Isolated, dual output, variable voltage flyback design

Posted by: Zeprom on 03/30/2018

Dear PI Experts,

We'd like to implement the following design : 230Vac to dual ouptut (P<5W) with the following features:
- Isolated flyback
- Vo1 = +5Vdc
- Vo2 = +15Vdc
Besides, we'd like the outputs to be selectable to either:
- Vo1 = +5Vdc / +12Vdc
- Vo2 = +15Vdc / 36Vdc

We intended to reuse a design we've been successfully using - inserting a mosfet and a series resistor in parrallel with the feedback bridge to select the voltage - and a LinkSwitch XT2 driver.
In the meantime, we are appealed by the LinkSwitch-CV and its primary side regulation that can improve cross-regulation for the dual-output design, and remove the burden of tuning the feedback loop with the optocoupler and so on...
In this case we would need using optocoupler in saturated mode to select the ouptut voltages from the primary side, but these have saturation voltage in the orders of 100' of mV...
Do you recommend such architecture? Or shall we stick with secondary side regulation?

Best Regards,

Comments

Hi Zeprom,
I recommend you use Innoswitch/Innoswitch3 product. It is an isolated flyback topology which uses voltage divider as output sense and does not require an optocoupler and instead use patented Fluxlink technology to provide feedback from secondary to primary. In this case you just need to vary the resistor value of the voltage divider directly. You just need to design component rating with the highest voltage selection such as the secondary rectifier and reflected voltage of the primary winding in order not to exceed the Drain voltage rating of the primary switch.

I recommend to use online guide design in the link below for your customized design.
https://piexpertonline.power.com/site/login

Regards.

Hi,
Innoswitch seems to be a perfect match for our design goals. However, I did not mention the whole supply design has to fit a L*l = 40*31mm PCB area... Although most of the functions are integrated, I am afraid the InSOP24 package might be too challenging for this design.
I think we might have to stick with the legacy SMD8/SO8 packages... I am reckoning the design to be based on the LNK3604 + secondary side regulation to be the best solution given our constraints....

Hi Zeprom,
I think you should use what you are used to do as this are already proven.
But you could also explore primary switching as theoretically this would work.
Things you need to check with this configuration is the cross regulation during no load on one output and full load at one output. As opposed to secondary control you could prevent one output flying high by using weighted voltage divider for the two outputs.

Regards.

Hi,
Actually we will typically be in that case: one output would source most of the power to the load, while the other one will only supply a few mW for analog biasing and MCU supply...
So, according to what you said, we'd better go for secondary side regulation as a way of improving cross regulation?

Thanks for your advices.

Hi Zeprom,
Yes, you will have more control using secondary side regulation.

Regards.

Thank you for your answer !