Forums

AC-fed SMPS topologies or the lack thereof

Started by Piotr Wyderski September 2, 2018
Why there seem to be no AC-fed SMPS topologies? Google is surprisingly
silent about that. There are some attempts to bulid a bridgeless PFC,
but they still have the high-voltage electrolytic capacitor in the 
middle and in most cases they do not address the inrush current issues
at all, as the PFC is a more or less typical buck, i.e. a series 
connection of L->D->C. The lack of this storage capacitor together
with a reasonable phase factor would either require another storage
technology (a bulky inductor) or high output ripple tolerance. But
we were used to the latter, any iron core trafo-based PSU had to
contain a big output capacitor bank and sometimes a choke.

I understand all this, if the directly AC-fed topologies were a niche,
I wouldn't ask this question. But they are not even a fringe and I find
this mysterious. So what's wrong with basically chopped sines?

	Best regards, Piotr


On a sunny day (Sun, 2 Sep 2018 09:48:02 +0200) it happened Piotr Wyderski
<peter.pan@neverland.mil> wrote in <pmg4jk$hnf$1@node2.news.atman.pl>:

>Why there seem to be no AC-fed SMPS topologies? Google is surprisingly >silent about that. There are some attempts to bulid a bridgeless PFC, >but they still have the high-voltage electrolytic capacitor in the >middle and in most cases they do not address the inrush current issues >at all, as the PFC is a more or less typical buck, i.e. a series >connection of L->D->C. The lack of this storage capacitor together >with a reasonable phase factor would either require another storage >technology (a bulky inductor) or high output ripple tolerance. But >we were used to the latter, any iron core trafo-based PSU had to >contain a big output capacitor bank and sometimes a choke. > >I understand all this, if the directly AC-fed topologies were a niche, >I wouldn't ask this question. But they are not even a fringe and I find >this mysterious. So what's wrong with basically chopped sines? > > Best regards, Piotr
Rectified 3 phase AC gives some percents ripple, then one does not need the storage capacitor, but just RF decoupling on the AC side.
>
For why?

If you want to make a switching buck equivalent of a VARIAC, you can.  You 
need two bidirectional switches, driven with isolated gate drivers, in 
synchronous buck mode, plus filtering.  The filtering is tricky because it 
will consume VARs at line frequency; you can't have a zero-current off state 
like you can at DC.

In that case, the output is AC, in phase with the input.

A single phase PFC isn't supplied with continuous power, but delivers DC. 
The input dips to zero periodically.  A capacitor is required.  That's 
obvious enough, but it was a point of confusion among a lot of participants 
in the Google Little Box challenge (which, it turned out, was about half 
inverter and half ripple management).

To do anything else, you need multiphase, at minimum two phase (90 degrees), 
of course three phase (120 degrees) is more traditional these days.  To get 
DC output from that, you need four quadrant converters, with each 
converter's output rotating in phase with the mains, to form the difference 
frequency (0Hz = DC).  Just as a DC-to-AC inverter (VFD, say) must deliver a 
rotating output vector.  It's doable, but that doesn't mean you want to do 
it...

