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Topic: Understanding MOSFET datasheets (Read 15856 times) previous topic - next topic

Boardburner2

I just noticed you are not the op.
wrong thread for understanding dadtshets.

Start your own please.

localbroadcast

lol??? He's not the op, but the thread IS called undestanding mosfet datasheets..

Since the generator voltage output is so similar to AC mains from your standard house wall plug of 120vrms, I am basically building this power supply to work when plugged into the wall.  I'll get it working off of this power source, and then will deal with any issues that comes from powering it with my generator.  The generator as the power source is the end objective, but I'm going to get it running off of standard 120vac first.. just to keep things simple.

I really didn't want to use a transformer to step down the voltage.. I didn't want to have the losses associated with it.  I wanted to use strictly pwm to do it, but it seems that the size of capacitor needed to filter such a gap in voltages would be unrealistic.. It wouldn't be impossible, just too large.  So I think I'm going to cut the voltage in half by using a 2:1 wound transformer to step down from 120vac to 60vac, rectify and filter to a nice smooth DC, PWM and filter to a nice smooth 36vdc, feed the load.

caps and trannys on the way.  already got my mosfets and leds.

Boardburner2

#62
Feb 22, 2015, 10:35 am Last Edit: Feb 22, 2015, 10:46 am by Boardburner2
This may be of interest
Lots of fets there


http://www.ijcee.org/papers/278-E747.pdf

Some ambiguities though.

Some important details missing also

What it does is convert the output from a voltage to a current scource which makes the switching easier

Boardburner2


http://forum.arduino.cc/index.php?topic=302331.0

An ac wall outlet is normally considered to be a voltage source.

A generator can be either but in the particular case of imag is best used as a current scource to avoid excitation collapse.

This is why standard buck converters (voltage source ) cause collapse as mentioned by rockwallaby.

localbroadcast

Hmm..
Boardburner2...

Your post about current sources and voltage sources intrigues me.. I have heard mentions of a difference between constant current output and constant voltage output of power supplies.. and current sources instead of voltage sources.. but I never really took the time to actually research the meaning of the two.

I've always assumed that a circuit will have a source voltage.. determined by the voltage of batteries used, or the voltage of the wall outlet...  And based on this voltage, the circuit will draw the amount of current that it needs based on the resistance, reactance, etc. of the combined total of all components of the circuit.  More resistance, more amps.  As long as the power source can provide the amperage being drawn, there's no issues.  If the power supply is capable of providing more current, it won't just put more current into the circuit... it only provides the amount of current that the circuit draws, or "asks politely for".

Always using this method of thinking to understand how circuits work, it kind of stresses out my brain to think of a circuit who's power supply provides a constant current, and the voltage used depends on what the components of the circuit draw, or "ask for politely".

Maybe I'm not understanding it quite correctly.. maybe you can clue me in to what you're talking about with the difference of a current source vs. voltage source...

As for if my generator will be able to handle the load and still keep on generating...
I haven't really felt like discussing it here or arguing the point of if it will do the job or not, because I'm going to get it working off of 120vac 60hz from the wall first.. and then tweak it to run on my generator output.

I am confident in my generator as the supply for a couple reasons..  The main reason is that it's not wired quite the same as what most people imagine a self excited induction generator to be wired like.. most assume they only work at a constant speed, or they suck balls at any other speed.. mine works quite well at a range of speeds.... but since it's fed by a stream that speed is pretty consistent.  Attached is a diagram of BASICALLY how my generator is rigged up.  Not exactly connected this way, but almost exactly like this..

I think you guys would get a kick out of seeing one of the schematics from when I built VFD's.. I'm not sure if I'm allowed to show other people the schematics, but since I haven't been employed there for over 5 years and the customers are given a copy when they buy the vfd, I don't see there being any risk.  Just let me know if it's worth scanning... I won't do it unless someone is interested in seeing it.


localbroadcast

#65
Feb 22, 2015, 03:43 pm Last Edit: Feb 22, 2015, 03:44 pm by localbroadcast
Here is a PDF that describes in more detail how my generator is wired up..  I used this document as reference when building my generator system.  I don't really wish to discuss it anymore because I have it working fine and I need more assistance with the power supply project I am working on at the moment.




Boardburner2

#66
Feb 22, 2015, 08:31 pm Last Edit: Feb 22, 2015, 08:57 pm by Boardburner2
I spent a couple of days researching terminology to try and explain .
This is the best I could find but it takes some digesting.

http://cds.cern.ch/record/987498/files/p13.pdfhttp://cds.cern.ch/record/987498/files/p13.pdf

The pdf you posted appears to be the same I posted in my earlier link so it appears we are both working to the same hymn sheet.

