AN-6861

www.fairchildsemi.com AN-6861 Applying FAN6861 to a Flyback Power Supply with Peak Load Current Profile 1. Introduction Highly integrated PWM controller, FAN6861, is optimized for applications with motor load, such as printers and scanners, that inherently impose some kind of overload condition on the power supply during acceleration mode. The two-level OCP function allows the SMPS to stably deliver peak power during the motor acceleration without causing premature shutdown, while protecting the SMPS from overload condition. The green-mode and burst-mode functions with a low operating current (2.2mA maximum in green mode) maximize the light-load efficiency so that the power supply can meet stringent standby power regulations. The frequency-hopping function reduces electro-magnetic interference (EMI) of a power supply by spreading the energy over a wider frequency range. The constant power limit function minimizes the component stress in abnormal condition and helps to optimize the power stage. Protection functions; such as OCP, OLP, OVP, and OTP are fully integrated into FAN6861, which improves the SMPS reliability without increasing system cost. This application note presents design considerations to apply FAN6861 to a flyback power supply with peak load current profile. It covers designing the transformer, selecting the components, and closing the feedback loop. Figure 1 shows a typical application circuit using FAN6861. L EMI AC input RSN1 + RSN2 CSN2 CSN1 D OUT VIN DDD1 RSTART 5 + + Filter VO + VO - N CIN COUT 1 R DAMP + DDD 2 DSN COUT 2 CDD 1 CDD 2 RG VDD 3 RT GATE 6 SENSE 4 GND 1 2 FB RCSF CCSF C FB RCS FAN6861 RBIAS R1 RDB CF R2 KA431 Figure 1. Typical Application © 2009 Fairchild Semiconductor Corporation Rev. 1.0.1 • 6/9/09 www.fairchildsemi.com AN-6861 APPLICATION NOTE 2. Design Considerations Flyback converters have two operation modes; continuous conduction mode (CCM) and discontinuous conduction mode (DCM). CCM and DCM have their own advantages and disadvantages, respectively. In general, DCM provides better switching conditions for the rectifier diodes, since the diodes are operating at zero current just before becoming reverse biased and the reverse recovery loss is minimized. The transformer size can be reduced using DCM because the average energy storage is low compared to CCM. However, DCM inherently causes high RMS current, which increases the conduction loss of the MOSFET severely for low line condition. Thus, especially for applications with peak load profile, such as printer and scanner; it is typical to design the converter such that the converter operates in CCM for low line and peak load condition to maximize efficiency. In this section, a design procedure is presented using the schematic of Figure 1 as a reference. An off line SMPS with 20W/32V nominal output power and 50W/32V peak output power has been selected as a design example. (Design Example) The specifications of the target system are: • VLINEMIN =90VRMS VLINEMAX)=264VRMS • Line frequency (fL) = 60Hz • Nominal output power (PNO) = 20W (32V/0.625A) • Peak output power (PPO) = 50W (32V/1.56A) • Peak load duration (tPO) < 500ms • Estimated efficiency: ηN = 0.87 and ηP = 0.82 P 50 PINP = PO = = 61W η P 0.82 P 20 PINN = NO = = 23W η N 0.87 FAN6861 can be used for this application because the peak load duration is less than the OCP delay time of 780ms. [STEP-2] Determine the Input Capacitor (CIN) and the Input Voltage Range [STEP-1] Define the System Specifications Designing a power supply with peak load current profile, the following