Tim

-- 
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

"Piotr Wyderski" <peter.pan@neverland.mil> wrote in message 
news:pmg4jk$hnf$1@node2.news.atman.pl...
> Why there seem to be no AC-fed SMPS topologies? Google is surprisingly > silent about that. There are some attempts to bulid a bridgeless PFC, > but they still have the high-voltage electrolytic capacitor in the middle > and in most cases they do not address the inrush current issues > at all, as the PFC is a more or less typical buck, i.e. a series > connection of L->D->C. The lack of this storage capacitor together > with a reasonable phase factor would either require another storage > technology (a bulky inductor) or high output ripple tolerance. But > we were used to the latter, any iron core trafo-based PSU had to > contain a big output capacitor bank and sometimes a choke. > > I understand all this, if the directly AC-fed topologies were a niche, > I wouldn't ask this question. But they are not even a fringe and I find > this mysterious. So what's wrong with basically chopped sines? > > Best regards, Piotr > >
On Sunday, 2 September 2018 12:30:45 UTC+1, Tim Williams  wrote:
> "Piotr Wyderski" <peter.pan@neverland.mil> wrote in message > news:pmg4jk$hnf$1@node2.news.atman.pl...
> > Why there seem to be no AC-fed SMPS topologies?
There are if you accept a BR on the input. Otherwise you'd need to duplicate the control circuitry, and have it tolerate reverse voltage. A BR is cheaper.
> > Google is surprisingly > > silent about that. There are some attempts to bulid a bridgeless PFC, > > but they still have the high-voltage electrolytic capacitor in the middle > > and in most cases they do not address the inrush current issues > > at all, as the PFC is a more or less typical buck, i.e. a series > > connection of L->D->C. The lack of this storage capacitor together > > with a reasonable phase factor would either require another storage > > technology (a bulky inductor) or high output ripple tolerance. But > > we were used to the latter, any iron core trafo-based PSU had to > > contain a big output capacitor bank and sometimes a choke. > > > > I understand all this, if the directly AC-fed topologies were a niche, > > I wouldn't ask this question. But they are not even a fringe and I find > > this mysterious. So what's wrong with basically chopped sines? > > > > Best regards, Piotr > > > >
> For why? > > If you want to make a switching buck equivalent of a VARIAC, you can. You > need two bidirectional switches, driven with isolated gate drivers, in > synchronous buck mode, plus filtering. The filtering is tricky because it > will consume VARs at line frequency; you can't have a zero-current off state > like you can at DC. > > In that case, the output is AC, in phase with the input. > > A single phase PFC isn't supplied with continuous power, but delivers DC. > The input dips to zero periodically. A capacitor is required. That's > obvious enough, but it was a point of confusion among a lot of participants > in the Google Little Box challenge (which, it turned out, was about half > inverter and half ripple management).
Actually that's not the case. I've encountered numerous lighting SMPSUs that use no reservoir cap. Some output a hf modulated sine wave, some output dc with bits chopped out at each crossover. Good for batery chargers too. In principle one could even run a micro like that for some cost cutting apps, with it going to sleep during each chopped-out bit of power. NT
On 02/09/2018 08:48, Piotr Wyderski wrote:
> Why there seem to be no AC-fed SMPS topologies? Google is surprisingly > silent about that. There are some attempts to bulid a bridgeless PFC, > but they still have the high-voltage electrolytic capacitor in the > middle and in most cases they do not address the inrush current issues > at all, as the PFC is a more or less typical buck, i.e. a series > connection of L->D->C. The lack of this storage capacitor together > with a reasonable phase factor would either require another storage > technology (a bulky inductor) or high output ripple tolerance. But > we were used to the latter, any iron core trafo-based PSU had to > contain a big output capacitor bank and sometimes a choke. > > I understand all this, if the directly AC-fed topologies were a niche, > I wouldn't ask this question. But they are not even a fringe and I find > this mysterious. So what's wrong with basically chopped sines? > > Best regards, Piotr > >
The latest generation of "pure sine" stage lighting dimmers are basically just that: a synchronous buck convertor where both switch and "catch diode" are implemented by series back to back FETs. Effectively a Variac, input and output at 50/60Hz but internally at 40kHz+ piglet
<tabbypurr@gmail.com> wrote in message 
news:4590480b-6c60-4381-be1e-28fae4210e30@googlegroups.com...
>> A single phase PFC isn't supplied with continuous power, but >> delivers DC.
^ ^ ^ ^ ^ ^
>Actually that's not the case. I've encountered numerous lighting SMPSUs >that use no reservoir cap. Some output a hf modulated sine wave, some >output dc with bits chopped out at each crossover. Good for batery chargers >too. In principle one could even run a micro like that for some cost >cutting apps, with it going to sleep during each chopped-out bit of power. >
Those don't deliver (continuous, stable) DC... Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
Tim Williams wrote:

