Inductive Switching Problem
Inductive Switching Problem
(OP)
Hi all,
I have undertaken a project to build a 4 axis router type machine. I am attempting to build the stepper motor cicuits myself. I had already built one using the L297 + L298 chips from ST Semi but found that something was going wrong with the drive chip and it kept on blowing. I have now decided to use 4 mosfets to drive the motor in a simple unipolar mode. The 4 coils are clamped with a UF5400 diode (Ultra Fast).
The problem seems to be a huge amount of noise on the main 30V DC supply, it almost appears as though the diode does nothing and the mosfet clamps the back emf. The mosfet also gets hot with about only 1 Amp of current, it is rated to take 10 Amps and its RDS On is 0.1 Ohm.
The Mosfet is a logic level device from ST also.
Can anybody give some suggestions....
Many Thanks
Mark
I have undertaken a project to build a 4 axis router type machine. I am attempting to build the stepper motor cicuits myself. I had already built one using the L297 + L298 chips from ST Semi but found that something was going wrong with the drive chip and it kept on blowing. I have now decided to use 4 mosfets to drive the motor in a simple unipolar mode. The 4 coils are clamped with a UF5400 diode (Ultra Fast).
The problem seems to be a huge amount of noise on the main 30V DC supply, it almost appears as though the diode does nothing and the mosfet clamps the back emf. The mosfet also gets hot with about only 1 Amp of current, it is rated to take 10 Amps and its RDS On is 0.1 Ohm.
The Mosfet is a logic level device from ST also.
Can anybody give some suggestions....
Many Thanks
Mark
RE: Inductive Switching Problem
RE: Inductive Switching Problem
RE: Inductive Switching Problem
Thanks for a quick reply to my question, I feel I may have the answer. The circuit uses a sense resistor on the source pin of the mosfet. The voltage on the pin will rise with increasing current (2V for 4Amps) therefore is it possible that my 5V logic level input to the logic level mosfet is now going from 5V down to 3V. I am basing this on my understanding that the Input is with referance to the Source.
Do you think this is correct.
Mark Lewry
RE: Inductive Switching Problem
Can you move the sense resistor to the Drain of the FET? This should still allow for over current detect (which is why I am assuming you are using the sense resistor), and will eliminate the souce voltage modification.
Also, you might want to make sure that the clamping mechanism on your pre-driver works while the IC is not powered or you must use self-clamping FET's. If your circuit is ever unpowered and needs to dissapate the energy from dropping out the low side of an inductive load (creating a very large positive voltage spike), then your FET should disapate the energy. Some pre-driver's can detect the over-voltage condition and turn on the FET to disapate the energy. To be safe, it would probably be a good idea to use self-clamping FET's. This will prevent damage to the pre-driver, FET or inductive load if your pre-driver does not operate while the IC is unpowered.
Hope this helps!
RE: Inductive Switching Problem
Also, the previous post stated that you should use self-clamping FET's. However, if you use the recirculation diode, there should not be any volatge spikes higher than the diode drop. Therefore, the previous post is more of an FYI.
RE: Inductive Switching Problem
I am using the mosfet to control the low side as you assumed. The mosfet is an SGS-Thompson Omnifet VNP10N07 and is a fully auto protected logic level FET.
I like your idea about putting the sense resistor on the drain side but I fear that the voltage will rise when the mosfet is turned of and will damage the L297 controller chip. I had also tried using an external 10V supply to pull up the logic level of the 74LS08 AND gate (driven by the L297's Inhibit and Phase lines) that drives the mosfet. When no supply is present to the coil the gate input rises to the 10V but when the supply is present the gate input gets squashed down to about 4V ??? I cant understand this.
Anyway tonight I will try some other things, please keep your suggestions coming I do appreciate them.
Mark
RE: Inductive Switching Problem
Could you email me a schematic of what you are doing? I must be missing something because I just can't picture what your circuit looks like.
Thanks,
Mark_Melone@yahoo.com
RE: Inductive Switching Problem