# Smps step up converter

#### S_lannan

Joined Jun 20, 2007
246
Hi. I want to design a small smps that steps up 12vDC to 300vDC @ 50ma.

I'm at a little trouble trying to figure out what is the best topology to choose.

Is it possible to use a non isolated boost converter to step it up that much or is there a practical limit to the step up ratio of that topology?

It's a really clever design, however i would like to design my own converter!!

http://diyaudioprojects.com/Tubes/12AX7_Preamp/index.htm

Anybody got any suggestions?

#### GS3

Joined Sep 21, 2007
408
I suppose that if you want simplicity a non isolated boost converter with a single inductance would work but I think that a circuit using a transformer, whether forward or flyback would be better and would also provide isolation (which you may or may not need). You can easily use a transformer from a 110/220 Vac to 12 Vdc SMPS. A flyback configuration has the advantage that the turns ratio is not critical but it has the disadvantage that the inductance is more important whereas it is not critical in a forward mode converter.

Now that I take a closer look at the circuit you linked to I notice it is not a single coil but actually a flyback transformer with 12 + 80 turns = 92 so that the straight voltage conversion would be to 92 V and up to 300 with the flyback effect. BTW, that circuit with such a coil resembles exactly the traditional ignition circuit of an automobile (where the points were in place of the FET).

By providing an isolating transformer you can have an isolated output which is not the case in that circuit.

If you are going to wind your own transformer I would get the turns ratio closer to the voltage ratio, say 12 on the primary and 300 on the secondary.

#### Papabravo

Joined Feb 24, 2006
20,404
Back of the Envelope Calculation:
Output Power : 300 VDC @ 50 mA = 15 Watts
Efficiency : Assume 80%
Input Power : 15 W / 0.8 = 18.75 Watts
Input Current : 18.75 Watts / 12 VDC ≈ 1.57 A
Peak Inductor Current : 2 * Input Current = 3.14

Be sure when selecting or sizing your inductors or transformers that the wire size can handle the current without excessive temperature rise AND that the core material will take the field without saturating. Ignore these points at your peril.

#### S_lannan

Joined Jun 20, 2007
246
If you are going to wind your own transformer I would get the turns ratio closer to the voltage ratio, say 12 on the primary and 300 on the secondary.
So for this "flyback boost" converter the transformer that was used was an etd29.

Does it matter if there is no air gap in this transformer?
I thought you had to use an air gap in a flyback transformer?

#### GS3

Joined Sep 21, 2007
408
So for this "flyback boost" converter the transformer that was used was an etd29.

Does it matter if there is no air gap in this transformer?
I thought you had to use an air gap in a flyback transformer?
Why do you say that? What makes you think that? What effect do you think the air gap has on the characteristics of the inductance/transformer? How does that relate to flyback transformers?

If you are thinking of designing a flyback SMPS the inductance/transformer is an essential part which you will need to understand well. There are many issues to deal with but the one thing you need to avoid at all costs is saturation.

#### S_lannan

Joined Jun 20, 2007
246
well as far as i know flyback transformers dont just transfer energy. They work like inductors aswell and store energy. thats why there are no output inductors on a flyback supply.

blah blah blah and so forth.

what this means to me is the transformer has to have a certain primary inductance to regulate voltage according to a varying duty cycle.

air gaps? most off-line flybacks i've seen use them to avoid core saturation problems which seem to plague this topology, as every offline flyback i've dissasembled has an airgap in the chopper transformer core.

#### GS3

Joined Sep 21, 2007
408
well as far as i know flyback transformers dont just transfer energy. They work like inductors aswell and store energy.
They store energy during one part of the cycle and they release that energy to the output on another, distinct part of the cycle. Correct.
thats why there are no output inductors on a flyback supply.
This is factually wrong as many (most?) flyback power supplies do have inductors in the output filter. Check any ATX power supply and you will see some pretty big inductors on the way out. In any case, it is also totally unrelated to whether a PS is of the switched mode type or not, flyback or not. SMPS just lend themselves better to using chokes in the filters because of the high frequencies involved.
blah blah blah and so forth.
It's difficult to argue with that. I admire the depth of your perception on such a crucial issue.
what this means to me is the transformer has to have a certain primary inductance to regulate voltage according to a varying duty cycle.
That statement needs to be parsed very carefully. "the transformer has to have a certain primary inductance", with that I can agree. "to regulate voltage...". No, not really. Not at all. Output voltage is regulated by a very roundabout process. What the controller is *really* controlling is amount of energy transferred, which in turn results in voltage variation which is the variable being ultimately controlled.
air gaps? most off-line flybacks i've seen use them to avoid core saturation problems which seem to plague this topology, as every offline flyback i've dissasembled has an airgap in the chopper transformer core.
Air gap radically changes several crucial characteristics of an inductance and the main one is that it decreases the value of the inductance. As I said, understanding and designing inductances is quite complex and I am no expert. I just know enough to realise how little I know and how much I still have to learn.

#### S_lannan

Joined Jun 20, 2007
246
sure i see what you mean, yes most flyback supplies do have inductors on the output but it's only to suppress hf and not to regulate the output according to a varying duty cycle, isnt it.

I did a bit of reading on flyback supplies and i must agree there is a lot i have to learn on the design of the magnetic elements .

However i cannot agree that computer power supplies are flyback, i've disassembled quite a few to get the high power components.
Definately all half bridge converters.

However, I can do a 2 output flyback (6.3v @ 1 amp & 300v @ 50ma) with simple switchers, that leaves me plenty of time to design the transformer and get my head around the theory .

cheers, steve