UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 24th Mar 2023, 7:20 pm   #1
dougietamson
Hexode
 
dougietamson's Avatar
 
Join Date: Mar 2019
Location: Edinburgh, Scotland, UK.
Posts: 341
Default Low cost DC-DC boost converter 400v

I bought one of those very low cost variable DC-DC boost converters with the idea of maybe using it in a valve tester. I bought the -400v 0 +400v version.

I thought I'd try it out on a pre-amp, the pre amp worked well but unless the voltage is set to minimum or maximum it has a nasty 10Hz sawtooth wave on top the DC, about 1vp-p.
Any ideas about filtering it?

I could set the output to max and use some extra RC filtering with dropping resisters to set the voltage.

Doug
Attached Thumbnails
Click image for larger version

Name:	400vDC-DC-boost.jpg
Views:	109
Size:	159.3 KB
ID:	275685  
dougietamson is offline  
Old 24th Mar 2023, 7:42 pm   #2
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Low cost DC-DC boost converter 400v

I'm probably not going to be able to suggest a simple fix for this. But other people might, in which case I suspect that they'll want to know how much current this device is going to be delivering. What current is the device rated for ?

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline  
Old 24th Mar 2023, 7:50 pm   #3
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
Default Re: Low cost DC-DC boost converter 400v

Try a bit of load on it and see if it goes stable. It looks like a feedback loop just hunting off load.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 24th Mar 2023, 7:58 pm   #4
dougietamson
Hexode
 
dougietamson's Avatar
 
Join Date: Mar 2019
Location: Edinburgh, Scotland, UK.
Posts: 341
Default Re: Low cost DC-DC boost converter 400v

Quote:
Originally Posted by GrimJosef View Post
I'm probably not going to be able to suggest a simple fix for this. But other people might, in which case I suspect that they'll want to know how much current this device is going to be delivering. What current is the device rated for ?

Cheers,

GJ
I'll give it some work to do, maybe try some 15W bulbs from scrapped microwave ovens.
dougietamson is offline  
Old 24th Mar 2023, 8:00 pm   #5
dougietamson
Hexode
 
dougietamson's Avatar
 
Join Date: Mar 2019
Location: Edinburgh, Scotland, UK.
Posts: 341
Default Re: Low cost DC-DC boost converter 400v

Quote:
Originally Posted by Radio Wrangler View Post
Try a bit of load on it and see if it goes stable. It looks like a feedback loop just hunting off load.

David
Yes, defo looks like the feedback loop, at about 50v output the waveform is visible but not enough to trigger the small scope, about 65v it is able to measure it.
dougietamson is offline  
Old 24th Mar 2023, 8:38 pm   #6
dougietamson
Hexode
 
dougietamson's Avatar
 
Join Date: Mar 2019
Location: Edinburgh, Scotland, UK.
Posts: 341
Default Re: Low cost DC-DC boost converter 400v

I'm not expecting to power much as they are about £7 and quite small, 60mmx50mmx20mm.
dougietamson is offline  
Old 24th Mar 2023, 8:43 pm   #7
dougietamson
Hexode
 
dougietamson's Avatar
 
Join Date: Mar 2019
Location: Edinburgh, Scotland, UK.
Posts: 341
Default Re: Low cost DC-DC boost converter 400v

Photo added
Attached Thumbnails
Click image for larger version

Name:	DCDCBoost400v.jpg
Views:	163
Size:	55.8 KB
ID:	275692  
dougietamson is offline  
Old 24th Mar 2023, 10:49 pm   #8
trh01uk
Octode
 
trh01uk's Avatar
 
Join Date: Nov 2010
Location: Ross-on-Wye, Herefordshire, UK.
Posts: 1,648
Default Re: Low cost DC-DC boost converter 400v

Quote:
Originally Posted by Radio Wrangler View Post
Try a bit of load on it and see if it goes stable. It looks like a feedback loop just hunting off load.

David
Does it? To me that exponential waveform looks like a typical capacitor discharge/charge cycle. In which case the remedy is more capacitance on that rail, surely?


