[SOLVED] Flyback with optical coupler vs without one

There are two isolated DC/DC flyback converter topologies: with optical coupler vs without one.

Advantages and disadvantages of each of them?
What about load regulation?
Does topology without optical coupler suits for dynamic load applications?

-tantudaisu-

Opto-coupled Pro's:
- Tight output regulation (i.e., load regulation is great)
- Possible to run in CCM, providing fixed frequency operation (EMI is thus at a predictable frequency)
- Potentially faster load response because output voltage is always being measured

Cons:
- Increased cost of opto-coupler
- Increased complexity of compensating around 431 and opto-coupler transfer functions
- If running in CCM, increased difficulty of compensating around RHP zero
- Reliability concerns with opto aging


Non Opto-Coupled Pro's:
- Simple to implement
- Compensation is simpler—due partly to lack of RHP zero, and partly to lack of 431+opto
- Inexpensive
- Lower parts count

Cons:
- Output load regulation not as good, and requires temperature compensation
- Load regulation more dependent on transformer turns ratio, and output diode characteristics
- Minimum load is required to maintain regulation
- Load response is slower due to slow sampling period, so the output capacitor may need to be larger

Zeke has included a very comprehensive list of attributes of both topologies.

The ONLY thing that I would add, is that non-optocoupled topologies require a tertiary feedback winding. This makes the transformer a little more complex. However, this winding can also provide DC power to the primary controller, thus it is a favorite among designers if you can design and build custom transformers.

The ONLY thing that I would add, is that non-optocoupled topologies require a tertiary feedback winding.

Click to expand...

This is not necessarily true; non-optocoupled flybacks have been produced which sense the output voltage from the primary winding, such as this chip. It does require that the primary-to-secondary turns ratio is precise.

The need for a third winding is mostly in applications with a high input voltage, which necessitates a way of producing low voltage to power the chip (for efficiency reasons). In these circumstances, the third winding is required regardless if sensing is done with or without an optocoupler.

Zeke, your observation is correct, there are some chips that don't require a tertiary feedback winding. If I remember properly,other companies (Power Integrations???) also have some devices that can be powered from the AC mains that don't require feedback.

But in general and a large majority of cases, my comment is also correct.

Flyback with optical coupler vs without one , this depend on 3 conditions:
1) output power
2) purpose
3) cost
if your application is (low output power , charger , low cost ) is better to use flyback without optical coupler (PSR) , otherwise you must use one with.

Thanks for answers, I found them useful.