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Topic: Energizing a mosfet (Read 3086 times) previous topic - next topic

dc42


Wouldn't the gate still rise to +15v when the NPN transistor is turned off?

The rating we are talking about is the gate to source voltage and the source is tied to +15v. So having the gate at +15v is no problem because then the gate to source voltage is zero. On the other hand, when the 2n2222 turns on, its collector is close to 0v so Vgs is around -15v. The point of the voltage divider is to increase the gate voltage to e.g. 5v so that Vgs is limited to (5 - 15) volts = -10v instead of -15v.
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Docedison

Resonance is defined as the frequency where XL and XC are equal, at that point they cancel out and the resultant is just the resistance of the circuit. The formula are somewhat difficult for one not familiar with the concept. In your description of combined reactances, inductive and capacitive the values for XL and XC are opposite in sign relative to each other and add algebraically. If for example if the XC was -180 ohms and XL was 90 ohms them the resultant would be -90 ohms. -reactances are capacitive and +reactances are inductive, so if XC =1/(2 X 6.28 X F X C) then 1/C = XC/6.28/F or if positive (inductive) L = XL/6.28/F  (C=Farads and L = Henry's). Frequently small value reactances are used to negate other reactance's... one example is power factor correction where typically the load might be inductive in nature (lots of big motors) and provide a mismatched load. Remember that max power transfer occurs where Xload = Xsource (for ac circuits) a capacitor equal to XL (load) is placed in parallel with the load to cancel out the inductive part of the load. The technique used in driving capacitive mosfet gates is to just 'swamp' them out i.e. provide a drive impedance so low as to force them into a minor consideration. Another perhaps better analogy would be parallel resistances. Assume a 10K resistor in circuit and the value might need to be 2K2 ohms and since (1/Rt) = (1/R1) + (1/R2) {Formula for the parallel equivalent resistance of 2 resistors).
We can say that (1/Rpar) = (1/Rreq) - (1/Rcir)... `(1/2200) = .0004545... and 1/10000 = .0001... therefore .0004545... - .0001 - .0003545... = .0003545...Rreq = 1/.0003454... = 2K82 ohms and the proof is 1/Rt = 1/10000 + 1/2820 = 2K2 ohms. The only difference between DC and AC is the sign of the reactance. Long and complicated until you have done it a few times but trivial with a little experience. Thus ends Basic AC theory lesson 1.6.2... or some such number. I didn't write the book and I remember a few pages, fuzzily. In Closing capacitive reactances I.E. Mosfet Gates are usually driven with a generator that is at least 1/10 the impedance of the gate being driven. The math and descriptions would occupy several pages and would need to come from 3 different books, Much too long, I think that if you were able to follow my reasoning so far you have some third year electronics theory education. I also think that If you are reading this sentence you are either glassy eyed or seriously interested in the topic of impedance matching. This subject is very complicated as the available gate drive current is a function of the Rise Time of The Driving Pulse not it's Amplitude since we are driving a capacitor. Remember that Rise time is piece-wise equivalent to frequency and that XC (Gate impedance) is an inverse function of frequency.

Doc



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smeezekitty

Basically the circuit will be driving a capacitor in parallel with an unknown load which could be capacitive, inductive or resistive or a mixture. I was simply not clear enough.

Resonance is defined as the frequency where XL and XC are equal, at that point they cancel out and the resultant is just the resistance of the circuit. The formula are somewhat difficult for one not familiar with the concept. In your description of combined reactances, inductive and capacitive the values for XL and XC are opposite in sign relative to each other and add algebraically. If for example if the XC was -180 ohms and XL was 90 ohms them the resultant would be -90 ohms. -reactances are capacitive and +reactances are inductive, so if XC =1/(2 X 6.28 X F X C) then 1/C = XC/6.28/F or if positive (inductive) L = XL/6.28/F  (C=Farads and L = Henry's). Frequently small value reactances are used to negate other reactance's... one example is power factor correction where typically the load might be inductive in nature (lots of big motors) and provide a mismatched load. Remember that max power transfer occurs where Xload = Xsource (for ac circuits) a capacitor equal to XL (load) is placed in parallel with the load to cancel out the inductive part of the load.

