® TM TinySwitch Flyback Design Methodology Application Note AN-23 Introduction Design Flow This document describes a simple Design Methodology for flyback power supply design using the TinySwitch family of integrated off-line switchers. The objective of this Design Methodology is to provide power supply engineers a handy tool that not only eases the design task but also delivers design optimization in cost and efficiency for most applications. Figures 2A, B and C present a design flow chart showing the complete design procedure in 22 steps. The logic behind this Design Methodology can be summarized as follows: Basic Circuit Configuration 2. Design for discontinuous mode operation using this calculated VOR. If necessary, increase VOR. 1. Calculate minimum reflected voltage, VOR, allowed by a given output diode. Because of the high level integration of TinySwitch, flyback power supply design is greatly simplified. As a result, the basic circuit configuration of TinySwitch flyback power supplies remains unchanged from application to application. Application specific issues outside this basic configuration such as constant current, constant power outputs, etc. are beyond the scope of this document. 3. At VOR = 150 V, select bigger TinySwitch to stay in discontinuous mode or go to continuous mode design. 4. Design transformer using EE16 core. 5. Select feedback circuit and other components to complete the design. Figure 1 shows the basic circuit configuration in a typical TinySwitch flyback design using TNY253. Output capacitor Output post filter L, C + VO D TNY253 EN BP Fusible CIN S VAC 0.1 µF Feedback Sense Circuit Snubber Circuit Bypass pin capacitor PI-2336-112098 Figure 1. Typical TinySwitch Flyback Power Supply. July 1999 AN-23 1. System Requirements VACMIN, VACMAX, fL, VO, PO, η 2. Select Output Diode Based on VO & η Estimate Diode Loss 3. Select Clamp/Snubber Circuit 4. Estimate Efficiency η 5. Determine VMIN, VMAX & CIN 6. Determine PIV Calculate VOR from VMAX, VO, VD, & PIV N VOR < 150 V Y 7. Choose TinySwitch Based on VMIN ,VMAX & PO To Step 8 PI-2345-111398 Figure 2A. TinySwitch Flyback Design Flowchart Steps 1 to 7. 2 A 7/99 AN-23 From Step 7 8. Calculate DMAX from VMIN, PO & IP 9. Calculate KDP from VMIN, VOR & DMAX 10. Fully Disc? N N 11A. Fully Disc Required? Y 12. Mostly Disc? Y Y N 13. Continuous OK? N 11B. Set KDP = (1- DMAX)/(0.67 - DMAX), 13B. Set KDP = 1, Recalculate VOR Recalculate VOR Y 14. Calculate DMAX from VMIN, VOR Y 15. Calculate KRP from VMIN, PO, IP & DMAX VOR < 150 V 16B. Set KRP = 0.6, N Recalculate DMAX N 16A. KRP ≥ 0.6 Y Back to Step 7 N VOR < 150 V 16C. Recalculate VOR from VMIN, DMAX Y Discontinuous 17A. Calculate LP Continuous 17B. Calculate LP To Step 18 To Step 18 PI-2351-111398 Figure 2B. TinySwitch Flyback Design Flowchart Steps 8 to 17A, B. A 7/99 3 AN-23 From Step 17A, B 18. Design Transformer NP, NS, Lg Discontinuous? N Y 19A. Discontinuous Calculate Currents IRMS, ISRMS 19B. Continuous Calculate Currents IRMS, ISRMS 20. Determine Output Short Circuit Current IOS N 21. Determine Output Capacitor COUT 22. Determine Feedback Circuit and Post Filter Design Complete PI-2347-112098 Figure 2C. TinySwitch Flyback Design Flowchart Steps 18-22. 4 A 7/99 AN-23 Step by Step Design Procedure Output Diode Symbols and parameters used in this design procedure are defined in Application Note “TOPSwitch Flyback Design Methodology” (AN-16). 1N5819 1N5822 Schottky MBR745 MBR1045 MBR1645 UF4002 MUR110 MUR120 UF4003 BYV27-200 UF5401 UFR UF5402 MUR410 MUR420 MUR810 MUR820 BYW29-200 BYV32-200 Step 1. Determine system requirements: VACMIN, VACMAX, fL, VO, PO, η • Determine input voltage range from Table 1. Input (VAC) 100/115 230 Universal VACMIN (VAC) 85 VACMAX (VAC) 132 195 85 265 265 Table 1. Input Voltage Range. Step 2. Select output diode. Estimate associated efficiency loss. VR (V) ID (A) 40 40 45 45 45 100 100 200 200 200 100 200 100 200 100 200 200 200 1.0 3.0 7.5 10.0 16.0 1.0 1.0 1.0 1.0 2.0 3.0 3.0 4.0 4.0 8.0 8.0 8.0 20.0 Manufacturer Motorola Motorola Motorola Motorola Motorola GI Motorola Motorola GI Philips, GI GI GI Motorola Motorola Motorola Motorola Philips, GI Philips Table 3. Output diodes. • The output diode can be selected based on expected power supply efficiency and cost (see Table 2). - Use a Schottky diode for