A varying-frequency control Flyback rectifier for constant current LED driving in Boundary-Conduction Mode

[Patrick_66]

Greetings everyone, there are a few questions that I'm hoping that you guys can help me to solve. What does it actually mean from the title that I had written above? Is it varying the input voltage while the output current remain the same at all times ? What is actually the use of a feedback controller is it to maintain a constant current by varying the frequency of the switch or is the controller used to ensure that the whole system is always running in BCM mode? Sorry for my ignorance guys. I hope that someone can help me. Thank you.

 

[Patrick_66]

Hello, Sir. It is me again. After seeing the datasheet I have a few unsolved questions that I need your help with. I did not manage to find the inductance value for the transformer. Maybe the datasheet inside got mentioned indirectly but I'm unable to catch on. Sir, can you help me with that? Based on the pdf file below I have used some of the equations to obtain the value for duty cycle, Rsense and Iout (max) but for the Rsense I did not manage to get the same value as your design based on the input voltage of 110 Vac, output voltage of 80 Vdc and I out = 333mA as you mention before. Furthermore, how did you get the resistance and capacitance value that I had circled based on the pdf file below because the datasheet did not mention how to calculate it. Other than that, if the external error amplifier connects to the IC at the primary side does this system still have isolation properties? The external error amplifier will replace the function of COMP in the IC if I'm not mistaken. Based on the simulation that you showed me, if not mistaken the system did not include dimming feature, PFC, Open and short LED indication right? Sorry for asking so many questions. Hope that you can help me. Thank you for your time and effort to teach a person like me. Hope to hear from you soon.

--- Updated Dec 7, 2023 ---

Question to ask:-

1. I have tried looking at the datasheet but did not manage to find the inductance value for the transformer.

2. As for the transformer can I use the predesigned transformer as stated in the datasheet? like model 750811291?

3. The datasheet taught me how to calculate the duty cycle, Rsense and Iout(max) but I did not manage to get the same Rsense value as your simulation.

4. How did you manage to get the resistance and capacitance value for the circled part based on the pdf file below because the datasheet did not mention that or is it based on the prebuild flyback converter designed that the datasheet suggested based on the output load condition?

5. Does the system that you proposed still have isolation properties since the output is connected to the input at the external error amplifier part?

6. Will the external error amplifier replace the function of COMP in the IC?

7. The simulation that you showed me doesn't include dimming feature, PFC, open and short LED indication right?


I have summarised the text above into question form. Thank you for helping me even though you are busy. Very sorry for the trouble.

 

[KlausST]

can I use the predesigned transformer as stated in the datasheet?

if you don´t trust a datasheet ... then you will have hard times in designing electronics.

For me the datasheet is the most trustworthy source of information.


Klaus

 

[Patrick_66]

if you don´t trust a datasheet ... then you will have hard times in designing electronics.

For me the datasheet is the most trustworthy source of information.


Klaus

Hello Sir, yeah I do trust a datasheet I'm thinking of using the predesigned transformer to build my circuit using 750811291 transformer since its output is close to my desired specification. I was curious what are the values for the capacitance and resistance for the circled part in the pdf file that I had uploaded previously because I did not manage to find how they calculate the values or maybe the datasheet did mention but I did not catch on. Thank you for your time.

 

[Patrick_66]

hi, can you complete the sentence in a few posts above first though?

Hello Sir, I have done changing my question into sentence form. Hope to hear from you soon. Thank you for your time, Sir.

 

[Patrick_66]

hi, can you complete the sentence in a few posts above first though?

Hello Sir, hope you are well. May I ask even though I changed the inductance value of the transformer that follows the specification of transformer model 750811291, I was still unable to get an initial output voltage of 85V. How come the output voltage still remains unchanged ?

 

[Patrick_66]

hi, can you complete the sentence in a few posts above first though?

Hope to hear from you soon Sir.

