LYT 4326 Dimmable Led Driver Problem

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I'm R&D Engineer from Turkey. I have a problem with my Led Driver. I designed Dimmable Led Driver with using referance design of DER-396 application note. And then I calculated Feedback Resistor value and Transformer Parameters with using PIXIs Designer. So I designed 800mA, 36V, 28.8W dimmable led driver.It works normaly.But an interesting problem occured. I'm using VİKO Dimmer Swicth to control my circuit. When ı turn switch to max. value slowly , current raises to max. value which is  800mA(Expected). But when ı turn switch to max. value fastly , current does not raises to max. value. It  sets at 315 mA. How can I solve this problem? Looking forward to hearing from you.



One possibility is the device is going from full-mode to reduced mode. Please make sure that you use the proper BP capacitor. For full mode, use 4.7uF capacitor. For reduced mode, use 47uF capacitor (ecap recommended) or 100uF ceramic cap.

Please also check that the conduction angle of the dimmer is the same regardless of how fast you turn the switch in order to eliminate it as the possible cause.

Please check that the switching frequency is around 132kHz in both cases. If one is operating at 66kHz, then please check whether the FB trace was routed near the drain. This could be noise issue and must be addressed through layout.

Thanks for your response. We have solved problem by changing BP capacitor with 4.7uF electrolitic cap.Problem has occured because of wrong note.However we add 100 pf ceramic capacitor between FB and ground  for preventing noise issue.Circuit is working  properly now. But we still have a little problem too.When dimmer level is very low(%10-20),under 30mA we see flickering issue. We estimate that the current drawn by the lamp is below the holding current of the triac within the dimmer. How can we solve this issue? 


Please monitor what's happening to the input voltage and current under this condition. If the conduction angle is varying randomly, then it's probably the holding current. Sometimes, the triac will completely turn-off, but might be turned-on due to leakage input voltage.

When the triac fires, please check if there's a huge ringing at the input current. this can be solved by putting  heavier damper resistor and increasing the value of R-C bleeder. However, this could have negative impact in the holding current towards the end of the conduction cycle. This is why it's important to know exactly where the flickering is coming from.


You can also monitor the BP capacitor if it's dropping below 5V and initiating auto-restart. If this is the case, then you probably need to increase the bias supply.


hı again,

I monitor input current, when the triac fires,ı see maximum 7,8A as ringing.İs it normal value? ı use 840 ohm damper resistor.However when ı monitor input voltage ı see some ıssue at negative alternans. What is the reason of this? Flickering is occured when this issue occur.And sometimes mode is changing again. I did not understand of this reason.Can you give me circuit values for 800mA,36V , 28.8W as DER-396 topology. So ı can compare your value with mine.

And how can ı send image to you? I want to send error image. 



You should be able to find "File attachments" below this text box. I recommend starting your design with similar circuit values as DER-396 with respect to input stage (damper, bleeder, emi). You are operating at a higher power so the default values may be good enough. However, I don't really know what's happening to your circuit and you can send me an image (input/output waveforms)

Sorry ı did not see file attachment. Now ı add image about input voltage(yellow) and current(turquois) waveform.ı have already started my design with similar circuit values as DER396. But ı changed Bleeder and Damper circuıt value because of my power.But ı have still some problems as ı say previous message. My 3  question are:

1. Why ı see this voltage waveform at 80 and above conduction angle as attachment image?

2. I have changed BP cap. with 4.7uF electrolitic cap. But why I am stıll seeing mode changing sometimes?

3. As I see in note DER396(500mA), ringing current is maximum 800mA. But when I monitor my circuit, I see 8A ringing current when the triac fires. İs it normal?

I am waiting for your response. Thanks... 


Here's my response to your questions:

1. The triac looks to be multi-firing. It can be due to negative ringing on the input due to reactive components in the triac as well as on your circuit. The ringing is worst at 90deg phase angle.


2. How did you know it's the mode? Under what condition can you see this? The device won't just changed from fullmode to reduce mode, unless the BP cap has increased in value. It could be something else but I don't know if it's layout-related or your circuit. I can take a look at your schematic if you can send it.


3.  8A is too much for a normal triac. Please use other triac and check the performance. It may be that you just encounter a "difficult" triac dimmer and it would be better to just drop it from compatibility list and focus on other dimmers.