LNK306 skipping cycles
Hello,
I am designing a non isolated step down converter with 4W using the LNK306.
Input voltage is 400VDC output is 48V.
The output voltage is used to drive a electromagnetic valve with 4W.
For the first 500ms the output voltage is 52V, then it is reduced to 48V. This is to create a bit more power when opening the valve under high pressure.
Actuallys it works fine, but...
I choose 2*470µH inductor, which should magnetize up to 500mA in 1,2µs and demagnetize in 9,8µs. At 66khz operation it should be able to transfer 7,75W. One cycle takes 11µs, so there should be 4µs left until the next cycle starts.
But the LNK306 starts to skip every second cycle and the output voltage drops to 45V...
Why is that?
Circuit and scope attached.
Yellow (Ch1): current measured with a 10R resistor in line with the inductor
Green (Ch2): output voltage
Thank you!
Comments
You can see in app note 37 about the process to regulate the output voltage. It is normal the controller to skip cycles. When you see the controller is not skipping any more cycles, it mean you are loading it to its MAX PO Load condition. You do not want your power supply, when running in steady state, to be operating in MAX PO region.
http://www.powerint.com/sites/default/files/product-docs/an37.pdf
Hello,
thank you for your reply.
My Problem was that the LNK306 is skipping cycles although it is already at maximum output !
I thought about it and as I said the output voltage drops from 48 to 45V when overloaded.
Normally the LNK306 should attempt to restart under this condition, but it didnt! So I took a closer look at the feedback voltage (S-FB) and it looked terrible - spikes and high ringing...
I replaced the CNY17-3 with a type with lower CTR (SFH6165-3) and now its working fine.
Attached you find a scope where you can see:
Ch1: inductor current (1V/100mA)
Ch2: feedback voltage
Ch3: output voltage
At 5W output (calculated maximum is 7,75W) the LNK306 is still skipping a cycles but reaches maximum frequency now!
Thank you!
At high Load the controller should still skipping cycles. The load conditions at MAX PO(no skipping cycles) is not a steady state condition
How do I attach 2 images?
Here is the circuit...
Tank you!