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Flybuck converter funnies

So I built this board for my fire prevention equipment customer. (An excellent outfit BTW: Argus Fire Control.) It needed two isolated 5-V supplies, one for the analogue stuff and one for RS-485.

A few months ago we were discussing those supplies in this very boutique, in the "Coupled Inductors--how coupled is coupled?" thread. I wound up with a flybuck design using an LM3103 sync buck chip.

Using a simple fixed duty-cycle model with realistic resistances for the switches and inductors, it simulated very nicely. However this switcher chip doesn't work very well at all in the application, apparently on account of its DCM mode, which makes it stutter unless there's a serious load on the non-isolated side (as in a normal buck).

When it stutters, the feedback voltage stays stable, but the isolated outputs collapse, of course--I need the output FETS to keep on switching, and they don't. There's no apparent way to turn this feature off.

Looks like I'll have to use something simpler.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs
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Yeah, the block diagrams of modern switcher controller chips look like the aerial view of Baltimore (with the burning bits too!)

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

The leading contenders so far are the Richtek RT7272 and RT2862A.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

On Tue, 26 Apr 2016 15:11:01 -0400, Phil Hobbs Gave us:

Instead of StarBuck Flying...

It could be FlyBuck Smoking!

Reply to
DecadentLinuxUserNumeroUno

Yes, it's more of a Skybuck converter at the moment. (The poor guy has had a lot of broken hardware over the years.)

Cheers

Phil Hobbs

Reply to
Phil Hobbs

On Wed, 27 Apr 2016 04:41:44 -0700 (PDT), Phil Hobbs Gave us:

Between his concept of ESD and his obsession with taking apart live gear, it is no wonder.

Shame he hasn't touched the AC feed terminals what with their double the voltage of here thing. One in each hand.

Just a shame. :-)

Reply to
DecadentLinuxUserNumeroUno

Well, I tried, and both stutter at light loads too. The RT2862 datasheet makes no such claim, but does it anyway. The plus side is that when I load down the primary, they produce heaps of isolated power, so the magnetics and so forth are working fine.

Anybody got a favourite sync buck that'll handle at least 36V input and

400 mA, and keeps on PWMing at light loads?

'Cuz otherwise I'm going to have to do it the fully-manual way, with a bridge driver and so on (yuck).

&*$%%!!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Was about to say TPS54233 or something like that, but I immediately realized those miss exactly those specs (30V and pulse skipping)...

LTC3810 is the only controller I know offhand, but that's rather a lot of brain (and additional circuit) for present levels. I don't know if there's a smaller, cheaper, integrated version out there, but hopefully it's a hint at least.

Suppose I might suggest dropping the sync requirement, since the max voltage is high enough not to matter much. But you'd know better than I..

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

The flybuck architecture (which I understand to be a buck with a second winding, to make a negative voltage) needs a pushpull drive to work over the full load range.

I've used a diode-capacitor voltage doubler off the switcher node to make a negative supply, similar idea. A catch-diode type buck switcher won't make the negative voltage if the positive supply is unloaded, or if the switcher chip decides to stutter.

These days, I mostly buy cheap SIP or DIP converters to make a negative or bipolar power supply.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

Not strictly, but you do need more load on the normal output. Otherwise, the "flyback" doesn't fly all the way back because it's being clamped by the negative output's load.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

It's a flybuck converter, so I need to use the bottom FET to dump the voltseconds into the secondaries. Normal bucks can generate isolated power too, but only if the main output carries most of the load.

In this design, the 'main' output (the usual buck converter output point) isn't loaded at all--the voltseconds all go to the secondary winding of the coupled inductors.

The Microchip MCP16312 looks like a good possibility, though the 32V abs max rating is a bit skimpy--it's running off a 24V supply, so that doesn't leave too much room for the transzorb to operate. I'll give it a whack though.

Other possibilities solicited!

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Or think off-the-wall...

"OldStyleBuckSwitcherWithAddedNegativeOutput.pdf" on the S.E.D/Schematics Page of my website. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
           The touchstone of liberalism is intolerance
Reply to
Jim Thompson

Buy a cheap potted dc/dc converter?

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

I'm making two isolated +5V from +24.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Sorry Phil, can you give the specs again.

36V in, you want two 5V outputs at 400 mA each? Or is one of those negative? And I believe you've got a price point too, how much money?

For $4-5 you can get made pre-packaged DC-DC converters on DK. (ROF-78E5.0-0.5)

George H.

Reply to
George Herold

They're ratiometric, though, and may or may not have the required product life. (Plus the application is cost-sensitive and high enough in volume to justify a certain amount of assing around getting there.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Thanks. The cheap ones aren't isolated (yours), or aren't regulated (John's CUI ones). Once I get this working, it'll be about $2 per side all told.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

Some are regulated, but they cost a little more. But there are lots of multi-sourced parts, especially the SIPs.

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

I bought some cui's that were regulated and isolated (more $).. I think the max V spec was32 and not 36V... As long as you've looked those over.

George H.

Reply to
George Herold

How will you get regulation and isolation from a flybuck converter? I guess I don't understand the topology.

Are the +-5 mutually isolated?

--

John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

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