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Blowing FETs




 
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ka1tdq
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« on: March 03, 2017, 01:46:53 PM »

I replaced all the FETs on my 75 meter class E rig to be on the safe side, I turned it on and the same two FETs blew again.  Each FET has its own IXDD driver, so I'm thinking that something's up with those two drivers.

So, I'm going to completely re-do the heat sink assembly.  Luckily when I built it, I made it a bolt-on assembly so that it can come off the chassis.  I can build it at the desk and then reattach it.  Judging from a lot of my other projects, I must've counted on this blowing up at some point.

Jon


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steve_qix
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« Reply #1 on: March 03, 2017, 02:19:37 PM »

You should never blow fets.  So, tell us more about the drivers.  How are the drivers driven?  Is there an approximately 200 ohm resistor in series with the input pin on EACH driver, right at the input pin?  This is very important to prevent parasitics, which are the most likely cause of your problem.

Another possibility would be a severe imbalance between FETs on the same bus.  This is not likely, but is possible.  Look at the gate waveforms.  Pick one FET to be a "control", and then on a dual trace scope, make sure all of the other FETs on that bus have the same gate waveform, and most importantly, the rise and fall times of the waveforms for all fets are identical and in lock step.

Also, you could have too much drain "flyback" voltage.  How high does the drain voltage rise on peaks?

Regards,  Steve
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ka1tdq
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« Reply #2 on: March 03, 2017, 06:22:14 PM »

There are 8 FETs and a driver for each one.  We talked before about not using an input bus for the drivers since they're prone to parasitics.  I had already built it at that point with the busses.

At the center of each driver bus is another IXDD614 to drive the bus.  All in all, there's ten IXDD614's in this transmitter. 

The center IXDD614 gets its signal from a WA1FFL tube grid driver kit.  Each phase has one.  I did this so that I could use a small signal DDS VFO to drive the rig.  I key power sequentially to the WA1FFL kits for the drive, and the VFO is on all the time.  I don't hear the VFO during receive.

The output of each WA1FFL kit is fed to the center IXDD614 via twisted pair wire about 20 inches long. 

I don't have a 200 ohm resistor in each IXDD input lead.

The ground bus, driver bus and drain bus are all made out of brass.  I know that's a no-no, but it's what I did and it did work for quite a while.

I'm going to re-do this with all copper.  I'm going to reduce the number of drivers in half.  I'll put the 200 ohm resistor in line with the IXDD driver inputs. 

Jon
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ka1tdq
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« Reply #3 on: March 03, 2017, 09:16:48 PM »

I found the problem.  When I disassembled the heat sink assembly, I saw that the IXDD drivers for the 2 FETs that blew up were cracked in half.  I never would've seen it without complete disassembly.  Because of the way it was constructed, a total rebuild was necessary anyway.  The good thing is that the heat sink unbolts from the chassis, so this shouldn't be too bad.

Jon


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« Reply #4 on: March 03, 2017, 11:51:54 PM »

Sounds like progress.  While you're in there, might as well put the resistors in series with each input lead.  I have found it can make a huge difference in keeping parasitics as bay !

Regards,  Steve
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« Reply #5 on: March 04, 2017, 01:09:08 AM »

I replaced all the FETs on my 75 meter class E rig to be on the safe side, I turned it on and the same two FETs blew again.  Each FET has its own IXDD driver, so I'm thinking that something's up with those two drivers   ....

Jon

Hi Jon,

Looking at the pic of the FET mounting with those flathead screws,   seems to me that this could possibly damage the plastic encapsulation of these devices,   if adequately torqued.   And,   if not adequately torqued,   heat transfer would most likely suffer.

Could not see the drivers,   but if these also use flathead screws,   the drivers might also be at risk.

It is customary to see a flat washer,  and a pan-head screw used to mount this type of package to a heat sink.

This is probably  not  the cause of the issue that you are seeing,   but just   IMO,   FWIW.    Vic
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WD5JKO
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« Reply #6 on: March 04, 2017, 08:05:13 AM »


Jon,

   Perhaps you can post a schematic of that Wa1FFL VFO amplifier. I am wondering if those units are phase stable, and if not, your two drive signals might not be phased properly.

Jim
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ka1tdq
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« Reply #7 on: March 05, 2017, 03:39:45 PM »

I bought some #4 washers and now I'm using flat head screws.

I'm not sure what you mean by phase stable.  I'm using the same Chinese VFO as before, and there's a setting to have the phases 180 degrees out.  When I added these two kits (one for each phase), I just lowered the output level of the VFO and let these do the work.  They just spit out what they're given. 

My initial explosion was caused by an audio over-driving condition.  I don't think it was a phase issue. 

I have the new heat sink assembly almost done.  I'm hoping to test within a week.

Jon
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ka1tdq
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« Reply #8 on: March 06, 2017, 02:20:04 AM »

I finished the rebuild and here's the drain waveforms at 400 watts.  Side A waveform looks choppy and side B looks pretty clean. 

What could cause all that choppiness?  I built both sides identical.

Jon


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« Reply #9 on: March 06, 2017, 07:22:47 AM »



Jon,

   It appears that the drive to one side is coming on early. The yellow waveform shows "back porching" which indicates the drive comes on while the drain waveform is decaying from it's peak. This is a dissipative condition, and puts that group of FET's at risk of blowing.

