New primary side regulation constant voltage solution in LED driver
Introduction
Nowadays, a growing trend in the LED lighting is no line frequency and low ripple current while maintains high PF. The Japanese market requiresthe ripple ratio of the lamp current should be less than 1.3 and ripple frequency should be larger than 100Hz, Energy star has similar requirement, output operating frequency =120 Hz [1].
Meanwhile, the tendency in a LED lighting driver is a PSR constant current (PSR-CC) with Single-flyback topology due to its simple circuit and low cost. Nothing comes for free, the biggest drawback of the PSR-CC solution is line frequency ripple current and it needs a large output capacitor to suppress the current ripple (Figure 2).
CH3: VLED, CH4: ILED
Figure 2 Line frequency ripple current in 17W single stage
In order to have high a PF, single stage PFC PSR-CC doesn’t have bulk E-cap, therefore it cannot eliminate the line frequency ripple current. In some high end lighting application, single stage PSR-CC cannot be used. The traditional solution is to use a three-stage approach: PFC stage, Flyback converter stage and secondary DCDC stage to solve this issue, as show in Figure 3.
Obviously, the circuit is too complex and it has more components. The circuit is also not cost effective and space saving even though the performance is good.
In this article, a two stage solution is derived by adopting PSR technology, it saves one stage, cost, while maintaining high performance – no line frequency ripple current and high PF. The circuit performance analyses of PSR-CC and PSR-CV solution are listed in section 1; how to achieve two stage PSR-CV solution are given in section 2. The experimental results of two-stage PST-CV solution prototype are given in section 3; the last section summarized the conclusions drawn from the investigation.
1. Two-stage solution
In terms of the PSR technique it depends on your control target which has two control methods: constant voltage regulation and constant current regulation. Because the LED current determines luminous intensity, the single stage PFC PSR-CC used in lighting application is steady .
In order to remove a line frequency ripple current and meet strict standards we must use a multi-stage solution. Since the PSR technique does not need a secondary feedback loop and opto-coupler then its tight regulation makes a two-stage solution a reality.
Based on three-stage solution, we can achieve a two-stage solution with PSR technique through two different ways:
1.1 Two-stage PSR-CC
The first two-stage PSR solution combines a Flyback converter stage and a secondary DCDC stage to manage isolation, driving and the dimming LED function. The left stage is the PFC stage. As we can see in Figure 4, the LED current is controlled in the primary side so the solution is also described as a two-stage PSR-CC.
A two-stage PSR-CC solution is well accepted in the market, especially in the phase cut dimming area – where a dimming signal comes from AC line. But in analog or PWM dimming, things are different. Since the dimming function is done on the primary side, considering safety, this solution needs a transformer to isolate the dimming signal. And because it is primary dimming, the dimming control is a little complex and not easy to achieve. Another point is the relatively poor CC regulation due to the PSR-CC being a weak point in high end lighting applications. Dimming range is another concern for this topology.
1.2 Two-stage PSR-CV
Based on a single stage solution and a two stage PSR-CC solution, a new two stage structure is appearing which is suitable for no line frequency current ripple application. The key point is that it combines a PFC stage and a Flyback convert stage in a single stage to achieve PFC and isolation functions [2]. The big difference compared to PSR-CC is that this single stage only controls the secondary output voltage and not the output current, hence it is called PSR-CV. The right stage is the DCDC stage which is used to drive or dim the LED. As show in Figure 5, two-stage PSR CV has a very clear function stage: the PFC function is achieved in the primary side and LED driving is implemented in the secondary side which will reduce circuit difficulty and is easy to design.
The two-stage PSR-CV solution maintains two stage PSR-CC merits and has some extra advantages. First, LED current control is simple and it has a more accurate LED current due to the secondary DCDC stage control LED current being handled directly. Secondly, for the dimming application, whatever 0~10V analog dimming or PWM dimming it can be easily implemented in secondary DCDC stage and without any isolation. Third, cost may be lower than PSR-CC. Compared with PSR-CC, we can consider it moves the PFC stage to secondary DCDC stage. As we all know, the PFC stage contain high voltage components. But in the secondary DCDC stage, these are low voltage components. Lastly, the PSR-CV solution gives us more flexibility. For example, we can add a standby power function in the PSR-CV stage to get low standby power once the LED is not connected. Or we can choose a suitable DCDC for multi-strings application.
