Originally Posted by chrugel  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 |
The 100K resistors give a reference ground for the upper Mosfet capacitors to bootstrap circuit while the low side Mosfet is OFF( high impedance). It solves a sudden death Mosfet driver problem at power up as per International Rectifier tech notes.
On your realization that the original UHU snubber was arbitrarily calculated, that is my first impression when I saw it. The resistor value is too high for any useful work. Since I don't know the original designer, I will probably never know the reason.
About the original gate circuit; it is designed to give asymmetric turn on-off times, but the Mosfet driver already has asymmetric current handling, so it could be eliminated in favor of faster turn on-off times and lower switching losses because we control dv/dt at the motor's terminals (with the RLC snubber).
Best regards,
kreutz.