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Class D 80 meter rig progressing




 
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N4LTA
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« Reply #50 on: January 22, 2020, 10:24:09 AM »

It is for 75 meters. MOV said that he had to add more inductance when he used this circuit on 75 meters. that is why I went with type 3. It is rated up to 5 MHz, but maybe I m pushing it. I'll probably wind another one this afternoon with type 2. He said he had to use two separate transformers on 75. I'll do some experimenting. The PWM is ready to test also - so I am close.

This tangle is a test rig I lashed together to test the layout. I have a positive film ready to make a pc board for the same circuit with 6 devices and drivers. I have a 4" type 2 core  for it if the 3l doesn't work.

I'll post some gate and drain photos later. This is with my digital 200 Mhz scope. Not a great photograph. The fuzz seems to be frequency jitter from my Rigol frequency generator This is a one shot image with the scope. Looks reasonable.This is the gate and drain superimposed.

I raised the voltage to 75 volts and got 60 watts output with nothing nasty happening. The mosfets were not even warm. I may make a small transmitter with this board for local use and run at at 150 volts in to a  PWM modulator  That would give me about 60 watts carrier and 240 watts PEP with a couple of $4 Mosfets.

Thanks

Pat
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« Reply #51 on: January 22, 2020, 04:12:37 PM »

The waveforms looks fine to me....
Duty Cycle about 43% on both  ...... all good...


Wayne
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« Reply #52 on: January 23, 2020, 08:02:35 PM »

Decided to try to to get the two MOSFET rig ready to use by the weekend and in a more compact package.

Designed a power supply board with ell the voltage regulators needed and a regulated 150 volt main supply with current limit at approximately 2.5 amps. Also added a 74F74 flip flop to generate the two phase drive signal for the RF board. I have a 400VA transformer that is more compact and lighter. Hopefully it will all fit together in a small 300 watt or so PEP transmitter to test with.

I am waiting for a larger piece of positive resist PC board to arrive to make the larger 6 device board. Should be here before the weekend. I have a completed film positive waiting for it.

I have attached a couple of photos . The board is complete except for a couple of fuses and a bridge rectifier that will be here tomorrow. The MOSFET will need a heatsink. If I choose the proper zeners, it will dissipate  approximately 1.8 amps x 7 volts or  12.6 Watts max, probably a bit less. I am also working on a crystal oscillator board for 80 meters with a phase splitter that uses an inverter and buffer to invert the signal without creating significant delay between the two signals. Hopefully that will allow me to use some old 75 meter crystals with the transmitter.

Pat
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« Reply #53 on: January 25, 2020, 01:09:26 PM »

Blew a MOSFET out this morning. Not sure what caused it.

I wound a new output transformer on a T300A-2 core. I used 8 turns but the ractance was still not 5 x   50 ohms. I noticed that Nigels 350 watt transfromer was a yellow -red core which  is very similar to the type 3 material I used originally.  The transformer put out a little more power, but as the voltage was increased at about 50 volts the power dropped rather than rising. I was running the drivers at 8 volts.  The fet blew up and the GFI breaker tripped. I am a little suspect of my power supply. The 74F74 phase inverter also seems to have blown and  this could have cause the problem initially. I have not tested the drivers or the other fet. . I will test the drivers and repair the 74F74 chip and set it all up again.

M0VRF  you mentioned using two transformers rather than the single one on 80 meters to get sufficient reactance.  I assume that one it to feed Vdd to the two drains as per Nigel's transformer.you mentioned the other as a 1:1 if I remember correctly.  I am not sure how the primary connects on this one - I assume it connects across the two drains??? -

Thanks for any help or ideas.

Pat
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« Reply #54 on: January 31, 2020, 01:56:07 AM »

Blew a MOSFET out this morning. Not sure what caused it.

I wound a new output transformer on a T300A-2 core. I used 8 turns but the ractance was still not 5 x   50 ohms. I noticed that Nigels 350 watt transfromer was a yellow -red core which  is very similar to the type 3 material I used originally.  The transformer put out a little more power, but as the voltage was increased at about 50 volts the power dropped rather than rising. I was running the drivers at 8 volts.  The fet blew up and the GFI breaker tripped. I am a little suspect of my power supply. The 74F74 phase inverter also seems to have blown and  this could have cause the problem initially. I have not tested the drivers or the other fet. . I will test the drivers and repair the 74F74 chip and set it all up again.

M0VRF  you mentioned using two transformers rather than the single one on 80 meters to get sufficient reactance.  I assume that one it to feed Vdd to the two drains as per Nigel's transformer.you mentioned the other as a 1:1 if I remember correctly.  I am not sure how the primary connects on this one - I assume it connects across the two drains??? -

Thanks for any help or ideas.

