The AM Forum
July 15, 2024, 03:58:47 PM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: 1 [2]   Go Down
  Print  
Author Topic: Looking for a board from Rod's PWM design  (Read 7178 times)
0 Members and 1 Guest are viewing this topic.
vk3alk
Member

Offline Offline

Posts: 251


« Reply #25 on: December 27, 2023, 04:55:44 PM »

Thanks for those comments Steve and was hoping you would post a comment on that diode...
I read an article about this diode in a broadcast transmitter service manual and was looking amongst all my stuff but couldn't find it...
I did mention that I used a MUR160 but missed out on a 6 as it was the MUR1660 one....

With HAM transmitters we only use them at odd times so probably you could get away with not using one but 24/7 type TXs its a must really...
Even something like a PL259 connector which often comes loose could be a source of this back EMF voltage problem causing TX modules to suddeningly go that away...

No amount of sequencing will stop this event....

There are some really good high voltage fast diodes around now  Smiley but to be honest not sure whether a 600 volt one is high enough....

I suppose there is no protection circuits that are 100% but you can minimise .....

Wayne

Logged
vk3alk
Member

Offline Offline

Posts: 251


« Reply #26 on: December 28, 2023, 02:40:06 AM »

I just thought of something thats worth doing with the UCC chip ( PWM Generator )

SS pin 14 on the UCC35701/2 meaning Soft Start....
Its a very good feature and worth while experimenting with.....

I would use different capacitor values and see the effect on the duty cycle on startup...
You don't need the generator connected to the output circuit just play with the values and see what happens  Smiley
Would take no more than 5 minutes of mucking around...

What this means is .... you maybe able to achieve say a startup time of 250mS which would mean that no sequencers would be required at all...

I suppose I do use a squencer in a way as my regulator has a deadtime control pin on the TL598 and using a time constant have made a delay of appox 250mS before full output....

I do have one of Steves generators made up in my spare parts so could try myself I suppose but this would be a good learning thing when you test your boards etc:


Wayne




Logged
w9jsw
Two shots of Whisky
Contributing
Member
*
Offline Offline

Posts: 599



« Reply #27 on: December 28, 2023, 07:28:12 AM »

Using SS is an interesting idea to explore.

Logged
steve_qix
Contributing
Member
*
Offline Offline

Posts: 2593


Bap!


WWW
« Reply #28 on: December 28, 2023, 09:32:32 AM »

Wayne, proper sequencing will be needed, soft start or no soft start.

The biggest issue is the transition from transmit to receive.  The transmitter is in full output mode at the start of this transition.

Therefore, the drive to the RF amplifier, and the load ON the RF amplifier MUST remain constant for some number of milliseconds after the PWM drive to the modulator MOSFETs is stopped.  There will be energy remaining in the PWM filter, RF bypass capacitors, etc. which must be drained away before the RF amplifier drive and load can be safely removed.  This doesn't take very long to happen - maybe 100ms, but that transition time is rather important to ensure that the RF amplifier is putting out no power before the drive and load are removed.

At least in my designs, there is a de-facto soft start due to the PWM generator design.  The PWM waveform ramps up in width at a rate that falls within the mid audio spectrum.  However, the system is sequenced when transitioning from receive to transmit, as well as going the other way.  When coming up, the RF drive and antenna relay are switched first, and then 50ms later, the PWM is enabled.

It may "work" without sequencing, but this is definitely not optimal.  I like to have all elements of the system in a known state at all times.

If the transition from receive to transmit is too long (more than 50ms in my humble opinion), it gives way for doubling, because people don't know that you are going to transmit due to too long a delay in the receive-to-transmit sequence/transition time.  At least around here, you have way less than a second to get your transmitter on the air, or you will likely end up doubling :-) :-)  It's tough out there !!!
Logged

High Power, Broadcast Audio and Low Cost?  Check out the class E web site at: http://www.classeradio.org
w9jsw
Two shots of Whisky
Contributing
Member
*
Offline Offline

Posts: 599



« Reply #29 on: December 28, 2023, 11:32:55 AM »

A while back, I decided to make a board that combined T/R relays, a relay for a RX only antenna, and a relay for sequencing bias, etc. The board contains a 2 relay sequencer that gives me control of the antennas separate from the transmitter. It is the same sequencer circuit I used in my 813 plate modulated rig. I also included a small 24V power supply if needed for control voltages.

It is turning out to be a very handy board. I plan to use it on this project.


* CM TR Schematic v1.0.pdf (82.12 KB - downloaded 61 times.)
Logged
steve_qix
Contributing
Member
*
Offline Offline

Posts: 2593


Bap!


WWW
« Reply #30 on: December 28, 2023, 02:00:52 PM »

A while back, I decided to make a board that combined T/R relays, a relay for a RX only antenna, and a relay for sequencing bias, etc. The board contains a 2 relay sequencer that gives me control of the antennas separate from the transmitter. It is the same sequencer circuit I used in my 813 plate modulated rig. I also included a small 24V power supply if needed for control voltages.

It is turning out to be a very handy board. I plan to use it on this project.



That's a good plan.  In my station, since I am changing transmitters all the time, I ended up (about 50 years ago) building a master sequencer.  There are 2 lines coming from this sequencer - the antenna/driver/receiver mute line and the transmit enable line.

To these lines connect various relays that are involved in the functional aspects of controlling the various pieces of equipment.

One thing I also suggest is somewhere in the RF driver, configure a relay that closes when the MOSFETs have drive.  In my case, I use a 1N34A diode connected to one of the gate drivers, working into a 1k resistor and then a .001uF capacitor to ground.  This drives the base of a 2N3904 (through a 10k resistor), which pulls in a 12VDC relay.

The contacts of the 12V relay are in series with the relay coils that enable the modulator / power supply.  No drive?  No key up of the modulator and power supply.  This is a PREVENTATIVE circuit - in other words, the relay in the RF deck not pulled in prevents the modulator from working. 

The modulator/power supply itself is keyed in the 2nd sequence, whereas the RF drive for the gates of the MOSFETs is keyed during the 1st sequence.  However, once the 2nd sequence is active, the modulator/power supply won't actually start doing anything unless RF drive is detected and the associated relay is operational. 

I am a fan of preventative circuitry - where some part of the system cannot operate unless all of the dependencies (RF drive, etc.) have been first satisfied.
Logged

High Power, Broadcast Audio and Low Cost?  Check out the class E web site at: http://www.classeradio.org
vk3alk
Member

Offline Offline

Posts: 251


« Reply #31 on: December 28, 2023, 05:47:55 PM »

Thanks for the remarks regarding Sequencing etc:

I did use them in my Class E days but since going over to Half Bridge Modulators and H Bridge TXs haven't bothered really...

Have printed out your post and will read over later this morning when I have a cappuccino coffee at the  shops... Smiley



Wayne
Logged
Pages: 1 [2]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.053 seconds with 19 queries.