Linkswitch-PL dimming

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Hello all,


I'm looking at the datasheet and app note for a Linkswitch-PL flyback LED driver. It states a few times that the threshold voltage on its feedback pin (for constant current operation) is around 290mV.


However, this threshold value changes when the LED is dimmed with a TRIAC dimmer (I plan to use the leading-edge variety).


It is my understanding that the Linkswitch senses zero crossings when the DC voltage on its drain pin falls below 19V- but how does this tie into the current sense threshold? Is there some guideline available that relates conduction angle at TRIAC turn-on to the Linkswitch IC's current-sense threshold?







In trying to find an answer to my original question, I collected some data on the Linkswitch PL dimming functionality and the results were somewhat startling.


I connected a leading-edge TRIAC dimmer before the input of a LinkSwitch-PL flyback supply. The attached graph shows how the LinkSwitch adjusts its feedback voltage threshold as the triac conduction angle changes (expressed as milliseconds here).


Can someone tell me if this is what I should be seeing?


The huge downward slope around 5.5ms is disconcerting. I was expecting the FB voltage to follow a more reasonable trend.




The dimming characteristic of LinkSwitch PL is based from the real conduction angle of the TRIAC.    Figure 6 of the data sheet shows the relationship of the feedback voltage from the conduction angle.At full conduction angle (180deg) the VFB is 290mV.  Approximately 135deg conduction the VFB will go down in proportion.

Make sure that the bulk voltage is able to discharge per half line cycle so that the IC can provide the correct VFB.

The zero crossing level will depend on the voltage seen by the IC ground and drain pin. Meaning for bulk topology, the zero crossing is Vout plus this threshold.  Make sure to discharege the bulk  down to this level so that dimming is more stable.




Thank you for your response.


I will verify that the drain voltage goes below the 19V threshold. Just to clarify: I will measure this between the Drain and Source pins on the IC.


I'm confused by what you mean when you say that "Meaning for bulk topology, the zero crossing is Vout plus this threshold". Does "bulk" voltage refer to the voltage across the output capacitor? Does this mean that the output voltage must drop past a certain level in addition to the 19V zero crossing threshold at the drain?



Also, in case something might jump out at you, attached is the schematic for my circuit.


The TRIAC is connected between the AC hot and F1.


Good you supported your inquiry with schematic and data.  Since it is a flyback topology then do not worry about subracting the output for zero crossing. 

As for the dimming curve of VFB>> there is a minimum angle that the controller can clearly measure and that is above 45degrees. Below this level the contoller will limit the power delivered during start-up to protect the LED.  The curve you have illustrated is from off position to full brightness condition.  This is the best way we can have to protect the LED or driver in case there is shorted output.

But then, from full brightness down to full dim, it will be smooth transition untill the BP supply collapsed.

Your feedback is highly appreciated so that we can develop a more suitable controller for you in the near future. 

Thanks and regards,


Thanks for the info.


So, are you saying that the graph I've shown might be correct?


I checked with a scope and the IC drain voltage certainly drops below 19V.


Also, the voltage on the bypass cap seems to be on the high side - 6.18 volts. Does this indicate any issues?


One more question: the 22k resistor shown in my schematic, titled Res in the app note, supplies current to the bypass cap. How do I calculate the minimum and maximum values for this resistor?


Also, is it required that the bypass capacitor is a ceramic type?


Thanks again 


Yes, your graph is correct.  It is the curve during turn on from full dimming to full brightness.

Thanks for the measurement, check also if the bullk voltage drops down to 20V to guarantee dimming angle measurement.

Around 6.2V BP voltage is correct with auxilliary supply.

2mA is the current to be used to calculate the series resistor.  Measure the auxilliary voltage at full dimming then subrtract 6.2V.  Divide by 2mA to the volage drop on the resistor to calculate the resistance.

Ceramic capacitor is acceptable to use.  Just consider the drop in capacitance when voltage is applied.

Enjoy testing, dimming is interesting design.