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Old 14th June 2004, 10:00 AM   #31
ekaerin is offline ekaerin  Sweden
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Chris,
The tail was from an IGBT. I have personally never seen it on FETs at least
not to that extent. I dont think the tail will contribute to the pumping of
the supply.
Do you get the pumping in spice with an ideal voltage source ?

On my bench at work I use a LeCroy LT344 waverunner but even that
is to slow when looking at diode recovery. Then I need to use a
4ch 1GHz sample / 2ch 2GHz sample scope to get it nice. But I could also
have used the old analog Tek we used to have. The best thing with the
digital scopes is the wave storage for further analysis.
If you want to get a scope, my personal advice is to go for an old Tectronix scope.
Nowadays people go for digital but the old analog scopes can be really
fast, and probably sheap too. No menus and crap, all is there on the front panel.
But even with the best of scopes, measuring anything switchmode is not
an easy thing to do. You need to be careful where you ground the probes
if you're not using isolated diffprobes. Also, common mode noise to give you
fake ringings etc.

/ Mattias
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Old 14th June 2004, 10:04 AM   #32
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Hi,

Quote:
Do you get the pumping in spice with an ideal voltage source ?
Yeah it really looks like it, sounds silly doesn't it? I woulnd't go so far as to say that's what's causing it yet, so for now I'll blame it on the DC offset which isn't acceptable at .5V. Same sort of effect though.

Chris
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Old 14th June 2004, 03:10 PM   #33
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Default tail in MOSFET

Hi !

I was also not aware of a tail current in MOSFETS.
In EVAs screen shot it would be interesting to see the drain current.
I am not sure if the delayed voltage
of the FET might be caused by some parasitic capacitance.
It might be that the current drops without much delay, but only the voltage swing is delayed.
Of course it is hard to measure fast voltages and currents correctly related.
Normally the current measurement equipment ( current probe + amp) delays the current trace longer than the voltage trace.
This gap can easily be around 100ns ... 200ns, already a long signal
wire between current measurement equipment and scope gives some ns delay.....
And not everyone has a LECROY at home that allows to adjust that timing! (@eakerin: I am trying hard not to get envious )

@ Chris!
Yes, it is a good idea to buy a scope.
Simulation and reality typically do not match perfectly at first approach.
Usually you have measure the real thing and the differences against
the simulation will give you hints about several parasitic effects.
In second step you may try to add these undesired effects to your simulation.
After this the simulation may again help to get rid of such troubles.

Cheers
Markus
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Old 14th June 2004, 07:47 PM   #34
Pafi is offline Pafi  Hungary
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Ekaerin!

Please let me know where did you measure these waveforms! (E.g. by schematic. Plz indicate the type of IGBT too!) I don't know what does tail exactly mean.
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Old 15th June 2004, 12:19 PM   #35
ekaerin is offline ekaerin  Sweden
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Marcus and Pafi,

1st. Marcus, I whish I had thoose scopes at home but I have them only
at work.....

2nd. Pafi, I can not show you the schematic. But it is from a DC/DC fullbridge
included in a 3phase input rectifier with true 3phase PFC as input boost.
The DC/DC runs from about 380Vdc from high side to output.
The IGBTs where tested to see if was possible to use them because of
lower cost compared to MOS-FETs. However, dispite theese where the
fastest IGBTs for the application the tailing losses became too much at
high temperatures. So we could not use them more than up to an
ambient temperature of 50C. That was about 140C on the IGBTs case
when runaway started to show up.
The IGBTs wereWarp-2 series IRGP50B60PD1 from International Rectifier.
The device is aimed towards 150kHz and although we used 50kHz we
could not use it in our design. But it is anice device anyway.

/ Regards / Mattias
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Old 15th June 2004, 09:03 PM   #36
Pafi is offline Pafi  Hungary
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Ekaerin!

It's OK, but still I don't know what does 1,2,3,4 waveforms represent, so please tell me!

How much current did you expect to switch with that IGBT?
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Old 16th June 2004, 08:01 AM   #37
ekaerin is offline ekaerin  Sweden
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Pafi,
The scaling, offsets and timing are not perfectly aligned but the traces are:
Ch1: C-E voltage of lower switch
Ch2: C-E voltage of upper switch
Ch3: Current of lower switch (turning on)
Ch4: Current of upper switch (turning off and shows the tail)

The peak current is about 25Ap in each IGBT.
Current probe used is a LeCroy AP015 with a 50:1 ratio small toroid core
around the leg of the transistors. (very cheap but very good)

/ Mattias
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Old 16th June 2004, 11:40 AM   #38
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Hi everyone,

I haven't been able to improve the DC offset at all, any sugestions? I'm stuck with -0.5 volts on the output. Powering comparator with 40mA and the output resistor Rb is 30 ohms for 20mA output....input is going straight to the base, any sort of grounding scheme (divider...10pF cap...) makes no difference.

Of course it's spice so the transistors are perfectly matched. The diodes that seem to work best for me are 1n914's and transistors are 2n5401 and 2n5551.

The pwm output seems centered about zero until it starts riding on the inverse of the input which looks like power supply pumping, but that can't be it as there isn't one it's just VDC sources, while the sine wave output is what has the -.5 dc offset.

Any ideas would be well appreciated.
Thanks,

Chris
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Old 16th June 2004, 02:00 PM   #39
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Hi,

Comparator circuit with output wave, input was 50mV 1khz. Even the graph of power over the load is riding an offset which I seriously don't understand, could this be a spice failure? I'll demonstrate more clearly what I mean by that in the third pic.

Thanks,
Chris

PS: I'll try to squeeze in the full circuit as well
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File Type: gif comparator.gif (5.5 KB, 120 views)
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Old 16th June 2004, 02:02 PM   #40
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Output waves, ignore the filename that's the filtered output over the load
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File Type: gif comparatoroutput.gif (12.4 KB, 96 views)
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