Hello again, I've been looking at this project, off and on, when I get the time, but still struggling with it. DC/DC boost conversion to power a Valve/Tube is not a trivial exercise, well not for me at least. This is potentially going to be a long post, as I have a few questions. Some might find that offensive, in which case move on, but I'm simply trying to understand a bit better.
Previously 'T3sl4co1l' mentioned a flyback design and I looked at that and it makes sense, as the turns ratio of the flyback transformer might get me close to the Voltage I need. In this design I'm trying to power a Valve pre-Amp so the output Voltage is sort of fixed at 250V DC. The input Voltage is not fixed, but I've mentioned 9V DC, as guitar effects pedals are usually powered by a 9V DC mains plug adapter. A lot of the analog pedals use less then 100mA, but I'm certain to break that by a bit. Now as previously mentioned 9V to 240V might well be asking too much so I can up the input Voltage to 18V DC or even 24V, using a Laptop power supply specifically to power this design. With that in mind I got a Boost IC to try breadboard this, selecting the ON Semiconductor's NCV887103 [1], for no other reason then is has a fairly wide input voltage range 3.2v-40V DC. That gives me a bit of wiggle room with input Voltage.
Having selected a Boost IC the next thing is a flyback transformer. I'm just browsing through the local farnell to get ideas. I came across the coil craft NA5920-ALD [2]. With a turns ration of 1:24 I'm thinking that's gotta get me close to 250V output. It seems to be rated for 100-400V so plenty good enough. It's only when I look at the data sheet [3] and see that the input voltage is 2V - 8V that I'm a bit surprised. In order to use that part I'd have to actually step down the 9V input voltage? I wasn't expecting that. I'll keep looking, but does that appear to be quite a low input voltage. I'll be the first to admit that I'm only at the wrong end of a learning curve when it comes to boost converters, and clearly I have assumptions that aren't valid. 8V might be a huge input voltage. Another question I have about that transformer's datasheet [3] is that it is 'designed to operate at 140KHz'. I got the NCV887103 variant of the Boost [1], because it can potentially run the duty cycle up to 93%, however it's opeating at 340KHz, unless I add other circuitary to feed it the 140KHz that the flyback transformer is 'designed for'.
I should forget that transformer and maybe go for the FA2470-AL_ [4]. It doesn't mention a 'designed for' frequency, and whilst it only has a turns ratio of 1:8 it does have an input voltage of 8.5V - 12V, (forget the laptop power supply)
I might leave this at that for the moment. I've read and re-read the datasheet [1] numerous times and I don't think it's a classic. Maybe it's just me, but it's not too bad until you get to the compensation network. Both figure 12 and 13, of [1], show a compensation circuit of a Cap C2 in parallel with a Resistor and a Cap, R2 & C1. No idea what that does but I'm hoping it becomes clearer when I have the other values and plug some numbers in the equations given in table 2. The obvious answer to that question is that those three passives create a compensation network. Like I say hopefully it gets clearer. Because the compensation circuit in the datasheet mentioned 'poles and zeros' I assumed the circuit was a filter of some description, but it's not one I recognise.
Thanks for making it this far. Apologies for the number of questions, as I have mentioned I'm at the wrong end of a learning curve.
[1]
https://www.farnell.com/datasheets/2255395.pdf[2]
https://ie.farnell.com/coilcraft/na5920-ald/flyback-transformer-1-24-100-400v/dp/2526826?st=flyback%20transformer[3]
https://www.farnell.com/datasheets/2017944.pdf[4]
https://www.farnell.com/datasheets/1870409.pdf