> For why?
Robustness + higher efficiency as a side effect. The bridge itself can waste considerable amount of power, 2-5W in the mid power range. You don't have the inrush current/charging spikes related problems either. It is also cos(phi)=~1 at no cost. So the input stage is far superior to a traditional switcher. The "only" problem is what to do with the chopped sine afterwards.
> If you want to make a switching buck equivalent of a VARIAC, you can. > You need two bidirectional switches, driven with isolated gate drivers, > in synchronous buck mode, plus filtering.
Complexity comparable to, say, a 2T synchronous isolated forward. And now you have GaN parts without the body diode, which, in fact, is the reason I asked this question.
> In that case, the output is AC, in phase with the input.
Or rectified AC, and then you basically end up with a high tech version of a normal transformer. IMHO useful enough not to be *that* rare. But the VARIAC analogy is a good one, thanks.
> A single phase PFC isn't supplied with continuous power, but delivers > DC. The input dips to zero periodically.
Sure.
> To do anything else, you need multiphase, at minimum two phase (90 > degrees), of course three phase (120 degrees) is more traditional these > days.
But then it's cheating, as it is extremely easy to have continuous power source in the multiphase case. You can even use the wye/star trick to effectively double the already high ripple frequency. BTW, do they get rid of the big input capacitors in that case? Best regards, Piotr
On Sunday, 2 September 2018 14:06:34 UTC+1, Tim Williams  wrote:
> <tabbypurr> wrote in message > news:4590480b-6c60-4381-be1e-28fae4210e30@googlegroups.com... > >> A single phase PFC isn't supplied with continuous power, but > >> delivers DC. > ^ ^ ^ ^ ^ ^ > >Actually that's not the case. I've encountered numerous lighting SMPSUs > >that use no reservoir cap. Some output a hf modulated sine wave, some > >output dc with bits chopped out at each crossover. Good for batery chargers > >too. In principle one could even run a micro like that for some cost > >cutting apps, with it going to sleep during each chopped-out bit of power. > > > > Those don't deliver (continuous, stable) DC... > > Tim
no, but it is dc. NT
"Piotr Wyderski" <peter.pan@neverland.mil> wrote in message 
news:pmgprr$5rf$1@node2.news.atman.pl...
> Robustness + higher efficiency as a side effect. The bridge > itself can waste considerable amount of power, 2-5W in the > mid power range. You don't have the inrush current/charging > spikes related problems either. It is also cos(phi)=~1 at no > cost. So the input stage is far superior to a traditional > switcher. The "only" problem is what to do with the chopped sine > afterwards.
You can do an active FWB just as well, which is probably simpler to support with off the shelf controllers. Direct conversion PFC is well known, I think the usual case is two transistors and two diodes. You can change the diodes to sync FETs quite easily. You'll still get inrush due to body diodes. A flyback or SEPIC configuration could be used to avoid inrush, or a buck-boost, or whatever else you like. I don't see that any of these are "no cost" though...
> Complexity comparable to, say, a 2T synchronous isolated forward. > And now you have GaN parts without the body diode, which, in fact, > is the reason I asked this question.
Yeah, but that's a lie, sort of. They might not have a body diode as such, but they de facto have the same behavior. If Vgs(min) were opposite to Vds(max) you could use it like a JFET, but with the extremely stringent limit (0-6V usually) you can't get more than about -2V D-S. So it acts like a higher Vf schottky, or really, not far from an SiC schottky with SFA capacitance. So you don't have any choice but to use them back-to-back for true AC applications.
> But then it's cheating, as it is extremely easy to have continuous power > source in the multiphase case. You can even use the wye/star trick to > effectively double the already high ripple frequency. BTW, do they get > rid of the big input capacitors in that case?
Don't know what "they" do, it's probably a matter of hold-up or ride-through time. The induction heaters I made, didn't bother, just a few 10s of uF to deal with switching ripple. Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website: https://www.seventransistorlabs.com/
On Sun, 2 Sep 2018 09:48:02 +0200, Piotr Wyderski
<peter.pan@neverland.mil> wrote:

>Why there seem to be no AC-fed SMPS topologies? Google is surprisingly >silent about that. There are some attempts to bulid a bridgeless PFC, >but they still have the high-voltage electrolytic capacitor in the >middle and in most cases they do not address the inrush current issues >at all, as the PFC is a more or less typical buck, i.e. a series >connection of L->D->C. The lack of this storage capacitor together >with a reasonable phase factor would either require another storage >technology (a bulky inductor) or high output ripple tolerance. But >we were used to the latter, any iron core trafo-based PSU had to >contain a big output capacitor bank and sometimes a choke. > >I understand all this, if the directly AC-fed topologies were a niche, >I wouldn't ask this question. But they are not even a fringe and I find >this mysterious. So what's wrong with basically chopped sines? > > Best regards, Piotr >
If you mean AC to DC, the now-classic PFC converter (boost switch into a big cap, than an isolated dc-dc switcher) is it. It has no inrush issue, and the line current is a sine. Any ac/dc converter needs energy storage, and a high-voltage electrolytic cap is the best way to do that.' -- John Larkin Highland Technology, Inc trk jlarkin att highlandtechnology dott com http://www.highlandtechnology.com