Dear all,
hi, my name is george (yioryos in greek) dimitriadis, and this is my first post. I would be grateful if everyone is interested to reply to my post and please excuse me for any non-conformities according to the newsgroup policy.... I will do my best to be as typical as possible.
I am trying to build a stepper microstepping driver for my small cnc mill. I am not new to microstepping and I am aware of chips such as lmd18245 or the popular a3977 from Allegro. I am also aware of the L6207 with dual bridge and built-in monostables and comparators. The thing is that since I am a hobbyist and passionate with designing my own projects, I like building from scratch in order to both learn and have a tailor-made system according to my needs.
I have experimented with L6205, a 7555 monostable triggered by a pic comparator, (pic comparator used stand-alone), at slow decay mode that worked fine. When I chose to put the comparator output directly, (without the monostable), to one of the 2 bridge logic inputs for slow decay again, or at enable input for fast decay, it also worked well but the currents should be kept low otherwise due to the relatively high Ron and the resulting high frequency losses, the chip's temperature would rise significantly.
So, I decided to build my own h bridge with low Ron mosfets, being able to achieve high frequencies and currents, utilising fast or slow decay, in order to have fine chopper control of coil current, necessary for microstepping. (The circuits attached, are not intended as the stepper motor final drivers, they are just for the development and debugging of the chopper function).-> the circuit schematics are attached at a post I sent at
**alt.binaries.schematics.electronic**with the same subject.I found that there was a very handy chip, HIP4081A, that would drive properly an n-channel fet h-bridge using charge pumps and bootstrap capacitors. Furthermore, deadband delays are implemented and input logic has been taken care of, by means that the bridge can be fully controlled by the lower side inputs.(ie if the lower inputs are high, the higher inputs are "not care", thus by having the higher inputs tied at logic high, the full bridge is controlled via the lower inputs).
But, there was a problem.... Although I think I was careful in setting up the circuit; proper bootstrap capacitors according to mosfet input capacitance,shotky diodes at bootstrap circuit, 1n5822 shotky diodes at mosfets, (I was amazed that both the hip4081a application note and datasheet does not mention anything about diode protection), decoupling caps in all chips and using the same succesful strategy as with L6205, the results were not as expected.
In L6205, the Vref I was setting for the current control,(fed in the non-inverting input of the pic comparator),was the upper magnitude limit of the chopper frequency measured at Rsense with my oscilloscope. The target current was always met with L6205.It was also dynamically linked with Vref, ie when I was changing Vref by turning the preset, current limiting was following the theory. With the HIP4081A though, the waveform at Rsense was very ugly: Instead of the usual chopper trapezoidal ripple with a magnitude=Vref voltage, there was a spiky waveform with no trapezoidal form at all, and 2 important drawbacks: I had a magnitude much higher than the one set by Vref, there was no whatsoever dynamic link between changing Vref, (it was locked at that magnitude although I was changing Vref), and the spike was extending in the negative region as well at approximatelly 1/3 of the positive spike magnitude.
One thing is for sure: I used the exact same strategy and working hardware as the one that succesfully operated with L6205 but I am missing something....
Although I hoped I would have an easy to read post for my first one, I ended up having all these.I am sorry for this.
I would be grateful if anyone is interested to tell me his/her opinion/advice considering the problem I am facing. Please excuse both my english and drawing quality.
Hope to hear from anyone out there!
Thanks,
yioryos