Pat
N4LTA

My study on the Nigel's transformer seems like the transformer is close to the point of resonance with MOSFET capacitance plus SHUNT Capacitors. If the circuit is at about resonant frequency, Current or Voltage Zero Crossing will occur. If you use rule of thumbs 5 x 50 ohm calculation that is the choke approximation and actually this is not, it is resonance circuit. If you intend to function as a choke as in the W1VD current mode class D circuit  you need a tank circuit to form waves at the resonance (or near) frequency.

My guess cause of you problem is that the left leg and the right leg switcher interact each other, energy that has not been used up on the left side is absorbed by the right side or vice versa. That is why RF output power decreases. in this condition the Mosfet is excess power dissipated and damaged. I recommend to check with the 2 CH Oscilloscope on the left side drain and right side drain at the same time and watch the waveform, make sure there is no part of waveform overlap one another. how to set? set the size of the C shunt, set the inductance of transformer, set the duty cycle of driver, check how proper you drive Mosfet in term of peak Voltage, over drive sometimes creating anomalies in Drain waveform.

My 2 cents.

BR

agus..
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« Reply #55 on: January 31, 2020, 03:59:15 AM »

8V on the drivers is not enough (for SiC, fine for GaN). The FETs will still be 'resistive' and dissipate a lot.
Try 12V.

J.
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« Reply #56 on: January 31, 2020, 06:34:24 AM »

Hi all....

8 volts does seem a little low not that I have used those FETs before....

And Agus .... I think your 2 cents is too cheap and agree with your comments.....
You might have 180 degrees phase difference at the gates but not at the Drains...
There is a lot of energy swinging to and throw across the output transformer when switching between each push pull module etc:

Also Pat have built up a TX module that maybe of interest to you......
Its just for your information thats all.....and may give you ideas and stuff..

The module is H Bridge design and definitely does not have the querks of Push Pull TXs....
The load is 13R so its more High Voltage low current unit....
So at appox 50 volts applied the input power is 204 watts and the output is 189 watts ...appox 92%...
This is a little short of what you wanted but building 2 modules and using a Combiner is easy to do......
You could leave it going all night and day most probably.....

Its a different way of doing things but this is good for forums etc:


Wayne


* 75M H Bridge 13R.JPG (481.93 KB, 1600x1200 - viewed 258 times.)
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N4LTA
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« Reply #57 on: January 31, 2020, 02:03:08 PM »

Wayne

What MOSFETS are you using this design. I'd love to see more of the design.

Pat
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« Reply #58 on: January 31, 2020, 05:07:53 PM »

Hi Pat...

There IRFP240s ....... around $2.50 each .......

To be honest not many FETs work well in H Bridge and of those all probably would work in Push Pull circuits....
These were in my box labelled " there worth a try "......
They needed a lot more drive power then the other FETs I use but they ended up working well.....
I think ....... there is too much induction using 4 cores as using only 2 the efficiency is slightly better...but they run at a higher temperature of 52 degrees which isn't too hot .....
I might order some FT114-43 cores and stack them and make my own core if you know what I mean.....


Wayne
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« Reply #59 on: February 05, 2020, 09:00:43 PM »

Wayne

What are you using in the CMCD transmitter for the tuned circuit between the drains?  W1VD shows a 1000 volt ATC porcelain and a air coil inductor in parallel. I have not had any luck in the past finding a source for the high Q,  high voltage, low ESR capacitors which I am told are required. In some pictures that you posted, it looks like you may be using a Russian silver mica cap. If those work that would be excellent.

My board layout lends itself to connecting a CMCD configuration.

if I can find a workable parallel tuned set of components, i'd like to try it.

Thanks

Pat
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« Reply #60 on: February 06, 2020, 07:25:23 AM »

Hi Pat....

Yes there those Russian Silver Mica Capacitors....
I used 2 in parallel .... a 750pF and 1500pF....
The coil gets very very hot so 1.8 to 2mm ECW is best when everything is finished...
I used 1.2mm for testing ... easier to adjust etc:
Had to play around there a bit .... changing the capacitance and expanding / squeezing the coil to get the efficiency right....
The FETs are STW20NM50s ..
If you use different FETs or number of FETs the values will change ... probably not by much...

I rewound the transformer with coax .... can post another photo if you like...

I wound the transformer to give a 6 ohm load to the PWM .... the efficiency was in the low 90s .... winding it to a load of 3 ohms gave slightly better efficiency.
Noticed that Jays CMCD has a very low loading as well...
Cannot explain why but its what my FETs liked....

