Hi everyone,
I have not touched electronics in quite a while so I am pretty rusty. So, I've taken upon myself to just try and measure out some of the characteristics of a common Mosfet.
So far, I have been able to just get a mosfet plug it into a breadboard of sorts and was able to measure out Rdson, Vgth, and BVDSS, but I am having heck of a time trying to begin to measure Gate Charge.
I've been reading everywhere..appnotes and datasheets, and it just left me confused and have so much information I just need someone to sort out my thoughts the right order for this puzzle to make sense to me.
Attached is a test circuit that I found through one of the app notes online. I figiured it looked pretty simple to put together and test a mosfet. I just have a voltage supply on Vdrain and I have a voltage supply on Vgate. I don't understand why the image shows that I apply constant current on the gate? From what I read, the gate charge is dependant upon Vdd and Vgs. I have a o-scope as well, and i was hoping I could catch the Vgs Vs Gate Charge waveform as it shows on every datasheet. This is where I really don't know how. I understand some of the theory behind it, but I would love to have someone show me how to actually measure this or tell me a step process.
I understand that the Fet charge has three stages. One which it slowly charges the input capacitance (Cgd ..which is Crss on datasheet) and after that it reaches a plateau called "Miller", after an x amount of time the voltage ramps up again. I remember that Q = CV, but in order to find this Q for a particular point in Vgs, I have to know the actual input capacitance. I also know that Q = current x time. If i can find how long it takes for capacitor to charge and considering that Ig is the current on the Gate then I can get Q and from there then I can apply it to Q=CV and find the input capacitance?
My head is wrapped around so much stuff, I confuse myself. Anyone who can help sort me out on how to measure Coss, Ciss, Crss and total Qg?
Much thanks.
I have not touched electronics in quite a while so I am pretty rusty. So, I've taken upon myself to just try and measure out some of the characteristics of a common Mosfet.
So far, I have been able to just get a mosfet plug it into a breadboard of sorts and was able to measure out Rdson, Vgth, and BVDSS, but I am having heck of a time trying to begin to measure Gate Charge.
I've been reading everywhere..appnotes and datasheets, and it just left me confused and have so much information I just need someone to sort out my thoughts the right order for this puzzle to make sense to me.
Attached is a test circuit that I found through one of the app notes online. I figiured it looked pretty simple to put together and test a mosfet. I just have a voltage supply on Vdrain and I have a voltage supply on Vgate. I don't understand why the image shows that I apply constant current on the gate? From what I read, the gate charge is dependant upon Vdd and Vgs. I have a o-scope as well, and i was hoping I could catch the Vgs Vs Gate Charge waveform as it shows on every datasheet. This is where I really don't know how. I understand some of the theory behind it, but I would love to have someone show me how to actually measure this or tell me a step process.
I understand that the Fet charge has three stages. One which it slowly charges the input capacitance (Cgd ..which is Crss on datasheet) and after that it reaches a plateau called "Miller", after an x amount of time the voltage ramps up again. I remember that Q = CV, but in order to find this Q for a particular point in Vgs, I have to know the actual input capacitance. I also know that Q = current x time. If i can find how long it takes for capacitor to charge and considering that Ig is the current on the Gate then I can get Q and from there then I can apply it to Q=CV and find the input capacitance?
My head is wrapped around so much stuff, I confuse myself. Anyone who can help sort me out on how to measure Coss, Ciss, Crss and total Qg?
Much thanks.
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