THD improvement in constant current LED driver for street lighting applications

Authors: Aman Jha & Manoj Kumar, System LAB, STMicroelectronics, Greater Noida, INDIA

The better light quality, high efficacy, environment friendly and long lifetime are major criteria for selection of LED (Light Emitting Diode) as a possible replacement of existing lighting [1-2]. It is also known as a lighting of twenty first century, as power levels are increasing and costs decreasing for LEDs. Power LED is now becoming viable for street lighting application. But use of LED as a load in such power ranges  is governed by  regulatory norms like IEC61000-3-2 for power quality issues [3].There are more stringent norms related to total harmonic distortion (THD) for LED lighting in India [4-5].  In order to meet these norms, a normal Diode Bridge Rectifier (DBR) followed by bulky capacitor is not recommended. There is a possibility to run magnetics in discontinuous conduction mode (DCM) to achieve low THD and high power factor. In DCM mode, PFC switch does not operate in zero voltage condition, so efficiency is not good. Another issue is that this switch operates at high peak current so EMI filter size will increase. The critical conduction mode (CRM) is the best option for high power factor without compromising the efficiency. There are several CRM mode commercial analog ICs available in market which are recommended for high power factor and low cost with high efficiency. These commercial ICs fulfil the IEC 1000-3-2 Class C norms but LED driver solution THD remains in the order of 20% or higher. These do not meet new regulatory requirements for LED lighting [4-5]. In order to improve THD without compromising efficiency, feedback pin injection control scheme is proposed in this scheme. In the proposed scheme, a feedback from line voltage is injected after bridge converter on the feedback pin to improve THD without compromising efficiency and load regulation. The proposed converter works on primary sensing technique for high transient response and fast correction. The single stage and single switch technique results in small form factor and lower cost.  The proposed converter prototype is experimentally verified using commercial IC HVLED001A [6] with 80W LED streetlight. The test results are well within the new regulatory norms [4-5].   The power factor and THD recorded are >0.97 and <10% for entire source voltage range with high current regulation (3%). The converter operates with high efficiency (>92%). The converter is thermally stable so it can be readily fitted with existing casing for street lighting application.

Proposed LED Driver Schme

Figure 1 shows proposed new scheme for LED driver. In this proposed scheme input voltage switching waveform is fed to feedback loop to follow current in phase of line voltage. A high performance operational amplifier is used to meet the constant current requirement for LED driver. Figure2 show the constant current controller block for current control for LED module. The proposed LED driver is functionally divided into five blocks. The first two blocks – line filter and DBR are conventional which are similar to other power convertor. The flyback converter is working on zero voltage switching to improve efficiency without compromising high frequency requirement for low form factor of LED driver. The primary sensing feature of LED driver improves the fast dynamic response in constant voltage mode. It also enables the short circuit protection and over-voltage protection in case of LED short and open case.

The proposed converter prototype is tested with 97-112VDC LED module of 0.7A current rating. The hardware results are well within the stringent norms set for LED lighting [4-5].The THD for entire line cycle measured is less than 10% and power factor more than 0.97. Thus the proposed converter possesses excellent power quality parameters. The current regulation measured for entire line cycle is less than 2%. The achieved efficiency for proposed driver is more than 91%. Thus the proposed topology achieves good power quality and high efficiency for LED driver in street lighting applications.

Control Scheme

The control system of LED driver is based on the QR based peak current mode flyback PFC and constant LED current control scheme. Figure 2 shows the control scheme for flyback PFC controller for proposed LED driver. The PFC scheme is based on turning “OFF” the PFC switch when the set peak current is reached and turned “ON” the switch after reaching first resonant valley for the primary side demagnetization at zero current detection circuit. The primary sensing controller is first starting up using high voltage start-up signal coming from DC bus voltage. During start-up the controller provide necessary gate driving pulse and LED driver provide necessary voltage to output and auxiliary power supply to controller. The zero current detection (ZCD) circuitry coming into operation and detects the zero crossing of active switch. The SR flip-flop set signal for gate driver is coming from ZCD signal. The reset signal of flip-flop depends upon difference of voltage from feedback signal and current sensing signal in multiplication. For achieving low THD the line voltage envelope is injected feedback signal.

The constant current requirement of LED driver is achieved by sensing LED current and controlling the gate drive at feedback voltage using opto-isolator. Figure 3 shows block diagram of constant current controller. The LED current feedback voltage signal from LED string is coming at current sense input. The reference current is set using potential divider at voltage reference signal and set the current reference signal. The voltage difference between current reference and current feedback signal is same for better current regulation.

Hardware Result and Discussion

The proposed LED driver is tested with 80W LED load of 97V-112V, 0.7A current rating. The commercial HVLED001A controller is used for PFC and DC-DC converter [6]. The requirement of constant current is achieved by using commercial analog controller TSM101A [7]. Table 1 shows major active components selected for this proposed design. The board picture of proposed LED driver is shown in Figure4. The test result shows good THD (<10%), high power factor, high efficiency (>90%) and good current regulation (<3%) for entire line and load condition. Figure5 shows the test set up picture showing 7% THD at full load and 300V AC input voltage. Figure 6 shows power quality parameters and overall system performance of LED driver.   Figure6 (a) shows THD and PF versus input voltage. The THD and PF are found to be well within acceptable range (<10%) for LED driver for streetlight application [5-7]. Figure6 (b) shows the efficiency and current regulation for LED driver. The test results show overall efficiency is 92% at nominal input voltage and minimum 90% for entire line cycle.  The current regulation is <3% for entire input voltage range.

Table 1: Major Selected Components

Conclusion

A low THD LED driver has been designed and validated in this work for street lighting application. The LED driver has high efficiency and good current regulation for wide range input voltage. The startup, steady state and dynamic performance is good. It is experimentally verified. The proposed driver is suitable for large area street lighting application. It is suitable for stringent norms for LED street lighting application.

References

• Li, S.Tan, C. Lee, E. Waffenschmidt ,S.Y.R Hui and C.K. Tse, “A Survey, Classification and Critical Review of Light-Emitting Diode Drivers,” IEEE Trans. Power Electron., vol.31, no.2, pp.1503-1516, Feb. 2016.
• K. Jha and B. Singh, “A PFC Modified Landsman Converter-Based PWM-Dimmable RGB HB-LED Driver for Large Area Projection Applications” , IEEE Trans.on Indus. App., vol. 53, no. 2, pp. 1552-1561, Mar.-Apr. 2017.
• Harmonic Current Emissions Limit for Equipment Source Current ≤16A per phase, International Standard IEC 61000-3-2, 2003.
• India announces critical standards on LED lighting,[Online] http://www.theclimategroup.org/what-we-do/news-and-blogs/india-announces, May. 2017
• Functional Requirement Specification for Energy Efficient LED Based Luminaire Unit for Indoor Lights to be Used in Indian Railway Offices andBuildings [Online]: http://www.rdso.indianrailways.gov.in/,May. 2017.
• Datasheet, “ HVLED001 Offline controller for LED lighting with constant voltage primary-sensing and high power factor” [Online] st.com/ web/en/catalog/sense_power/FM142 /CL1854/SC1572/SS 1656/PF261186, May. 2017.
• Datasheet, “TSM101 Voltage And Current Controller” [Online] http://www.st.com/web/catalog/sense_power/FM142/ CL1454/ SC276 /PF65510?s_searchtype=partnumber, May. 2017.