Originally Posted by kreutz  I don't know about the PCB you are using, changing the output Mosfets by IRF640 is a wise decision, also replacing the bootstrap diodes by HER105. You might get a big improvement just by doing that. Add a couple of 100K 1/2 W carbon composition resistors from the high side mosfets' sources to power ground.
I recommend using the same output snubber used with the HP_UHU it was calculated for that voltage range. Current won't be a problem. Reduce the gate resistance to 51 ohms and get rid of the diode resistor combination at once. You can leave the 15 V zeners on the gates. Do another test. Resetting and UHU chip losing eeprom contents depends a lot on the ground noise and that is dependent on the PCB design.
If you were able to completely suppress cross-conduction, ground noise is not going to become a big deal, but all depends on the logic ground and power ground being connected only at one point near the power ground terminal.
Good Luck!
Snubbers act as "dissipative" high dv/dt energy suppressors, in a certain way they act as low pass (dissipative) filters. The calculations are completely different to "normal" low pass filters.
Kreutz. |
Thank you for the respons kreutz!
What is the purpose of the 100k resistors from the output to Vmotor?
Why do you recomend replacing the whole gatenetwork with one resistor?
Isn't the purpose of this network to create higher turn-on-times and faster turn-off-times?
The following part of the post will be about my understanding of snubbers and the original UHU snubber. It is going to be long and boring, so I completly understand if you don't continue reading beyond this point. ..
Today I've spent quite some time trying to understand the concept of "snubbers". Found some good pdf files online the explained them quite well.
Basically a snubber absorbs energy, either in the form of controlling voltage or current. Instead of looking at the frequency respons of a filter we are here looking at the impulse respons of the same type of circuts.
What i dont understand is the snubber-netvork on the original UHU design with the values 1k8 and 100n.
These values don't fit innto any of the formulas I've found so fare.
The rule of thumb i found for choosing the C value was 4 times the output capacitance of the Transistor, in this case about 6nF. The R value on the other hand is harder since this is best placed equal to the impedance of the resonant circut, in this case the output capacitance and stray inductance circut. This way the resonant circut will be critically damped. This is impossible to achive without trial and error in the specific controller - motor configuration and a digital storidge. But a rule of thumb for the mximum size here is to make Tau, the timeconstant for the RC snubber, nothing less than 10 times bigger than 1/f. And a rule of thumb on the lower limit is to not allow discharge currents thru the transistor at higher rates than the peak current capabilities of the transistors.
In the original UHU case with IRFP260 transistor this yealds C = 6nF and R between 2 an 800ohm probably in the 20 to 100ohm range.
The timeconstant in the original UHU is 4 times lower than the period time. For all practical purposes this will give a powerloss in the snubber-resistor that is half the supplyvoltage^2 / R. At 40V this is 0,4W. and 60V it is 2 W!
I have to get my hand on a storage oscilloscope so i can make som serious testing here!
Anyway thats all for now.
chris