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Class D Switching Power Amplifiers and Power D/A conversion

Massive ringing with IPP320N20N3
Massive ringing with IPP320N20N3
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Old 19th November 2019, 06:30 PM   #21
JLesterP is offline JLesterP  Philippines
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Quote:
Originally Posted by Baldin View Post
Interesting!

Tried antiparallel diodes on gate resistors. No change.

What is your switching freq?
All with no snubbers?
So you also use gate buffers in this design?
Post a picture?
Switching freq around 320kHz. Snubber 10R + 100pf.
bypass / decoupling caps right at the mosfet pins.
Green PCB output mosfet is IPP320N20N3, no gate buffers, only 10R + 1n4148 + 4.7R for gate turn off.
Scope traces are from the green PCB.

I used ZXGD on the blue PCB prototype, but it uses IRFB4227. Also I don't have scope traces of the blue PCB because I already sent it on the other side of the globe for evaluation.
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Old 19th November 2019, 06:46 PM   #22
Baldin is offline Baldin  Denmark
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Waooo nice result.
The first yellow cap from left is that decoupling from rail to rail?

I'm even using SMD ....
My results must then be down to layout and not enough decoupling :|
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Old 19th November 2019, 06:51 PM   #23
JLesterP is offline JLesterP  Philippines
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Yes yellow is for rail to rail decoupling.
Might also be a problem with your scope probe, did you try to test other class-d amps?
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Old 19th November 2019, 07:47 PM   #24
Baldin is offline Baldin  Denmark
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I need to try to connect a cap directly over the fets from rail to rail .... could that be it?
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Old 2nd December 2019, 10:16 PM   #25
Eva is offline Eva  Spain
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Of course the dent in d-s capacitance vs. Vd-s in newer faster MOSFET will awaken any hidden demons (resonances) in the layout. But these chips switch fast and abrupt.

Then there is the chasing ghosts phenomena, a 10:1 or 100:1 probe getting substantial ground loop ringing, or a 1:1 causing ringing itself due to good 50~100pF of input capacitance.

And there are plenty of slower chips for slower layouts.
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Old 2nd December 2019, 10:42 PM   #26
Eva is offline Eva  Spain
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Lets remember some switching speed. 2 Layer PCB. SMD on both layers. TO-220 IRFB4227. IR drivers. Zetex buffers. Gate OFF: 1500W 33% duty shown. Gate ON: 4500W 33% duty shown. Reaching 150V/7ns transients.

(But be aware that even though this was for a product that has been producing money for 10 years, the company refused to continue sharing the money.)
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Last edited by Eva; 2nd December 2019 at 10:54 PM.
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Old 3rd December 2019, 08:08 AM   #27
NMOS is offline NMOS  Germany
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How about to add feriite beads to solve problem ?
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Old 3rd December 2019, 08:13 AM   #28
kASD is offline kASD
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Massive ringing with IPP320N20N3
Quote:
Originally Posted by Eva View Post
Of course the dent in d-s capacitance vs. Vd-s in newer faster MOSFET will awaken any hidden demons (resonances) in the layout. But these chips switch fast and abrupt.
The Ultra Junction[superjunction] Mosfet design team at IXYS confirmed, it has nothing to do with the PCB layout, no matter how tight you make it, problem is inherent to Gate construction, the noise is much higher than planar mosfets.

Last edited by kASD; 3rd December 2019 at 08:15 AM.
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Old 3rd December 2019, 08:30 AM   #29
SemperFi is offline SemperFi  Wake Island
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Interesting thread! How do we identify if a MOSFET is planar? Manufacturers have all kinds of superduper names for their devices.
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Old 3rd December 2019, 09:18 AM   #30
SemperFi is offline SemperFi  Wake Island
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Quote:
Originally Posted by Baldin View Post
Thought I had found the ultimate MosFet for class D
Have been experimenting IPP320N20N3 instead of my usual IRFB5620.

I really though one of my boards was fried as I en the first test got massive ringing.

I mounted another board up with IRFB5620. Only limited ringing (before mounting of snubbers).
Then changed to IPP320N20N3 ...... see pictures!!!

Boards based on IRS2092 with ZXGD3003E6 buffers and 4R7 gate resistors.
Pre + post filter fb.

Data sheet would suggest this is a real good MosFet, fast, lower Rds(on), higher current capability, low Ciss ... all in all better than IRFB5620 !!

Anyone having experience with these Fets?
What am I missing here?
So same circuitboard (layout) same resistor values, only two different MOSFETs.
The parasitic inductances are the same. Ringing is caused by excitation of inductance, the ringing frequency is a combination of capacitances with the inductance. Ringing amplitude (or overshoot) is due to L*di/dt. So if the di/dt is greater, the peak is greater. If the ringing lasts longer, the capacitance is larger. The difference is therefore due to a device that pulls the edges up and down faster, which must mean it has higher gm, and also has more parasitic capacitance.
The par-capacitance may be similar at high voltages, but at the lower voltages where it gets extremely non linear and goes sky high is perhaps where the major differences are. And all MOSFETs are always shown with different graphs, some log-log, some log-lin, all at different zooms, etc.

The interesting part is taming it. According to kASD it is inherent in the MOSFET, at least in some MOSFETs, and not controllable thru Rg very interesting.

EVA also has a very important point in that probing these fast edges is almost impossible to do with 100% certanty.
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