100VDC to 13.5VDC simple switcher

[mri_bob]

i want to build a dc-dc converter that will handle higher input voltages
than the common 72v limit of modules or the 60v limit of the "simple
switcher" line. i would prefer not to need a preregulator stage, so what i
really want is an LM2576HV that would work with an input range of 30v-100v
and provide 13.5VDC @1-2A. I would much prefer a boost converter topology
using a torroid over a transformer i have to wind. any ideas? and thanks
in advance.

 

[Genome]

i want to build a dc-dc converter that will handle higher input voltages
than the common 72v limit of modules or the 60v limit of the "simple
switcher" line. i would prefer not to need a preregulator stage, so what i
really want is an LM2576HV that would work with an input range of 30v-100v
and provide 13.5VDC @1-2A. I would much prefer a boost converter topology
using a torroid over a transformer i have to wind. any ideas? and thanks
in advance.

I recently realised that the orgasm thing is over rated and you are just
reinforcing the concept.

DNA

 

[Rich Grise]

i want to build a dc-dc converter that will handle higher input voltages
than the common 72v limit of modules or the 60v limit of the "simple
switcher" line. i would prefer not to need a preregulator stage, so what i
really want is an LM2576HV that would work with an input range of 30v-100v
and provide 13.5VDC @1-2A. I would much prefer a boost converter topology
using a torroid over a transformer i have to wind. any ideas? and thanks
in advance.

Pardon my ignorance, but what exactly is the advantage of using a boost
converter to go from 30~100VDC to 13.5VDC?

I thought "boost" meant to _increase_ the voltage above the input voltage.
Or does the choice of topology have something to do with line
regulation/input voltage range/something like that?

Thanks,
Rich

 

[John Popelish]

Pardon my ignorance, but what exactly is the advantage of using a boost
converter to go from 30~100VDC to 13.5VDC?

I thought "boost" meant to _increase_ the voltage above the input voltage.
Or does the choice of topology have something to do with line
regulation/input voltage range/something like that?

The boost configuration is one that stores energy in an
inductor's magnetic field and then dumps it to the output.
If the inductor is also a transformer or autotransformer,
the output may have a higher or lower voltage than the
source. One reason you might use a boost (flyback)
configuration rather than a buck converter to step a voltage
down, is that the step down transformer can also provide
input to output isolation. And the turns ratio of the
transformer/inductor gives you a degree of freedom you don't
have with a simple buck converter.

However, I seldom find that the boost configuration is the
overall best (all warts considered), except for low power
applications. The peak currents are at least 4 times the
average current.

 

[mri_bob]

thanks guys, of course there is no advantage to using boost topology to
drop voltage like this, i meant buck. i have been writing about buck/boost
regulators and i mistyped. the rest of my post makes clear what i want to
do. I want to use a simple buck regulator to get 13.5v on my electric bike
which runs at up to 85v to run 12v accessories without unbalancing my
batteries and all the simple solutions i have found top out at 60v. i
would like to find a part just like National's simple switcher line
LM2576HV that just needs a diode a couple of caps and a torroid, but i
need it to work up to 90v or so in and i can't find any solutions that
won't require me to add a pre-regulator stage or to use a chip made for an
off-line switcher that uses a special transformer. sorry about the
buck/boost confusion. i do know the difference.

-bob

 

[Nico Coesel]

i want to build a dc-dc converter that will handle higher input voltages
than the common 72v limit of modules or the 60v limit of the "simple
switcher" line. i would prefer not to need a preregulator stage, so what i
really want is an LM2576HV that would work with an input range of 30v-100v
and provide 13.5VDC @1-2A. I would much prefer a boost converter topology
using a torroid over a transformer i have to wind. any ideas? and thanks
in advance.

Look at the topswitch devices from power integrations.
(www.powerint.com) IIRC, these can be made to start at lower voltages.

 

[mri_bob]

thanks, i have looked at devices like this that are meant for offline
switchers, but they all require an expensive bulky transformer, while the
buck converters like the LM2576 only need a single wind torroid. I can
easily get a couple of amps at 13.5v out of a small inexpensive torroid
with this simple topology, but the devices just don't go up to the 85v
input i need.

