5KW LED DRIVER. High Power White LED. LED Driver Requirement. Topology selection: Design Specifications

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1 5KW LED DRIVER High Power White LED Enormous energy can be saved by using efficient equiments along with effective control and careful design. The use of energy efficient lighting has been gaining oularity and LED offer one of the alternatives owing to their low ower consumtion. Particularly white LED s are suitable for lighting as they rovide lighting condition similar to the conventional light sources. The lighting intensity roduced by it deends on the current through it at a secified forward voltage. LED Driver Requirement LED driver is a regulated ower suly designed to match the electrical characteristics of an LED or an array of LED s in the alication. Its rimary function is to maintain consistent forward current and forward voltage required for oeration of the LEDs over varying conditions. Change of current changes the brightness / luminosity of the LED hence it is imortant for the driver to rovide a constant current even in the resence of disturbance and arameter variation which can be obtained through using feedback comensation. However the LED can also be fed with ulsating current due to its fast light to current resonse, neglecting the small color shift. Toology selection: Various toologies can be used, buck, boost, buck-boost for LED driver circuit but the flyback converter are the favorites for the ower levels below 100W for all kind of alications and also referred due to following advantages: a. LED driver based on flyback converter is simle and chea (number of comonent is less) b. Provide isolated outut for safety of LED lams (LED s and AC line are electrically isolated) c. Can add PFC function without greatly increasing the cost of the system. d. Addition of TRIAC can enable dimming function of LED light. e. Inherits the advantages of isolated DC-DC converters over non-isolated( Design Secifications The LED s are non linear and have electrical characteristics similar to diode. White LED oerates at a minimum forward voltage and through driver a constant current has to be maintained. The forward voltage dro of white LED (V f ) = 3V Parameter Symbol Value Inut Voltage Vin 0 V(RMS) at 50 Hz Outut Power Pout 5 W LED string average current ILED 300 ma The LED devices can be connected in arallel, series and arallel/series arrangements. However the series connection has certain advantages over arallel connection as it does not require current limiting resistors/functions in each branch. Constant current driver to rovide constant load current at required outut voltage would do the task.

2 Let the LED s are connected in series with same current and sharing the outut voltage among them. V Outut Voltage = out Pout 5W = = I 0.300A LED =16.66V There will be the limitation in the number of LED s that can be connected in series. umber of LED s in the string = n V out = = Vf With these secifications we design the fly back converter with roer devices and comonents suitable for the said alication. FlYBACK COVERTER The isolated Flyback converter is a Buck-Boost Derived toology with an isolation winding. Hence it has a transfer function of the form Vout Vin D = 1 D s The toological structure of flyback converter consists of a flyback transformer or couled inductor. When the switch is O the energy is stored in the rimary side (magnetizing inductor) and when the switch is OFF the energy is transferred to the secondary side by mutual induction. However the inut and outut currents have high rile content. AC-DC Bridge Rectifier A flyback transformer is actually an inductor with multile windings having characteristics of both inductor (store and release energy) and transformer (isolation/change of voltage level). It stores energy taken from inut during one switching instant and releases/delivers to the outut in the subsequent interval. Energy is stored in the gaed core and the air ga (higher reluctance) inductance reresents most of the magnetization inductance. Hence the flyback transformer is modeled as a magnetizing inductance in arallel to an ideal transformer. The leakage inductance of the flyback