specifications should be determined first: Line voltage range (VLINEMIN and VLINEMAX) Line frequency (fL). Nominal output power (PNO) Peak output power (PPO) and its duration (tPO) Estimated efficiencies for nominal load (ηN) and peak load (ηP): The power conversion efficiency must be estimated to calculate the input powers for each condition. Typically, the efficiency at peak load condition is lower than that of nominal load since most of the components of power supply are selected for nominal load condition. If no reference data is available, set ηN = 0.7~0.75 and ηP = 0.65~0.7 for low-voltage output applications and ηN = 0.8~0.85 and ηP = 0.75~0.8 for high-voltage output applications. With the estimated efficiency, the input power for peak load condition is given by: It is typical to select the input capacitor as 1.5~2μF per watt of peak input power for universal input range (85-265VRMS) and 0.7~0.8μF per watt of peak input power for European input range (195V-265VRMS). With the input capacitor chosen, the minimum input capacitor voltage at peak load condition is obtained as: VINP MIN = 2 ⋅ (VLINE MIN ) 2 − PINP ⋅ (1 − DCH ) C IN ⋅ f L (3) The minimum input capacitor voltage at nominal load condition is obtained as: VINN MIN = 2 ⋅ (VLINE MIN ) 2 − PINN ⋅ (1 − DCH ) C IN ⋅ f L (4) where DCH is the input capacitor charging duty ratio defined as shown in Figure 2, which is typically about 0.2. The maximum input capacitor voltage is given as VIN MAX = 2VLINE MAX (5) PINP = PPO ηP (1) The input power for nominal load condition is given by: PINN = PNO ηN (2) Figure 2. Input Capacitor Voltage Waveform © 2009 Fairchild Semiconductor Corporation Rev. 1.0.1 • 6/9/09 www.fairchildsemi.com 2 AN-6861 APPLICATION NOTE (Design Example) By choosing a 100μF capacitor for the input capacitor, the minimum input voltages for peak and nominal load are obtained, respectively, as: VINP MIN = 2 ⋅ (VLINE MIN ) 2 − = 2 ⋅ (90) 2 − PINP ⋅ (1 − DCH ) C IN ⋅ f L 61 ⋅ (1 − 0.2) = 90V 100 × 10 −6 ⋅ 60 PINN ⋅ (1 − DCH ) C IN ⋅ f L As can be seen in Equation (7), the voltage stress across the MOSFET can be reduced by reducing VRO. However, this increases the voltage stresses on the rectifier diodes in the secondary side. Therefore, VRO should be determined by a trade-off between the voltage stresses of MOSFET and diode. Because the actual drain voltage rises above the nominal MOSFET voltage due to the leakage inductance of the transformer, as shown in Figure 3, it is typical to set VRO around 70~100V so that VDSNOM is 430~450V for 600V MOSFET (73~78% of MOSFET voltage rating). (Design Example) By determining VRO as 100V: VINN MIN = 2 ⋅ (VLINE MIN ) 2 − = 2 ⋅ (90) 2 − 23 ⋅ (1 − 0.2) = 115V 100 × 10 −6 ⋅ 60 DMAX = VRO VRO 100 = = 0.53 MIN 100 + 90 + VINP The maximum input voltage is obtained as VIN MAX = 2 ⋅ VLINE MAX = 2 ⋅ 264 = 373V [STEP-3] Determine the Reflected Output Voltage (VRO) VDS NOM = VIN MAX + VRO = 273 + 100 = 473V When the MOSFET is turned off, the input voltage (VIN), together with the output voltage reflected to the primary, (VRO) are imposed across the MOSFET, as shown in Figure 3. With a given VRO, the maximum duty cycle (DMAX) and the maximum nominal MOSFET voltage (VDSNOM) are obtained as: [STEP-4] Determine Inductance (LM) the Transformer Primary-Side The transformer primary-side inductance is determined for the minimum input voltage and peak load condition. With the DMAX from Step-3, the primary-side inductance (LM) of the transformer is obtained as DMAX = VRO VRO + VINP MIN (7) (6) LM = VDS NOM = VIN MAX + VRO (VINP MIN ⋅ DMAX ) 2 2 P INP