Should I use a SG3525 ?

[Pooh Bear]

Hi.

I'm designing a forward converter. Probably be full bridge as opposed to
half - at the several kilowatt power level - probably @ 100kHz or maybe
a bit more.

Plan to use dedicated gate drivers btw.

For the controller, the SG3525 seem to be a popular choice. Multi
sourced and a classic workhorse. Used in some current products I've
looked at ( in the pro-audio market ).

It's been around a while though. Just wondering if I'm missing an
alternative that is superior in some ways.


Graham

 

[CBarn24050]

Subject: Should I use a SG3525 ?
From: Pooh Bear rabbitsfriendsandrelations@hotmail.com
Date: 16/11/2004 21:22 GMT Standard Time
Message-id: <419A6F8B.F88B884C@hotmail.com

Hi.

I'm designing a forward converter. Probably be full bridge as opposed to
half - at the several kilowatt power level - probably @ 100kHz or maybe
a bit more.

Plan to use dedicated gate drivers btw.

For the controller, the SG3525 seem to be a popular choice. Multi
sourced and a classic workhorse. Used in some current products I've
looked at ( in the pro-audio market ).

It's been around a while though. Just wondering if I'm missing an
alternative that is superior in some ways.


Graham

Hi, if your using a full bridge then you can use a phase shift control
stratergy which makes your drivers much simpler, especialy if you plan to use
drive transformes. Unitrode make a chip, the number escapes me, or you can use
a PIC there is an application note somewhere on the microchip site.

 

[ChrisGibboGibson]

cbarn24050@aol.com wrote:

Subject: Should I use a SG3525 ?
From: Pooh Bear rabbitsfriendsandrelations@hotmail.com
Date: 16/11/2004 21:22 GMT Standard Time
Message-id: <419A6F8B.F88B884C@hotmail.com

Hi.

I'm designing a forward converter. Probably be full bridge as opposed to
half - at the several kilowatt power level - probably @ 100kHz or maybe
a bit more.

Plan to use dedicated gate drivers btw.

For the controller, the SG3525 seem to be a popular choice. Multi
sourced and a classic workhorse. Used in some current products I've
looked at ( in the pro-audio market ).

It's been around a while though. Just wondering if I'm missing an
alternative that is superior in some ways.


Graham


Hi, if your using a full bridge then you can use a phase shift control
stratergy which makes your drivers much simpler, especialy if you plan to use
drive transformes. Unitrode make a chip, the number escapes me, or you can
use
a PIC there is an application note somewhere on the microchip site.

Yes, a PIC at a buck and a half each plus S/W development time or a 3525 at 10
cents each and *no* S/W development time.

Why is it when people don't know the real answer they always say "or you could
use a PIC" ?

Gibbo

 

[Pooh Bear]

ChrisGibboGibson wrote:

Why is it when people don't know the real answer they always say "or you could
use a PIC" ?

Quasi-religious faith ? Looked at them - single source - uhuh !

Graham

 

[Phil Allison]

"Pooh Bear"

I'm designing a forward converter. Probably be full bridge as opposed to
half - at the several kilowatt power level - probably @ 100kHz or maybe
a bit more.

Plan to use dedicated gate drivers btw.

For the controller, the SG3525 seem to be a popular choice. Multi
sourced and a classic workhorse. Used in some current products I've
looked at ( in the pro-audio market ).

** The folk at QSC do seem to like them a lot.

It's been around a while though. Just wondering if I'm missing an
alternative that is superior in some ways.

** At the the "several kilowatt" level you ought to be considering using a
PFC SMPS design - gives you regulated DC rails at well.

Might pay to get a SMPS expert to do that one for you.





................ Phil

 

[John Walton]

You can get 1.5kW out of an outlet for a toaster, but not for an SMPS power
supply. (that is to say, a standard 117 VAC residential outlet in the U.S.)

Under certain circumstances, both halves of some controller chips can
conduct --



"Phil Allison" <philallison@tpg.com.au> wrote in message
news:419abf45@dnews.tpgi.com.au...

"Pooh Bear"

I'm designing a forward converter. Probably be full bridge as opposed to
half - at the several kilowatt power level - probably @ 100kHz or maybe
a bit more.

Plan to use dedicated gate drivers btw.

For the controller, the SG3525 seem to be a popular choice. Multi
sourced and a classic workhorse. Used in some current products I've
looked at ( in the pro-audio market ).


** The folk at QSC do seem to like them a lot.



It's been around a while though. Just wondering if I'm missing an
alternative that is superior in some ways.


** At the the "several kilowatt" level you ought to be considering using
a
PFC SMPS design - gives you regulated DC rails at well.

Might pay to get a SMPS expert to do that one for you.





............... Phil

 

[Phil Allison]

"John Walton" <jdwalton@comcast.net

You can get 1.5kW out of an outlet for a toaster, but not for an SMPS
power
supply. (that is to say, a standard 117 VAC residential outlet in the
U.S.)

** Pooh in in the UK - ie 230 volts at up to 15 amps continuous.

Using a PFC corrected SMPS design up to 3 kW of continuous DC power is
possible.





............... Phil

 

[legg]

On Wed, 17 Nov 2004 02:21:23 +0000, Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote:

Hi, if your using a full bridge then you can use a phase shift control
stratergy which makes your drivers much simpler, especialy if you plan to use
drive transformes.

Would you mind elaborating on that ?

A phase modulator normally drives it's switches with a ~constant 50%

duty, allowing reduced drive transformer and ancilliary circuit
complexity.