   My reference of phase offset on the drive from the Haggerty amplifier is a concern. The digital drive signals need to be exactly 180 degrees out of phase, and the same pulse width. Use your 10X probes (adj. compensation with common GND point), and look at the drive signals.

Jim
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ka1tdq
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« Reply #10 on: March 06, 2017, 10:02:58 AM »

As it turns out, my homemade 1000x scope leads were noisy.  I connected to the drains using the factory probes and here's the result at 400 watts.

You're right about the tube driver kits.  When I stuck it inline, the blue trace got noisier and there was a current imbalance.  When I just use the VFO to drive the IXDD's, it's even current and the traces are cleaner. 

It's a bummer that I have to go back to two-handed operation going from TX/RX, but it's certainly manageable.

Jon


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ka1tdq
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« Reply #11 on: March 08, 2017, 01:30:49 PM »

...and, I made a relay box that's keyed sequentially just like the tube driver kits were.  This time, during receive the VFO outputs have 50 ohm loads across them and switched to the transmitter during TX.  It works our really well!  Again, I can't hear the VFO during receive.

Jon


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WD5JKO
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« Reply #12 on: March 08, 2017, 07:33:12 PM »

Jon, 

Are you back on the air now? If so, when do you operate?

Jim
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ka1tdq
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« Reply #13 on: March 08, 2017, 08:47:34 PM »

I just finished all my testing today and everything seems fine.  I'm actually a little leary now.  I've had so many explosions, bangs, fires in the shack this year that I key the transmitter with some hesitation.

I'm listening to 3870 right now to both the AM net and the SSB net operating on the same frequency.  I might jump in there a little later once more AMers get on. 

But overall, I don't get on very much.  I'm like a guy with a '68 Corvette in the garage and take it out twice a year. 

Not to get off-topic, but I just bought the RSP2 SDR receiver and I'm using the SDRuno software.  Very nice stuff!  I can memory jump between 40 meters and 75 meters AM with the click of a button. 
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WD5JKO
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« Reply #14 on: March 08, 2017, 11:03:08 PM »


Jon,

   As to the blow ups, is your power supply current limited, or are you using some of the other protection circuitry available on Steve's class E website? What about SWR protect?

  Around here 3875-3880 is usually open in the early evenings. Perhaps we could make a sched this weekend. PM me if interested. I can muster about 180 watts AM (~1KW PEP) with my Gonset G-76 and Gonset GSB-201 amplifier.

  I too have been off on a SDR fling. Just got a new to me Flex 5000 with 2nd RCVR, and ATU. Plenty to play with there. That is an amazing device that went into production way back in 2007.

Jim
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ka1tdq
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« Reply #15 on: March 09, 2017, 09:29:20 AM »

The only protection circuity I have is a few TransZorbs on the drains.  The power supply is unregulated and I use a meter to monitor SWR. 

I joke about the blowups, but this rig is actually very stable.  I used it for a long time with no problems.  When I overdrove the audio, that's when things went bad.  I got on the AM net last night and again had no problems with normal audio drive. 

Jon
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« Reply #16 on: March 09, 2017, 09:38:50 AM »

John doesn't wear protection.  He uses the std of class E, Heising.....  Lol.  Steve and I have been on him for some time to build a pwm.

When the rig works, all his rigs sound great.  I've only had a handful of qso's with Jon, but his audio is great.

Catching Jon on.....  Lol.  That's about as easy as catching me on lately.

If you guys make a schedule this weekend, I'll listen in and see if Tejas can 'make the trip'.


--Shane
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ka1tdq
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« Reply #17 on: March 09, 2017, 09:50:10 AM »

I sent a PM, but if others are interested I'm thinking Sunday evening around 6:30pm Arizona time.  I have to help out with the kids all weekend until then when reinforcements arrive.  An open frequency in the AM window at that time would work.  

Who needs all that protection anyway?  My version of class E is about as basic as you can get.  Heising modulation, a regular old power supply and a purchased DDS VFO.  Throw some MOSFETs in the middle and you've got yourself a FB transmitter!  

Jon
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WD5JKO
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« Reply #18 on: March 09, 2017, 01:55:57 PM »

Who needs all that protection anyway?


 Grin

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ka1tdq
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« Reply #19 on: March 09, 2017, 07:49:21 PM »

The show must go on!
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KD6VXI
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« Reply #20 on: March 12, 2017, 11:26:02 AM »

I sent a PM, but if others are interested I'm thinking Sunday evening around 6:30pm Arizona time.  I have to help out with the kids all weekend until then when reinforcements arrive.  An open frequency in the AM window at that time would work.  

Jon

It's 825 am now, and 1525 GMT.

This thing a go, and if so  is this the time of year (last day, actually) where we are the same as your time, Jon?

--Shane
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ka1tdq
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« Reply #21 on: March 12, 2017, 12:00:27 PM »

According to my great and mighty iPhone, California is on the same time as Arizona right now. I'm going to look for a clear frequency at 6:30pm and text Jim.

I'm also texting an ex-coworker/friend in Gilbert. He'll give me a signal report from there. It'll be interesting because we're close, but there's a mountain in between us.

To make it easier, you can PM me your phone number and I'll text you as well.

Jon 
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« Reply #22 on: March 12, 2017, 01:49:29 PM »

That will work.  I'll be on the ssb station, so no good audio, but will be listening..

Will txt my cell to you.

--Shane
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