Richard
trh01uk is offline  
Old 24th Mar 2023, 11:30 pm   #9
cmjones01
Nonode
 
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
Default Re: Low cost DC-DC boost converter 400v

If the ripple is at 10Hz, it's definitely going to be the converter's feedback loop being unstable. Adding capacitance is unlikely to fix it. It'll just slow it down. Adding a bit of load may well help, as has been suggested above.

That said, boost (and by extension, flyback, as used in very many small mains switch-mode power supplies) converters are notoriously difficult to get to be stable under varying load conditions. This doesn't usually matter when they're powering a more or less constant load that isn't too fussy about ripple and noise. Data sheets for (good) boost converter chips will typically go in to some detail about the frequency compensation arrangements needed in the feedback loop to keep them stable.

The fundamental problem is that trying to increase the duty cycle of a boost converter because the output voltage is low actually reduces the output voltage for a few cycles, because the decreased off-time of the switch allows less time for the energy in the inductor's magnetic field to flow out in to the load. The output power only starts to increase once a new steady state is found. This delayed-action effect plays havoc with the feedback loop's attempts to keep a constant output under varying load.

Chris
__________________
What's going on in the workshop? http://martin-jones.com/

Last edited by cmjones01; 24th Mar 2023 at 11:36 pm.
cmjones01 is offline  
Old 24th Mar 2023, 11:59 pm   #10
trobbins
Heptode
 
trobbins's Avatar
 
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: Low cost DC-DC boost converter 400v

Have you tried alternative input supply and local buffer cap. A weak supply may be another cause of what is being observed. I will see if I have a schematic and notes, as that PCB looks similar to what I tested a few years ago ( but populated for just single output).
trobbins is offline  
Old 25th Mar 2023, 2:38 pm   #11
trobbins
Heptode
 
trobbins's Avatar
 
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: Low cost DC-DC boost converter 400v

I had a look at that pcb back in 2016 - the link below includes a schematic. I haven't used that smps since then as it had pulse skipping and burst type modes depending on the loading.

https://www.diyaudio.com/community/t...ol-ics.297112/

Since then I have found the 12V dc-ac inverter modules much more convenient, as they just pump out power without any quirky operating modes, but do need a rectifier and filter added to an appropriate secondary tapping. Ebay typically has them with a header description something like "DC 12V to AC 110V 220V 150W Inverter Boost Transformer Power Adapter" - they can come in different layouts, and are based on the SG3525A control IC.

Some links that may be relevant:
https://www.diyaudio.com/community/t...-for-b.313234/

https://www.aggh.net/discussion/index.php?topic=51615.0

Last edited by trobbins; 25th Mar 2023 at 2:43 pm.
trobbins is offline  
Old 25th Mar 2023, 3:03 pm   #12
dougietamson
Hexode
 
dougietamson's Avatar
 
Join Date: Mar 2019
Location: Edinburgh, Scotland, UK.
Posts: 341
Default Re: Low cost DC-DC boost converter 400v

Quote:
Originally Posted by trobbins View Post
I had a look at that pcb back in 2016 - the link below includes a schematic. I haven't used that smps since then as it had pulse skipping and burst type modes depending on the loading.

https://www.diyaudio.com/community/t...ol-ics.297112/

Since then I have found the 12V dc-ac inverter modules much more convenient, as they just pump out power without any quirky operating modes, but do need a rectifier and filter added to an appropriate secondary tapping. Ebay typically has them with a header description something like "DC 12V to AC 110V 220V 150W Inverter Boost Transformer Power Adapter" - they can come in different layouts, and are based on the SG3525A control IC.

Some links that may be relevant:
https://www.diyaudio.com/community/t...-for-b.313234/

https://www.aggh.net/discussion/index.php?topic=51615.0
Good work, thanks Tim

Doug
dougietamson is offline  
Old 26th Mar 2023, 1:42 am   #13
trobbins
Heptode
 
trobbins's Avatar
 
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: Low cost DC-DC boost converter 400v

Doug, a recent restoration of just a Ferrograph 2A/N amp chassis from 1955 needed a new B+ and heater supply as they weren't on the original amp chassis. I used an external 12Vdc 5A plugpak and retrofitted a 12V dc to ac inverter pcb (set up for 300Vdc B+) on the amp chassis - quite simple and convenient as the amp chassis had plenty of spare room. Ciao, Tim
https://dalmura.com.au/static/Ferrog...2A-N%20amp.pdf

