Auto-restart with no input power

Posted by: jive74 on 05/19/2015

Hi,

I'm writing to this forum because in my design the auto-restart feature of LNK364 is causing a long power-off transient.

The chip is used both in isolated and non-isolated mode and feedback is taken on the non-isolated side as a partition of regulated voltage (15 V DC).

Schematics is attached. (DC+ - DC-) is the rectified input voltage.

Input AC power is between 115 and 230 V rms and is rectified. DC rectified voltage is maintained by 2/3 large electrolytic capacitors with a total capacitance between about 1000 to 1500 uF. When input power is removed DC voltage across capacitors drops gradually and a condition is reached in which input voltage is too low to keep the LNK364 on without overpower protection, but high enough to allow the the auto-restart feature to operate.

The effect is that display and LEDs of the board are blinking for a time which is long enough to be noticed by user, that could think of this as a failure of the board.

Is there a built-in feature or a recommended design to disable auto-restart when input power is removed?

Ideally, the LNK364 should be kept on until voltage across the input capacitors has dropped to a safe value in case of un-assembly of the board from its enclosing.

Best regards

Giovanni Casoli

(Electronic Designer at DAB Pumps Italy)

Files

flyback_diy.PNG (38.05 KB)

Comments

Hi,

LNK364 does not have a input voltage detection feature however some of our other parts such as the TinySwitch,TOPSwitch and LinkSwitch-HP have a line voltage detection feature which will prevent the auto-restart if the input voltage drops below the operating voltage range.

Best Regards,

PI-sarek

Thank you,

I will investigate on the possibility of replacing the component with a superior model, but replacement is not well-seen by design owners, since it requires to introduce a new component and the LNK364 is used in other designs already.

I found a workaround, by modifying the lower resistor of FB pin to a higher value when DC voltage on electrolytic capacitors drops below an appropriate threshold.

However, one of the possible schematics of this circuit needs a voltage reference and a suitable voltage would be that on the BP pin.

I have therefore attached a resistor of about 200 kOhm in parallel to the bypass capacitor and the circuit seems to work correctly. On the datasheet I found an upper limit to the current that can be sunk by the BP pin.

Is there a limit to the current that can be sourcet by the same pin? Can 29 uA be a tolerable current for the internal regulator of the LNK364?

Best regards

Giovanni Casoli

Normally, we do not recommend that any load be applied to the BP pin however I would think that 29uA should be OK.

It will be better for you to provide me a schematic so I will understand exactly how much the load will be.

Regards,

PI-Sarek