h bridge problem (for brushless motor)

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usually you want a Vgs of about 10-12v to get the rated Rds-on but I don't know if you have available any voltage that high.
The Pmosfet is rated 0.117 ohm @10v, that is the close to the best you can get, if you want lower you have to change mosfet
 

alexan_e said:
usually you want a Vgs of about 10-12v to get the rated Rds-on but I don't know if you have available any voltage that high.
The Pmosfet is rated 0.117 ohm @10v, that is the close to the best you can get, if you want lower you have to change mosfet
Click to expand...

My smps like any other has 12 and 5v supply.so that's not a problem.

Do both the Ids sshould be same.i.e. Should they be set equal by changing Vgs of each.or can they different? So that Ids depends on the least resistance FET
 

The current has to go through the two mosfets and the motor so the drain current is limited by the load, you don't want to be anywhere near the saturation region of the mosfet (where the output current is limited by the mosfet) because you start get high voltage drop and heat increases.
You apply a high Vgs to get a low Rds-on and stay in the linear region of the mosfet for the working current, if you can apply 12v you are fine.

Alex
 


So you suggest I make 12v transistor switches an control the current using Pwm?
In that way the load is, the main resistance an all power
Drops across it.
 

I haven't used H bridge to drive a motor like yours, I have only used it in stepper motors so I can't tell you exactly what to so but driving the mosfets with 12v is a very good start because you will get a very low Rds-on.
PWM is probably a good way to control the current but I think you need some feedback system too that measures the current and changes the PWM accordingly.

Also for fast pulses you 'll have to use a proper driver, there are ready IC mosfet drivers like TC4427 **broken link removed**

Alex
 


ok,so i'll use 6 2n2222 and switch between 12V and then use PWM.for the current sensing i thought of using a resistor in series with motor,but this would limit the current.how do i overcome that.
 

I have no experience with motors like this , I have only used stepper motors of about 1 ohm where a 20m ohm sense resistor was not a problem.

Alex
 

i think I'm spoiling the mosfets.i removed one fet and connected the motor windings between 5V and drain of fet.i grounded the fet's source.then through a voltage divider i i gave 0~5V to gate.at first it worked fine.(the coil i'm using now takes 600mA at 5v).when i increased the gate voltage the current increased.then i changed the voltage divider voltage to 12V and then gave the divided voltage to gate,still the fet worked,but after somtime when i switched on the power,a max. of 140mA flows in the coil and decreases as the voltage increases.i'm not understanding what's wrong.can you give me a cleaner circiut that actually does the job,to first test the fet biasing?
 

Why did you use a voltage divider for the gate, you want to drive it with 12v not 5v.
If you are asking me for a discrete driver it is not a very simple circuit, a proper driver would look like



an intermediate driver is


and the most simple driver can be like

with a power supply of 12v and without the gate resistor but only the first schematic can provide a high enough current for fast switching, an IC solution is easier

Alex
 

i was actually thinking of using the third circuit you gave. but has a said, these circuits are not.so I think I'll use the ic you suggested for now. i hope i can find the ic. later,i'll build the circuits myself.

and what about the fet behaviour? have ie spoilt the fet? is there any way to test the fet?


Thank you
 

thank you. i'll test all the fets and rebuild circuit, first rate transistor switches and and then with the ic and after that i get back to you with the results.
 

Maybe you can find more info in RC forums, I assume that you will find many hobbyist that use motors like this in airplanes etc so they may have easier solution to drive them, maybe an IC that can do all the job.

Alex
 

As you many rc forums are involved in brushless electronic speed controller design. i'll try whatever you said and then see what to do.
 

i contacted other forums but they said the same thing you said:to turn the fets completely on.so i used an L293D which supplies 1.5A.also i replaced all the components and then tried again.the current was proper,but the p side is getting heated badly.i noticed a blunder i was doing.i did'nt use a fluback diode.but i also noticed that the mosfets have an internal diode.is it causing the heating and burning?
 

The P mosfet on resistance is 6 times higher that the motor resistance, this is probably why you get the increased heat.

Alex
 

I got the error I was doing. I had interchanged the reference and supply voltages of l293d.i corrected it and the motor ran with full current.now the current is just 1.25 amps, no heat at all. but somehow the circuit is getting spoilt.and always the problem is with the p side.suddenly the fets start heating up and eventually burn out.gate and source will be shorted out after this.any ideas why it happens?
on the brighter side I now know the basics of switching used in esc(s).thank you.
 

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