f SW K RF (8) where fSW is the switching frequency and KRF is the ripple factor at peak load and minimum input voltage condition, as shown in Figure 4. The ripple factor is closely related to the transformer size and the RMS value of the MOSFET current. Even though the conduction loss in the MOSFET can be reduced by reducing the ripple factor, too small a ripple factor forces an increase in transformer size. From practical point of view, it is reasonable to set KRF = 0.3~0.6 for the universal input range and KRF = 0.4~0.8 for the European input range. Once LM is calculated by determining KRF from Equation (8), the peak current and RMS current of the MOSFET for minimum input voltage and peak load condition are obtained as: I DS PK = I EDC + ΔI 2 (9) (10) (11) (12) www.fairchildsemi.com ΔI ⎤ D ⎡ I DS RMS = ⎢3( I EDC ) 2 + ( ) 2 ⎥ MAX 2⎦3 ⎣ PINP where: I EDC = MIN VINP ⋅ DMAX Figure 3. The Output Voltage Reflected to the Primary © 2009 Fairchild Semiconductor Corporation Rev. 1.0.1 • 6/9/09 and ΔI = VINP MIN DMAX LM f SW 3 AN-6861 APPLICATION NOTE K RF ΔI ΔI = 2 I EDC The peak drain current at minimum input voltage and peak load condition was obtained from Equation (9) in Step-4. The peak drain current at minimum input voltage and nominal load condition is given as: I DS . N PK = PINN ⋅ (VIN MIN + VRO ) VINN MIN ⋅ VRO + VINN ⋅ VRO 2 LM f SW ⋅ (VINN MIN + VRO ) MIN (13) I DS PK Figure 4. MOSFET Current and Ripple Factor (KRF) : CCM I DS . N PK = 2 ⋅ PINN f SW ⋅ LM : DCM (14) (Design Example) Determining the ripple factor as 0.57 LM = (VINP ⋅ DMAX ) (90 ⋅ 0.53) = 2 P INP f SW K RF 2 ⋅ 61 ⋅ 65 × 103 ⋅ 0.57 MIN 2 2 Whether the converter operates in CCM or DCM at minimum input voltage and nominal load condition is determined by: = 503μ H I EDC = PINP 61 = = 1.28 A VINP MIN ⋅ DMAX 90 ⋅ 0.53 2 PINN LM f SW ⋅ 2 PINN LM f SW (VINN MIN + VRO ) > 1 : CCM VINN MIN ⋅ VRO (15) ΔI = VINP MIN DMAX 90 ⋅ 0.53 = = 1.46 A LM f SW 503 × 10 −6 ⋅ 65 × 103 ΔI = 1.28 + 0.73 = 2.01A 2 (V MIN + V ) ⋅ INN MIN RO < 1 : DCM VINN ⋅ VRO I DS PK = I EDC + The condition for the sensing resistor is given as: I DS RMS ΔI ⎤ D ⎡ = ⎢3( I EDC ) 2 + ( ) 2 ⎥ MAX 2⎦3 ⎣ RCS < RCS 0.5 I DS . N PK (16) 0.53 ⎡ ⎤ = ⎣3(1.28) 2 + (0.73) 2 ⎦ = 0.98 A 3 [STEP-5] Determine the Sensing Resistor Value 0.89 < I DS PK (Design Example) For minimum input voltage and nominal load condition, the operation mode is DCM as: The current sensing resistor value should be determined considering the over-current protection threshold and the pulse-by-pulse current limit threshold, as shown in Figure 5. The peak value of current sensing voltage (VCS) should be lower than the pulse-by-pulse current limit level for peak load condition. It should be lower than the OCP threshold for nominal load conditions to prevent false triggering of OCP protection during normal operation. 2 PINN LM f SW ⋅ (VINN MIN + VRO ) VINN MIN ⋅ VRO (115 + 100) N P min Setting VDD* as 12.5V, the number of turns for the auxiliary winding is obtained as: V * + VF 12.5 + 1 ⋅ NS = ⋅ 20 = 8 N A = DD 32 + 1 VO + VFA www.fairchildsemi.com 5 © 2009 Fairchild Semiconductor Corporation Rev. 1.0.1 • 6/9/09 AN-6861 APPLICATION NOTE [STEP-8] Determine the Wire Diameter for Each Winding Based on the RMS Current of Winding. The maximum RMS current of the secondary winding is obtained as: I SEC RMS = n ⋅ I DS RMS 1 − DMAX DMAX (20) (Design Example) The diode voltage and current are calculated as: V MAX 373 VDO = VO + IN = 32 + = 154V 3.05 n 1 − DMAX I DO RMS = n ⋅ I DS RMS DMAX The current density is typically 6~10A/mm2 when the wire is long (>1m). When the wire is short with a small number of turns, a current density of 8~14A/mm2 is also acceptable. These current densities are based on the peak load condition and therefore almost twice of conventional power supply design. Avoid using wire with a diameter larger than 1mm to avoid severe eddy current losses and to make winding easier. For high current output, use parallel windings with multiple strands of thinner wire to minimize skin effect. (Design Example) The RMS current of the primary- 1 − 0.53 = 2.8 A 0.53 10A and 200V diode is selected assuming very small heat-sink is used for the diode = 3.05 ⋅ 0.98 [STEP-10] Feedback Circuit Configuration side winding is obtained from Step-4 as 1.01A. The RMS current of the secondary-side winding is calculated as: The FAN6861 employs peak-current-mode control as shown in Figure 8 . A current-to-voltage conversion is accomplished externally with current-sense resistor RCS. Under normal operation, the FB level controls the peak inductor current as: I SEC RMS = n ⋅ I DS RMS 1 − DMAX DMAX I DS ⋅ RCS + VSLOPE = I DS ⋅ RCS + 0.35 ⋅ D = VFB - 1.2 4 (25) 1 − 0.53 = 3.05 ⋅ 0.98 = 2.8 A 0.53 0.4mm (8A/mm2) and 0.55mm (12A/mm2) diameter wires are selected for primary and secondary windings, respectively. where VFB is the voltage of FB pin, VSLOPE is synchronized positive-going ramp, and D is duty cycle ratio. [STEP-9] Choose the Rectifier Diode in the SecondarySide Based on the Voltage and Current Ratings. The maximum reverse voltage and the RMS current of the rectifier diode are obtained as: VDO = VO + VIN MAX n (21) I DO RMS = n ⋅ I DS RMS 1 − DMAX DMAX (22) Figure 8. Peak Current Mode Circuit The typical voltage and current margins for the rectifier diode are as: (23) VRRM > 1.3 ⋅ VDO I F > 1.5 ⋅ I DO RMS (24) where VRRM is the maximum reverse voltage and IF is the current rating of the diode. Figure 9 is a typical feedback circuit mainly consisting of a shunt regulator and a photo-coupler. R1 and R2 form a voltage divider for output voltage regulation. RF and CF are adjusted for control-loop compensation. A small-value RC filter (e.g. RFB= 100Ω, CFB= 1nF) placed from the FB pin to GND can increase stability substantially. The maximum source current of the FB pin is about 325μA. The phototransistor must be capable of sinking this current to pull the FB level down at no load. The value of the biasing resistor, RBIAS, is determined as: © 2009 Fairchild Semiconductor Corporation Rev. 1.0.1 • 6/9/09 www.fairchildsemi.com 6 AN-6861 APPLICATION NOTE VO − VOPD − VKA ⋅ CTR > 325 × 10 −6 RBIAS (26) [STEP-11] Design the Startup Circuit where VOPD is the drop voltage of photodiode, about 1.2V: VKA is the minimum cathode to anode voltage of shunt regulator (2.5V); and CTR is the current transfer rate of the opto-coupler. Figure 10 shows the typical startup circuit for FAN6861. Connecting startup resistor to AC line allows the reset of latch protection by unplugging the AC line from the power supply. Two-stage hold-up capacitor configuration (CDD1 and CDD2) is typically used to increase the hold-up time while minimizing the startup time. Initially, FAN6861 consumes only startup current (maximum 15μA) before it begins normal switching operation. Therefore, the current supplied through the startup resistor (RSTART) can charge capacitor CDD1 while supplying the startup current to FAN6861. When VDD reaches turn-on voltage of 17.5V (VDD-ON), FAN6861 begins switching operation and the current consumed by FAN6861 increases to normal operating current (typical 3mA). Then, the current required by FAN6861 