Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

H-bridge sources low current to motor???

Status
Not open for further replies.

Los Frijoles

Full Member level 1
Joined
Oct 5, 2006
Messages
98
Helped
7
Reputation
14
Reaction score
3
Trophy points
1,288
Location
Provo, Utah, United States
Activity points
2,252
I have created an H-bridge controlled by an ATMega164P which uses P-channel MOSFETs on the top and N-Channel MOSFETs on the bottom. It draws its power from a NiCd battery from an R/C racing car and feeds the motor from said car. Both the N-channel and P-channel FETs have ~33milliOhm RdsOn and the battery is able to source full current to the motor when hooked up directly. However, when I plug the motor and battery into the H-bridge and have it run at 100% duty cycle, the motor takes a few seconds before it even starts turning and then it never gets up to full speed (I have a tachometer attached to the motor).

I am direct driving the MOSFETs from the microcontroller, so the gates are only seeing 5V even though the voltage into the h-bridge is 7.2V. However, according to my oscilloscope the MOSFETs are letting the full voltage through when I insert a 330Ohm resistor in the place of the motor.

I have no real idea as to why the h-bridge is unable to source the current to the motor, so if anyone here as any thoughts I would appreciate them.
 

dick_freebird

Advanced Member level 5
Joined
Mar 4, 2008
Messages
6,914
Helped
2,024
Reputation
4,052
Reaction score
1,864
Trophy points
1,393
Location
USA
Activity points
55,367
A low voltage microcontroller and battery may not be
able to give the MOSFETs the gate charge they need to
fully enhance the channel. If you have standard power
FETs, not "logic level", they want 8-10V to hit the spec
RON values and may be subthreshold at 3V.

I don't know this uC byt power MOSFETs want amps of
transient gate drive to turn on in under a uS. Hit it with
a 10mA output buffer and you'll be waiting longer than
your period perhaps, and never even get to fully switched.

Start with the fine print, I guess. That, and oscilloscope.
 

snafflekid

Full Member level 4
Joined
May 9, 2007
Messages
214
Helped
31
Reputation
62
Reaction score
8
Trophy points
1,298
Location
USA
Activity points
2,644
Like dick_freebird says probably the mosfets aren't turned on strong enough. You need a couple of half-bridge mosfet drivers to interface the microcontroller to the mosfets. This half-bridge driver can be powered from the 7.2V and level shifts the logic signals from the microcontroller to proper gate voltages.
 

Status
Not open for further replies.
Toggle Sidebar

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top