AN4496 Application note How to get a high power factor with the HVLED815PF device Stefano Signoria Introduction The standard application of the HVLED8xxPF (HVLED807PF, HVLED815PF) is a constant current (CC) LED driver. The average output current IOUT, as described in the HVLED815PF datasheet (section 4.5 Constant current operation), does not depend on the value or the waveform of the input voltage VIN, then can be used in standard or high power factor implementation. Two methods of implementing the high power factor (HPF) based on the HVLED8xxPF family are current in use: the current sense modulation and ILED modulation. In either case the input voltage after rectification is not smoothed and the waveform on the bulk capacitor is a semi-sinusoidal waveform. The voltage on the bulk capacitor, VRECT, contains information about the phase and waveform of VIN. With the first method, the additional circuitry in Figure 1 applies a DC offset and a modulation, both proportional to the VRECT, to the CS pin and permits to obtain an HPF. This solution is described in detail in 1. of Section 3. The second one, shown in Figure 2, applies a modulation of the ILED pin proportional to the VRECT and features HPF in a single range application (refer to the AN4129 for further details). This application note describes how to modify the basic circuit to support wide input range applications. 1 Figure 1. CS HPF; additional components Figure 2. ILED HPF; additional component 9UHFW 9UHFW 5$5% 5S 5S &DF 526 &6 ,/(' 5 53) &26 &S &FV 5S 6285&( +9/('[[3) WR5VHQVH +9/('[[3) WR$X[LOLDU\ZLQGLQJ $0 September 2014 DocID026370 Rev 1 &/('UHPRY HG $0 1/18 www.st.com 18 Contents AN4496 Contents 1 2 ILED pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 DC analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 High power factor modulation analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 High power factor implementation in wide input range application . . . . . . 6 Designing a high PF wide range LED driver with the HVLED815PF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.1 Input specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.2 Operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.3 Transformer design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.4 Drain source breakdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.5 Current sense resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.6 Primary inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.7 Secondary/auxiliary turn ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.8 Feed forward . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.9 OVP protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.10 AC modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 3 Supporting material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2/18 DocID026370 Rev 1 AN4496 1 ILED pin ILED pin The ILED pin voltage (VILED) is the constant current (CC) loop reference. This voltage divided by two is used as the reference for the MOSFET's peak drain current during the CC regulation. An external capacitor is used for filtering the ILED pin current with an appropriate time constants (ILED >> 1 / 2 * * fIN). In this configuration the peak of the drain current remains constant during the semi-period but the tON of the primary MOSFET increases when the instantaneous input voltage decreases and with the mains near the zero-crossing the MOSFET remains in the ON state until the mains voltage becomes enough to source the peak drain current. Then near the zero-crossing the mains current has a peak as in Figure 3 A. Figure 3. ILED voltage and input current waveform $0 1.1 DC analysis With a DC input voltage (VIN) and the device working in the transition mode (TM), the voltage on the ILED, VILED, is: Equation 1 This voltage changes in function of mains voltage to maintain constant the output current and VILED reaches the maximum value at the minimum mains voltage. DocID026370 Rev 1 3/18 18 ILED pin AN4496 In case of sinusoidal mains voltage, if the