 

[Patrick_66]

hi, can you complete the sentence in a few posts above first though?

Hello sir, can I change the circuit in this way as shown in the figure below when I simulate the output current still maintained at 333mA as what you designed. I have one more question to ask, how did you set the initial output voltage to 80Vdc because I changed the inductance value and the auxiliary winding based on the transformer model 750811291 but the initial output voltage did not change to 85Vdc. Hope that Sir, can help me and once again sorry for the trouble.

 

[D.A.(Tony)Stewart]

The secondary peak voltage is limited by the LED string of 24 white LED. 80V/24=3.33V / LED.

You can raise Vout by raising the current sense R or adding more LEDs.

 

[Patrick_66]

The secondary peak voltage is limited by the LED string of 24 white LED. 80V/24=3.33V / LED.

You can raise Vout by raising the current sense R or adding more LEDs.

Hello Sir, I tried increasing the number of LEDs and the output voltage did when up but the initial output voltage when t=0 still remained at 80Vdc.

 

[D.A.(Tony)Stewart]

Raising the initial voltage may increase start current if started on a peak AC voltage. Why do you want this ?
It requires 16 ms in order to store that extra energy in case of a momentary power outage or half cycle or so. Not sure. There are many conditions for power interruption that ought to be simulated. A 2~3 cycle delay for startup seems reasonable, but could be boosted at some expense if needed.

 

[Patrick_66]

Raising the initial voltage will increase start current. Why do you want this ?

Hello sir, I wanted to use this transformer model 750811291 for my design and based on the figure below the target application is 85Vdc and 0.4A so I thought if I changed the inductance value to follow the transformer model I would be able to get output voltage 85Vdc but after changing it the output voltage when t=0 is still at 80Vdc which is the same design specification as cupoftea did even after changing the inductance value. I was curious why changing the specification of the transformer did not change anything in the circuit. Sorry sir for my ignorance.

 

[BradtheRad]

I'm running a flyback simulation with transformer 1:1. I'm unable to achieve satisfactory performance unless I set the step-down ratio 1:0.4 or 1:0.3.

If it's really 1:1 as in the listing, then why not use a plain buck instead? You'd have one coil to make life simpler. I thought the advantage of a flyback is that a transformer lets you have the sizes of wire you need in primary and secondary, as well as a reduced number of turns in the secondary. I start to wonder if some in that list of transformers are available for small customers after the really big manufacturer found them unsuitable for a big project.

For the spec 400 uH primary I find a frequency of several hundred kHz is okay.

 

[D.A.(Tony)Stewart]

I asked you why do you want to change it and got no reply.
Let me explain.
Many variables affect initial and final and this is close enough.
If you want to test for every variable, use the STEP parameter for AC phase, Vf of LEDs, N=#LEDs, 24.3V DC supply (?) etc
The purpose of a CC supply is to produce variable voltage and to be within 10% is fine.
You need to perform tolerance tests to see more.

I don't know the why there is 24.3Vdc or the efficiency or temp rise or limits to this design, but your initial voltage is fine.

You ought to be more concerned about Irms in caps and temp rise in parts.

 

[Patrick_66]

I asked you why do you want to change it and got no reply.
Let me explain.
Many variables affect initial and final and this is close enough.
If you want to test for every variable, use the STEP parameter for AC phase, Vf of LEDs, N=#LEDs, 24.3V DC supply (?) etc
The purpose of a CC supply is to produce variable voltage and to be within 10% is fine.
You need to perform tolerance tests to see more.

I don't know the why there is 24.3Vdc or the efficiency or temp rise or limits to this design, but your initial voltage is fine.

You ought to be more concerned about Irms in caps and temp rise in parts.

Hello Sir, I think I will try to create a transformer based on my specification and create the transformer on my own instead of buying the predesigned version. As for the equation to create the transformer I'm thinking of using the method based on this paper but I don't know whether the equation provided is complete to form the whole transformer.