highest efficiency for output voltages up to 7.5 V. - For output voltages beyond 7.5 V use an Ultra Fast PNdiode. - If efficiency is not a concern (or cost is paramount), use a Fast PN-diode. - The Schottky and Ultrafast may be used with continuous mode of operation. The Fast PN-diode should be used only with discontinuous mode of operation. - Choose output diode type. Table 2 shows approximate forward voltage (VD) for types of output diode discussed above. • Output diode efficiency loss is the power supply efficiency reduction (in percentage) caused by the diode. Diode Type VD (V) Step 3. Select clamp/snubber circuit and determine associated efficiency loss. • Clamp/snubber circuit is required at DRAIN to keep DRAIN voltage below rated BV: - A snubber alone may be used at low power (< 3 W with Universal input) and will provide lower video noise and superior EMI performance. - An RCD clamp may be used for power levels < 3 W for higher efficiency and is required at power levels > 3 W with Universal input. • Table 4 shows the approximate efficiency loss due to clamp/ snubber circuits. Schottky 0.5 Efficiency Loss (0.5/VO) × 100% Ultrafast-PN 1.0 (1.0/VO) × 100% Clamp/Snubber RC Snubber Fast-PN 1.0 (1.0/VO) × 100% RCD clamp Table 2. Diode forward voltage (VD) and efficiency loss. • The estimated efficiency loss due to the output diode is also shown in Table 2. • Table 3 shows some commonly used output diodes. VR is the diode reverse voltage rating. ID is the diode DC current rating. • The final diode current rating is to be determined in Step 20 to accommodate continuous short circuit current IOS. PO 0 to 3 W Efficiency Loss 20% >3W 15% Table 4. Clamp/Snubber efficiency loss. Step 4. Estimate power supply efficiency η. • Total efficiency loss is the sum of the output diode efficiency loss (from Step 2) and the clamp/snubber efficiency loss (from Step 3). A 7/99 5 AN-23 • Calculate overall power supply efficiency as: η = 100% total efficiency loss. Step 5. Determine maximum and minimum DC input voltages VMAX, VMIN and input storage capacitance CIN. (see AN-16 for more detail) • If VOR > 150 V, go back to Step 2 and choose a different diode for higher VR. • Refer to Table 6 for approximate VR range for different types of diodes. • Calculate the maximum VMAX as: 100/115 230 1 2-3 Universal > 200 VMIN (V) ≥ 90 ≥ 240 Step 7. Choose TinySwitch based on input voltage range and output power PO. • Select appropriate TinySwitch according to Table 7 based on output power PO and input voltage range (from Step 1). ≥ 90 Output Power Capability (W) Table 5. CIN Range. • Set bridge rectifier conduction time tc = 3 ms. • Derive minimum DC input voltage, VMIN VMIN 40-45 100-200 Table 6. Diode reverse voltage range. • Choose input storage capacitor, CIN per Table 5. CIN (µF/Watt) 2-3 VR (V) Schottky UltraFast-PN Fast-PN VMAX = 2 × VACMAX Input Voltage Diode Type 1 − tC 2 × PO × 2 × fL = (2 × VACMIN 2 ) − η × CIN Device PO for Single PO for Universal Voltage* Voltage 5.0 TNY253 2.5 TNY254 8.0 5.0 10 TNY255 7.5 Recommended Power Range for Lowest Cost** (W) PO for Single PO for Universal Voltage* 0-2.5 Voltage 0-1.5 2.0-5.0 6.0-10 1.0-4.0 3.5-6.5 Table 7. TinySwitch output power (PO) capability where CIN : input capacitance fL : line frequency tc : diode conduction time Step 6. Determine output diode peak inverse voltage PIV. Calculate reflected output voltage VOR based on VMAX, VO, VD and PIV. • Look up output diode reverse voltage VR from diode data sheet or Table 3 in Step 2. • Calculate maximum peak inverse voltage PIV. The maximum recommended PIV is 80% of the reverse voltage rating VR. PIV = 0.8 × VR • Calculate reflected output voltage VOR: VOR 6 A 7/99 V × (VO + VD ) = MAX PIV − VO * Single Voltage 100/115 VAC with voltage doubler, or Single Voltage 230 VAC without doubler ** Based on EE16 core transformer • For Universal input voltage and an output power range of 1W to 1.5 W, TNY254 is usually a better choice than TNY253 except for applications requiring low video noise. • For Universal input voltage and an output power range of 3.5W to 4 W, TNY255 usually results in smaller transformer size and higher efficiency than TNY254. Step 8. Determine primary peak current IP. Calculate maximum duty cycle DMAX for discontinuous mode of operation based on VMIN, PO and IP. • Primary peak current is 90% of minimum ILIMIT from the data sheet of the selected TinySwitch. 