input current is in phase and with the same waveform of the input voltage (i.e.: HPF and THD > 30%), the Equation 1 becomes: Equation 2 Where VILEDavg is the average voltage on the ILED pin, obtained by integration (ILED >> 1 / 2 * * fIN) of the ILED current in an external capacitor. Figure 4 shows the calculated and measured values of the voltage VILED vs. VIN with VR = 102 V; the measurements are made on the board EVLHVLED815W10F at VOUT nominal (22 V, VR = 102 V). The difference between the curves, when the VIN increases, is due to increasing of switching frequency and then greater power transferred in valley skipping (not TM) . Figure 5 shows the calculated and measured values of the voltage VILED vs. VIN with VR = 70 V and VR = 122 V; the measurements are made on the same board EVLHVLED815W10F changing the number of the LED and then VOUT and VR. Figure 4. VILED at VR = 102 V (VOUT= 22 V) Figure 5. VILED at VR = 70 V (VOUT = 15 V) and VILED at VR = 122 V (VOUT = 26 V) 9,/(' >P9@ 9,/(' >P9@ 0HDVXUHG YDOXH DW9 5 9 &DOFXODWHG YDOXHDW9 5 9 9,1 >9@ 0HDVXUHG YDOXH DW9 5 9 &DOFXODWHG YDOXHDW9 5 9 9,1 >9@ $0 4/18 0HDVXUHG YDOXH DW9 5 9 &DOFXODWHG YDOXHDW9 5 9 DocID026370 Rev 1 $0 AN4496 1.2 ILED pin High power factor modulation analysis A solution to obtain the high power factor and low THD, as shown in Figure 2 on page 1, is the AC coupling of the ILED pin with the rectified voltage. In this way the average of the voltage VILEDavg is generated by the internal loop, which regulates the average output current, while the waveform is modulated through the voltage divider by VRECT (see Figure 3 B). The average output voltage of an ideal single-phase full wave rectifier is: Equation 3 Then the AC modulation is optimal when: Equation 4 To ensure accurate regulation, the peak voltage on the ILED pin (VILEDpeak) must be smaller than its maximum headroom voltage VILEDx (1.5 V). From Equation 2 and Equation 4 can be estimated the maximum reflected voltage: Equation 5 The effect of a reflected voltage greater of VRmax is the reduction of the output current when VIN is lower than: Equation 6 The equivalent input resistance of the ILED pin (RinILED) is 50 K when VILEDavg = 1 V and the voltage divider must drive the ILED pin with equivalent resistance RAC << RinILED. The phase rotation, at the input frequency, introduced from the capacitor CAC impacts on the power factor, then the value of the capacitor must be: Equation 7 DocID026370 Rev 1 5/18 18 ILED pin AN4496 The selected value of VR and VINmin imposes the maximum value of: Equation 8 Where VINmin is the minimum RMS input voltage and is the efficiency at the VIN used in the equation. The correct AC modulation is obtained setting the voltage divider ratio (KAC) equal to: Equation 9 Where Vdrp is the input drop voltage between VIN and VRECT (input bridge, input filter, fuse, etc.). Referring Figure 2 on page 1, because KAC is >> 1, then Rp3 can be set equal to RAC: Equation 10 Rp1 + Rp2 = Rp3 * (KAC - 1) 1.3 High power factor implementation in wide input range application With the value of the components chosen in the previous paragraph, the AC modulation is optimized at the minimum input voltage. When the voltage increases, the ILED pin average voltage decreases, but the AC modulation increases and forces to zero the primary current for longer time near the zero crossing (see Figure 3 B). The effect is the generation on the input current a zero crossing distortion, that initially, reduces the harmonics (and THD) of the flyback configuration, but when the input voltage reaches a value around 1.25 * VINmin, the distortion begins again to increase. If is desired a THD under 20 - 25%, the ratio between the maximum and minimum of VIN must be lower than 1.5. In Figure 6 is presented a solution that permit to cover wide-range application with THD under 20 - 25% and high PF. The circuitry uses in the low range (USA and Japan) a different voltage divider ratio (KAC), than in the high range (European). The transistor Qr is used to change the voltage divider ratio between the value KACL in the US and Japanese and KACH in the European range. 