3 transformer is unwanted arasitic comonent and is neglected. So the main aim is designing features of core, ga and winding considering the roblems associated due to saturation and loss (core/hysteresis). Selection of Switching Frequency: Various erformance criteria are affected by the selection of switching frequency. Though higher switching frequency reduces imroves the transient resonse and reduces the size of converter but that comes with a trade in decrease in efficiency and increase of heat roduction. Hence a otimum switching frequency has to be used and for this case it is selected as 100 KHz. Time eriod( Ts ) = 1/ fs = sec (1) FLYBACK COVERTER DESIG: The inut AC voltage is fed to a diode bridge rectifier and the outut through the DC link caacitor or smoothing caacitor is given to the flyback converter. The circuit can be divided into converters AC-DC/rectifier and DC-DC converter. If the overall efficiency If the overall efficiency of the converter is assumed to be 90% then the inut ower drawn from AC source is: η = P P out in () 5W P in = 0.9 = 5.5 W AC-DC Converter The DC value of the rectified voltage using full bridge diode rectifier is given as: V v dc eak 1 = 1 V frlc k ( rect ) = V and V = V -V rms k(rect) eak diode If the caacitor filter comonent is very large the dc voltage at the filter end is equal to the eak voltage of the suly as the caacitor gets charged and cannot discharge in small time eriod due to large time constant associated with equivalent load resistance and caacitance. Generally the caacitor is selected on the basis of -3μF /watt of inut ower. However, to simlify the Flyback converter design the DC Link Cacacitor is selected to 0.01F such that the voltage inut to DC-DC converter would be constant i.e the rile voltage would be negligible. C, 0.01F f in = (3)

4 DC-DC Converter Oerating Mode of Converter: Both the CCM and DCM mode have their share of advantages and disadvantages and the comonent and control design deends on the oerating mode of the converter. The advantages of CCM are higher efficiency and lower current stress while the DCM mode does not have RHP zero which simlifies control rocedure. The CCM mode will be used and the issues with it will be discussed. CCM Mode of Oeration: The design of flyback transformer consists of the main work in the flyback converter design. Let us make following assumtions as secifications for the flyback transformer design. Magnetizing current rile (I mr ) = 0% of DC comonent of magnetization current. Duty cycle (D) =0.4 (has to be selected carefully to not to cause high voltage stress on Switching Devices) Coer loss (Cu loss ) = 1.5 W (Core loss and other losses are neglected) Maximum flux Density (B max )= 0.5 T (To revent transformer from being driven to saturation). Vo s D =, where s /, is the secondary to rimary turns ratio. V 1 D in Ł / (n) = 1.4: (4) s The DC comonent of the magnetizing current (I ) = m s 1 V 1 D R out eq, ( V / R e I ) out q out = Am The magnetizing current rile is Im = 0% of Im =0.008 Am Maximum value of magnetizing current Im,max = Im + Im = Am The magnetizing inductance (L m ) required roducing the given rile inductor current is as: L m VinDT = i M s = H

5 The RMS value of the rimary winding current is given I 1 i M = IM D 1+ 3 I M = Am Similarly the secondary winding RMS current is, 1 P i M IS = IM D ' 1+ S 3 I M = Am Total RMS winding current is equal to I = I + I S tot P S P = Am The outut caacitor required is larger in case of flyback converter due to absence of inductor this is the only filter/energy storage comonent resent in the outut side. The value of caacitor is based on the rile requirement on outut voltage and the outut rile is determined by the ESR of the caacitor. For an aluminum electrolytic caacitor we have ESL of the caacitor is zero( below 300KHz) and ESR is r C Let the outut rile voltage be V rr = 0.5 ESR caacitance roduct ( C0r C ) = FΩ (5) Vrr = Irr rc I D I V r VO = + C f V + V eak O ds RO C O S O F, where ( VF is voltage dro across secondary diode=0v) D V = V and I = max eak in RO DC min ds 1 Dmax VDC min Dmax Using the above equation along with the values we have P VRO 0.4 = 310 = 06 V the, voltage reflected on the rimary side during switch off time. 0.6 eak Ids 5.5 = = A the maximum eak drain current of the switch

