100VDC to 13.5VDC simple switcher

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

DNA

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

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.

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

Look at the topswitch devices from power integrations.

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

How about this:

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

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.

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.

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

using a

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

Ah! Got it! :-)

Thanks! Rich

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

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.