-bob

 

[Ken Smith]

i want to build a dc-dc converter that will handle higher input voltages
than the common 72v limit of modules or the 60v limit of the "simple
switcher" line. i would prefer not to need a preregulator stage, so what i
really want is an LM2576HV that would work with an input range of 30v-100v
and provide 13.5VDC @1-2A. I would much prefer a boost converter topology
using a torroid over a transformer i have to wind. any ideas? and thanks
in advance.

How about this:



----------+-------------------------
! !
\ !
/ !!-
\ ----/\/\----------!!- N MOSFET
----+ ! Vx !!-
! +---+--------- !
--- ! ! !
--- /-/ --------------------
! ^ ! Booster chip !
! ! A---!fb !
----+ ! !
! --------------------
! Vsw ! Vout
--------------------+-----------+--)))))))-----------
!
---
^
!
GND
Vx
! !
/ / ---/\/\---
\ \ ! !
/ / ! Vx !
GND ! ! ! ! !
------/\/\/-+------+---!-\ !
! ! \ !
! ! >--+---A
Vout ! ! /
------/\/\/----+---+---!+/
! !
\ Vsw
/
\
!
Vsw

 

[Nico Coesel]

thanks, i have looked at devices like this that are meant for offline
switchers, but they all require an expensive bulky transformer, while the
buck converters like the LM2576 only need a single wind torroid. I can
easily get a couple of amps at 13.5v out of a small inexpensive torroid
with this simple topology, but the devices just don't go up to the 85v
input i need.

Can't you try to be creative with an off-line switcher? Like using a
zener diode contraption to start the chip and after start-up, power it
from the secondary side? Shouldn't be hard to do... Designing
electronics is not just a matter of connection the right application
note / example circuit to another.

An offline switcher doesn't need a transformer if you don't want the
isolation. First try to define the duty cycle control range you need
and then map this onto an offline switcher and see if it can do what
you want.

 

[Jamie]

thanks, i have looked at devices like this that are meant for offline
switchers, but they all require an expensive bulky transformer, while the
buck converters like the LM2576 only need a single wind torroid. I can
easily get a couple of amps at 13.5v out of a small inexpensive torroid
with this simple topology, but the devices just don't go up to the 85v
input i need.

-bob

sounds like you need a simple switcher using High-V transistors
on the primary side of a high-F xformer. Feed back to a coupler
to regulate the output.
i've actually seen a computer power supply hacked out simply
by connecting into the bridge +/- side to the high voltage
battery source.

 

[mri_bob]

thanks, i find your circuit almost impossible to read but i get the general
point, and as i said i would prefer not to have to build a preregulator. i
take minor offense to the post that said there is more to engineering than
just hooking up parts or something to that effect. Of course there is. i
have space, weight, cost, and complexity constraints that make it
important I get the smallest simplest solution and part of engineering is
also looking around to see what parts are available before reinventing the
wheel.

-bob

 

[Paul Mathews]

zener diode contraption to start the chip and after start-up, power it
from the secondary side? Shouldn't be hard to do... Designing
electronics is not just a matter of connection the right application
note / example circuit to another.

An offline switcher doesn't need a transformer if you don't want the
isolation. First try to define the duty cycle control range you need
and then map this onto an offline switcher and see if it can do what
you want.

Power Integrations TopswitchGX series, among many others, works just
fine as a non-isolated converter down to about 30 vdc.
Paul Mathews

 

[Rich Grise]

The boost configuration is one that stores energy in an
inductor's magnetic field and then dumps it to the output.
If the inductor is also a transformer or autotransformer,
the output may have a higher or lower voltage than the
source. One reason you might use a boost (flyback)
configuration rather than a buck converter to step a voltage
down, is that the step down transformer can also provide
input to output isolation. And the turns ratio of the
transformer/inductor gives you a degree of freedom you don't
have with a simple buck converter.

However, I seldom find that the boost configuration is the
overall best (all warts considered), except for low power
applications. The peak currents are at least 4 times the
average current.

Ah! Got it!

Thanks!
Rich

 

[Fred Bartoli]

Rich Grise a écrit :
....but that you have from a pushpull or a forward, which are nothing
else but a buck with a front transformer.

 

[Ken Smith]

thanks, i find your circuit almost impossible to read but i get the general
point, and as i said i would prefer not to have to build a preregulator.

The Zener section only needs to power the chip's internals and the
op-amp. The common gate power stage has no average gate current so the
capacitor on the Zener provides enough.