Are there any 24VAC to 3.3VDC/1.5A SMPS?

[Joerg]

Gents,

Does anyone know of a little AC/DC switch mode converter module that can
take 24VAC and put out well regulated 3.3V at 1.5A or more? $25 in 1k
qty would be nice, availability in the US would be even nicer.

All I see is 24VAC to 12VDC and placing two in a row costs effiency.
Sure, I could roll my own again but this time we want to avoid NRE and
agency hassle (EMC et cetera).

 

[[email protected]]

Gents,

Does anyone know of a little AC/DC switch mode converter module that can
take 24VAC and put out well regulated 3.3V at 1.5A or more? $25 in 1k
qty would be nice, availability in the US would be even nicer.

Check out Intersil's new buck regulators. They'll go up to 42V and up to 10A,
IIRC.

All I see is 24VAC to 12VDC and placing two in a row costs effiency.

Regulatory problems, too.

 

[Joerg]

Check out Intersil's new buck regulators. They'll go up to 42V and up to 10A,
IIRC.


Regulatory problems, too.

Yes, I'd could roll my own buck with some automotive chip. Sorry, forgot
to mention one minor detail: Must be isolated, or should be (we may be
able to live without if it really has to be).

Thing is, with one of those ubiquitous 24VAC -> 12VDC switchers plus
another buck we'd be at 60% overall efficiency or less and we can't
quite afford that.

 

[Ross Herbert]

:Gents,
:
oes anyone know of a little AC/DC switch mode converter module that can
:take 24VAC and put out well regulated 3.3V at 1.5A or more? $25 in 1k
:qty would be nice, availability in the US would be even nicer.
:
:All I see is 24VAC to 12VDC and placing two in a row costs effiency.
:Sure, I could roll my own again but this time we want to avoid NRE and
:agency hassle (EMC et cetera).


I don't think there are any 24Vac input converters off the shelf - almost zero
demand I would think.

Why not use one of the 6 - 10W, 18-36V input DC-DC converters (eg. V-Infinity)?
Provide a bridge rectifier and bulk filter cap in front of it. A few bucks for
the BR and cap would keep it under $20 per unit in 1K quantity.

 

[Hammy]

Gents,

Does anyone know of a little AC/DC switch mode converter module that can
take 24VAC and put out well regulated 3.3V at 1.5A or more? $25 in 1k
qty would be nice, availability in the US would be even nicer.

All I see is 24VAC to 12VDC and placing two in a row costs effiency.
Sure, I could roll my own again but this time we want to avoid NRE and
agency hassle (EMC et cetera).

You could just add a bridge rectifier and a filter cap. This would
widen your selection to several hundred of the SMPS's that have a max
input to 36 or 75 Vin. Cheapest one I could find with a quick search
is

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=445-3306-ND

 

[Spehro Pefhany]

Gents,

Does anyone know of a little AC/DC switch mode converter module that can
take 24VAC and put out well regulated 3.3V at 1.5A or more? $25 in 1k
qty would be nice, availability in the US would be even nicer.

All I see is 24VAC to 12VDC and placing two in a row costs effiency.
Sure, I could roll my own again but this time we want to avoid NRE and
agency hassle (EMC et cetera).

24VAC is not a very common voltage (24VDC is much more popular in
industrial applications).

I second the suggestion of adding a bridge and filter cap upstream of
a converter, along with maybe a CM choke and some film or ceramic
caps.

You didn't specify whether you needed isolated or not, that's going to
make a difference.

When you read the detailed app notes, most of the modules require some
filtering ahead of them to meet EMC requrements anyway (small caps and
CM choke maybe), so it's not really "extra" work.



Best regards,
Spehro Pefhany

 

[Joerg]

Why not use a transformer to take the 24V AC up to 110V and then use any bog
standard switcher to get your 3.3V ?
Michael Kellett

Not enough space :-(

Plus on this one it could get us into regulatory hot water.

 

[Joerg]

:Gents,
:
oes anyone know of a little AC/DC switch mode converter module that can
:take 24VAC and put out well regulated 3.3V at 1.5A or more? $25 in 1k
:qty would be nice, availability in the US would be even nicer.
:
:All I see is 24VAC to 12VDC and placing two in a row costs effiency.
:Sure, I could roll my own again but this time we want to avoid NRE and
:agency hassle (EMC et cetera).


I don't think there are any 24Vac input converters off the shelf - almost zero
demand I would think.

Why not use one of the 6 - 10W, 18-36V input DC-DC converters (eg. V-Infinity)?
Provide a bridge rectifier and bulk filter cap in front of it. A few bucks for
the BR and cap would keep it under $20 per unit in 1K quantity.