Using a imag connected to a transformer then a Buck convertor works.

However it can be very lossy and I was using a motor which I think was too small.

Using a series inductance works but only for a given frequency which is not much good for a wind generator.

I thought as you clearly do that a switching method was the solution but I lacked the experience at the time when building a dc dc convertor was adventurous and building an ac to dc invertor was for the brave.

I looked up Z invertor which was a new term for me but. quote

The source can be either a voltage source or a current source. The DC source of a ZSI can either be a battery, a diode rectifier or a thyristor converter, a fuel cell stack or a combination of these.
The main circuit of a ZSI can either be the traditional VSI or the traditional CSI.
Works as a buck-boost inverter.
The load of a ZSC can either be inductive or capacitive or another Z-Source network.

unquote.

I think you need the current scource type.  

That pdf omits some things.

It does not explore the performance wrt frequency of the input signal which is strange considering it is for wind power.

For the dc link L4 is shown but the text describes an inductive capacitive circuit without explaining detail.

Also it does not describe the size of motor and inductor but the text mentions it does not work well for small motors without qualifying it.
I think this is probably where I fell down.

It also mentions torque discontinuities without elaboration which I take to mean whine or vibration.
I think that may be the onset of excitation failure but that's a guess.


Hmm..


I've always assumed that a circuit will have a source voltage.. determined by the voltage of batteries used, or the voltage of the wall outlet...  And based on this voltage, the circuit will draw the amount of current that it needs based on the resistance, reactance, etc. of the combined total of all components of the circuit.  More resistance, more amps.  As long as the power source can provide the amperage being drawn, there's no issues.  If the power supply is capable of providing more current, it won't just put more current into the circuit... it only provides the amount of current that the circuit draws, or "asks politely for".




That's true of the grid supply which to all intents and purposes is infinite.
IMAG however has its own ideas.

A generator can be considered either a voltage source (fixed frequency) or a current scource (variable frequency).
In the case of IMAG it can be both.

I believe it defies conventional analysis requiring iterative means to describe its behaviour.



it kind of stresses out my brain to think of

Oh yes.

I tried thinking what does the motor 'see' and what does the input circuit 'see'

Went around in circles for months there

Boardburner2

#67
Feb 22, 2015, 08:37 pm Last Edit: Feb 23, 2015, 01:30 am by Boardburner2
It appears you are taking up where I left off.
I will be very interested to see what your results and conclusions are.

I will attempt to answer any specific questions if I can but I think I have explained my experience as best I can

Edit

My suspicion is that you will encounter a problem between the uncontrolled rectifier and the Z input convertor depending on weather the input to the Z convertor is capacitive, inductive or resistive.

I think it will work off AC mains in any case but not off an IMAG source.

I would be delighted if you could prove me wrong but I do not have sufficient hardware to test myself.
I think I would need several sizes of motor and inductors to test and they are plain expensive.

localbroadcast

Well.. Like I said... that diagram is a basic starting point of how I designed my generator system.. The actual way that I built it was not a "z source invertor".. As I don't know what that is either.. But it's a simple 6 mosfet inverter with the gates triggered with PWM to create 120vAC at 60hz.  You can buy motor control ICs that have all 6 of these mosfets already built in as one chip.  Between the rectifier and the inverter is what's called the DC BUS.  Its a simple capacitor in parallel and inductor in series which makes what's coming out of the rectifier a pretty smooth DC signal.

It's the basic design of a VFD but the rectifier and the inverter are reversed. 

localbroadcast

I'm still working on this project, it's coming along slowly but surely.
Here's something I would like to have clarified...
In the attached circuit diagram, I would like to switch the diode "D" with a mosfet in order to improve efficiency, making it a synchronous constant current DC - DC converter.

Should I use an N-type mosfet or a P-type mosfet for this?  How should it be triggered ON / OFF?

If I use an N-type mosfet here, when the main mosfet 'M' is OFF, if I apply a +10vdc signal at the gate pin to turn it on, the inductor will be producing a voltage which will be entering the SOURCE pin of this mosfet.  Since VGS needs to be about +10vdc to turn the mosfet on, if I have a voltage at the gate pin from the inductor 'freewheeling', then my +10vdc at the gate pin isn't going to turn on the mosfet.

Any help is much appreciated!

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