High voltage low power regular UHU controller.

[chrugel]

Hi,
I've just buldt three regular UHU controller and I use two of them for 24V motors and i want to use the third one for a 140V motor rated 1 A.
When i turn op the voltage to about 70V the snubber resistor on the output gives of quite a lot of smoke. With snubbere resistor i mean the 1k8 2W in series with a 100nf cap across the motor output terminals.
Can i remove this? what happens if i increas the resistor to 4k 4W?


I must say these controller are ingenious!! they are working great and it was actually quite easy to set the parameters. I was unable to get the system to oscillate so what i did was to use the go command and set the parameters so the overshoot became as small as possible. Is this the right way to do it?

best regards
chris

[chrugel]

This is what I'm going to try:
First Iæm going to try and replace the resistor with a higher value and higher power handlig capabilities. probably 3k4 6W since that what i have lying arround.
The next thing I'm going to try is to use the values from the HP UHU board which also includes an inductor. I wont be needing an inductor capable of handeling 20A, probably 5 A is sufficient. This will however require heavier modifications of the PCB.
My biggest concern when increasing the voltage is that the turn-off time on the transistors gets higher that than the delay and turn on time. This will cause severe power loss in the output stage. Guess i just have to try and see what happens.

regards
chris

[chrugel]

Without knowing what happend i just blew a UHU chip . . ...
The problem is that one of the encoder inputs seems to be irresponsive on the chip. I have three identical setups and if i move the chip to another board the error follows the chip.
Also the corresponding gate in the 74LS14 chip is blown together with 3 74ACT245 IC on the BOB.
And i have no idea how this could happen!!!!!!


Anyway i just replaced the capacitor with one on 22n instead of the 100n one with no effect on the heat in the resistor. Very strange!
I am going to try with a inductor there in series before the RC filter, hope that helps. I need to double the voltage .. .

regards
Chris

[chrugel]

Hello again!

Today i decided to do the math on the output on the high voltage UHU controller. I officially dont see the purpose of the original filtercomponents: 100nF and 1k8 this gives a -3dB point at about 1khz with a pwm signal at 20kHz you could probably just short the cap without any effect.

On the HP UHU controller the -3db point is at about 2.3MHz which is probably moore resonable.

I have decided to create a new filter on my controller with the following values:
Cap : 6 nF
Res: 25 Ohm
Ind: 5 uH

This will result in a -3db pint at one MHz wich i feel is moore resonable. I only need 3 amps of current capability in the inductor since my controller is limited to about 1,5 amps.

anyway thats all for now.
Feel free to comment. . . . .

regards
chris

[contactirfu]

Hi Chris - we left all the calculations to the experts - I feel difficult to comment - but I think it would be easier to get one of the HP UHU kits and build it.

just my 2c

[chrugel]

Originally Posted by contactirfu Hi Chris - we left all the calculations to the experts - I feel difficult to comment - but I think it would be easier to get one of the HP UHU kits and build it. just my 2c

If i knew then, what i know now i probably would have done so. My problem now is that I probably doubled my budget and cant afford to spend much more on this. I probably got to eat cheap the couple of days just to get another UHU controller chip!

It has been stated the it should be possible with up to 150 v on the regular UHU with the IRFP260 transistors. These are rated 200V so 140 which I'm aiming for should not be a problem, i think . . .
I know that the reason for the HP UHU design was problems with handeling powers excess of 500w which I'm not even nearing. I have to try and make it works . . . .
I was hoping someone had done this before, running the regular UHU design with excess of 100V.

regards
Chris

[kreutz]

Originally Posted by chrugel If i knew then, what i know now i probably would have done so. My problem now is that I probably doubled my budget and cant afford to spend much more on this. I probably got to eat cheap the couple of days just to get another UHU controller chip! It has been stated the it should be possible with up to 150 v on the regular UHU with the IRFP260 transistors. These are rated 200V so 140 which I'm aiming for should not be a problem, i think . . . I know that the reason for the HP UHU design was problems with handeling powers excess of 500w which I'm not even nearing. I have to try and make it works . . . . I was hoping someone had done this before, running the regular UHU design with excess of 100V. regards Chris

The reason of the HP_UHU design was the cross-conduction above 70-80Volts independent of the load. The RLC filter at the output is not a low pass filter, it is a snubber.

[chrugel]

Originally Posted by kreutz The reason of the HP_UHU design was the cross-conduction above 70-80Volts independent of the load. The RLC filter at the output is not a low pass filter, it is a snubber.

It's been quite a while since i had this at school so please be patient with me!
Isn't the function of the components in the RC or RLC filter at the output to limit the dv/dt going to the motor? in other words limit the higher frequencies towards the motor? Again is this not a lowpass filter that acts as a snubber??

cross-conductin, isn't the turn on time of a MOSFET dependent on the current thru it? Or have I got it all wrong?

kreutz, first of all I must say you have done a great job on the HP-UHU. I've been thru most of the posts concering HP-UHU, which is a lot, and you most certainly know what you are doing!
Is there no simple way for med to use the controller i have for this low power application? Do I really need the HP-UHU?

regards
Chris

[chrugel]

This will be my next move:

Replace the 1n4007 diodes in the gate network with HER105.

This beacase the HER105 is a super fast diode compared to the slow 1n4007, trr of 50nS compared to 2000nS

Replace the IRFP260 transistors with IRF640.

The gate capacitance of the 260 is 5200pF while that of the 640 is1300pF. The original UHU was designed for IRF540 which has a capacitance of 1700pF

Create a RLC snubber network on the output with these values:
R 25 Ohm
L 5uH
C 6nF

Since it seems to be nearly impossible to obtain this inductor at the time beeing this will be omitted and there will only be a RC snubber there for preliminary tests.
I dont think this should be any problem since the C is so small, of cause this will yeld higher dv/dt rates at the motor.
(frantically searching the web for diy inductor design formulas . . . )

Just to be clear, my goal is not a HP UHU controller just a high voltage version without the snubber heating problem and cross-conduction.
Ultimate goal: 150V 3 A

regards
chris

[kreutz]

Originally Posted by chrugel This will be my next move: Replace the 1n4007 diodes in the gate network with HER105. This beacase the HER105 is a super fast diode compared to the slow 1n4007, trr of 50nS compared to 2000nS Replace the IRFP260 transistors with IRF640. The gate capacitance of the 260 is 5200pF while that of the 640 is1300pF. The original UHU was designed for IRF540 which has a capacitance of 1700pF Create a RLC snubber network on the output with these values: R 25 Ohm L 5uH C 6nF Since it seems to be nearly impossible to obtain this inductor at the time beeing this will be omitted and there will only be a RC snubber there for preliminary tests. I dont think this should be any problem since the C is so small, of cause this will yeld higher dv/dt rates at the motor. (frantically searching the web for diy inductor design formulas . . . ) Just to be clear, my goal is not a HP UHU controller just a high voltage version without the snubber heating problem and cross-conduction. Ultimate goal: 150V 3 A regards chris

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.

[chrugel]

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

[kreutz]

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.