Flyback Converter question

[Jay Carp]

Hey all. I have a quick question: is it possible to build an offline 5W flyback converter? I know the flyback is ideal for low powers. I want to build a small offline converter to power some stuff that I am working on ( mainly other control ICs, OPAMPS, etc). How popular is it to build a 5W converter? Would most people just use a 60hz line transformer to supply a voltage regulator? Thanks!

---------- Post added at 01:08 AM ---------- Previous post was at 01:06 AM ----------

What I meant by the last par is would most people in this situation just use a voltage regulator IC instead of designing a small SMPS?

 

[russ]

Yes, u can built a 5W flyback converter. Flyback with active reset can be used all the way up-to 300W. Check out powerint.com for flyback reference design. Flybacks are used because they are much more efficient (greater than 80%) than linear PS. No you cannot use a 60Hz line transformer as a flyback SMPS transformer. They are totally different transformer used for different purposes.

 

[Jay Carp]

Thanks Russ. I just wasn't sure if the power was too low to even bother designing a flyback. Thanks for the help!

 

[rvionics]

You can use Flyback design for 5W without any problems, there are lot of chips available to simplify your design & reduce your no load power (Chips from Power integration or On semi etc)
All the new generation mobile chargers are based on flyback design where in some cases power is even less than 5W

 

[Tahmid]

Hi,
You can use a dedicated controller for your purpose. Some suitable converters with high efficiency, wide operating voltage (85-260V) and low parts count include the TNY and LNK series of controllers as well as the TOP series of controllers from Power Integrations. I think you should go to Welcome To Power Integrations | Power Integrations and see which controller fits your requirement. The PIExpert software does all necessary circuit design calculations for you if you use the TNY/LNK/TOP/DPA series controllers from Power Integrations.

Hope this helps.
Tahmid.

 

[Jay Carp]

Tahmid, ravionics, and russ: Thank you guys so much for helping me out. I actually ended up building a 10W current mode controlled offline flyback converter using the UC3842 with the TL431 as opto feedback. It works like a charm. I have to say though, after all I have read about the TL431, I still don't think I quite understand fully how it works. It is a complicated little circuit (atleast to me). Bottom line is that I hooked it up right and compensated the system using the TL431 as a type 2 amplifier. Anyways, thanks guys

 

[eem2am]

well TL431 is just a slow op-amp feeding into a NPN BJT, and the bjt acts as a curent sink to pull current through the opto diode inproportion to the amount by which your vout is away from its intended value.

but it is magic, becasue if you do it with a discreet opamp and npn then its hard to make it wirk...because the opamps tend to have too high bandwidth amd yuou end up going unstable

 

[Tahmid]

Hi jaycarp,
I think that if you have no problem (if you do not want to intend to use it commercial or other reason) maybe you could post the schematic and some design notes here and share it with the members. Maybe some one will find it useful.

 

[Joel-Tang]

No you cannot use a 60Hz line transformer as a flyback SMPS transformer. They are totally different transformer used for different purposes.

What is the reason behind this? I've been trying to learn this and so far all I have found is that the 60Hz transformers will be physically larger and not suitable for say a PCB.

 

[mtwieg]

What is the reason behind this? I've been trying to learn this and so far all I have found is that the 60Hz transformers will be physically larger and not suitable for say a PCB.

60Hz transformers are made with laminated steel and iron, which will have very high losses at switching frequencies more than a few KHz.

 

[Joel-Tang]

I always thought the whole reason to use higher frequencies was to reduce the size/inductance of the coils. If size isn't an issue, can't we operate at the lower frequencies with larger a inductance to compensate allowing the use of laminated steel and iron cores?

 

[mtwieg]

Well if size, weight, and efficiency aren't an issue then sure you don't even need a SMPS. I highly doubt using a 60Hz transformer in a SMPS will give good results, even if you turn down the frequency. If will have very poor transient response, and it's efficiency will probably be very poor, much like the first ever SMPS which were also made using steel transformers.