I built up Steves Duty Cycle circuit as well to obtain appox 43% or so....

I much prefer High Voltage low current though....


Wayne
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« Reply #61 on: February 06, 2020, 09:14:29 AM »

Another photo would be great. I have some Russian SM caps. I have all the stuff to make Jays transformers also. I may have even some already made. I have three transformers wound. I used a lot of Jay's design's back when I was doing the high powered LF stuff with the experimental license.

Thanks again for all of your help.

Pat
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« Reply #62 on: February 11, 2020, 09:24:39 AM »

I replaced the blown MOSFET and driver and re-wound the output transformer. I also built up a new phase splitter board using Rod's design. I have a prototype board coming this week for Steve's circuit. I did this one 2 sided so I had to send it off for manufacturing. 2 sided is more trouble for me to do than I like. Steve's circuit allows for duty cycle adjustment.

I also built a Butterworth filter board and a test design for using a crystal at the fundamental frequency, doubling the frequency and then cleaning up the doubled square wave to get a clean 2 x squarewave that will work with the splitter circuits, allowing me to uses a few of my older crystals on 75 meters. This is all SMT and I will have to wait a day or so on the chips.
I have be experimenting with SMT a good bit. I have used a low cost plate heater and a hot air station and have had fair results. I bought an oven from Amazon and am modifying it and changing the software. If it works as expected, I may do less through hole work and save the drilling.

I have used RCA type jacks for most signal leads lately on my boards. Amazon sells RCA to BNC adapters cheap. Probably not great for return loss but cheap and easy for low level digital signals.

I'll build the filter today and may try to generate some RF.



Pat
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« Reply #63 on: February 13, 2020, 10:11:59 AM »

Got the filter done and changed the bad driver. No drive with the new driver. Oh well must be a bad MOSFET - changed it. No drive on that same side. Checked with a magnifier. Everything looked fine.

Got some new PC boards for 4 MOSFETS, so I assembled 4 drivers and all of driver parts and installed a MOSFET on each side. Plugged in a the drive circuit and put 9 volts on the four drivers.
No drive at the output of any of the four MOSFETS.  What???  Shut down and went to read spec sheets. Nothing. The truth table was as I designed it so I am puzzled.

Went to watch TV and then remembered on thing! I couldn't have done that could I? Yes I did.  The drivers come in an A and B series. The A is 5 vollt TTL and CMOS compatable. The B series is based on the voltage feeding the drivers - I had installed B series drivers .

Went back to the old test board with two MOSFETS. It was late, but I removed the B driver with Quick Chip ( I love that stuff) and installed a A driver. Drive on the gate as expected. Installed a new MOSFET and fired up the board at 6 volts - about 5 watts, a nice sine wave. Shut down and put the heatsink back on.  50 watts with 25 volts, 100 watts with 60 volt - nice smooth adjustment. At about 110 watts, the heatsink was barely warm.

THEN  -  it was late - lets do something stupid - I raised the Vcc on the drivers on the fly to about 12 volts - power went up slightly - running about 110 watts - then 0 watts.

Lowered the driver voltage and turned off the main DC to the board and turned it back on.  It started working again??? repeated the above and it tripped again at 12 volts on the drivers and 110 watts.

Tried it again and this time it tripped at about 50 watts and is now dead - Drivers are gone on both sides, not sure about the MOSFETS.

Went to bed on that!  

Good things learned    - the output transformer worked very well - the output filter worked very well - it appears that the input impedance for the two MOSFET rig is about 30 ohms  so the 4 MOSFET rig will be about 15 ohms. That will make filter inductors for the PWM output smaller and more managable.the driver.

I am still very puzzled as to how the drivers shut down several times before failing? Any ideas? I can find nothing in the specs that describes a overload or any type of shutdown designed in

Pat
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« Reply #64 on: February 15, 2020, 03:08:55 PM »

Nothing stupid today so far !

Got the 4 MOSFET deck built with a larger transformer. Here is what I got at first RF:

25 volts   1.1 amps   21 watts on my Daiwa CN 901 wattmeter

30 vots   1.3 amps    30 watts

35 volts   1.45 amps   34 watts

40 volts  1.53 amps  55 watts

50 volts   1.8 amps   86 watts

60 volts   2.14 amps  122 watts

70 volts    2.7 amps   175 watts     this is where I am shooting for carrier to give me 700 watts PEP  - may have to drop it a bit depending on my power transformer output current. Looks like about 25 ohms at the power supply so that is what I will design the PWM filter for. Not bad though for 4 Mosfets.