0.9 is the over temperature derating factor for ILIMIT: AN-23 IP = 0.9 × minimum I LIMIT • Calculate maximum duty cycle DMAX for discontinuous mode of operation as: DMAX = 2 × PO η × VMIN × IP Step 9. Calculate KDP from VMIN, VOR and DMAX. • KDP is the ratio between the off-time of the switch and the reset time of the core: K DP = VOR × (1 − DMAX ) VMIN × DMAX Step 10, 11A, 11B, 12, 13A, 13B. Check KDP to ensure discontinuous mode of operation. Raise VOR if necessary. The mode of operation can vary depending on power supply requirements. However, discontinuous mode of operation is always recommended wherever it is possible. • With discontinuous mode of operation, generally, the output filter is smaller, output rectifier is cheaper with PN junction diode, EMI and video noise are lower. • Fully discontinuous mode of operation (discontinuous under all conditions) may be necessary in some applications to meet specific requirements such as very low video noise, very low output ripple voltage. Use of RC snubber, and/or PN junction diode as output rectifier also demand fully discontinuous mode of operation. This can be accomplished by raising VOR higher if necessary until KDP ≥ (1- DMAX)/(0.67 - DMAX). To keep the worst case DRAIN voltage below the recommended level of 650 V, VOR should be kept below 150V. • Mostly discontinuous mode of operation (KDP ≥ 1 ) refers to a design operating in discontinuous mode under most situations, but do have the possibility of operating in continuous mode occasionally. • Continuous mode operation (KDP < 1 ) provides higher output power. In this mode a Schottky output diode should be used to prevent long diode reverse recovery times that could exceed leading edge blanking period (tLEB). Step 10. Check for fully discontinuous operation. • KDP ≥ (1- DMAX)/(0.67 - DMAX): Fully discontinuous. Go to Step 17A. • KDP < (1- DMAX)/(0.67 - DMAX): Go to Step 11. • 0.67 is the reciprocal of the percentage of duty cycle relaxation caused by various parameters such as the tolerance in TinySwitch current limit and frequency. Step 11A, B. Determine if fully discontinuous is necessary. • If yes, set KDP = (1- DMAX)/(0.67 - DMAX). Recalculate VOR as VOR = K DP × VMIN × DMAX 1 − DMAX - If VOR < 150 V, go to Step 17A. - If VOR > 150 V, go back to Step 7 and select higher current TinySwitch. • If not, go to Step 12. Step 12. Check for mostly discontinuous. • KDP ≥ 1. Operation is mostly discontinuous. Go to Step 17A. • KDP < 1. Go to Step 13. Step 13A, B. Determine if continuous is acceptable for the application. • If yes, go to Step 14. • If not, set KDP = 1. Recalculate VOR as: VOR = K DP × VMIN × DMAX 1 − DMAX - If VOR < 150 V, go to Step 17A. - If VOR > 150 V, go back to Step 7 and select higher current TinySwitch. A 7/99 7 AN-23 Step 14. Recalculate DMAX for continuous mode of operation from VMIN and VOR. • Start continuous mode design. • Recalculate DMAX as: DMAX = VOR VOR + VMIN Step 15. Calculate KRP from VMIN, PO, η, IP, and DMAX. • KRP is the ratio between the primary ripple current IR and primary peak current IP. And IP is 90% of minimum ILIMIT. • From AN-16, and IP = I AVG K RP (1 − ) × DMAX 2 I AVG = PO η × VMIN • By combining the above equations, KRP can be expressed as: K RP = 2 × ( IP × DMAX × η × VMIN − PO ) IP × DMAX × η × VMIN Step 16A, B, C. Check KRP against 0.6. • KRP ≥ 0.6, go to Step 17B. • KRP < 0.6, set KRP = 0.6. - Recalculate DMAX using Step 15 equation. Recalculate VOR using Step 14 equation. If VOR < 150 V, go to Step 17B. If VOR > 150 V, go back to Step 7and select higher current TinySwitch. Step 17A, B. Calculate primary inductance LP. • Discontinuous mode: LP = Z × (1 − η) + η 10 6 × PO × 1 1 2 η × × I P × fS 2 0.9 • Continuous mode: 10 6 × PO LP = 1 K K RP × (1 − RP ) × × I P 2 × fS 2 0.9 Z × (1 − η) + η × η • IP is 90% of minimum ILIMIT from TinySwitch data sheet as previously defined in Step 8. • fS is minimum switching frequency from TinySwitch data sheet. • Please note the cancellation effect between the over temperature variations of IP and fS resulting in the additional 1/0.9 term. • Z is loss allocation factor. If Z = 0, all losses are on the primary side. If Z = 1, all losses are on the secondary side. • Since output