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Old 7th January 2008, 04:06 PM   #1
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Default Cross Conduction

I SIM this simple classD output stage. It's like Behringer output stage, with turnoff transistors.
The driver is idealized Vpulse, +/-10V p-p, rise/fall time=10ns, 2uS period and 0.5uS on-time (to reflect working under load). The load is 1.5ohm.
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Old 7th January 2008, 04:10 PM   #2
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"Vss#branch" is Vss current.
It shows severe cross conduction between 0->15A (lower mosfet) and -10->-25A (upper mosfet).

What/who makes this cross conduction? Is it caused by dead time?
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Old 7th January 2008, 04:47 PM   #3
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It looks like the reverse recovery transient and it corresponds with the off time of about 100ns plus the reverse recovery time of 100ns in the IRF530 datasheet.
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Old 7th January 2008, 05:01 PM   #4
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Hi, BWRX,

Thanks for the input Is there any trick to eliminate this cross conduction? I tried with IRF540-9540 and they are worse.

Why this cross conduction don't show up if I make on-time 1uS (50% of 2uS), and become worse with smaller dead time?
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Old 7th January 2008, 05:21 PM   #5
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I'm really curious and make this SIM. Upper mosfet is driven by ideal square wave V1, off=35V, on=45V.
Lower mosfet is driven by V2, off=-45V, on=-35V.
Both T=2uS, Ton=0.5uS, rise/fall time=10ns.
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Old 7th January 2008, 05:24 PM   #6
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and I still get this :
There are 2 cross conductions now.

How can we advoid this "mosfet inherent recovery time" problem?
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Old 7th January 2008, 05:48 PM   #7
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Adding a snubber at the switching node may help some.
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Old 7th January 2008, 07:22 PM   #8
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Try adding a 200ohm resistor in series with D2 and an RC snubber (R=10ohms, C=0.22uF) between VDD and SW. That helps get rid of some of the cross conduction but not all of it. I can post the switchercad files of the simulation I cobbled together in a little bit.
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Old 8th January 2008, 01:35 AM   #9
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Hi David,

R1,R2 should be atleast 1K or even less to speedup turn-off and insert the Rg as said by BWRX to slow turn-On time.
Trr=100nS is enough fast if the SWF=300KHZ even, in Class-D operation

Kanwar
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Old 8th January 2008, 03:56 AM   #10
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There is a good paper on body dioda Qrr, IR AN-967.
It's really a problem in high speed switching. This cross conduction won't show up in idle, but appears in modulated/loaded condition.

I wonder how Fumac get away with this in Mhz modulation, this Trr will be very big compared to the Mhz timing period.

BWRX, Kanwar, I will try to sim your suggestions, thanks.
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