That's solution B, if I really, really have to. In this case that would
require some regulatory scrutiny though because the input side would now
not be "taken care of by module" anymore. If that wasn't an issue I'd
just roll my own because then I won't have to deal with those dreaded
50C or 60C derating issues that "professional" switchers often exhibit.

 

[Joerg]

You could just add a bridge rectifier and a filter cap. This would
widen your selection to several hundred of the SMPS's that have a max
input to 36 or 75 Vin. ...

That's what Ross suggested and it's a solution I had planned for, but
only in case there really is no other option.

... Cheapest one I could find with a quick search is

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=445-3306-ND

We could probably get one from CUI for under $15, offsets the cost for a
good 105C/5000h or similar electrolytic up front.

 

[Joerg]

24VAC is not a very common voltage (24VDC is much more popular in
industrial applications).

It used to be very popular in Europe but I haven't seen any industrial
switch gear there in some 15 years. All the power contactors for huge
motors and such were run on 24VAC. Up to 30-50A. Larger ones still had
24VAC "control contactors" but they'd drive huge 230VAC contactors, the
kind that sounds like a gunshot when they engage.

While studying for my masters I assembled gear like that because those
jobs paid quite well, compared to bussing tables

I still have a discarded supply transformer for a cabinet that was sold
to me at scrap metal price. 24V, one kilowatt (!). That was from an
older cabinet two stories high where you needed a ladder to get to some
of the contactors. The secondary of that transformer consists of
soaked-cloth insulated copper bar that was formed around the primary
using a wooden mallet.

I second the suggestion of adding a bridge and filter cap upstream of
a converter, along with maybe a CM choke and some film or ceramic
caps.

Looks like I hafta :-(

That's why I posted, was hoping for a silver bullet that lets me avoid it.

You didn't specify whether you needed isolated or not, that's going to
make a difference.

Yes, sorry, should be isolated but we could make it work without. AC/DC
converters are generally isolated though because they use bridge rectifiers.

When you read the detailed app notes, most of the modules require some
filtering ahead of them to meet EMC requrements anyway (small caps and
CM choke maybe), so it's not really "extra" work.

Yes, and we'd put one on there anyhow because it affords some surge
protection.

 

[Hammy]

That's what Ross suggested and it's a solution I had planned for, but
only in case there really is no other option.

We responded simultaneously. I had my secretary proof read for grammar
and spelling.This explains the minor discrepancy in time stamp She's a
bit slow but I keep her around because she's a good lay.

We could probably get one from CUI for under $15, offsets the cost for a
good 105C/5000h or similar electrolytic up front.

Yea good 105c caps are pricey particularliy for 250VDC or better. I
just paid 7 bucks for two of these for a off line SMPS.

http://search.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=P11629-ND

It's hard being a mad scientist on a budget.;>)

 

[[email protected]]

Hammy wrote:

[ 24VAC input, 3.3V 1.5A output required]

We could probably get one from CUI for under $15, offsets the cost for a
good 105C/5000h or similar electrolytic up front.

Value wrong! If you only need 5 watts, the 33V filter cap ought to be
about 200 uF, not 5000 uF. Or does '5000h' mean something other
than the capacitance value?

I assumed 5k hours (@105C), not 5000 henries. ;-)

 

[Joerg]

We responded simultaneously. I had my secretary proof read for grammar
and spelling.This explains the minor discrepancy in time stamp She's a
bit slow but I keep her around because she's a good lay.

Now, now ...

Yea good 105c caps are pricey particularliy for 250VDC or better. I
just paid 7 bucks for two of these for a off line SMPS.

http://search.digikey.com/scripts/DkSearch/dksus.dll?vendor=0&keywords=P11629-ND

That must be a SMPS in disguise

It's hard being a mad scientist on a budget.;>)

500mOhm Rdson can get a bit toasty. But I only need a 63V cap.

 

[Joerg]

Hammy wrote:

[ 24VAC input, 3.3V 1.5A output required]

You could just add a bridge rectifier and a filter cap. This would
widen your selection to several hundred of the SMPS's that have a max
input to 36 or 75 Vin. ...
We could probably get one from CUI for under $15, offsets the cost for a
good 105C/5000h or similar electrolytic up front.

Value wrong! If you only need 5 watts, the 33V filter cap ought to be
about 200 uF, not 5000 uF. Or does '5000h' mean something other
than the capacitance value?

I assumed 5k hours (@105C), not 5000 henries. ;-)

I always thought it was meggeehoitzes

 

[Spehro Pefhany]

Still trying to mount Lasers on sharks? ;-)

Even mutant sea bass run $25/lb.


Best regards,
Spehro Pefhany