6/18 DocID026370 Rev 1 AN4496 ILED pin Figure 6. Pin ILED modulation circuitry for wide input voltage range application 95(&7 5U 5S 5U 5S 'U 723,1,/(' 5SF 5SV &DF 4U &U 5U 5SH &S 5S $0 In the low range (88 V - 132 V) the transistor Qr is open and the circuit in Figure 6 becomes as that in Figure 7. The value of KACL is obtained from Equation 9. Equation 11 The sum Rp1 + Rp2 + Rps can be calculated by Equation 10 that becomes: Equation 12 Rp1 + Rp2 + Rps = Rp3 * (KACL - 1) Where Rp3 = RAC and Rph = Rp1 + Rp2. Rph resistor is splitted into Rp1 and Rp2 to satisfy the maximum voltage ranting of the case, then Rp1 = Rp2, and choosing Rps = Rp1 / 3 permits to use a high value of Rpe. Equation 13 Rp1 = Rp2 = 3 / 7 * Rp3 * (KACL - 1); DocID026370 Rev 1 Rps = 1 / 7 * Rp3 * (KACL - 1) 7/18 18 ILED pin AN4496 Figure 7. Equivalent pin ILED modulation circuitry in low input voltage range 95(&7 5U 5S 5U 5S 'U 95(&7 5S 5S 723,1,/(' 5SF 5SV &DF 723,1,/(' 5SV &DF 4U &U 5U 5SH 5S &S 5S &S $0 In the high range (185 V - 265 V) the transistor Qr is saturated and the circuit in Figure 6 becomes as that in Figure 8. The value of KACH is from Equation 9: Equation 14 Where VINminH is the high range minimum RMS input voltage and the value used for and Vdrp are the values at the VINminH. The value of Rpc + Rpe to obtain the selected value of KACH the will be: Equation 15 The resistors Rr1, Rr2 and Rr3 implement a voltage divider. In parallel to the Rr3 is placed a capacitor Cr3 that filters the voltage ripple. When this voltage is greater than the sum of Dr Zener voltage (VZ_Dr) and the base - emitter voltage of Qr (VBE_Qr), the transistor Qr starts to switch on. The emitter resistor reduces the gain of Qr and permits a linear transition between the low and the high range, that allows to work without hysteresis and reduces the THD in the transition range (132 V -175 V). 8/18 DocID026370 Rev 1 AN4496 ILED pin Equation 16 Where VINthL is the lower transition voltage and Rr3 << (1 + hFE_Qr) * Rpe. Figure 8. Equivalent pin ILED modulation circuitry in high input voltage range 95(&7 95(&7 5U 5S 5S 5U 5S 5S 'U 723,1,/(' 5SF 5SV 723,1,/(' 5SF 5SV &DF &DF 4U &U 5U 5SH 5S &S 5SH 5S &S $0 The higher transition voltage (VINthL ) is determined by Rpe but the base of Qr is always biased while the collector voltage drops near to zero and modelling is not simple. Starting from the below estimated values and the recommended value in Section 2.10 on page 16 further fine tuning of the final application can be done assuming that: Decreasing/increasing the Dz Zener voltage the lower transition voltage decreases/increases Decreasing/increasing the Rr3 resistor value the lower and the higher transition voltage increase/decrease Decreasing/increasing the Rr1, Rr2 resistor value the lower and the higher transition voltage decrease/increase Decreasing/increasing the Rpe resistor value the spread between the lower and the higher transition voltage decreases/increases. DocID026370 Rev 1 9/18 18 Designing a high PF wide range LED driver with the HVLED815PF 2 AN4496 Designing a high PF wide range LED driver with the HVLED815PF Main characteristics and circuit description The main characteristics of the LED driver are listed here: 2.1 Universal input mains range: 88 ÷ 265 VAC Output power 10 W continuous operation Output current: 460 mA at 22 V continuous operation Overall efficiency up to 85% Power factor higher than 0.95 Input specification The following is a possible design procedure for a high power factor LED driver using the HVLED815PF device. This design is referred to the schematics of Figure 9. First step is to define the design specification. Minimum mains voltage [VAC rms]: Equation 17 VIN min = 88 V Maximum mains voltage [VAC rms]: Equation 18 VIN max = 265 V Range (wide, US or European) = WIDE; in the European range maximum output power is 15 W, in an other case must be limited to 9 - 10 W. If the mains voltage range is a single range, the switch range network can be