6 = + C C hence C = µ F And the ESR will be, Load Side calculations r C = 0.44Ω The load in this case is LED string with a current sensor in series with the unit. The sensor resistor will form a dual urose as a current limiting resistor in case of fault in the circuit. In MATLAB we can use a single Diode with a turn O voltage of 5 LED or a DC voltage source of equivalent value. If the converter is designed for 16.6 V outut and LED string requiring 15.5 V to fully turn O. Voltage dro on resistance of LED and Sensor VR = Vout VF, LED = =1.1 V Let the turn on resistance of LED be 0.01 Ω, Iout R = V, Ł R = Ω Tot R Tot RTot = RO + Rsen = Ω Hence the sensor resistor is R Ω Design of Core sen The resence of core in flyback transformer will cause to concentrate the magnetic field. The cores of flyback transformer must be able to allow large rimary currents to flow without saturating the core. To revent magnetic saturation of core we need to use either a gaed core or owdered ermalloy core. In this design we will be using a gaed core which will increase number of amere turns though on the cost of reduced ermeability. The core geometrical constant is a figure of merit that describes the effective electrical size of the core and is a function various geometrical arameters and given as: A I c W ρl A m,max Kg = ( MLT) B maxpcuk u The above relation shows the deendence of core size on the system secifications and is used to design a core to attain a given coer loss. The given secifications are tabulated below used for calculation of the K g

7 Secifications Wire resistivity(ρ) Peak winding current( I max ) Magnetizing Inductance( L ) Coer Loss( P ) cu m Winding fill factor( K ) 0.3 u Value 1.74e-6 Ω cm A H 1.5 W Maximum oerating flux Density( B ) 0.5 Tesla max (0.0775) ( ) (0.048) Kg 10 (0.5) = cm 5 The smallest EE core along which satisfies the given inequality has the dimension is obtained from the table below:

8 The air ga length in the core is calculated as: µ L I l 10 0 M m,max 4 g = B max Ac = 7 4Π (0.048) 4 10 (0.5) (0.14) -3 = cm With the chosen core the minimum number of turns for the transformer rimary side to avoid the core saturation is given as: = L I, B A M M m ax max c = = 1063 turns (on rounding off) Then using the desired turns ration required turns in secondary side is as: = / turns For calculating the window area allocated to each winding, I α = = = Itot s Similarly for secondary side I s s α S = = = Itot The wire gauge determining the size of wire in terms of cross sectional area is required as: A α K W u A w = P = cm 1063 And for the secondary side winding -5

9 A WS α K W S u A = S = cm 86-4 The AWG value for rimary side is smaller and hence the conductor s hysical size is smaller and the value for rimary side AWG is below the lowest AWG value available. Based on the value obtained above for secondary side winding area the suitable AWG is COTROLLER DESIG The feedback comensation is required to maintain stable oeration and rescribed oerating condition/regulate outut and resond fast enough in the event of disturbance and arameter variation. The load (LED) requires secified amount of current as the change in current causes various characteristics of load to change. Voltage mode control will be used to maintain a secified voltage across a fixed sensor resistor which will ensure a constant current through LED. VMC is a single loo controller which will act to change the duty cycle based on error between the observed voltage and desired voltage at the sensor. Small Signal Analysis The small signal dynamic characteristics of PWM flyback converter can be derived using the small signal model: The control to outut transfer function is given as: s s nvorcωznωz (1 + )(1 ) VO ( s) ω ω =, d( s) (1 D)( R + rc) ω (1 + + ) zn z s s 0 ω0q ω0

10 1 1 n D( n 1) VF Where, ωzn =, ωz = n(1 D) R(1 ) + n(1 D) r Dr CrC LD n D( n 1) Vo ω = 0 [ ] { } 3 n (1 D) R + n D( n 1) r LC( R + rc) n D( n 1) { } 3 τ = n D CRrC + Cr R + rc + L n D n (1 ) ( ) [ ( 1)] [ ] ρ = LC( R + rc) n D( n 1) [ ] 3 γ = n D R + n D n (1 ) ( 1) τ and daming ratio( ζ ) = ρ γ Using MATLAB to calculate the control to outut transfer function, we have Controller Design Using SISOTOOL The control to outut transfer function is a second order with zeros one of which lies ω 0 = 853. rad / sec in RHP. The oen loo gain has a resonant eak at. The hase lot starts from 360 which is equivalent to 0, just the difference in the hase offset.

