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/etc... Amplifiers & Other Linear
/etc... Amplifiers & Other Linear Forum
Turning off/on Mosfets
What are the factors which affects turn off/on time of mosfets.In datasheets total gate charge is mentioned or curve showing Vgs vs Qg, I calulate time by Ig= Qg/T. I know gate charge by curve showing at particular Vgs. Current I know which I can provide via pin. By this I calculate time. Is this right method to calculate time. What are affects of input and output capacitance,reverse transfer cap on this. How do they affect the circuit. I an using DC regulated power supply for operation. Factors like turn on delay time,rise time,delay time, fall time. Are they minimum time required for operation.I have 10V dc source to be applied on Vgs of n channel mosfet & want to operate in 1ms. By curve I can find Qg=25nC at vgs =10V. Now current required to be supplied at gate terminal is Ig= 25nC/1ms. Is this method right?
Your assumptions are generally correct. The basic issue is charging the gate capacitance. It requires a certain amount of charge to change the voltage on the gate to a given voltage. There are additional factors that may influence the situation. For example, capacitance from drain to gate can inject a counteracting charge as the drain voltage changes (Miller effect). Thus, how the voltage changes on the drain (dependent on the drain load) will affect the charge required to change the gate voltage.
I think your questions would be better directed to a MOSFET manufacturer as they are likely to better know the detailed issued involved. Not all MOSFETs are the same in these behaviors.
You will find a good deal of MOSFET expertise on TI's Power Management forum, specifically the NexFET™ Power MOSFETs forum. We can provide general information about the MOSFETs here on the Amplifiers & Other Linear forum, but it is very likely the NexFETTM group can answer most any question about MOSFETs you might have.
PA - Linear Applications Engineering
Bruce & Kuehl,
Appreciate ur help.Will re-post it to suggested forum.
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