calculate the amount of current required to drive mosfet gate

[nour]

I have decided to build a test circuit  for testing mosfet characteristics free running and under load using signal generator and oscilloscope and a couple of DMs   
I think that would help me to understand more about how different mosfet behave under different conditions and compare them to the basic theoretical calculations

I have more than 10 different discrete mosfets laying around so I will go through them one by one and will do a table for comparison later
I am not in a hurry and I think that is better than buying stuff that wouldn't work for me later

later I will decide if I will use gate driver for my H-bridge or not and decide how much current I will need for such a thing

I will open new topic for that very soon after building my basic circuit and get my hands dirty in the experiment
hope nothing smoke 

if someone would think that's waste of time, please advice me and tell me why

thanks

[nour]

later I will decide if I will use gate driver for my H-bridge or not and decide how much current I will need for such a thing

How can you drive an H bridge without gate driver? Logic shifting with high dv/dt middle point is a PITA if done yourself. 2 integrated half bridge driver takes care that issue for you.

I meant by gate driver an integrated circuit driver not discrete components, may be I used the wrong terms there 
actually if possible I would like to do the gate driver from discrete components because I have a lot of components on my shelf and want to do something with them 
may be gate driver with integrated circuit would be much simpler and easier(indend it is) but as I have said, I want to make use of the components that I have (I have a lot of discrete transistors and other stuff)

[nour]

Logic shifting with high dv/dt middle point is a PITA if done yourself.

would you please explain what did you meant by "dv/dt middle point is a pita" ?

[nour]

Logic shifting with high dv/dt middle point is a PITA if done yourself.

would you please explain what did you meant by "dv/dt middle point is a pita" ?

If it is a half bridge, then the source pin of the top mosfet is switching at a very high dv/dt, as intended. You need a method to level shift your ground referenced gate signal to the switching virtual ground, which is not easy to do.

I will try to solve this issue after finishing the test circuit that I have mentioned
thanks

[nour]

I will try to solve this issue after finishing the test circuit that I have mentioned
thanks

An integrated half bridge gate driver has this integrated. You do not have to worry about it if you use one.
This only becomes your concern if you roll out your own gate driver, or you are switching at an insanely high dv/dt.

yes I know that, but as I have mentioned before I will try first to do this from discrete parts because  I have a lot of them
also I will need a lot of this drivers IC
I am willing to build around 4 drivers for bipolar steppers, every motor has 2 H-bridge which means 4 motors X (2 H-bridge X 2 Drivers per bridge) = 16 drivers X 0.7$ = 11.2 $

If I can use my current resources to build a driver that will do the shift level without spending those 11$ it would be good
also I am willing to build after finish this another 4 BLDC drivers, so that will help later

I Don't have much information about how to do it or whether or not it is easy to build, but later I will decide about it
if you can help me with a schematic for doing that, it would be very nice

[hamdi.tn]

I think for this particular application , you may use some of the shelf circuit , a simple one and used a lot by hobbyiste is the L298 double H-bridge so you will need 4 of them, with high voltage and high current capability. should be easy to interface with any micro-controller and can get the job done.

[nour]

I think for this particular application , you may use some of the shelf circuit , a simple one and used a lot by hobbyiste is the L298 double H-bridge so you will need 4 of them, with high voltage and high current capability. should be easy to interface with any micro-controller and can get the job done.

I manage to blow up like 3 of those L298 until now  , they are so fragile, that's one of the reasons that made me decide to use external mosfets

[uncle_bob]

I just set a maximum frequency for the driver to handle not for the motors
for the motors I really don't have enough experience to say for sure how much switching that would make a specific motor runs smoothly and fast

may be you are right, couple of hundreds hz would be enough, still need to do tests to understand how efficient would that be

Hi

Ok, let's back off a bit. If you are buying a driver, simply going for overkill is fine. The cost isn't much and they have already taken care of the all the nasty issues.

For an application specific driver, that's not a good way to go:

Your end device needs to move at X rate to be useful. You have a mechanical advantage of Y between the motor and what you are moving. The device weights something and in a CNC you need force to hold it in place. Those are all numbers that are a direct result of your basic design.

You then head off to see if you need a giant motor or a little tiny motor. They all have torque and hold data as part of the spec sheets. Different motor designs have different curves. It's *always* a "runs out of torque here" sort of decision. The max step rate only applies to a motor with absolutely nothing attached. You then take a look at the price of this or that motor. If they are $900 each and you need four of them that might not be inside the budget. Back you go and take a look at the basic design.

Looping through the basic design, you find that high mechanical advantage (gear way down) gives you high resolution but forces you to a very high step rate. It also impacts torque, but you need it at a higher speed so likely you loose there as well. Back through the design, and "can you get the resolution at the price and speed" is the question.

On all of the systems I've built, the mechanical stuff has cost *way* more than any of the electronics.

Bob

[nour]

If you just want a rugged and faster L298N, why not try DRV8844? For $1.75, you get 4 half bridges (2 H bridges), rated at 2.5A peak, 1.75A RMS, 60V at 25 deg C. It is PWM ready with only 90ns dead time.

unfortunately this one is not available at TME, currently I just buy from them because they have the lowest shipping rates (and very fast) for me

anyway, I want to do it from discrete components for the sake of power dissipation and better heat transfer over time
when I was using the L298, it was getting too hot very quickly which makes it not idle for long term running (of course I used heatsink for it)

[nour]

On all of the systems I've built, the mechanical stuff has cost *way* more than any of the electronics.