omitted (R13, D4, D7, Q2, R21, R20, R4). In a wide mains voltage range the network switches smoothly between the US range and European range. Minimum mains frequency [Hz]: Equation 19 FIN min = 47 Hz Mean output current [mA]: Equation 20 IOUT = 460 mA Output current ripple [%]: Equation 21 IOUT = 140 % 10/18 DocID026370 Rev 1 AN4496 Designing a high PF wide range LED driver with the HVLED815PF Mean output voltage [V]: Equation 22 VOUT = 21.7 V The mean voltage LED string drop is the output voltage. Overvoltage protection [V]: Equation 23 VOVP = 29 V The output voltage VOUT has a ripple at twice the line frequency and whose amplitude is proportional to the output current and reverse proportional to output capacitance. This application has a high ripple voltage (due to the high power factor and small bulk capacitor). With a mean output voltage of 22 V the peak is at 25 V. For reliability the output capacitor has a rated voltage of 35 V. The selected OVP threshold of 29 V is set between these two limits (the peak output voltage and output capacitor rated voltage). Supply voltage of the device [V]: Equation 24 VCC = 12 V In this design the supply voltage Vcc is low. That value is selected, because in this design the output voltage is fixed to 22 V. A higher value of Vcc is recommended (until 18 - 21 V) in case of application with variable output voltage [example: Iout = 460 mA, Vout = 14 V ~ 22 V)]. The efficiency is better if the supply current is sourced from the auxiliary winding rather than from the high voltage startup. DocID026370 Rev 1 11/18 18 Designing a high PF wide range LED driver with the HVLED815PF AN4496 Figure 9. Schematic 5OPLQ 9RXW &RXW 6HF 'RXW 9$8; 3ULB'UDLQ 5GPJ 5I E '0* 3ULB5HFW 7 'Y FF 5Y FF 8 '5$,1 6285&( '5$,1 &6 '5$,1 9&& '5$,1 *1' ,/(' 'VQ '0* &VQ &203 1$ 5VQ 5V 9&& &Y FF &DF &S &I 5I &S 5SV 5S +9/('3) 5S 5S 5U 5SF 4U 5SH 'U 5U &LQ 5U &U /LQ 5SO 5SO /LQ 9,1 &LQ %ULGJHGLRGH ) $0 12/18 DocID026370 Rev 1 AN4496 2.2 Designing a high PF wide range LED driver with the HVLED815PF Operating conditions The first step is to verify: Maximum power output [W]: Equation 25 POUT = IOUT * VOUT POUT_MAX 2.3 In the European range (VIN min > 175 V) POUT max < 15 W In the US and Japanese range (VIN min < 175 V) POUT max < 10 W. Transformer design The voltage at the ILED pin must be limited at 1.5 V. Then for the best performance the optimal reflected voltage VRopt must be set for using all the dynamics of the ILED pin at minimum mains. Equation 26 Where: 2.4 _VINmin = 80% efficiency at minimum input voltage VILEDx = 1.5 V pin ILED maximum voltage Drain source breakdown Reflected voltage can be limited by drain-source breakdown voltage. Equation 27 VRbrk = V(BR)DSS - 2 * VINmax -VSpike - Vtol = 800 - 2 * 265 - 150 - 80 = 200 V Where Vtol is a margin for the components tolerance. The value of VR lower than VRopt and VRbrk is used to calculate the primary/secondary turns ratio n: Equation 28 Where VFsec = secondary diode forward drop voltage. When VR is lower than VRopt, the dynamic is not optimized but the HVLED815PF device is working without problem. Real value used: n = 4.52 VR = 100 V DocID026370 Rev 1 13/18 18 Designing a high PF wide range LED driver with the HVLED815PF 2.5 AN4496 Current sense resistor Current sense resistor value is determined by the average LED current IOUT. Equation 29 This formula is exact if: The peak voltage on the ILED pin is smaller than the maximum headroom voltage for all mains voltage range. OVP protection is set 20% over the maximum output peak voltage. Perfect transformer coupling. The LED driver works in the TM mode for over an half of the semi-period. For different designs, fine tuning may be needed, but once the final values are selected, repeatability from unit to unit is excellent. Real value used: 2.6 RS = 1 Primary inductance The primary inductance Lp sets the working frequency. For the best regulation is better to limit the minimum frequency at maximum mains voltage (265 V) to 90 KHz, in this way when the voltage is around the peak of the semi-period (and the instantaneous power is higher), the LED driver operates in transition mode (TM). In wide range application, at minimum mains voltage (88 V), this frequency drops to the minimum fmin 40 KHz. Equation 30 The increase fmin reduces the primary inductance and the transformer can be smaller. In this case KRs can be used to compensate divergence of IOUT from the case with optimal frequency. Real value used: 14/18 LP = 1.5 mH DocID026370 Rev 1 AN4496 2.7 Designing a high PF wide range LED driver with the HVLED815PF Secondary/auxiliary turn ratio The operating range of VCC is between 11.5 V and 23 V. The mid voltage is around 17 V. The drop voltage on the limiting resistor Rlim (R9) and the auxiliary rectifier diode (D2) at the nominal operating point is set to 1 V. Equation 31 Vdrop AUX = VFaux + VRlim = VFaux + ICC * Rlim Then is defined the secondary/auxiliary turn ratio. Equation 32 Two conditions must be checked: 1. In case of an open circuit the output voltage is a bit upper at the threshold VOUT OVP, in this situation Rlim must limit the current to 25 mA. Equation 33 The higher value of Rlim must be used in the design, if greater Rlim is determined for the open circuit conditions, the secondary/auxiliary turn ratio to compensate the drop on Rlim must be recalculated. 2. If requested, wide range output voltage at the constant current VCC must be increased to 21 V. To demonstrate the high voltage startup functionality in this design VCC is lower, then: Real value used: 2.8 NS / NAUX = 1.75 VCC = 12 V Feed forward Equation 34 Real value used: Rdmg = 91 k DocID026370 Rev 1 15/18 18 Designing a high PF wide range LED driver with the HVLED815PF 2.9 AN4496 OVP protection In Section 2.1 on page 10 the selected value VOUT_OVP = 29 V, then: Equation 35 Real value used: 2.10 Rfb = 16k AC modulation To obtain the high power factor and low THD, a signal proportional to rectified mains is applied to the ILED pin. The other components of the network that switching the range can be calculated, but the non-linear behavior of Qr due the modulation of the Qr collector requires a fine tuning of the network; using the next recommended value for the components, the only value to be calculated is R3 to optimize the modulation at 85 - 90 V. Equation 36 With: Rp1 = Rp2 = 180 k Rps = 120 k Rpc = 51 k Rpe = 15 k Rr1 = Rr2 = 120 k Rr3 = 62 k Cr3 = 4.7 µF Dr = BZV55-C20 Qr = MMBTA42 16/18 DocID026370 Rev 1 AN4496 3 Supporting material Supporting material Documentation 4 1. HVLED815PF - “Offline LED driver with primary-sensing and high power factor up to 15 W” datasheet. 2. AN4129 - “STEVAL-ILL044V1: 9 W Triac dimmable, high power factor, isolated LED driver based on the HVLED815PF (for US market)”. Revision history Table 1. Document revision history Date Revision 08-Sep-2014 1 Changes Initial release. DocID026370 Rev 1 17/18 18 AN4496 IMPORTANT NOTICE – PLEASE READ CAREFULLY STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, enhancements, modifications, and improvements to ST products and/or to this document at any time without notice. Purchasers should obtain the latest relevant information on ST products before placing orders. ST products are sold pursuant to ST’s terms and conditions of sale in place at the time of order acknowledgement. Purchasers are solely responsible for the choice, selection, and use of ST products and ST assumes no liability for application assistance or the design of Purchasers’ products. No license, express or implied, to any intellectual property right is granted by ST herein. Resale of ST products with provisions different from the information set forth herein shall void any warranty granted by ST for such product. ST and the ST logo are trademarks of ST. All other product or service names are the property of their respective owners. Information in this document supersedes and replaces information previously supplied in any prior versions of this document. © 2014 STMicroelectronics – All rights reserved 18/18 DocID026370 Rev 1

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