Hi Mario,
Funny thing, I tried the coupled inductor you linked to already, a couple of years ago when I was designing an isolated DC-DC supply for laser drivers. I only needed 5 watts at 8 volts and it worked fine as a transformer in flyback mode. I ended up using a smaller and cheaper one since that one was bigger than I needed. I don't think you can get 30 watts through one of them, even at high frequency as a forward converter with an added output inductor. I was only running at 30-40KHz (I think) since I was trying to limit any conducted EMI above 150KHz with minimum parts count. The mantra here is always "smaller-cheaper-faster" and what I did was pretty cheap and small enough since the power requirement was low.
There are plenty of true transformers for switching supplies available at Digikey etc, but they tend to be a bit spendy. Something like this might work well for you in either flyback or forward mode(probably originally intended to be used in forward or push-pull topology):
http://katalog.we-online.de/pbs/datasheet/750311771.pdf
It's meant to be step-down but no reason not to use it backwards in forward or flyback mode. They probably have one with a higher ratio, you should try a couple of different ones.
To your other question, no reason you couldn't do a two-stage circuit like you also described, if you don't mind the extra complexity and lower efficiency. With an autotransformer it's just as easy to make it negative or positive output depending on which leads you put the power and switch to.
Friendly regards, Bob
________________________________________
From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of Mario
Sent: Friday, February 16, 2018 7:45 AM
To: Microcontroller discussion list - Public.; Microcontroller discussion list - Public.
Subject: Re: [EE] 7-16V to 350-400V SMPS boost IC?
Hi Bob,
I have studied more, and what I found that I could implement my 30-40W
15uF 400V capacitor-charge circuit in two different ways:
One is using a 2 transistors typical push-pull configuration (like in a
Royer oscillator, but I'd rather use a PIC to drive the the transistors)
but this is not well suited to the kind of load.
The other way would be a classic boost (or better, buck-boost) converter,
limited to about 1:5, but with a coupled inductor, and taking the flyback
on the secondary (inverted).
I have simulated this and noticed that it's just like a typical buck-boost
converter but, in substance, when the (single, as usual in buck-boost
converters) transistor is ON, current builds in the primary, when the
transistor switches OFF current disappears from the primary (and this
is the difference with a normal, single inductor converter) but it appears
(and starts decaying) at the secondary, so as long as there's a diode and
a load there, the energy gets transferred to the other side anyway.
This second type is what I want to implement. It looks very easy and as I
have some experience with boost converters, also very familiar. But I have
what seem abnormal difficulties in sourcing the transformer for it, and
here it comes my new question (as I don't want to make my own transformer,
but to source them from some producers):
At 18:23 2018-02-13, Bob Blick wrote:
>On the other hand, I have made plenty of little transformers out of
>cheap inductors. You could wind a second layer on top of a typical
>"spool" inductor and you'd have yourself a fine little transformer
>with your hand-wound primary. Start with something like this:
>
>https://www.digikey.com/product-detail/en/bourns-inc/RLB9012-471KL/RLB
>9012-471KL-ND/1969612
>
>Peel the heatshrink off it first, and use thicker wire for your
>primary. And keep fingers crossed about high voltage breakdown.
If I found a coupled inductor (with different enough inductances, of
course), would it work? Like this one, for example (this can't handle
the currents I need, but is just an example of the type):
https://www.mouser.com/ds/2/414/HA78D-1223833.pdf
Or would I have better chances by searching among autotransformers instead?
Would an autotransformer give me inverted polarily?
Or would it simply add the input voltage to the output?
I need to source a suitable transformer from Mouser or DigiKey. This is
the only component of the whole circuit that I have problems with, being
the rest very familiar (it's just like a boost converter, so to speak).
I do understand that it will be very hard to find the ideal transformer,
but the problem I have is that I am not able to identify the type of
transformer or coupled inductor that I need.
I hope anyone can shed some light in this argument. Until now I thought
transformers were all equal, with differences of course in construction
to privilege some aspects (low capacitance, saturation, etc..), but I
now reckon that open and closed core behave quite differently, although
they follow the same principle (electromagnetic induction, as found by
Faraday), then there are things like leakage inductance to take into
account, etc..
What I need to do is so simple (just the usual boost converter but taking
flyback from another, inverted, winding, with say 1:10 turns ratio), but
sourcing the transformer is being so weirdly difficult. Maybe I just don't
know how to identify the right type? I can't believe they aren't made in
the usual variety of any other component.
Thank you.
With kind regards,
Mario
>
>I've also salvaged transformers from electric bug swatters and photo
>flashes, but they need to operate at audible frequencies.
>
>Cheerful regards,
>
>Bob
>
>
>
>________________________________________
>From: piclist-***@mit.edu <piclist-***@mit.edu> on behalf of Mario
>Sent: Tuesday, February 13, 2018 2:33 AM
>To: Microcontroller discussion list - Public.
>Subject: [EE] 7-16V to 350-400V SMPS boost IC?
>
>
>Hello,
>could you advice me some SMPS IC (possibly with integrated MOSFET) to
>boost from around 7-16V to around 350-400V with at least 1A average
>input current please?
>
>Thank you.
>
>Kind regards,
>Mario
>
>--
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