Transformer is a 600VA toroidal at 116 volts so it should be good for a minimum of 5 amps . Supposed to be good for 20% overload.

78.4 volts at  3.26 amps    250 watts - as high as I  could go with my variable bench suoply - it went into current limit at 3.3 amps

The drivers and the heatsink were just barely warm at 122 watts.

But am getting there. The Daiwa meter is fairly close to what I was observing on the scope. The scope after the filter was a nice sine wave.  I need a slug for the Bird for 250 or 500 watts at 2-30 MHz.

Pardon the very cluttered bench.

Pat
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« Reply #65 on: February 15, 2020, 06:47:06 PM »

Hi Pat....

Things are starting to come along well......
The efficiency is good too so things look OK....

The load impedance ...... if it was my TX would like it to be lower.....28R is rather high for power levels limited by your Power Supply...
And on the Power Supply ... 116VAC only gives you appox 160 volts.....
You have resistive losses in the transformer too and also your regulator ....
I would work around 150 volts under load to the PWM......
Remember as well the PWM works on a Mark to Space ratio best set to 60:40 to allow head room for modulation peaks etc:
So I would work around a maximum of 60 volts to the TX....

Maybe you could comment on that Rod.... Smiley

For my TXs that H Bridge a few posts back gave 13R load....its perfect for my stuff....
Develops quite some power at 50 volts with the power supplies I have....etc:

I would play around with the turns ratio on your output transformer just to bring it down a bit...

What power levels are you after ....

Wayne
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« Reply #66 on: February 15, 2020, 07:09:18 PM »

Yes I'm with you, Wayne - 28 ohms is much higher than usual.  Depending upon what power he's hoping for, the power supply may be a challenge.

Am also puzzled by Pat's pix - they show only one transformer.  I was under the impression he was going to follow the topology I have which requires two transformers with their secondaries connected in series.

Oh well, nothing wrong with new circuits and more experiments...

Rod
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« Reply #67 on: February 15, 2020, 10:39:21 PM »

My boards were layed out with two parallelled FETs on each side so I used Nigel's single transformer. Nigel's load for three FETs on each side was 13 ohms and he got 350 watts out at 70 volts. That made for a legal limit transmitter. I am looking for  about half that power with the same power supply voltage so 25 ohms seems close. I am using SiC 1200 volt mosfets as Nigel used.

My power supply is regulated at 150 volts, as was Nigel's and so the impedance is going to be higher. I was not using my PWM supply  - I was using a bench supply for testing. My PWM supply supplying the modulator deck  is 150 volts regulated at about 5 amps maximum. I have tested it at 3.8 amps for several hours, so that I know the supply can handle that much load. The transformer is good for about 5 amps. Voltage drops to 157 volts and 3.5 amps - so I may need to find out what it drops to at 5 amps and regulate it a bit lower by changing the zeners - but that makes the regulator FET dissipate more heat. May have to change the regulator.

Nigel ran his carrier level at 70 volts and about 350 watts with 6 FETs. At 70 volts, I am getting about 175-180 watts with 4 fets which is a bit low.

I am shooting for 700-750 out PEP.

Using Jim Tonnes paper on PWM filters, The L values are not unreasonable at approx 750 uH and 500uH . Lowering the impedance would lower them.

I need to play with the secondary turns. I am running the drivers at 9 volts

Shooting for about 60 volts on the carrier like Wayne suggests is probably a good starting point

As things are, I am feeling pretty good as the efficiency is good. I may try to excite the deck at 40 meters and see what that does to the efficiency.

I put a bank of power resistor totaling about 25 ohms The power supply voltage dropped to about 144 .7 volts at a current that was about 5.6 amps as read on my Fluke DC clamp on an d the power supply analog meter. That works out to over 800 watts, so the supply can handle my projected load. The supply was run at the level for over 10 minutes. Voltage stayed within 1.3 volts. That was the raw loaded supply.  The load was a couple of 100 ohm plate resistors and a 600 watt dry 50 ohm dummy load. 10 minutes did not overheat anything. The transformer was  just warm. The power supply is a 50 ohm heat sinked bridge rectifier and 10,000uF of filter capacitors.

My voltage regulator will have to have the zener string changed. It is set at 150 volts. A good target is 140  volts. The pass MOSFET will need a larger heatsink.   4.7 volts at say 6 amps will be the likely worst case so a bit under 30 watts is a good bit of heat to get rid of. I may clip lead it in and see how hot it gets.

I am assuming that if I set the carrier level at  60 volts or about 120 watts, that the power supply needs to deliver 480 watts minimum  at 100 percent modulation.


