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Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
The fact that connecting it to earth ground made a difference is very suspicious, is your scope properly floating? Keep in mind that on most scopes the ground clip is in fact grounded.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
... wrote ...
The fact that connecting it to earth ground made a difference is very suspicious, is your scope properly floating? Keep in mind that on most scopes the ground clip is in fact grounded.
No, the scope is connected to the ground. However, I wasn't using the ground clips. I was connecting one probe tip to the gate; the other to the the emitter. I was using the ADD/INVERT function, so I did not think having the scope connected to ground mattered. Should I use a plug adapter to remove the ground connection, or do I have to use an isolated 120v source to power the scope?
As far as the voltage supplies, the tl494 is run off of 15v/0v; the fod3180 is connected to another 15v/0v. Both voltage supplies come from a single transformer that has two separate secondary windings. Could this be an issue and the emi is crossing over to the other winding? Should I use two separate transformers to supply the dual 15v?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Those scope shots look plenty good enough for government work. As others have pointed out, your scope isn't necessarily seeing what the actual gate sees.
The inter-winding capacitance of transformers can be an issue as regards letting fast pulses through. Toroidal transformers are the worst: I've seen them with interwinding capacitance of 1000pF. That's enough to give you little tingles at 50Hz, let alone 50kHz.
You need an E-I transformer with a double chambered bobbin. And even with one of those, there will be quite a lot of capacitance (and not terribly good isolation) between the two secondaries, because they're usually in the same chamber. Two separate transformers might be a better idea.
For work like this, I like the little resin encapsulated transformers with the 3kV isolation rating. That rating tells me that there'll be lots of insulation between primary and secondary, hence low interwinding capacitance. It also gives me a little more confidence that it won't break down when running a high-side driver. I've never seen one that specified an isolation rating between the two secondaries, so I've never risked it.
You can measure interwinding capacitance with a capacitance meter, just connect all the terminals of one winding together and connect them to one terminal of the meter, and likewise for the other winding.
Registered Member #2481
Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
IamSmooth wrote ...
No, the scope is connected to the ground. However, I wasn't using the ground clips. I was connecting one probe tip to the gate; the other to the the emitter. I was using the ADD/INVERT function, so I did not think having the scope connected to ground mattered.
That's not a good idea to measure fast signals... A slight difference in delay of acquisition chain of the two channels will give you a big error with fast signals...
You should use a differential probe for this kind of measurements.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I connected the clips to ground with the ADD/INVERTER. I then checked again using my differential probes. Both changes cleaned things up a lot.
However, while it cleaned up the waveform, allowing me to get to higher voltages, the problem is still there. It just is not as bad. If I go to 100v, the oscillations grow. I know if I got to 200v I'm going to have problems. Here are the images. This time I used my good camera.
I removed the snubber and the problem was still there. I put it back for the pictures. I tried a diode across the gate resistor in both directions. The problem is still there.
Could changing the IGBT help?
Image of gate waveform without any voltage across the IGBT C-E
About 100v across C-E junction. Duty is the same (probably under 5%)
Voltage is still 100v, but duty has been increased. Notice oscillations are reduced.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
IamSmooth wrote ...
I connected the clips to ground with the ADD/INVERTER. I then checked again using my differential probes. Both changes cleaned things up a lot.
However, while it cleaned up the waveform, allowing me to get to higher voltages, the problem is still there. It just is not as bad. If I go to 100v, the oscillations grow. I know if I got to 200v I'm going to have problems. Here are the images. This time I used my good camera.
I removed the snubber and the problem was still there. I put it back for the pictures. I tried a diode across the gate resistor in both directions. The problem is still there.
Could changing the IGBT help?
Each voltage square on the Y-axis is 5V. X-axis divisions are 500ns
Image of gate waveform without any voltage across the IGBT C-E
About 100v across C-E junction. Duty is the same (probably under 5%)
Voltage is still 100v, but duty has been increased. Notice oscillations are reduced.
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Help with cleaning up gate drive to IGBT buck converter