The only drawback is PSR-CV output voltage regulation which is not very tight, but we can choose wide input range secondary DCDC to overcome this problem.
2. PSR-CV operation principle
Currently, the PSR-CV is implemented through controlling voltage on auxiliary winding. Once auxiliary winding voltage is controlled, the output voltage is set via transformer coupling. Therefore, in order to have accurate output voltage, we need to control auxiliary winding terminal voltage directly as we can see from Figure 6.
Figure 7 Simplified accurate PSR-CV control
During the rectifier diode conduction time, the sum of output voltage and diode forward-voltage drop is reflected to the auxiliary winding side as (Vo+VF) · Naux / Ns. Since the diode forward-voltage drop decreases as current decreases, the auxiliary winding terminal voltage reflects the output voltage best at the end of diode conduction time, where the diode current diminishes to zero. By sampling the winding voltage at the end of the diode conduction time, the more accurate output voltage information can be obtained.
Because auxiliary winding terminal voltage will move up and down in one switching period, we need to find out sampling point first, then using sampling/holding circuit, , after that compare sensing voltage to internal precise reference. Anyhow, the control logic is a little complex.
Another easy and feasible way is we can control rectified auxiliary winding voltage through PFC controller error amplifier as Figure 8 shows. The drawback is not accurate output voltage.
However, it is a tradeoff simply between control and accurate output voltage. In the two-stage PSR-CV solution, first stage output voltage accuracy is not a big issue, we can choose a wide input voltage range for the secondary DCDC to overcome this.
3. Test results and waveform measurement
An evaluation board is made based on single PFC controller FL6961 which has OVP function can be changed to implement PSR-CV and high voltage buck controller FL7701[3] which has extremely wide input range.
Thanks to the PSR-CV solution, we also can easily generate a secondary CV supply to other accessories like MCU by applying another auxiliary wind-ing at the secondary side.
With PSR-CV Vcc regulation, we can see the CV accuracy than can achieve ±4.25% CV tolerance in whole output load range. If we eliminate the light load voltage drift (caused by the burst mode), the CV accuracy will be better by ±1.1%.
With FL7701 we can easily help build up the BUCK DCDC with the analog dimming function inside. The total solution comes out with an extremely low ripple current.
Conclusion
This article introduces and develops a new two stage PSR-CV solution which offers no line frequency current ripple and high PF and maintains simple circuit, easy to design merits. Experimental results have proved the proposed two-stage PSR-CV solution is an excellent candidate for high end LED analog dimming and PWM dimming application. In the future, we can add new features such as standby-power at primary side to achieve low standby-power in order to meet LED driver development tendency.
Reference
[1] Energy Star, Energy Star Program Requirements Product Specification for Luminaries (Light Fixtures)
[2] Fairchild Semiconductor, AN-9737 (Design Guideline for Single-Stage Flyback AC-DC Converter Using FL6961 for LED Lighting), 2012.
[3] Fairchild Semiconductor, FL7701MX (Smart Non-isolated PFC Buck LED driver), 2012
About the authors
Jason Tao is a field applications engineer at Fairchild Semiconductor for the East China region, focusing on AC-DC and lighting applications. Prior to working at Fairchild, he served as a design engineer for AC-DC adapters at Delta Shanghai. Jason Tao graduated from the Zhejiang University, with a bachelors degree in power electronics.
Justin Zhu is a field applications engineer at Fairchild Semiconductor for the East China region, focusing on AC-DC, lighting and industrial applications. Prior to working at Fairchild, he served as an applications engineer for the high current DC-DC group at MPS Hangzhou and field applications engineer at MPS Europe. Justin Zhu graduated from the Nanjing University of Aeronautics and Astronautics, with a master degree in power electronics.
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