DPA424R low voltage startup
I am trying to get a DPA424R device to startup at 10V drain voltage. I am finding that the device starts up as advertised at about 15VDC with no external current source connected to the Control pin.
I've tried connecting the Floating Constant Current Source circuit recommended by PI to the Control pin of the device. I have the floating current source circuit set to supply 2mA and have verified this value empirically. The floating current source output is connected to the Control pin of the device. This node also connects to the control pin capacitor (0.1uF), the emitter of the feedback optocoupler output transistor, and a series RC network consisting of a 10 ohm resistor and a 47uF cap.
I am finding that, when the floating current source circuit is connected to the control node, the converter will not start at all instead of starting at a lower voltage as expected. I've tried reducing the floating current source to 1mA to no avail.
Can you provide any insight into this problem?
Do you have any example circuits that show the connection of an external current source connected to the Control pin to lower the startup voltage?
Comments
The datasheet for the DPS424 on page 2-19 indicates
"Input voltages below 16V are possible, but since the internal startup current source is not specified at voltages below 16V, an external chip supply current should be fed into the CONTROL pin approximately equal to but less than Icd1"
How should this supply current be fed into CONTROL pin as mentioned in the datasheet? Icd1 for the DPA424R is 3.0mA from the datasheet. I've tried injecting 2.0mA and 1.0mA and neither works. A schematic showing the recommended circuit to reduce the startup voltage would be helpful.
Is it actually possible to reduce the startup voltage as the datasheet indicates?
I am sorry, which page do you mean? On page 2 of the data sheet is just the document index. Can you please tell me the page I can find the text
" Input voltages below 16V are possible, but since the internal startup current source is not specified at voltages below 16V, an external chip supply current should be fed into the CONTROL pin approximately equal to but less than Icd1"
Also Please take a look to page 25 Note D of the DPA data sheet
http://www.powerint.com/sites/default/files/product-docs/dpa_family_datasheet.pdf .
I'm sorry, I am using the printed version of the 2007 Data Book and Design Guide. The text I mentioned in on page 17 of the DPA data sheet. It is listed as item 3 under the heading "Issues Affecting Dissipation".
I understand the tradeoffs mentioned in Note D on page 25. I would like to understand how I can reduce the startup voltage as it indicates on page 17. If current needs to be fed into the CONTROL pin, how should this be done? An example circuit would be helpful.
Further investigation reveals that simply removing the UV and OV limits by connecting the LINE pin to the SOURCE causes the DPA Switch to start up at about 10VDC input. The turn-on is pretty clean as the input voltage rises, but the turn-off, when reducing the input voltage, isn't quite as clean. Adding a UV limit configured for 10V cleans up the turn-on and turn-off.
This additional capability apparently comes with a cost. According to Note D at the end of the DPA Switch datasheet parameter table, the startup time, auto-restart frequency, and auto-restart duty cycle are affected by starting up below 16VDC.
These issues can, apparently, be remedied as indicated on page 17 of the datasheet as follows:
"Input voltages below 16V are possible, but since the internal startup current source is not specified at voltages below 16V, an external chip supply current should be fed into the CONTROL pin approximately equal to but less than Icd1."
The problem is how to feed this additional current into the DPA Switch CONTROL pin. The floating current source circuit offered by PI for the TinySwitch-III does not apply to the DPA Switch.
Does anyone know how this current should be fed into the CONTROL pin?
The (fccs) floating constant current source does not work for the DPA family. The fccs feeds current to the bypass pin of a device like the TNYSwitch. Not to Control pin as in the DPA family. The Bypass pin in TNYSwitch has only DC component. The Control Pin in the DPA had DC bias and feedback signal. Once you connect the FCCS to the DPS you are disturbing the feedback loop and the supply would not work.
Thank you for your interest in PI Producst.