RL

 

[Pooh Bear]

legg wrote:

On Wed, 17 Nov 2004 02:21:23 +0000, Pooh Bear
rabbitsfriendsandrelations@hotmail.com> wrote:

Hi, if your using a full bridge then you can use a phase shift control
stratergy which makes your drivers much simpler, especialy if you plan to use
drive transformes.

Would you mind elaborating on that ?

A phase modulator normally drives it's switches with a ~constant 50%
duty, allowing reduced drive transformer and ancilliary circuit
complexity.

Ok - I'm with you now. Seen the method described but never seen it in practice.

Since I'm planning on using gate driver ICs - the transformer issue doesn't arise.
Not sure I see how 'ancillary circuit complexity' would be reduced though.

Graham

 

[Pooh Bear]

John Walton wrote:

SNIP

Quite an interesting article in the November/December issue of QEX in which
an ATX supply is modded -- used in a ham transceiver which is operated both
in phone and CW modes -- for a widely ranging output current (1 to 20 amps
in this case) the most critical element is the inductor in the LC filter.
The value of the inductor is inversely related to the minimum current drawn.

At any rate, it seems that Class A amplifiers would like to mate with SMPS.

That figures. A nice steady current draw.


Graham

 

[Pooh Bear]

CBarn24050 wrote:

Subject: Should I use a SG3525 ?
From: Pooh Bear rabbitsfriendsandrelations@hotmail.com
Date: 16/11/2004 21:22 GMT Standard Time
Message-id: <419A6F8B.F88B884C@hotmail.com

Hi.

I'm designing a forward converter. Probably be full bridge as opposed to
half - at the several kilowatt power level - probably @ 100kHz or maybe
a bit more.

Plan to use dedicated gate drivers btw.

For the controller, the SG3525 seem to be a popular choice. Multi
sourced and a classic workhorse. Used in some current products I've
looked at ( in the pro-audio market ).

It's been around a while though. Just wondering if I'm missing an
alternative that is superior in some ways.


Graham

Hi, if your using a full bridge then you can use a phase shift control
stratergy which makes your drivers much simpler, especialy if you plan to use
drive transformes.

Would you mind elaborating on that ?

Unitrode make a chip, the number escapes me, or you can use
a PIC there is an application note somewhere on the microchip site.

I've also considered one of Philips' super fast 8051 variants. I'm a bit of an
8051 fan / small-time expert but have zero experience of PICs despite considering
them from far back ( and dislike single source too btw ).

Thanks, Graham

 

[Terry Given]

Pooh Bear wrote:

Hi.

I'm designing a forward converter. Probably be full bridge as opposed to
half - at the several kilowatt power level - probably @ 100kHz or maybe
a bit more.

Plan to use dedicated gate drivers btw.

For the controller, the SG3525 seem to be a popular choice. Multi
sourced and a classic workhorse. Used in some current products I've
looked at ( in the pro-audio market ).

It's been around a while though. Just wondering if I'm missing an
alternative that is superior in some ways.


Graham

Stick to peak current-mode control, rather than voltage mode control.
That peak primary current limit is a wonderful thing. Especially when
you fit a large saense resistor at the beginning, limiting the current
to a nice small amount (say 3x expected mag current).

UC3846 was the original part for this job, but unitrode have a much
nicer version now with better rise/fall times, faster current limit
comparator, more GBW opamp, much less current etc. I forget the number
though.

If you go voltage-mode and something goes wrong, KA-BOOM. A good CM-smps
ought to run forever into a dead short on the output.

Unitrode, Cherry etc all have nice app notes on how to do it. When you
have made it work at 50W, then start cranking the power up. Then when
you have made it work at several kW, go build a phase-shift version and
throw out all that primary switching loss (hint: same power stage etc.
just a different controller, and perhaps a transformer with more leakage
inductance). But beware the drop in duty cycle. And lookout for > 180
(or < 0) degrees too

Cheers
Terry

 

[Pooh Bear]

Terry Given wrote:

Pooh Bear wrote:

Hi.

I'm designing a forward converter. Probably be full bridge as opposed to
half - at the several kilowatt power level - probably @ 100kHz or maybe
a bit more.

Plan to use dedicated gate drivers btw.

For the controller, the SG3525 seem to be a popular choice. Multi
sourced and a classic workhorse. Used in some current products I've
looked at ( in the pro-audio market ).

It's been around a while though. Just wondering if I'm missing an
alternative that is superior in some ways.


Graham


Stick to peak current-mode control, rather than voltage mode control.
That peak primary current limit is a wonderful thing. Especially when
you fit a large saense resistor at the beginning, limiting the current
to a nice small amount (say 3x expected mag current).

UC3846 was the original part for this job, but unitrode have a much
nicer version now with better rise/fall times, faster current limit
comparator, more GBW opamp, much less current etc. I forget the number
though.

If you go voltage-mode and something goes wrong, KA-BOOM. A good CM-smps
ought to run forever into a dead short on the output.

Unitrode, Cherry etc all have nice app notes on how to do it.

Can you suggest ?

Not a mention of the 3525 here ( apart from the specs of the improved B
version parts )

http://www.unitrode.com/products/apps_prt.htm

I'll look at the 3846 though.

When you
have made it work at 50W, then start cranking the power up.

That's pretty much what I'm planning. Very much bit by bit.

Then when
you have made it work at several kW, go build a phase-shift version and
throw out all that primary switching loss (hint: same power stage etc.
just a different controller, and perhaps a transformer with more leakage
inductance). But beware the drop in duty cycle. And lookout for > 180
(or < 0) degrees too

Because audio amplifers have very rapid changes in load current beyond the
capability of the feedback loop to respond, popular practice seems to be to
run the controller @ max duty cycle anyway and accept that the output volts
will refelct variations in ac line ( PFC sorts this of course ).


Graham