The third generation of high-precision constant current LED driver isolation techniques

For LED lighting load characteristics of the current-isolated constant current drive power topology is basically Flyback (flyback) buck structure. This will isolate LED constant current control technology development is divided into three generations, to discuss control strategies to achieve the principle of constant development, analysis of the advantages and disadvantages of each generation, each generation what breakthrough progress. Shanghai-based company's DU8966 duty chip, introduced the latest generation of isolated closed loop current control technique, described in detail how groundbreaking this control strategy to improve 280w led high bay light output current accuracy, from open-loop to closed loop is the essence of the breakthrough.

Three isolated LED constant current control technology

All mentioned in this article are all based on isolated constant current driver Flyback (flyback) topology.

The first generation of isolated constant current drive technology:


The initial isolated constant current drive technology called SSR (Secondary Side Regulation), shown in Figure 1, a high-frequency transformer primary side, secondary side isolation, and use the optocoupler output current of the secondary side to the primary side feedback signal, to provide to the control chip, chip control output PWM duty cycle, and thus play a CV (Constant Voltage) and CC (Constant Current) role. Because the secondary side can use the precision voltage reference, and a real negative feedback, so this type of drive technology is a closed loop constant current control method can achieve high precision constant current.

SSR technology dominate the market for many years, but its surrounding optocoupler line the circuit becomes complicated, as the market develops, the isolation drive technology made new demands. Cost, size is a major consideration.

The second generation isolated constant current drive technology:

The last 10 years, one kind of AC / DC technology is widely used, this is what we often say that the PSR (Primary Side Regulation), PSR edge technology with the original sample and control to replace the primary side and the secondary side of the control sample, with the first generation SSR , the biggest advantage is omitted optocoupler, secondary side current sensing and voltage reference chip peripheral devices 431. This is a revolutionary advancement in Figure 2, because this technology saves system board space, reducing costs while increasing system reliability, for mobile phone charger and 280w led high bay light driver power supply so right volume demanding a deep wide market impact. Output current projection formula is as follows:


Ipk = Vcs / Rcs;
Ipks = N * Ipk;
Io = [Tons / (2 * Tsw)] * Ipks
= (N * Tons * Vcs) / (2 * Tsw * Rcs);
Ipk: primary peak current
Ipks: secondary side peak current
Io: The average output current
Tons: secondary side freewheeling diode conduction time
Tsw: switching cycles
Vcs: Chip galvanometer foot peak comparison site level
Rcs: primary-side current-sense resistor


From the above equation, we know that as long as the chip internally fixed a peak comparison level VCS, power system fixed turns ratio N, chip detects the secondary side freewheeling diode freewheeling through 0:00, and fixed Tons / TSW, you can guarantee Io fixed.

PSR technology compared to the SSR main advantage is greatly simplified lines, but it is an open-loop current control method, so current precision and SSR methods can not match.

The second generation PSR-isolated constant current drive technology, if you want to achieve the precision constant current, there are some technical difficulties, as follows:

Output current Io and secondary D 'about, you need to accurately set proportion chip for mass production of the chip, so there are some difficult high accuracy.

Is true zero crossing detection. Currently done 0:00 detection approach is "But first test + zero delay" to achieve, that is about to zero in the chip set before the comparison level. This is inaccurate.

Is and Ip, and if no closed loop Ip detection, how to achieve accurate current control?

Theoretically 280w led high bay light is an open loop control technique, open-loop current control, can not be closed as the current accuracy and dynamic response. From a development point of view, is still not an ultimate solution is to reduce costs, sacrificing performance transition program.

source from:www.lead-lighting.com