How to design power supply for 9V to 18V input with DPA

It is *possible* for you to create a DPA Switch design that runs from a 9-18V input. However, the control pin charging current is generated internally from the Drain pin voltage. For drain pin voltages less than 16 volts, the control pin charging current is quite limited and can impact a variety of device parameters.


You should be able to "force" the design in PIXls. It will give you a warning about the input voltage being too low. You can also use PI Expert but you will need to specify your input voltage as 16-18V. I would suggest disabling the UVLO so the PSU will come up for voltages less than 16V. However, you'll want to make sure and use a bias supply so that once the PSU starts up, the bias winding will provide the control pin charging current for input voltages less than 16V.

-The Traveler

Dear Traveler,

Thanks for your support,
could you please share some reference design for the same.

Could you please elaborate following

"For drain pin voltages less than 16 volts, the control pin charging current is quite limited and can impact a variety of device parameters."

Regards

Chinmay

Chinmay -


I don't have any DPA Switch reference designs for that low of an input voltage unfortunately. From what I can tell, most of our DPA Switch reference designs were really targeted at telco applications (36-48V DC input).


Regarding the drain pin minimum voltage of 16V:
http://www.powerint.com/sites/default/files/product-docs/dpa_family_datasheet.pdf
Check out page 25 note D and Page 29 Figure 45. There is also a great explanation of the control pin functionality in the PDF link above starting in Page 4.


The short version is that that besides being part of the feedback loop, the control pin provides the current necessary to run the internals of the IC. The control pin charging current (via the drain pin) is quite limited for drain pin voltages less than 16V. Features like soft-start, auto-restart, etc depend on the internal controller operating properly.


Having a bias winding in your transformer to supply the control pin with the neccessary voltage for proper operation should allow the IC to operate at input voltages below 16V. However, during start-up, before the bias supply is active, you might run into some issues.


Let me know if I can be of more help.

-The Traveler

However, during start-up, before the bias supply is active, you might run into some issues.
Could you please elaborate above sentence.

Is it recommended or not recommended to use DPA IC in 9V to 18V input application?

chinmay -

My apologies for not getting back to you sooner. I can only say that I've been incredibly swamped.

To address your question, DPA switch will work with input voltages less than 16V as called out in the datasheet. However, designs utilizing input voltages less than 16V are not officially supported.

The control pin (and the attached external capacitor) on DPA Switch is charged during start-up of the device. The charging current is provided by a shunt regulator internal to the IC that is connected to the drain pin. As you can see from the attached picture (Fig 45 from the DPA Switch datasheet), below 16V the charging current from the shunt regulator drops off sharply.

Depending on your design and application, this may not cause a significant problem. It's possible that the reduced charging current will only cause power-up times to increase. However, it's likely that auto-restart functionality (which relies on charging and discharging the control pin capacitor for timing purposes) could perform outside of datasheet values. This could affect the built-in fault protection circuitry.

My suggestion would be the following:
Put together your design in PIXLS using the input parameters you have (9-18V). PI XLS will likely give you a bunch of errors regarding the input voltage but go ahead and ignore them. Use the design software to finish your design and come up with a transformer design. Once you've got the board built, disable the UVLO and OVLO functionality by tying the L pin to Source. With that done, go ahead and bring up your power supply and see how it performed at the lower input voltages. If the start-up time and auto-restart timing isn't too far off and you can live with the trade-offs, I'd say you're good.

However, if you're running into problems where the control pin charging current is just too small to start the device, you can look into adding an external current source to supply a small amount of charging current into the control pin.

We have a Design Idea on our website for a TinySwitch-III design that uses a similar setup. http://www.powerint.com/sites/default/files/PDFFiles/di152.pdf
If it were my design, I'd probably try to avoid the added complexity of the circuit described in DI152 and use a programmable current source IC. Something like this might do the trick: http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000458.pdf

Using the datasheet parameters for DPASwitch, you can experiment with different values for the external current source. My guess is that if you have to use the external source, you'll want to keep the current as small as possible while still getting proper operation. This will probably be somewhere near the value of ICD1 or ICD2.

I hope all this helps, please let me know if you have any other questions.

-The Traveler