I wish my bench supply that goes up to 120 volts could do another amp or so, so I could see exactly what the deck requires at full modulation.

Pat
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« Reply #68 on: February 26, 2020, 12:29:02 PM »

Pat,

Instead of the zener string driving the gate of the regulator fet, why not try a LR8?

The LR8 will regulate to 400 plus volts DC, just at a very low current.  Not a problem when driving the gates of fets however.

Have the LR8 output drive the gate.  Take the resistor divider to the adj pin off the source.  That way you are sensing the output.

Think of them as a low power lm317 that will regulate from something like 4 to 400 volts.

I'm using one as a screen supply for a pair of 250B tubes now.  Two 840 fets as a pass regulator and a 50watt resistor string on the output to sink any negative screen current. I dump 25 watts as heat, but it's below the rf deck in a the pressurized compartment, so heat really isn't an issue.  This gives variable bias from 200 to 350 volts and can vary the power output quite nicely.

Of you'd like, I have a spare LR8 and a couple 840s I can send gratis.

--Shane
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« Reply #69 on: February 26, 2020, 03:58:53 PM »

Shane

Thanks for an excellent idea. I was not aware of the LR8.

It sure beats a string of 5 watt zeners and the dropping resistor heat. I have found that if you dissipate more than a couple of watts, a 5 watt zener may get hot enough to melt solder, and this is with long leads.

I just ordered ten and should get them early next week. I'll change the regulator circuit as soon as they arrive.

Thanks again! Also thanks for the offer.  They sound like something I need a lot of so I bought ten of them.

Pat
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« Reply #70 on: February 26, 2020, 07:36:32 PM »

The LR8is a Tl431 on some serious steroids.

Yeah, I also buy them by the 10 packs.

With the capabilities of the LR8, I may just use it to run my bias board instead of the 431.

--Shane
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« Reply #71 on: March 02, 2020, 06:36:54 PM »

Got some work done on the PWM this weekend. The problem was a bad IR2110. I pulled the old one off the board and replaced it and the inverter works fine now. A strange and not expected problem. Things look fine and I had designed a permanent double sided board which arrived today so I am building a new board. Changing parts had the original board in bad shape.

I'll complete it and test it tonight and tomorrow.

I also got the LR8s in today so I have some more work to do.

Pat
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« Reply #72 on: March 04, 2020, 10:15:15 PM »

The IR2110 is giving me fits. The input circuit works fine now after changing the chip.

The #1 pin (low side) works fine telling me the enable circuit is working. Pin #7 stays high at 12 volts all the time - no pulses when enable is low.

The relay circuitry works fine and the off delay seem to be working. Maybe an for the high side other bad chip but that seemed pretty unlikely - I changed it but have the same results.

The app notes show a F4007 as the FRED. I have used it and two other freds . The 10 uF electrolytic bootstrap cap  does not seem to be the best cap , but I ordered some special low ESR caps. I changed it to a 4.7uF MLCC and then used a low ESR Tantalum in parallel with a 1uF MLCC.

I still suspect the bootstrap circuit but not sure what to do next. The board is so simple around the chip there is not much to check. Polarity on the FRED has been checked 10 times and all the chip to part connections have been checked multiple times with an ohmeter.

Nothing has helped - It can't be that hard?

Pat
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« Reply #73 on: March 05, 2020, 01:03:18 AM »

I'd check the input.

JB.
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« Reply #74 on: March 05, 2020, 09:22:30 AM »

I found the problem. Something stupid. Looked at a TI Ap Note about how the bootstrap works and it was obvious. The previous bad IR2110 had blown out a MOSFET so I was trying to test with no MOSFET. Can't do that as the MOSFET is part of the bootstrap circuit. - Duh

Learned a few things also. I was feeding the board with 3 volt high logic, zero volt low from my function generator which works fine with 5 volt TTL logic. had worked fine with my other circuits.
I had fits with the 2N7000 mosfet inverter. It would not switch reliably with a 3 volt signal. Figured it out by bread boarding the circuit. Works fine at 5 volts high.

I ended up with a 100uF bypass on the IR2110 and a 10uF MLCC chip for the bootstrap capacitor.

With 50 volts on the output it does fine and the unsinked MOSFETs were cool feeding 30 watt 22 ohm resistor to smoking hot. They were so cool that I didn't think they were working and burned myself on the resistor. I  was feeding a square wave  at 100 kHz and could vary the duty cycle with the generator and the output followed the input.

Next I'll hook up the PWM generator and see how that goes.


Pat
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