Got that almost 100% The problem is that the circuit is kinda-sorta a type of "power supply" and you do not know if the output will be a 100 Henry inductor or 1 Megaohm resistor or a capacitor. If the inductor is big enough, it may completely negate the effect  of the capacitor.
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  The technique used in driving capacitive mosfet gates is to just 'swamp' them out i.e. provide a drive impedance so low as to force them into a minor consideration.

Wouldn't the 2N2222 driven to saturation provide sufficiently low impedance to have a decent rise time? I only need to run it at around 10KHz maximum.
Another perhaps better analogy would be parallel resistances. Assume a 10K resistor in circuit and the value might need to be 2K2 ohms and since (1/Rt) = (1/R1) + (1/R2) {Formula for the parallel equivalent resistance of 2 resistors).
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We can say that (1/Rpar) = (1/Rreq) - (1/Rcir)... `(1/2200) = .0004545... and 1/10000 = .0001... therefore .0004545... - .0001 - .0003545... = .0003545...Rreq = 1/.0003454... = 2K82 ohms and the proof is 1/Rt = 1/10000 + 1/2820 = 2K2 ohms.

Now I am confused. What does "2K2" and "2K82" resistors mean?
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The only difference between DC and AC is the sign of the reactance.

So basically inductors and capacitors are opposite from each other and what a capacitor does on AC, an inductor will do on DC and vice-versa.
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Long and complicated until you have done it a few times but trivial with a little experience.

Not all that complicated but slightly confusing.
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In Closing capacitive reactances I.E. Mosfet Gates are usually driven with a generator that is at least 1/10 the impedance of the gate being driven.

This should not be a problem.
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The math and descriptions would occupy several pages and would need to come from 3 different books, Much too long

Too long indeed LOL
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, I think that if you were able to follow my reasoning so far you have some third year electronics theory education.
I do not.
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I also think that If you are reading this sentence you are either glassy eyed or seriously interested in the topic of impedance matching.

Actually the latter is more true.
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This subject is very complicated as the available gate drive current is a function of the Rise Time of The Driving Pulse not it's Amplitude since we are driving a capacitor. Remember that Rise time is piece-wise equivalent to frequency and that XC (Gate impedance) is an inverse function of frequency.

In my experience, microcontroller pins usually have decently sharp rise time since I have put them on a scope before.

Now lets hope you have experience with OpAmps and current sensing since its another topic I am pretty new to and will probably start another thread soon.
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

Docedison

Yes I can handle that... It's not too difficult. The Mosfet thing was something I learned in an advanced theory course and had to re-remember when I first started working with the devices in the late 80's. I DO Appreciate you being able to understand what I wrote it's the first time I had occasion to try to put into words something that is almost instinctive for me. The work is almost like a bicycle once you learn it it applies to a lot of different concepts. My hardest subject was Fourier analysis or at least the frequency/time relationships... which in a nutshell say that if a pulse has 0 rise time it has infinite current, a good part of what I was trying to explain involve that concept. The Math is easy... and remember that reciprocal idea, it is applicable to a great deal of electronics especially for deriving solutions to network matching or network solutions. Finally thank you for a thoughtful review of what I wrote.

Doc
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smeezekitty


My hardest subject was Fourier analysis or at least the frequency/time relationships...

That subject tends confuses me as well.  which in a nutshell say that if a pulse has 0 rise time it has infinite current[/quote]
But in practice a pulse of zero rise time does not exist since everything has resistance.
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The Math is easy... and remember that reciprocal idea, it is applicable to a great deal of electronics especially for deriving solutions to network matching or network solutions.

I do my best to remember it.
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Finally thank you for a thoughtful review of what I wrote.

I am not 100% on all concepts yet but I am learning. The concept is not completely foreign to me.
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

Docedison

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That subject tends confuses me as well.  which in a nutshell say that if a pulse has 0 rise time it has infinite current

But in practice a pulse of zero rise time does not exist since everything has resistance.[/quote]
The basic formula deals only with the time/energy transform.
Basically what is says is that the faster the rise time the more power there is in the step and that it will repeat in harmonics... at the rate of the rise time... so if you had a regular 100nS pulse transforming it once would show a 10 Mhz waveform modulated by the repetition rate and the various combinations of the pulse and rise time mix.
when the signal is presented to a filter it will show that information... as the filter will spread it out due to a number of reasons in both the energy and time domains. The simple way to think of it is to consider a sieve...
The theory does conform to and in one sense re-confirms the law of conservation of energy...

Doc
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

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