diode loss and clamp/snubber loss are both secondary losses, Z = 1 is a reasonable starting point. Step 18. Design Transformer. • Calculate turns ratio NP/NS: NP VOR = NS VO + VD • Selecting core and bobbin - With triple insulated secondary wire and no margin winding, EE16 core is suitable for most TinySwitch applications. - To accommodate margin winding, EEL16 core must be used. - In below 2 W and/or space constrained applications, EE13 or EF13 cores with special bobbin meeting safety requirements may be used. • Calculate primary and secondary number of turns for peak flux density (BP) not to exceed 3000 gauss. Limit BP to 2500 gauss for low audio noise designs. Use the lowest practical value of BP for the greatest reduction in auido noise. See AN-24 for additional information. • Calculate primary number of turns (NP) N P = 100 × IP′ × LP BP × Ae where I’P equals to maximum ILIMIT 8 A 7/99 AN-23 • Calculate secondary number of turns NS: NS = N P × (VO + VD ) VOR • Calculate gap length (Lg). Gap length should be larger than 0.1 mm to ensure manufacturability. NP 2 1 Lg = 40 × π × Ae − 1000 × LP AL • Please refer to Power Integrations Web site www.powerint.com for audio noise suppression techniques applicable to transformer design. Step 19A, B. Calculate primary RMS current I RMS and secondary RMS current I . SRMS - Calculate primary RMS current IRMS I RMS = DMAX × IP′ 2 3 where I’P equals to maximum ILIMIT - Calculate secondary RMS current ISRMS 1 − DMAX = ISP × 3 × K DP where I = I ′ × SP P ISRMS = ISP × K RP 2 − D × − K RP + 1 1 ( MAX ) 3 where ISP = I P′ × NP NS and I’P equals to maximum ILIMIT • Choose wire gauge for primary and secondary windings based on IRMS and ISRMS. • In some designs, a lower guage (larger diameter) wire may be necessary to maintain transformer temperature within acceptable limits during continuous short circuit conditions. • Do not use wire thinner than 36 AWG to prevent excessive winding capacitance and to improve manufacturability. Step 20. Determine output short circuit current IOS. • Discontinuous mode: ISRMS - Calculate secondary RMS current ISRMS NP NS and I’P equals to maximum ILIMIT • Continuous mode: • Calculate maximum output short circuit current IOS from I’P and NP/NS, where I’P is the maximum ILIMIT from TinySwitch data sheet and NP/NS is the turns ratio from Step 18: IOS = IP′ × NP ×k NS where k is the peak to RMS current conversion factor • The value of k is determined based on empirical measurements: k = 0.9 for Schottky diode and k = 0.8 for PN junction diode. • Check IOS against diode DC current rating ID. If necessary, choose higher current diode (see Table 3). Step 21. Determine Output Capacitor COUT. • Calculate output ripple current: I RIPPLE = ISRMS 2 − IO 2 - Calculate primary RMS current IRMS K 2 I RMS = IP′ × DMAX × RP − K RP + 1 3 where I’P equals to maximum ILIMIT • Choose output capacitor with RMS current rating equal to or larger than output ripple current. • Use low ESR electrolytic capacitor rated for switching power supply use. • Examples are LXF series from UCC, PL series from Nichicon, and HFQ series from Panasonic. A 7/99 9 AN-23 Step 22. Determine feedback circuit and output post filter. • The output voltage of the TinySwitch flyback power supply should be sensed at the first output capacitor, which is before the output post LC filer. This way the output post LC filter is outside the feedback control loop and the resonant frequency of the output post LC filter can be as low as required to meet the output ripple specification requirement. • Use Zener diode in series with the optocoupler LED. 10 A 7/99 • Output voltage VO is determined by VO = VZ + VLED where VLED ≈ 1V • Replace the Zener with a TL431 for better output accuracy. • In non-isolated design, use a bipolar NPN transistor in place of the optocoupler. Replace the LED with the base emitter junction and connect the collector to the ENABLE pin of the TinySwitch. AN-23 A 7/99 11 AN-23 For the latest updates, visit our Web site: www.powerint.com Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. Power Integrations does not assume any liability arising from the use of any device or circuit described herein, nor does it convey any license under its patent rights or the rights of others. 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