Why don't they make power supplies like this...?

comprising a flyback step-up

implement :

I mean no more than

problems where

reservoir cap.

of supplies

haven't yet found anyone

benefit at negligible

Now you know why I end up rolling my own switcher almost every time :-)

--
Regards, Joerg

http://www.analogconsultants.com

comprising a flyback step-up

implement :

I mean no more than

What inrush current? For the line-side, this is usually handled by a cheap thermistor.

It's unusual to have inrush problems on the DC side as the capacitors tend to be small.

I had to add inrush limiting on an old huge Heathkit stereo-- big old

80,000 uF capacitors made the house lights dim. Heathkit often bobbled a small detail or two.

comprising a flyback step-up

implement :

I mean no more than

Look at the inrush spec on a typical 200W SMPSU - typically 10-20A at 230V Now run ten of these PSUs on the same mains power supply & watch the lights flicker when you hit the breaker to turn them all on.

100-200A peak on a nominally 8A supply can't be a good thing.

I'd look at the size of the capacitors after the first bridge rectifier. Typically about the size of a chapstick up to maybe a C cell. If the PS doesnt have anything to limit the current before this, and you switch on the power at the peak of the cycle, yes, you're going to get a current spike. Most power supplies use a 40 cent thermistor to limit this current.

If the PS has some power-factor correction circuitry before this point in the circuit, then you're right, you'd think that circuit could do a smart startup at a zero-crossing.

On a sunny day (Tue, 12 Dec 2006 09:30:29 GMT) it happened Mike Harrison wrote in :

comprising a flyback step-up

implement :

zero I mean no more than

flicker when you hit the

There is an other problem too, with many of those on one phase (cable), that is that the current these draw is not a sinewave.

More like this: . . . . . . . . .---0 . etc As losses in the cables and transformer (where it ultimately comes from) are proportional to i^R, a high peak current leads to a _VERY_ high heat generation in the cables. If you design your own switchmodes and expect many in parallel, better is to use the good old inductor (choke):

------ L ------- ~ + ------------ | AC bridge === rectifier --- | ---------------- ~ - ---------- This will give a more sine like current in the mains, but will make a heavy unit, large iron core, airgap...

On a sunny day (Tue, 12 Dec 2006 13:15:18 GMT) it happened Jan Panteltje wrote in :

Correction, no DC, no airgap, no.

Unfortunately the majority of PFC front ends use a boost converter as the front end. That means that the input supply is connected directly through to the bulk DC bus capacitor via the boost, or a bypass, diode at start up and there is therefore no inherent surge limiting.

A flyback stage in this location might achieve what you are looking for but its output voltage is negative with respect to the input which makes interfacing some things tricky. Input and output current are both discontinuous as well which makes noise problems worse.

Micro Linear (defunct) did have a method for doing things slightly differently but the main DC bus output voltage rode on top of the input rectified line so you end up with interface problems again. The method would avoid inrush currents and maintains continuous input current but the voltage rating of the power switch needs to be greater.

The only topology (well one that comes to mind) is the SEPIC converter. It provides a positive voltage referenced to the same ground as its control electronics. However it is slightly more complex/harder to design and places more stress on the components used. In particular the main switch sees a peak of (VIN+VOUT) and has to carry a peak of (IIN+IOUT).

It does overcome the major inrush current that would arise from charging the main DC bus capacitance but the coupling capacitor still has to be charged so there is still some inrush but it is of much shorter duration. If you have a look about the TI website you should be able to find a couple of papers by Lloyd Dixon that deal with the design and compenstaion of a SEPIC converter for use as a PFC front end.

DNA

comprising a flyback step-up

implement :

I mean no more than

problems where

reservoir cap.

of supplies

haven't yet found anyone

benefit at negligible

On a sunny day (Tue, 12 Dec 2006 13:18:49 GMT) it happened Jan Panteltje wrote in :

Oops, an other correction:

--- ~ + -- L ------ | AC bridge === rectifier --- | --- ~ - ----------

And yes an airgap in the inductor. Hope I got it right this time.

comprising a flyback step-up

:

mean no more than

problems where

reservoir cap.

of supplies

haven't yet found anyone

benefit at negligible

Fairchild have the ML4826 and the ML4841 which combine PFC with soft start.

