Hello all,

I'm trying to build my own H bridge, not wanting to start using an already made motor controller from the beginning. I've seen a lot of designs online, but there's no deciding which is better. Plus, most of those designs were not made for my desired voltage.

I will be using a 6V motor from a toy car. I don't know how much current it draws, but I'm guessing somewhere around 300mA tops. It's not a powerful motor. I would like a design that allows the motor as much power as possible, so basically something that will drop as little voltage as possible and won't dissipate much power.

From what I've read, using two N MOSFETs and two P MOSFETs as well as two NPNs works best. But I've also read that for this design to deliver the necessary current to the motor, the MOSFETs need to be above a certain voltage (Vgs?). Judging by some of the datasheets, that's not really happening at 6 volts.

Then there's the fact I'm not sure how to pair the N and P MOSFETs in the design if they DID work at 6 volts. Do they have to be rated for the same voltage or amperage? How do I choose them from the catalogue?

I hope that was somewhat coherent and I'm waiting for some answers

 

I assume you want to use the bridge to reverse the motor direction?
What is the signal you will use to control the bridge?

You need logic-level type MOSFETS.
Look in the data sheet for the Vgs (gate-source) voltage used to characterize the ON resistance of the MOSFET, Rds(on).
That Vgs voltage should be 5V or less.
This is not the MOSFET threshold voltage, which is the voltage where it just starts to turn on.

The N and P MOSFETs do not need to have the same voltage or current rating.

 

Yes, I want to be able to reverse direction just through a microcontroller. At first, the signal will just be a normal digital low or high to change direction. But I do want to be able to use PWM in the future as well.

The datasheet is a little confusing on the Vgs. Here's what I mean:

RDS(on) max. (Ω) VGS = -10 V | 0.80
Gate-Source Voltage : ± 20V
Continuous Drain Current: |VGS at -10 V| -6.5A
Gate-Source Threshold Voltage: -2V

±20V means you can't have a bigger voltage drop than ±20 between gate and source, right?
-6.5A would be more than enough, but at -10V from what I'm seeing. It would be helpful to know how much current the transistor could provide at -5 or -6 volts. This is what's bugging me with the datasheets.

 

Okay, so after a little research I found that logic level MOSFET products have a "IRL" prefix. I used that to narrow down my search. I also found the Gate to Source voltage graph that showed how many amps it could output at what voltage. I think I found my N type. It's the IRL3713. It's got a very small Rds, at about 3mΩ. Is it good?

What about P types? Do I also search for those with smaller Rds? I see these values are a lot larger than N types.

 

If your motor needs only 300mA full load, then even an IRLML series in sot-23 could be sufficient and withstand the stall current that may be ten times higher i.e. 3A, but I will suggest a larger package like the IRLL in sot-223 asi it should be much more reliable. A 30V mosfet should be more than enough. A DPAK (IRLR) would be even more foolproof if you have the board space for it.

 

I also found the Gate to Source voltage graph that showed how many amps it could output at what voltage.

Note that those are typical values, not worst-case values, so the one you buy may deliver either more or less current at a given Vgs as compared to that shown in the graph.
You never want to design to typical values.

 

Thanks for the answers.

Can you tell me what I should consider when browsing for the P MOSFETs and the bipolar transistors?

Also, can PWM allow MOSFETs enough voltage for them to output the necessary current? Is there a constraint on the frequency of the PWM signal that I should be aware of?

 

If wishing to do this with a micro controller there is two sources of schematics on the Picmicro site, one is the Mechatronics Demo board of which the manual shows all the schematics as well as the code.
There is also the Inverted Pendulum that used a similar circuit and also has schematics etc.
You may find some guidance there for your project.
Both use similar motors.
Max.

 

Your only REAL challenge in driving a homemade H bridge, is keeping the Hi and low fet from conducting at the same time.
You need a short delay between the time you turn one off and the other on.
Using a micro means you can implement in software, so that makes it easier.

 

..........
Also, can PWM allow MOSFETs enough voltage for them to output the necessary current? Is there a constraint on the frequency of the PWM signal that I should be aware of?

If the MOSFET is fully turned on when switching then it can output the necessary current.
The constraint on frequency is the rise and fall times of the switching which causes losses.
For this reason you don't want to use a PWM frequency any higher than necessary for the load you are driving.
For motors you probably don't need to go any higher than a few hundred Hertz.