How to shield my DC/DC converters ? - UK Vintage Radio Repair and Restoration Discussion Forum

georgesgiralt · 22nd Feb 2010, 2:59 pm

Hi !
In the last months I've made some DC/DC converter to power D series valve sets.
The better design I've found is the one I described in another thread (based on a Royer converter, designed by Prof Jirmann).
This device use an RM8 core gapped internally at about 0.9 mm. The device oscillate at about 26 KHz.
In order to make the sets as good as they can be I want to shield the 90V battery as well as I can. For now, enclosing all in a box made with scrap PCB soldered is not too bad, but I would like to shield the RM8 core in a better way in order to reduce the magnetic filed and electric field it radiate.
I would be glad to get your advice and design ideas.
Thanks !

Skywave · 22nd Feb 2010, 4:15 pm

The usual approach is more-or-less what you have done: enclose it all in a screened box. Personally, I wouldn't have used the PCB technique: I'd have gone for the more traditional aluminium or steel case enclosure. Now add to this thorough filtering of all lines (signal and power) that pass into the box, using feed-through capacitors plus additional capacitors and RF chokes on all these lines. If you really want to take screening to the extreme, take the above screened and filtered box and enclose it in another one with more filtering. This is what is usually done in quality signal generators and there are good reasons for going to this extra layer of screening enclosure.

HTH.

Al. / Skywave.

cmjones01 · 22nd Feb 2010, 4:56 pm

In my experience the primary source of interference from electronics is actually the connecting leads, so make sure that any wiring leaving the screened box is RF grounded as close as possible to where it leaves the box. I'm in the habit of putting 1nF ceramic capacitors from all DC wiring to ground at the edge of my PCB designs.

Chris

kalee20 · 23rd Feb 2010, 2:10 pm

Don't forget to wrap a turn of copper coil around the outside of the RM8-cored transformer (and connect to 0V). This will reduce radiated fields.

I've got mine running in an aluminium diecast box - in a Vidor CN420a attache-case portable, there's no interference. External connecting leads are unshielded. But the circuit layout is critical, precisely so that the connecting leads are RF-cold.

Whether this'll work with my Ever Ready K (where the converter will have to sit within the frame aerial - aargh!) remains to be seen. And the techniques that Al Skywave may be invoked...

georgesgiralt · 23rd Feb 2010, 4:50 pm

Hi !
Thank you for the advices ! I'll try them one by one and all together !
Something puzzle me : I've made 4 copies of the design using the same components bought in a package, on the same PCB. The only difference being the RM8 core gap (done by hand, as they are sold un-gapped).
One is into an aluminium TEKO box and radiate quite a lot (compared to the others) The one which are into a copper foil PCB box are less radiating, one of them giving no whistles on the BBC on 469 KHz.
I wonder why the copper seems more efficient than the aluminium case ? Is there a physical reason or is it a coincidence due to wiring or gap ?
Last but not least, Kalee20, should the turn of copper foil be short circuit ?

kalee20 · 23rd Feb 2010, 6:23 pm

Quote:

Originally Posted by georgesgiralt

Last but not least, Kalee20, should the turn of copper foil be short circuit ?

Yes, definitely, with as low a resistance as possible.

With the entire transformer sitting inside this shorted turn, there can be no net magnetic flux through the centre. So any lines of force which which are created within this shorted turn, have to have their return path within this shorted turn. In this way, the AC magnetic field is prevented from spreading outside of the shorted turn.

See the photo of the circuit board assembly here.

georgesgiralt · 23rd Feb 2010, 7:41 pm

So, Having a couple of 1nF caps rated 400 V on hand, I tried to solder them as close as I could on the PCB and on the output terminals.
Things have improved but very little. There is a lot of hiss and "vrooom" on the LW range.
The MW (BBC on 469 kHz ) is better.
As I had no copper foil to shield the transformer I could not try it, but I hope to find some before the end of the week.
I'll will tell you, stay tuned ;-)

22nd Feb 2010, 2:59 pm	#1
georgesgiralt Hexode Join Date: Jul 2008 Location: Toulouse, France Posts: 406	How to shield my DC/DC converters ? Hi ! In the last months I've made some DC/DC converter to power D series valve sets. The better design I've found is the one I described in another thread (based on a Royer converter, designed by Prof Jirmann). This device use an RM8 core gapped internally at about 0.9 mm. The device oscillate at about 26 KHz. In order to make the sets as good as they can be I want to shield the 90V battery as well as I can. For now, enclosing all in a box made with scrap PCB soldered is not too bad, but I would like to shield the RM8 core in a better way in order to reduce the magnetic filed and electric field it radiate. I would be glad to get your advice and design ideas. Thanks !

22nd Feb 2010, 4:15 pm	#2
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: How to shield my DC/DC converters ? The usual approach is more-or-less what you have done: enclose it all in a screened box. Personally, I wouldn't have used the PCB technique: I'd have gone for the more traditional aluminium or steel case enclosure. Now add to this thorough filtering of all lines (signal and power) that pass into the box, using feed-through capacitors plus additional capacitors and RF chokes on all these lines. If you really want to take screening to the extreme, take the above screened and filtered box and enclose it in another one with more filtering. This is what is usually done in quality signal generators and there are good reasons for going to this extra layer of screening enclosure. HTH. Al. / Skywave.

22nd Feb 2010, 4:56 pm	#3
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,678	Re: How to shield my DC/DC converters ? In my experience the primary source of interference from electronics is actually the connecting leads, so make sure that any wiring leaving the screened box is RF grounded as close as possible to where it leaves the box. I'm in the habit of putting 1nF ceramic capacitors from all DC wiring to ground at the edge of my PCB designs. Chris

23rd Feb 2010, 2:10 pm	#4
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	Re: How to shield my DC/DC converters ? Don't forget to wrap a turn of copper coil around the outside of the RM8-cored transformer (and connect to 0V). This will reduce radiated fields. I've got mine running in an aluminium diecast box - in a Vidor CN420a attache-case portable, there's no interference. External connecting leads are unshielded. But the circuit layout is critical, precisely so that the connecting leads are RF-cold. Whether this'll work with my Ever Ready K (where the converter will have to sit within the frame aerial - aargh!) remains to be seen. And the techniques that Al Skywave may be invoked...

23rd Feb 2010, 4:50 pm	#5
georgesgiralt Hexode Join Date: Jul 2008 Location: Toulouse, France Posts: 406	Re: How to shield my DC/DC converters ? Hi ! Thank you for the advices ! I'll try them one by one and all together ! Something puzzle me : I've made 4 copies of the design using the same components bought in a package, on the same PCB. The only difference being the RM8 core gap (done by hand, as they are sold un-gapped). One is into an aluminium TEKO box and radiate quite a lot (compared to the others) The one which are into a copper foil PCB box are less radiating, one of them giving no whistles on the BBC on 469 KHz. I wonder why the copper seems more efficient than the aluminium case ? Is there a physical reason or is it a coincidence due to wiring or gap ? Last but not least, Kalee20, should the turn of copper foil be short circuit ?

23rd Feb 2010, 7:41 pm	#7
georgesgiralt Hexode Join Date: Jul 2008 Location: Toulouse, France Posts: 406	Re: How to shield my DC/DC converters ? So, Having a couple of 1nF caps rated 400 V on hand, I tried to solder them as close as I could on the PCB and on the output terminals. Things have improved but very little. There is a lot of hiss and "vrooom" on the LW range. The MW (BBC on 469 kHz ) is better. As I had no copper foil to shield the transformer I could not try it, but I hope to find some before the end of the week. I'll will tell you, stay tuned ;-)