Flyback Converter for Constant Current LED driver

[Patrick_66]

Hello everyone, wanted to ask how to obtain all the capacitance and resistance values in this paper as the datasheet for the IC did not mention how to get it as well as the paper. Hope that someone can help with that.

 

[D.A.(Tony)Stewart]

Unless you are a big OEM like Meanwell or Philips, you may not likely find out, unless you can ask them for samples and a full datasheet with a signed NDA and you are capable of beaucoup volume demand.

They have several patents applied over 10 yrs ago. This chip is described as a 3 step current control flyback that starts conduction at the zero crossing. There are other chips with high PF and many others.

GENERAL LIGHTING APPLICATIONS​

FEATURES

• Low cost, high efficiency LED driver controllers
• Patented Free Dimming™ technology
• Over-voltage, over-current and LED short circuit protection
• Simple — works with any non-dimming on/off wall switch
• Broad Appeal — works with sockets that DO NOT have dimmers

• • Great Value — NO dimmer cost
  • Easy — NO dimmer installation
  • Lowest cost BOM with smallest footprint
  • Energy Efficient — higher efficiency than triac dimming
  • Superior Performance — NO flicker, audible hum, triac dimming compatibility issues

LED CONTROLLER ICs for GENERAL LIGHTING Applications - NON-ISOLATED​

FEATURES

• High Power Factor, 2-Step Dimming
• 3-Step Dimming
• High Power Factor, 3-Step Dimming
• Continuous Dimming
• High Power Factor, Color Control

LED CONTROLLER ICs for GENERAL LIGHTING Applications - ISOLATED​

FEATURES

• High Power Factor, PSR (Primary Side Regulation)
• High Power Factor, 3-Step Dimming

 

[Patrick_66]

Hello Sir, can you recommend any IC that have constant output current in BCM mode with examples of circuit. I only want to focus on the circuit that provide constant output current and operate in BCM mode. As for the other feature it is not my main priority. Hope to hear from you soon Sir.

 

[D.A.(Tony)Stewart]

Lots of examples around https://www.edaboard.com/threads/fl...rent-output-design-calculations.396202/page-2

 

[Patrick_66]

Lots of examples around https://www.edaboard.com/threads/fl...rent-output-design-calculations.396202/page-2

Hello Sir, the datasheet did not mention the capacitance and resistance value for the circled part (purple) based on the figure below. May I ask how to find all of those values? Thank you for your time.

 

[D.A.(Tony)Stewart]

My quick analysis is that it is just an RC snubber load so that when the diode turns off there will be some minimal load for the regulator rather than On Off with large transient voltage spikes. This is a half wave rectifier with constant current sensing operating at some fixed > 100kHz to 1MHz without looking at the IC datasheet.

Initially the load is the diode resistance and the Capacitor ESR until the large e-cap charges up and the LED string turns on and it reaches the regulated current across the 2 ohm resistor. Then the the flyback reverse the diode the RC clamp circuit maintains around a 100 ohm decaying load current to suppress the otherwise high voltage spikes on the circuit.

I made a quick simulation using Falstad's emulator to prove my point.

https://tinyurl.com/yl3qmnr7

--- Updated Dec 17, 2023 ---

Rev A https://tinyurl.com/yltlq39k click the switch after you see the diode voltage.
you can compute the RC current clamp result.

 

[BradtheRad]

The schematic consists of several sections. Each has its function in the overall system.

Once you choose your desired voltage and current levels, those various sections tend to adopt a range of values that go with those voltage and current levels. This is done with both the power supply (input) and the supply (output) to the final load.

 

[Patrick_66]

The schematic consists of several sections. Each has its function in the overall system.

Once you choose your desired voltage and current levels, those various sections tend to adopt a range of values that go with those voltage and current levels. This is done with both the power supply (input) and the supply (output) to the final load.

Hello Sir, if I want my Vout to be 85 Vdc and Iout 0.4A which is the same as the specification given by the predefined transformer model 750811291. How do I know the range of value to adopt based on my defined Vout and Iout? Thank you Sir for your time.

 

[D.A.(Tony)Stewart]

Run the simulation with extra LEDs if you want 85V.

Reduce the current sense R on steps and to test the AC input limits with a step range in the simulation.

Power dissipation is plotted with the Alt hot key when selecting a part.

What other parameters should you expect to change?

 

[Patrick_66]

Very sorry sir for asking the same thing . This is because I compared the circuit diagram shown in the datasheet for LT3799 and the parts that are circled in purple show different values when compared with 20W, 4W and 14W LED but the datasheet did not mention the equation to obtain the values for the circle part so I was confused how to get the values. Once again thank for sir for your patience towards me and willingness to teach a person like me.

 

[BradtheRad]

Vout to be 85 Vdc and Iout 0.4A which is the same as the specification given by the predefined transformer model 750811291. How do I know the range of value to adopt based on my defined Vout and Iout?

Flyback type is usually designed for 50% duty cycle. Your specs use about 40 Watts of power. The first half of a cycle must convey 40 Watts of power (on the primary side), and the same 40W for the other half of the cycle to your load.

Each time the mosfet turns on, the transformer primary shall be energized for a long enough time to rise from 0W to 80W. Applying mains voltage the current is almost 1 Ampere (through the primary). This sets a proper switching frequency (I suppose the IC automatically creates the correct frequency.)

Select resistor and capacitor values which can handle such power (Watts) while having their needed effect on voltage swings, at the switching frequency.

On both primary and secondary side often the aim is to minimize heat. In control circuitry it pays to use high ohm values to limit current (and reduce power dissipation). In power circuitry you want to maximize current with low ohms and low impedance. For example your mosfet must turn on fully so it presents very low ohms.

 

[D.A.(Tony)Stewart]

I have no LT3799 experience and they do not give the gain function for power out. They used 3.33V * 1 A per 3.33 W LED which could be worst case Vf(?) Their design is a mystery for ratio scaling power with turns ratios, I sense and CTRL1. Power is transferred by a product of primary V and current sense values in a table for flyback forward transfer. You can reduce the 20W design raising Rs. But I haven't studied long enough to know how linear CTRL1 is for power transfer.

I guess they want you to use the table values but that is not a universal design with a pot like using a Meanwell supply or the O2 chip.

 

[Patrick_66]

Thank you very much for all the information and guidance. I will have a look and study more in-depth.