11 Voltage across the sensor will be measured and comared with the reference voltage. For outut current of 300 ma at load branch (through LED) the voltage dro at sensor resistor should be Vsens, ref = 0.3A Ω =1.085 V The voltage across the sensor resistor will be comared and the error will be assed through the comensator which will be modulated by a sawtooth carrier to form a suitable gating signal to force sensor resistor voltage to reference value which will ensure desired current to flow through the LED. The Comensator is designed using SISOTOOL. Since the RHP zero is resent near the resonant frequency it comlicates the feedback comensation. The crossover frequency has to be limited below the theoretical limit available even in the case with RHP zero (1/5 th of RHP zero location). An integrator and a high frequency ole at ESR zero are added to obtain the desired shae of comensated loo gain ensuring stability and best available accurate tracking. The comensator transfer function in the Zero/Pole/Gain Format

12 The Crossover Frequency is very low and hence the settling time/time resonse of the closed loo system is not fast enough during disturbance. Ste Resonse Settling Time (t s ) = seconds. SIMULATIO RESULTS: The simulation and verification of above design is done using MATLAB/SIMULIK. Both the oen loo and closed loo simulation is done. A Universal Diode Bridge rectifier is used for rectification of AC and outut of which is fed to the flyback converter. The fly back transformer is imlemented using the linear transformer and an inductor (magnetization) in arallel to it. The LED is modeled as a diode of turn O voltage 15.5 V can also be modeled as a voltage source. The average value of the outut current and voltage is measured with the hel of simulink block MEA VALUE. The gating signal is rovided through PULSE GEERATOR block in oen loo simulation and PWM block using sawtooth carrier is used for the closed loo simulation. A disturbance voltage signal (ste u/down voltage) is added in series with caacitor to check the closed loo resonse/erformance in the event of disturbance.

13 OPE LOOP Average Values

14 CLOSED LOOP Average Values

15 OBSERVATIOS/COCLUSIOS Converter Design The duty cycle has to be selected carefully such that the turn s ratio should be small which determines the voltage stress on the ower transistor. P nom max Vmax = Vdc,max + ( Vout + Vdiode) Vds = VDC + VRO S Or So, the maximum voltage stress in the switch is around 530 V, Which can also be verified by the voltage waveform across the windings during simulation Unlike other converters there is only caacitor in the outut side (no inductor) so its value should be carefully selected, infact larger caacitor is used to ensure that it can suly the requirements of load during switch O time. The outut voltage and current are discontinuous due to ESR of the caacitor, however the average outut voltage and current are aroximately 16.6V and 300 ma. Pulsating current can also be fed to LED (without outut caacitor) as mentioned in the beginning but would increase load eak and RMS current and may cause EMI roblems. The voltage across the winding obeys the turn s ratio relation while the instantaneous winding current does not obey. VP / VS = P / S The outut current starts to flow after several switching cycle and for some initial cycles the current is 0 and voltage rises steadly. Controller Design Control to outut transfer function is a second order with zeros one of which lies in RHP. Hence the flyback converter s ower stage is a non-minimum hase system. The crossover frequency of closed loo system is limited due to RHP zero. The f is C should be within one fifth of the osition of RHP zero. However due to closeness of the RHP with the resonant oles the full available theoretical bandwidth could not be attained. For better erformance the Current mode control can be used which uses dual loo and the roblem of RHP zero can be negated and a faster resonsive system can be attained. The flyback can be oerated in the DCM mode which is also the referred mode of oeration for low owered alications and rovides simlified feedback comensation/higher bandwidth due to absence of RHP zero. Furthermore core size is also reduced and is suitable for high voltage low current alication like LED Driver.