Looking at the MOQ of 1000pcs @ $5.52 (ML4841) it is not hard to see why there are so few PC psu's with such functions. The PC manufacturers would prefer to use a much cheaper option to keep costs down. If that means providing only the PFC function as specified by the new requirements then that's all they need do to conform and the addition of the "nice to have but not necessary" soft start gets dispensed with.

comprising a flyback step-up

:

mean no more than

A typical PFC front end has a bridge, a small inductor, a diode then the main smoothing capacitor. There is nothing you can do with the PFC MOSFET to prevent inrush when the smoothing capacitor is initially discharged.

I don't know much about mains switchers, I presume the smoothing capacitor can be smaller with PFC because it gets charge over more of the mains cycle so inrush could be less anyway?

Ross, I see no soft start in those ML46XX ICs. that you pointed out. They are the standard boost topology of PFC ICs but a soft start can be attained by inserting an input current limiting resistor, like 25R in series with the low side of the bulk cap to ground. A power MOSFET is added in parallel with the resistor and turned on after the bulk cap has charged. Harry

comprising a flyback step-up

:

mean no more than

problems where

reservoir cap.

of supplies

haven't yet found anyone

benefit at negligible

The smartest thing is not to duplicate the AC front end n times, Have one power supply connected to the mains and run the others as dc-dc off of it's outputs.

You say these are 'distributed' around an 'installation'. Perhaps reviewing methods of distribution are warranted. Perhaps your intended method doesn't really create a inrush problem.

RL

comprising a flyback step-up

implement :

zero I mean no more than

flicker when you hit the

I think you're looking at this the wrong way.

You are designing the power interface for this equipment. It might be worth while doing a bit of research before just bringing in a bunch of

120VAC power supplies.

Everything does not run off of 120V, unless you make them do it.

RL

On Mon, 11 Dec 2006 22:29:20 GMT, Mike Harrison Gave us:

comprising a flyback step-up

:

mean no more than

Most well designed supplies do soft start.

Sequencing?

With soft start, shouldn't be needed. If you are running a circuit at close to it max rated spec, you should be placing some of your load(s) on a different mains circuit branch.

reservoir cap.

A PSU can be designed and configured to respond any way you want it to, within the reason of the technological limitations.

of supplies

haven't yet found anyone

You want a set of custom behaviors in an off-the-shelf mentality search mindset. You are going to have to compromise and get something that almost gets you there, or actually get a custom design requirements evaluation and quote on a customized off the shelf item, or a full custom all-for-you design.

benefit at negligible

Engineering time is not cheap, and YOU would STILL have to bear the initial tooling/NRE costs, if any in a custom design. It wouldn't matter what subsequent benefits the vendor may or may not reap later on.

Try

On 11 Dec 2006 14:56:06 -0800, "Ancient_Hacker" Gave us:

comprising a flyback step-up

implement :

I mean no more than

Exactly.

Only as large as needed by the power requisite and the noise figure ceiling.

Hey! You leave my favorite childhood company alone! :-]

I haven't actually used either of these parts so I am going on the info the manf provides.

The data sheet contains this info on p.10;

Soft Start

Start-up of the PWM is controlled by the selection of the external capacitor at SS. A current source of 50uA supplies the charging current for the capacitor, and start-up of the PWM begins at 1.5V. Start-up delay can be programmed by the following equation:

CSS = tDELAY x (50uA/1.5V )

where CSS is the required soft start capacitance, and tDELAY is the desired start-up delay. It is important that the time constant of the PWM soft-start allow the PFC time to generate sufficient output power for the PWM section. The PWM start-up delay should be at least

5ms. Solving for the minimum value of CSS:

Css = 5mS x (50uA/1.5V) = 157nF

Caution should be exercised when using this minimum soft start capacitance value because premature charging of the SS capacitor and activation of the PWM section can result if VFB is in the hysteresis band of the VIN OK comparator at start-up. The magnitude of VFB at start-up is related both to line voltage and nominal PFC output voltage. Typically, a

1.0ìF soft start capacitor will allow time for VFB and PFC out to reach their nominal values prior to activation of the PWM section at line voltages between 90Vrms and 265Vrms.

On Wed, 13 Dec 2006 14:08:11 GMT, Ross Herbert Gave us:

No. Thy retain soft start. NTBF goes down the tubes otherwise.

This refers to starting the PWM IC but the bulk cap has already been charged by the input supply, look at the schematic! Harry