Last edited by trobbins; 26th Mar 2023 at 2:10 am.
trobbins is offline  
Old 26th Mar 2023, 9:43 am   #14
dougietamson
Hexode
 
dougietamson's Avatar
 
Join Date: Mar 2019
Location: Edinburgh, Scotland, UK.
Posts: 341
Default Re: Low cost DC-DC boost converter 400v

Quote:
Originally Posted by trobbins View Post
Doug, a recent restoration of just a Ferrograph 2A/N amp chassis from 1955 needed a new B+ and heater supply as they weren't on the original amp chassis. I used an external 12Vdc 5A plugpak and retrofitted a 12V dc to ac inverter pcb (set up for 300Vdc B+) on the amp chassis - quite simple and convenient as the amp chassis had plenty of spare room. Ciao, Tim
https://dalmura.com.au/static/Ferrog...2A-N%20amp.pdf
Nice write-up.

Is this the one, photos attached,

Looks like couple of MOSFETS buzzing 20KHz into transformer primary with various taps on the secondary.

Only £6.50 so bought one to try, while I wait for it to arrive I might crack open an old car cigarette lighter adapter 12v-230v rated for 150W that used to power a laptop on the move from the late 1990s to find out what's inside.

Do you know if the pad at V1 is connected to a tap on the transformer?

The default output voltage wiring as follows:
V0-V2 = 110 V
V0-V3 = 172 V
V0-V4 = 220 V
V0-V5 = 220 V
V5-V4 = 20 V
V5-V3 = 45 V
V5-V2 = 110 V
V4-V3 = 28 V
V4-V2 = 90 V
V3-V2 = 62 V

Doug
Attached Thumbnails
Click image for larger version

Name:	12v-220v-inv-top.jpg
Views:	40
Size:	36.0 KB
ID:	275738   Click image for larger version

Name:	12v-220v-inv-bottom.jpg
Views:	35
Size:	30.9 KB
ID:	275739  
dougietamson is offline  
Old 26th Mar 2023, 11:17 am   #15
trobbins
Heptode
 
trobbins's Avatar
 
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: Low cost DC-DC boost converter 400v

Yup, that's the 'long' version I bought a few of. Photo of the rectifier & filter I used for the Ferrograph amp. V1 pin has no winding wire connected to it, but I snipped the pin off anyway as I used the V1 pad for the rectifier/filter wiring - which uses smt diodes and enough cap (100nF) for local smoothing of the switching frequency.

Because I could, I added a smt cap to bypass 3525A Vcc and Vc direct to 0V trace going to main cap. I added an smt cap directly across the main 2200uF cap pads and bolstered the gnd plane between FET sources. I sat the FET tabs flat and bolted to pcb to improve thermal conduction and lower resistance to transformer terminals - as this application didn't need additional heatsinking.

Timing resistor RT on 3525A pin 6 is 4.3kohm to 0V, to provide 25kHz switching frequency with timing cap CT of 5N6. RT can go down to 2k. I changed RT to 3k4.

As well as the 3525A app notes, some other forum threads that have some insight are:
https://www.diyaudio.com/community/t...-for-b.313234/
https://www.diyaudio.com/community/t...-37khz.311812/
Attached Thumbnails
Click image for larger version

Name:	Ferrograph pcb.jpg
Views:	43
Size:	83.0 KB
ID:	275745  
trobbins is offline  
Old 27th Mar 2023, 6:39 pm   #16
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
Default Re: Low cost DC-DC boost converter 400v

I've used one of those converters to replace the vibrator PSU [whose transformer had failed] in a R209 receiver.

See this thread: https://www.vintage-radio.net/forum/....php?p=1503469

Yes, they are intrinsically noisy, but with a fistful of small chokes and feedthrough-capacitors, along with some decent shielding [in my case a soldered-together box made of PCB material], they can be made quiet-enough to live inside the same case as a HF receiver.

I never did fully scope/analyse the waveforms involved, but I suspect they use several methods [both classic PWM and some sort of mode where they skip oscillation-cycles if the output voltage is already high enough] to do the required regulation.
__________________
I'm the Operator of my Pocket Calculator. -Kraftwerk.
G6Tanuki is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 8:12 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.