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2018-10-19, 23:34:07
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Author Topic: Controller No5 With Protection - Looking for Explosions  (Read 78736 times)

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Increasing the drains (bifilar coils) voltage from 24V to 42V still does not return the crackling sounds.
A mild whispering is all i can detect together with some shown oscillations on the scope.
I tried all kinds of combinations of frequency, pulse width and delays.

Could it be that the present setup with 900V Cree MOSFETs and 14A drivers is to beefy for the effect to occure (like avalanching or so)?

Itsu
   

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Buy me some coffee
Hi Itsu
I never tried the Cree Sics they do have quiet a high on resistance but not sure if that is enough to stop the crackling, do you see the large pulse appear when you do a sweep on the delay.
   

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First screenshot shows the gate signals (yellow and Blue) of the 2 MOSFETs (220ns delay), and the drain signals (green and purple) of the same MOSFETs with attached bifilar coils.
To be noticed is that allthough the gates are switched 220ns apart, the drains show a timed switching.
I would have expected that we would see also a delayed switching on the drains (220ns apart).
(yellow and green are MOSFET 1, blue and purple MOSFET 2).
 
Second screenshot shows more delayed gate signals (700ns apart) and again the timed drain signals.

So the delayed gate signals do not translate directly to a delayed drain signal, is it the tramsformer function of this bifilar coil causing this?


Itsu
   

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Hi Itsu
I never tried the Cree Sics they do have quiet a high on resistance but not sure if that is enough to stop the crackling, do you see the large pulse appear when you do a sweep on the delay.


you think 0.065 Ohm is quite high?       I am sweeping (+15), but no pulse is seen up till now.

Itsu
   

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Could it be that the present setup with 900V Cree MOSFETs and 14A drivers is to beefy for the effect to occur (like avalanching or so)?
Yes it could be.
Also, the drain pulse shape can be different (and its frequency content)
   

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Hmmm,   i went back to using the 2x IRFP260N MOSFETs driven my a single dual MOSFET driver IXDD604 with 42V on the drains, but no more crackling is heard here too.
It looks like boxing up Peters box has tamed the crackling as that is the only change i did.

I will continue to do some sweeping, but up till now no crackling pulses / oscillations are noted.


Itsu
   

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Bump.


since i boxed up Peters box as mentioned above, i never was able to create the crackling sounds i had.
I will go over this setup again to see why that is.

For now i have again the earlier setup using Peters box (Freq. 200Khz, Pwidth 64ns (max.), phase 220ns).
I have 18V on the MOSFET driver (dual output IXDD604) and 25V (2x batteries) on the drain of ONE MOSFET.

I use only ONE MOSFET now to see how this ONE MOSFET behaves
The other MOSFET has no drain voltage nor is the MOSFET driver input connected.

I use the max Pwidth from Peters box to get the max gate signal.
The both MOSFETs are loaded with a 100 Ohm resistor (so no (bifilar) coil).

The screenshot below shows in yellow the drain voltage of the working MOSFET and in purple the gate signal of
this MOSFET.

We see that the MOSFET opens quite fast/good, but it closes very  very late.
We also see that the gate signal (purple) is nowhere near the 18V that the driver is running on.
This causes the MOSFET to linger in off/on state to long i guess.
The Pwidth of Peters box (64ns) has been shown by the vertical cursors.

I think i have to improve on this simple resistive setup before going into the inductive bifilar coil setup.

Why does the gate signal stay on longer then the driving pulse (108ns versus 64ns)?
When using the required 32ns Pwidth, the gate signal is only about 7V, how to improve this to 18V (which is the driver Vcc)?
How to improve the MOSFET close time (adding a schottky diode across the gate resistor?)?

MOSFETs are irfp460's


Thanks,  Itsu
   

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Try measuring what this type of MOSFET does with these kind of pulses, like this:

   
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