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1KW - PWM - CLASS - E - 75meter - TRANSMITTER BUILD




 
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Author Topic: 1KW - PWM - CLASS - E - 75meter - TRANSMITTER BUILD  (Read 4326 times)
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K9MB
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« Reply #25 on: January 03, 2022, 08:27:36 PM »

One of the things we're looking at is to stack several MLC capacitors in parallel.  Possible 3 .047uF devices, or maybe a .068, a .047 and a .033 in parallel.  Still researching parts.

This is to increase the current handling capability of the RF bypass capacitors at the DC side of the RF output transformers.

Steve- Isn’t this cap in parallel with the PWM output LPF cap and form the total value of the output capacitance?
I am asking because I am planning to use WIMA high current, low inductance caps for the filter and it has never been clear to me whether the requirements for esr and rise time for these 0.15 uF caps has to be better in HF ESR than the WIMA puse caps.
In this case, you have two 0.15 UF caps in parallel across the PWM output LPF circuit, or 0.33uF total- right?

Please clarify the requirements for these caps- and whether high speed WIMA film caps can work well here. If so, it seems like straining at a gnat to try to stack these MLCC caps when bigger ones could be bootstrapped near the board. Please comment and also correct me in this matter. Thanks, Mike
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« Reply #26 on: January 04, 2022, 05:31:20 PM »

Hi Steve and all else
Please keep in mind that I am only doing 80meters so I hope the .22uf will be ok.
However, if the idea is to increase current, keep overall capacity at .15uf, fit nicely on Doug's boards, and be available for purchase then maybe the C4532NP02E683J230 would do the trick.

the pads on the board are made for a chip 2525 and these are 1812.
I already checked the layout and it would fit nicely with 2 of those C4532NP02E683J230 parts side by side on the pads. That would be .136uf pretty close to the target .15uf and I have a bunch in stock and I can see they are also available for purchase at Mouser.

What are everyone's thoughts on that possibility?

Thanks
Jeff
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« Reply #27 on: January 04, 2022, 08:32:04 PM »

Jeff -

The cap you are talking about does indeed become a part of the last stage in the LPF filter.  But it's effect is minimal as far as the filter characteristics are concerned.  Small variations in its value will have essentially no effect.  As Steve has pointed out, it's main purpose is to provide RF suppression at its point in the circuit.  As such, it's important to have a low ESR rating.  Distributing its capacitance between different caps is a plus since that divides the current surges.

Rod KQ6F
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« Reply #28 on: January 04, 2022, 10:07:01 PM »

so very interesting
So it sound like the .068 doubled up would meet the intended of .15uf value before the .22uf cap was used due to non-availability of the .15uf.

I have about 100 of the C4532NP02E683J230 and may try two of them in parallel

tks
jeff
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« Reply #29 on: January 05, 2022, 02:10:07 PM »

One of the things we're looking at is to stack several MLC capacitors in parallel.  Possible 3 .047uF devices, or maybe a .068, a .047 and a .033 in parallel.  Still researching parts.

This is to increase the current handling capability of the RF bypass capacitors at the DC side of the RF output transformers.

Steve- Isn’t this cap in parallel with the PWM output LPF cap and form the total value of the output capacitance?
I am asking because I am planning to use WIMA high current, low inductance caps for the filter and it has never been clear to me whether the requirements for esr and rise time for these 0.15 uF caps has to be better in HF ESR than the WIMA puse caps.
In this case, you have two 0.15 UF caps in parallel across the PWM output LPF circuit, or 0.33uF total- right?

Please clarify the requirements for these caps- and whether high speed WIMA film caps can work well here. If so, it seems like straining at a gnat to try to stack these MLCC caps when bigger ones could be bootstrapped near the board. Please comment and also correct me in this matter. Thanks, Mike

Hi Mike,

Any good cap can be used in the PWM filter.  We have not seen any issues in the field with various capacitors in this part of the circuit.

The reason that we can't put a lot of extra capacitance at the board is that the overload shutdown circuit, which is fairly fast and is located after the PWM filter, will start reacting to high audio frequencies from the modulator being fed to too much capacitance at the RF amplifier.  This will cause false overload triggers at high audio frequencies which is not desirable.

It was kind of a balancing act between getting sufficient bypass capacitance to do the job, without putting too much capacitance there where false overload triggers would be an issue.

Regards,  Steve
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« Reply #30 on: January 05, 2022, 04:38:38 PM »

One of the things we're looking at is to stack several MLC capacitors in parallel.  Possible 3 .047uF devices, or maybe a .068, a .047 and a .033 in parallel.  Still researching parts.

This is to increase the current handling capability of the RF bypass capacitors at the DC side of the RF output transformers.

