 19. Three configurations of a capacitance bridge are used, and then individual caps are algebraically determined.
that is a great App note you linked. Thanks A lot!
– Edwin Fairchild
Nov 8 at 4:37
add a comment |
up vote
8
down vote
accepted
You are measuring the lump capacitance in accord with this standard model,
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.
Brief description of methods used to characterize individual capacitances can be found in this Vishay Siliconix appnote AN-957, see Fig.17, 18, and 19. Three configurations of a capacitance bridge are used, and then individual caps are algebraically determined.
that is a great App note you linked. Thanks A lot!
– Edwin Fairchild
Nov 8 at 4:37
add a comment |
up vote
8
down vote
accepted
up vote
8
down vote
accepted
You are measuring the lump capacitance in accord with this standard model,
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.
Brief description of methods used to characterize individual capacitances can be found in this Vishay Siliconix appnote AN-957, see Fig.17, 18, and 19. Three configurations of a capacitance bridge are used, and then individual caps are algebraically determined.
You are measuring the lump capacitance in accord with this standard model,
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.
Brief description of methods used to characterize individual capacitances can be found in this Vishay Siliconix appnote AN-957, see Fig.17, 18, and 19. Three configurations of a capacitance bridge are used, and then individual caps are algebraically determined.
edited Nov 7 at 5:32
answered Nov 7 at 2:29
Ale..chenski
25.3k11859
25.3k11859
that is a great App note you linked. Thanks A lot!
– Edwin Fairchild
Nov 8 at 4:37
add a comment |
that is a great App note you linked. Thanks A lot!
– Edwin Fairchild
Nov 8 at 4:37
that is a great App note you linked. Thanks A lot!
– Edwin Fairchild
Nov 8 at 4:37
that is a great App note you linked. Thanks A lot!
– Edwin Fairchild
Nov 8 at 4:37
add a comment |
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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
Nov 7 at 1:42
2
S for Siemens. s for second.
– winny
Nov 7 at 7:06
Your calculations are only valid for a fast enough switch
– PlasmaHH
Nov 7 at 7:38