DCM & CCM DC-DC flyback converter help plz

[bn0246]

Hi everyone,

I need your help in designing and controlling a DC to DC flyback converter that operate in both CCM & DCM. where to start? and what is special about these kind of converters? and will i find a ready-made transformer for such a converter? how to choose the right MOSFET? what parameters will play the role of deciding the operation mode?
can you suggest a reference-design or a datasheet so i can start the job.

many thanks in advance

Regards

[sawreyrw]

I wouldn't do this. The control loops have to be far different.

[bn0246]

will consider using two control loops as i have to build it this way... any suggestions!!!

[Pwrmad]

You start with a complete list of specs/features for the power supply and design what works best to meet all requirements. What's special is that you might get away with only one custom magnetic device. What decides the operating mode is load, inductance, operating frequency and turns ratio. If you use current-mode control, the loops won't be so bad. Minimize the value of inductance. This maximizes the RHP zero frequency. It's important. Look at TI/Unitrode application notes. Read a book like Pressman, Erickson or Billings. Also, why do you have to use a DCM/CCM flyback? Why not BCM? Why not forward?

[dtproff]

If you do a current mode control CCM Flyback with a proper loop compensation and ramp compensation there is no issue running DCM.

You can look at any of the 65W adapter designs for notebook computers and they are predominantly CCM flyback.

I usually start the design using the followning input

Vmin (min Bulk cap voltage at bottom of the trough) usually about 65Vdc
Vo output voltage
65% duty cycle at peak loading
Popk Peak power to handle
Bmax (usually 370mT for 100C operation of PC44 material)
Iripple This is the ripple current that Bmax is based on. usually about 35% of Idc
Idc is Popk/(Vinmin*DutyCycle)
All the above will spit out your primary inductance and turns ratio

If you need more help than that then let me know.

I won't ask the question of topology choice since I suppose you have already been through the topology choices and have settled in on a CCM flyback. However, if you care to share why you chose a flyback it might help make sure that the solution we help you with will achieve the desired results you need from the PSU.

Tony

[dtproff]

http://www.onsemi.com/pub_link/Collateral/AND8461-D.PDF

Try this app note. It will help you.

Tony

[eem2am]

why do you do both ccm and dcm?
is it because the input voltage has a big variation?

[dtproff]

Originally Posted by eem2am

why do you do both ccm and dcm?
is it because the input voltage has a big variation?

As you decrease the load the Flyback will naturally move from CCM to DCM.

I have included my sample calculator for flybacks

Tony

[darkfenriz]

Try power integrations and forget getting ready made trnsformers from farnell, rs, digikey and such.

[dtproff]

Originally Posted by darkfenriz

Try power integrations and forget getting ready made trnsformers from farnell, rs, digikey and such.

I have had issues with PI in the past. First they voltage mode control and second they have a twitchy board layout.

Is that easier to handle than ramp compensation and current mode control? Not sure. I guess I am probably more comfortable with current mode control.

Totally agree with you on the transformer. Wind and gap them yourself. Theen you know what you have.

Tony