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[SOLVED] Determining Switch Current

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ktr

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Hello all,

I have some basic questions regarding capacitor and switch behavior.

Lets say Gate voltage of a transistor is 3.3V and I have bypass capacitors at the gate which makes 5uF in total(parallel connection).

Q = V. C (3.3V* 5uF) = 16.5u Coulomb charge is necessary to charge this C.

And I need to say switch on/off the PA with 100Hz rate, so that makes 16.5*2 of charge and discharge( so thats sink and source, a FET is needed)

33uC/10mSec= 3.3mA of current needed to charge up and down Is my calculation correct?

Best Regards,

ktr
 

Hi,

I recommend to draw your circuit.
Then add arrows that show the current you are talking about.

Then draw a diagram with the expected voltage waveform.

Mind: the current drawn by a capacitor (ideally) is proportional to the rise rate of the voltage. --> in times where the voltage is constant (independent of absolute value) the current will be zero.

Klaus
 

Hello,

Although poorly drawn, this is what I mean. I am designing a multi-stage PA, and want to be able to control the current it draws by manipulating gate voltages.

IMG_5841.jpg

Kind Regards,

ktr
 

Hi,

I miss the expected voltage waveform... with voltage levels and timing.

Without this we can not estimate the current.

Klaus
 

Hello,

I thought I said it in first post.

100Hz or so is my switch rate, and +3.3 to -1 Volt is my Gate voltage.

Maybe you mean something else?

ktr
 

Hi,

I thought I said it in first post.

100Hz or so is my switch rate, and +3.3 to -1 Volt is my Gate voltage.

Maybe you mean something else?

Definitely yes, as already mentioned: Waveform, rise rate, voltage levels, timing

It´s totally different whether you want 100Hz
* sine wave
* triangle
* square wave
* trapezoid
...or any other waveform

Klaus
 

Hi,



Definitely yes, as already mentioned: Waveform, rise rate, voltage levels, timing

It´s totally different whether you want 100Hz
* sine wave
* triangle
* square wave
* trapezoid
...or any other waveform

Klaus

Hmm, Square wave with aforementioned limits and
I'm not sure with rise time since it will depend on PLL lock time as well.
 

Hi,

square wave....
I assumed something like this.

now let´s do some example:
let´s say 100Hz, 50% duty cycle (5ms OFF, 5ms ON),rise rate: in 10us from -1V to 3.3V and the same time back
* Gate voltage is -1V (OFF) continously for 4.09ms --> drive current is 0mA
* Gate voltage is -1V to 3.3V (OFF to ON) = 4.3V in 10us --> this means an average current of 5uF * 4.3V / 10us = 2.15A during this 10us
* Gate voltage is 3.3 (ON) continously for 4.09ms --> drive current is 0mA
* Gate voltage is 3.3V to -1V (ON to OFF) = -4.3V in 10us --> this means an average current of 5uF * (-4.3V) / 10us = -2.15A during this 10us
..repeating

The average current is 0mA (at the capacitor, independent of waveform)
The peak current is 2150mA (at capacitor, gate driver, power supply)
The RMS current is 96mA (at the capacitor, gate driver)
The average power supply current is: 2.15A x 10us / 10ms = 2.15mA (assuming a single 4.3V supply, independent of waveform)

You will get totally different peak and RMS results with modifed rise_times / modified_waveform.

Btw: Don´t be too lazy to draw a waveform chart. It doesn´t only give us the information what you expect, it will in future help you to understand what´s happening.

Klaus
 
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    ktr

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Yeah you are correct, I should not be lazy about these stuff.
However, things get messy in mind from time to time. Thinking about gigahertz level matching and transistors then going back to low frequency switching and miller capacitances. I guess getting experience is a messy thing.

Anyway, thanks for the help. That was what I was failing to do, I was missing the rise time of the waveform, noticed when you write it just now.
 

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