Steve- Isn’t this cap in parallel with the PWM output LPF cap and form the total value of the output capacitance?
I am asking because I am planning to use WIMA high current, low inductance caps for the filter and it has never been clear to me whether the requirements for esr and rise time for these 0.15 uF caps has to be better in HF ESR than the WIMA puse caps.
In this case, you have two 0.15 UF caps in parallel across the PWM output LPF circuit, or 0.33uF total- right?

Please clarify the requirements for these caps- and whether high speed WIMA film caps can work well here. If so, it seems like straining at a gnat to try to stack these MLCC caps when bigger ones could be bootstrapped near the board. Please comment and also correct me in this matter. Thanks, Mike

Hi Mike,

Any good cap can be used in the PWM filter.  We have not seen any issues in the field with various capacitors in this part of the circuit.

The reason that we can't put a lot of extra capacitance at the board is that the overload shutdown circuit, which is fairly fast and is located after the PWM filter, will start reacting to high audio frequencies from the modulator being fed to too much capacitance at the RF amplifier.  This will cause false overload triggers at high audio frequencies which is not desirable.

It was kind of a balancing act between getting sufficient bypass capacitance to do the job, without putting too much capacitance there where false overload triggers would be an issue.

Regards,  Steve


Thank you for the explanation, Steve. It makes sense. I knew that you have used Orange drop caps there in the past, so the WIMA caps I have would be even faster than the orange drops, so it should work.
Interesting that the rise time spec is so fast that the bulk of the output filter capacitance is not “visible” in this time frame. Anybody who has done anything in RF or high speed buses knows that a trace to a “ground” or low impedance- looks like anything but. 😉

Just about the time I think I have peeled this onion to the middle, I learn that I know nothing much about it. It is great to have you as a resource.
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KK6IRZ
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« Reply #31 on: January 05, 2022, 06:16:46 PM »

quoted by steve_qix 1/5/22
Hi Mike,

Any good cap can be used in the PWM filter.  We have not seen any issues in the field with various capacitors in this part of the circuit.

The reason that we can't put a lot of extra capacitance at the board is that the overload shutdown circuit, which is fairly fast and is located after the PWM filter, will start reacting to high audio frequencies from the modulator being fed to too much capacitance at the RF amplifier.  This will cause false overload triggers at high audio frequencies which is not desirable.

It was kind of a balancing act between getting sufficient bypass capacitance to do the job, without putting too much capacitance there where false overload triggers would be an issue

Thanks for the info Steve
So the 2 caps I suggested again sound like a good choice to try.
With two .068uf in parallel the total capacity of .136uf is still higher than the low side tolerance of the .15uf original caps.
I may need to cherry pick a bit to ensure the .068uf caps are on the equal to high side of tolerance but seems like it is possible to be a good choice.

I am curious Steve if the issues that were seen connected to false overload trips could be related to operators trying to modulate beyond 100% and using the higher .22uf caps?
Also Steve were this seen issues while operating on 160 or 80 meter?

Thanks
Jeff

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K9MB
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« Reply #32 on: January 05, 2022, 08:39:01 PM »

I do not think there would be a difference between 160 and 80 meters for the bypass capacitors.
I suppose that if the Xc was marginally high, it would, but that should not be a factor in a choice between 0.15uF and 0.22uF.
Of course, the shunts on the drain bus are much more important to limit spikes that can toast the 11N90s. Those are indeed and specific and also bus length specific, in terms of number of devices.

For the overload shutdown circuit, high rise times are essential, so the larger value slows it down too much and affects function of the shutdown circuit.
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« Reply #33 on: January 05, 2022, 09:44:29 PM »

To answer Jeff's question - we actually haven't seen many false overload shutdown triggers at all.  Usually if the overload (properly adjusted,  of course) triggers, there is a real reason for it.

The way the overload shutdown works is with a window comparator.   The current is constantly compared to the voltage, even as the voltage moves around.  So, if the transmitter is modulating very high in the positive direction, the voltage is higher and therefore the allowable current is higher - because the current is compared to the voltage. 

This differs from essentially every other overload shutdown scheme I've seen, where typically only the current is sensed, and since the current varies with modulation, false triggers are highly likely if the overload shutdown is to actually be able to protect the transmitter in any real sense.

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« Reply #34 on: January 05, 2022, 10:49:15 PM »

Hey Steve sounds good
So it sound like the theory is if to much capacity is used at the bypass it has higher possibility of a false trigger but reality on the transmitters out there is that if everything is adjusted correctly there is little cause for alarm for possible false triggers using .15 or the .22uf caps,

The real issue in the field is the current spikes on the bypass caps and parallel caps would provide added capability in that area of the circuit.

With all this information I think I will try the .068uf in parallel
It will double the current capability and still keep the value close to original design.

thanks all
Jeff
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