is supplied from the auxiliary winding of transformer. The average current supplied through the startup resistor for minimum line voltage condition is 2VLINE MIN VDD −ON 1 (28) I RST = ( ) − > I DD − ST π 2 RSTART Figure 9. Feedback Circuit TSTART MAX = C DD1 I RST VDD −ON − I DD − ST MAX (29) ˆ vFB ω 1 + s / ω ZC =− I ⋅ ˆ vo s 1 + s / ω PC RB where ω I = R1 RDB C F 1 : ω pc = RB C FB The feedback compensation network transfer function of Figure 9 is obtained as: (27) , The maximum power dissipation of RSTART is: PRST ≅ (VLINE MAX ) 2 2 RSTART (30) ω ZC = 1 ( RF + R1 )CO , RB is the internal feedback bias resistor of FAN6861; and R1, RD, RF, CF, and CFB are shown in Figure 9. (Design Example) Assuming CTR is 100%; VO − VOPD − VKA ⋅ CTR > 325 × 10 −6 RBIAS RBIAS < 3kΩ resistor is selected for RDB. VO − VOPD − VKA 32 − 1.2 − 2.5 = = 87 k Ω 325 × 10 −6 325 × 10 −6 The voltage divider resistors for VO sensing are selected as 120kΩ and 10kΩ. Figure 10. Startup Circuit © 2009 Fairchild Semiconductor Corporation Rev. 1.0.1 • 6/9/09 www.fairchildsemi.com 7 AN-6861 APPLICATION NOTE (Design Example) 510kΩ resistor and 10μF capacitor Thermal Protection are selected for RSTART and CDD1, respectively. Then, the current through the startup resistor for minimum line voltage is: 2VLINE MIN VDD −ON 1 − I RST = ( ) π 2 RSTART =( 2 ⋅ 90 17.5 1 − = 62 μ A ) π 2 510 × 103 Then, the maximum startup time is obtained as Figure 12 shows the internal blocks for thermal protection. A constant current, IRT, of 99μA is provided from the RT pin. For over-temperature protection, an NTC thermistor in series with a resistor can be connected between the RT and GND pins. As temperature increases, the impedance of NTC thermister decreases and RT pin voltage drops. When the voltage of the RT pin is less than 1V longer than 17ms (tDOTP-LATCH), OTP is triggered. When RT pin voltage is less than 0.7V, OTP is triggered after the 100μs debounce time. If the thermal protection is not used, connect a small capacitor (around 0.47nF is recommended) from the RT pin to the GND pin to prevent noise interference. This RT capacitor should not be larger than 1nF; otherwise, the thermal protection is triggered before a successful build-up of output voltage. TSTART MAX = CDD1 I RST VDD −ON − I DD − ST MAX = 10 × 10 −6 17.5 = 3.7 sec (62 − 15) × 10 −6 The maximum power dissipation of RSTART is: PRST ≅ (VLINE MAX ) 2 2652 = = 68mW 2 RSTART 2 ⋅ 510 × 103 Leading-Edge Blanking (LEB) Each time the power MOSFET is switched on, a turn-on spike occurs across the sense resistor, caused by primaryside capacitance and secondary-side rectifier reverse recovery. To avoid premature termination of the switching pulse, a leading-edge blanking time is built in. During this blanking period (360ns), the PWM comparator is disabled and cannot switch off the gate driver. Thus, an RC filter with a small RC time constant is enough for current sensing (e.g. 200Ω + 470pF). A non-inductive resistor is recommended for RCS. Figure 12. Thermal Protection Circuit Figure 11. Current sensing © 2009 Fairchild Semiconductor Corporation Rev. 1.0.1 • 6/9/09 www.fairchildsemi.com 8 AN-6861 APPLICATION NOTE Printed Circuit Board (PCB) Layout PCB layout is a very important design issue for highfrequency switching current/voltage application. Good PCB layout minimizes excessive EMI and helps the power supply survive during surge/ESD tests. Guidelines: To get better EMI performance and reduce line frequency ripples, the output of the bridge rectifier should be connected to capacitor C1 first, then to the switching