Bob

can you recommend motors for me to work with in the range of sub 100$

I don't have any experience with motor's torque and how much would be enough
how to evaluate a motor torque whether or not it would be good for the job without spending money over a lot of motors to just try them
I have already a lot of small stepper motors vary between bipolar and unipolar and all of them came from various consumer printers, but I feel they can't do the job (I can't find enough info about them)
most of the time when I try to drive them with a basic drivers using arduino and discrete components they get hot after some time and soon enough they are getting ugly hot

another example the one that I have introduced, the sanyo denki one, the spices os over there
http://store.lipsiasoft.com/uploads/415640_103-H7126-1740.pdf

How I can evaluate that motor and have a good estimation about whether or not it is good enough for my application and it wouldn't be over-killing
it says in the datasheet the following
Holding torque:165 Ncm
Rotor Inertia: 350x10^-7  Kgm²
back E.M.F: 31 V/Krpm

I hope there is a guide line for something like this that would be easy enough to start with

I think we are drifting over here and the discussion is no longer related to the topic 

[hamdi.tn]

I think we are drifting over here and the discussion is no longer related to the topic 

i think coz everyone here think that you overthinking this ... did i say think 
and cause you missing some steps to the choice the drivers... including the steppers choice , you need to know your loads before choosing the switches and the drivers.
am not sure about the aspect of the project ,but i really recommend to choice the steppers and some basic H-bridge circuit, and go forward with the project. if you doing this for fun, it will be easy to salvage some steppers and circuit drivers from old printers, i personally did that before. and as been said before, the mechanical part of such project could be more challenging and expensive than the electronics.

[uncle_bob]

On all of the systems I've built, the mechanical stuff has cost *way* more than any of the electronics.

Bob

can you recommend motors for me to work with in the range of sub 100$

I don't have any experience with motor's torque and how much would be enough
how to evaluate a motor torque whether or not it would be good for the job without spending money over a lot of motors to just try them
I have already a lot of small stepper motors vary between bipolar and unipolar and all of them came from various consumer printers, but I feel they can't do the job (I can't find enough info about them)
most of the time when I try to drive them with a basic drivers using arduino and discrete components they get hot after some time and soon enough they are getting ugly hot

another example the one that I have introduced, the sanyo denki one, the spices os over there
http://store.lipsiasoft.com/uploads/415640_103-H7126-1740.pdf

How I can evaluate that motor and have a good estimation about whether or not it is good enough for my application and it wouldn't be over-killing
it says in the datasheet the following
Holding torque:165 Ncm
Rotor Inertia: 350x10^-7  Kgm²
back E.M.F: 31 V/Krpm

I hope there is a guide line for something like this that would be easy enough to start with

I think we are drifting over here and the discussion is no longer related to the topic 

Hi

Motors are part of the "mechanical" side of the system. You can easily spend $1,000 apiece on motors for some of this stuff. That same $1,000 motor runs fine with a pre-built driver that costs less than $50.

The sizing of the motors and what you "need" is mechanical engineering. You do a design, run through the math and that is what gives you your requirements.

You start with:

How fast does the head need to move in mm/s
How much does the head weigh
How much does the structure to hold the head weight
How much back force does the tool generate (you may have to measure this).

Next up:

How rigid does the system need to be
How accurate does it need to be
How repeatable does it need to be (different than accuracy)
How small a step do you need to get a good finish (different than the other two above)
All of these basically are "how good does it need to be".

Next:

How large an area does the machine need to traverse in X, Y, and Z
That's a biggie all by it's self. A machine that needs to go a meter in each of those can be "interesting".

With those inputs you can work out your aluminum budget and figure out how much that's all going to weigh. There are various drive approaches, screws and belts are the two main ones. Your accuracy budget will steer you to one or the other.

If one lead screw (or belt) does not get you to adequate positioning, you may need two (or three for interesting drive approaches).

Finally:

You can calculate the number of RPM you need to move your head a given distance. You can work out how fast you need to accelerate or decelerate. (right angle turn that is still fast enough). That will give you a torque at RPM number. That plus the friction and back force on your motor give you the likely torque range. I'd add at least 20% and probably 50% to whatever you come up with as a "first time I've done this" safety factor.

So now you run off and buy a bunch of aluminum straight piece parts, and all the fittings to rigidly attach them to each other. You come up with the drawings for your various mounting plates and send them off to the laser shop to have them cut. You order up a few (hundred) bearings. Out goes the order for belts or lead screws. If it's lead screws, they start at "expensive" and go up from there. Also get the traveler nuts for the lead screws from the same guy.

Figure that it all comes in and *something* isn't going to fit. Back for more mounting plates or some such thing.

Note that that the motors and electronics come in very late in the process. The other stuff needs to be worked out much earlier.

Bob

[Ghydda]

Whenever I wade into new and uncharted territories I find that answers come fastest  and with least effort by actually building a prototype of the subcircuit and using it to get a feeling or even confirmation on what works and what does not.

LTspice is great, I use it for analyzing switch behavior among other things, but it is very very difficult to get accurate results as the real world is never as idealized as a simulation.

Doing math is great to get in the ballpark of what values the components should have, but for finetuning math is a long and hard and tedious road on all but the simplest of circuits.

Mock-ups/prototypes is a fast way to get know new circuits, learn what parameters are more important than others (and which parameters dominate others), and how to evaluate the performance of said circuit.

Anyway, that would be my approach to nour's challenge.

Cheers
/Ghydda