Mosfet H Bridge

[terrapinlogo]

Hi I have designed a mosfet h bridge for a bot i am currently working on from parts i have in my parts bin and was wondering if it would work as i haven't really used fets before and if anyone had any suggestions on how to keep the magic smoke in. I know i probably need some resistors in there but where and what values?
Thanks in advance
Damian

 

[Torben]

Hi I have designed a mosfet h bridge for a bot i am currently working on from parts i have in my parts bin and was wondering if it would work as i haven't really used fets before and if anyone had any suggestions on how to keep the magic smoke in. I know i probably need some resistors in there but where and what values?
Thanks in advance
Damian

Kinda hard to comment without seeing your schematic.


Torben

 

[terrapinlogo]

Kinda hard to comment without seeing your schematic.


Torben

sorry i forgot to attach it when i posted fixed now

 

[Hero999]

It won't work because the gate voltage isn't high enough and there are no pull-down resistors.

 

[terrapinlogo]

Like i said in post 1 i know i need some resistors in there. where and what value? And what would a minimum gate voltage be?

 

[Hero999]

Like i said in post 1 i know i need some resistors in there. where and what value?

It depends on the switching speed, if it's just DC, i.e. you just want to reverse the motor, it's not important, just add a 10k pull-down to each MOSFET gate.

And what would a minimum gate voltage be?

Look at the datasheet to find out what gate voltage is required to pass the current required by the motor.

My guess is that you'll need to increase the voltage ot Q5 and Q6 to 12V and you'll need some additional level shifting to enable the PIC to control them.

 

[terrapinlogo]

looking at the datasheet it seems to me that it has a required gate voltage of 2-4v but im not exactly sure. is it the gate to source threshold voltage im looking at?

 

[smanches]

You need to look in the notes for the Rds(on) parameter and see what voltage they used to determine it. In your case it's 10V (which is typical of most MOSFETS) above the source pin. You will need a bootstrap supply to drive the high side FETs, or they will not turn on properly. It would be easier to get some actual MOSFET drivers at this point, as it will make the entire h-bridge much more functional and stable.

 

[mneary]

looking at the datasheet it seems to me that it has a required gate voltage of 2-4v but im not exactly sure. is it the gate to source threshold voltage im looking at?

I like to describe the threshold voltage as the point when the MOSFET begins to "leak".

As clarified above, the gate voltage that achieves the desired RdsON is what you should strive to get.

 

[terrapinlogo]

Ahh that explains alot. so at 2-4v the drain to source resistance is too high to be of much use?

 

[mneary]

At threshold, the drain current is 250 microamps. It's not a "resistance" as far as the physics is concerned.

But taking R=E/I you could consider it to be between 8,000 and 16,000 ohms.

 

[Hero999]

Why are you using a 500V MOSFET to drive a 3V motor?

There's nothing wrong with overrating components but higher voltage MOSFETs have a much higher drain resistance than lower voltage MOSFETs. Use a lower voltage MOSFET such as the IRL540.

You still haven't said what current the motor requires?

If it's <1.5A you'll be fine with a gate voltage of just 5V (see figure 6 on the datasheet) but that still means the gate voltage on the high side needs to be at least 8V, relative to 0V, because the motor is 3V.

 

[terrapinlogo]

because it is just what was in my parts bin.
0.2A no load
2A stall

 

[terrapinlogo]

If i swapped the mosfets for BD681 (NPN) and BD682 (PNP) transistors would it work?
would i need to add any resistors or other parts?

 

[Hero999]

The motor will turn slowly with those transistors because they have a high saturation voltage.

The transistor does not need to be able to pass 2A continuously, as long as it can withstand it for half a second or so.

The current will be higher than 0.2A with a load connected to the motor. How much current does the motor draw when the load you want to drive is connected?

 

[terrapinlogo]

Ok so what transistors would you reccomend? under normal loading it draws beetween .2A and .5A

 

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edited post

 

[Hero999]

Ok so what transistors would you reccomend? under normal loading it draws beetween .2A and .5A

See attached.
NPN transistors = BC548
PNP transistors = BC327

 

[terrapinlogo]

hero the BC548 only have a Ic max of 100ma so they wouldn't be able to cope with the current i would of thought?

 

[terrapinlogo]

OK I'm going to Jaycar tomorrow. Can some one provide me a schematic using only parts from Jacar that can drive a motor (3v 0.5A max load normal conditions) from 5v logic.