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Author Topic: A Clean Canvas - 24 Pill Class E Rig Construction  (Read 166885 times)
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K1JJ
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"Let's go kayaking, Tommy!" - Yaz


« Reply #250 on: June 09, 2010, 08:50:28 PM »

Frank,

Each vcc bus feeds three 414 drivers which in turn feeds six fets.  Can I use one diode per bus instead? (four diodes total for all twelve 414 drivers)   Or will this cause six fets and three drivers to blow if there is a failure?

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
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« Reply #251 on: June 09, 2010, 09:12:51 PM »

I put a transzorb across the VCC for the each group of IXDD414s.  A series diode in series, at least with each group of IXDD414s +VCC line is not a bad idea at all - or if you're really paranoid, a diode in series with each IXDD414's supply.

I'm guessing you have transzorbs across the drain busses as well.  You have an overload shutdown that kills the main power supply along with the pulse train.

There is nothing at all wrong with "belts and suspenders"  Cheesy

One thing that has really come far over the past few years is protection.  The old days of the catatrophic failures are behind us. Think about it - there would have to be a lot of things failed, and all at the same time, to get any sort of cascading situation going - and even then, you've taken steps (as outlined above and in posts by Frank and others) to stop it.   If something shorts out in the RF deck.  The overload shutdown immediately kills the DC to the RF amplifier before the real current gets going.  The instant the current rises faster than the voltage, the overload detect stops everything cold.

Those 18V transzorbs are amazing!  Bob K1KBW, using analog drive, had his driver putting out some 80 watts or so into an 8 MOSFET transmitter.  After a little while of this, the transzorbs finally gave up the ghost and shorted dead, but the gates were unscathed.  I've never had anything get through them under any circumstances..... but there's always a first time...

I use seperate power supplies for each module's IXDD414s in my designs, with 18V transzorbs on each power bus.

Never had a failure of any kind whatsoever the RF amplifier, modulator or driver despite my best efforts to transmit at full power into no load (many times), or on the wrong band or into a dead short or with no drive....  The only thing I've had to replace were 2 50 ampere bridge rectifiers in the power supply... and this was due to an open step-start resistor.  I put in a "real" resistor, and no more rectifier problems!!
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High Power, Broadcast Audio and Low Cost?  Check out the class E web site at: http://www.classeradio.org
K1JJ
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« Reply #252 on: June 09, 2010, 09:59:17 PM »

Thanks for the suggestions.  I'll add some diodes to the 414 busses the next time the RF deck is pulled out.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
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« Reply #253 on: June 10, 2010, 10:40:04 AM »

I've done a lot of lightning testing of transorbs. We hit 3 series 15KW transorbs with 1/2 million watts of WF5A pulse with the diodes at 85 degrees C and they hung in. I hit them 10 times with no effect.
OTOH they have a low steady state dissipation rating (few watts). Shorting the drain to the gate of a FET will place the DC supply on the transorb through the substrate of the driver. I bet the shutdown circuit isn't fast enough. I bet you have less then 500ns time to get it disconnected. Since this is a short you have to stop the pulse train in the modulator and discharge the filter.
Let's see a couple bucks worth of diodes or hope this never happens. Your rig...
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WA1GFZ
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« Reply #254 on: June 11, 2010, 08:36:11 AM »

On 75 last night I suggested a quicker shut down possible if the drive signal could be forced to zero on both phases. Jay suggested this could be done with the enable of the IXDD414. So the simple addition of one IXDD414 controlled by Steve's overload board could be used to control the enable of all the driver chips for a quicker shut down. Killing the PDM still leaves the caps in the filter charged so there is still energy to dissipate if the drive is still running.
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W1VD
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« Reply #255 on: June 11, 2010, 08:57:01 AM »

So then...as discussed...the 414 enable pins should have individual pull down resistors to ground and individual series diodes to the 'enable bus' just in case HV blasts through the FET and IXDD414?

Wish BP had disaster planning ...
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'Tnx Fer the Dope OM'.
WA1GFZ
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« Reply #256 on: June 11, 2010, 12:46:24 PM »

Well, if you really want a nuke cert you need to do the same thing on the inputs of the 414s. I would put a cap 100 pf to 1000pf  across each input  diode.
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W1IA
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« Reply #257 on: June 12, 2010, 06:30:43 PM »

An update. I rewired the SU-5 transformer thanks to Jay W1VD and got a quiet transformer. After I did the 20 amp brick rectifier took umbrage and shorted. Replaced with a 50 amp brick; now that the power supply was delivering its full current the step-start resistor opened up and I kept tripping the breaker. Replaced with a 10 ohm wire wound and problem solved. The transmitter was current starved and the audio actually cleaned up.

Brent W1IA
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