circuits. The high-frequency current loop is in C1 – transformer – MOSFET – RS – C1. The area enclosed by this current loop should be as small as possible. Keep the traces (especially 4 → 1) short, direct, and wide. High-voltage traces related to the drain of MOSFET and RCD snubber should be kept far way from control circuits to prevent unnecessary interference. If a heatsink is used for the MOSFET, connect this heatsink to ground. As indicated by 3, the ground of control circuits should be connected first, then to other circuitry. As indicated by 2, the area enclosed by transformer auxiliary winding, D1, C2, D2, and C3 should also be kept small. Place C3 close to the FAN6861 for good decoupling. Two suggestions with different pro and cons for ground connections are offered: GND3 → 2 → 4 → 1: This could avoid common impedance interference for sense signal. GND3 → 2 → 1 → 4: This could be better for ESD testing where the earth ground is not available on the power supply. Regarding the ESD discharge path, the charges go from secondary through the transformer stray capacitance to GND2 first. The charges then go from GND2 to GND1 and back to the mains. Note that control circuits should not be placed on the discharge path. Point discharge for common choke can decrease high-frequency impedance and increase ESD immunity. Should a Y-cap between primary and secondary be required, connect this Y-cap to the positive terminal of C1. If this Y-cap is connected to the primary GND, it should be connected to the negative terminal of C1 (GND1) directly. Point discharge of this Y-cap also helps for ESD. However, the creepage between these two pointed ends should be large enough to satisfy the requirements of applicable standards. L AC Input N Common-Mode Choke C3 5 + VDC C1 + D2 + D1 C2 RA RT RFB VDD 3 RT GATE 6 RG 2 FB GND 1 SENSE 4 RF CF RS Y-cap CFB FAN6861 Figure 13. © 2009 Fairchild Semiconductor Corporation Rev. 1.0.1 • 6/9/09 Layout Considerations www.fairchildsemi.com 9 AN-6861 APPLICATION NOTE Design Summary Figure 1 shows the final schematic of the 20W (50W peak) power supply of the design example. L EMI AC inpu t R SN1 + C IN 510k 100µ F 50 R S N2 C SN2 470pF 1.6µH C SN1 D O UT 10nF 10A/200 V Filter VIN D DD1 100k + + 470µ F VO + N C O UT 1 R DAMP + 5 D DD 2 DS N 220µ F C O UT 2 VO - R S T A RT 5 V DD 3 RT + C DD 1 10µ F C DD 2 100µ F 82k RG GA TE 6 50 300 4N60 2 FB C FB 1nF SE NSE 4 G ND 1 R CSF C CSF 470pF R CS 0.39 3k FAN 6861 R BIA S 120 k R1 R DB 2.2k RF 4.7k CF 10nF R2 KA431 10k Figure 14. Final Schematic of Design Example © 2009 Fairchild Semiconductor Corporation Rev. 1.0.1 • 6/9/09 www.fairchildsemi.com 10 AN-6861 APPLICATION NOTE Figure 14. Figure 15. Transformer Specification Winding Specification Pin N1 5 3 Insulation Tape Shielding lead to Pin 2 Insulation Tape Diameter / Thickness 0.4mm Turns 31 1 1 3 20 3 1 1 30 3 8 3 N2 6, 7 8, 9 Insulation Tape Shielding lead to Pin 2 Insulation Tape N3 3 4 Insulation Tape N4 1 2 Insulation Tape 0.55mm 0.4mm 0.2mm Core: EF25/13/11 (Ae=78 mm2) Bobbin: EF25/13/11 Inductance: 500μH © 2009 Fairchild Semiconductor Corporation Rev. 1.0.1 • 6/9/09 www.fairchildsemi.com 11 AN-6861 APPLICATION NOTE Related Datasheets FAN6861 — Low Cost and Highly Integrated Green-Mode PWM Controller for Peak Power Management DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION, OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. LIFE SUPPORT POLICY FAIRCHILD’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness. © 2009 Fairchild Semiconductor Corporation Rev. 1.0.1 • 6/9/09 www.fairchildsemi.com 12