DCM & CCM DC-DC flyback converter help plz

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bn0246

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Joined 2012
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
 
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Pwrmad

Member
Joined 2010
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?
 
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dtproff

Member
Joined 2008
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
 
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dtproff

Member
Joined 2008
darkfenriz said:
Try power integrations and forget getting ready made trnsformers from farnell, rs, digikey and such.
Click to expand...

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
 
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dtproff

Member
Joined 2008
I use a lot of controller in the 10-65W range for flybacks. In general I never use an integrated controller/FET. First it dilutes my purchasing power, and second if I have an efficiency/EMI problem then I can change out the FET (better FET in the first instance, cheaper FET in the second).

And my last reason for not using PI is that they do not own their own foundry. This puts them at a competitive disadvantage to those that do own their own foundry. At my volumes this is an issue.

Thanks for letting me get my $0.02 in.

Tony
 
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