TinySwitch. Flyback Design. Methodology Application Note AN-23. Design Flow. Introduction. Basic Circuit Configuration

Transcription

1 Tinywitch Methodology pplication ote -23 TM Flyback Design Introduction This document describes a simple Design Methodology for flyback power supply design using the Tinywitch 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 Basic Circuit Configuration Because of the high level integration of Tinywitch, flyback power supply design is greatly simplified s a result, the basic circuit configuration of Tinywitch flyback power supplies remains unchanged from application to application pplication specific issues outside this basic configuration such as constant current, constant power outputs, etc are beyond the scope of this document Figure 1 shows the basic circuit configuration in a typical Tinywitch flyback design using T253 Design Flow Figures 2, 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: 1 Calculate minimum reflected voltage, OR, allowed by a given output diode 2 Design for discontinuous mode operation using this calculated OR If necessary, increase OR 3 t OR 150, select bigger Tinywitch to stay in discontinuous mode or go to continuous mode design 4 Design transformer using EE16 core 5 elect feedback circuit and other components to complete the design Output capacitor Output post filter L, C + O - D T253 E B C Fusible C I 01 µf Feedback ense Circuit Figure 1 Typical Tinywitch Flyback ower upply nubber Circuit Bypass pin capacitor I July 1999

2 -23 1 ystem Requirements CMI, CMX, f L, O, O, η 2 elect Output Diode Based on O & η Estimate Diode Loss 3 elect Clamp/nubber Circuit 4 Estimate Efficiency η 5 Determine MI, MX & C I 6 Determine I Calculate OR from MX, O, D, & I OR < Choose Tinywitch Based on MI, MX & O To tep 8 I Figure 2 Tinywitch Flyback Design Flowchart teps 1 to 7 2

3 -23 From tep 7 8 Calculate D MX from MI, O & I 9 Calculate K D from MI, OR & D MX 10 Fully Disc? 11 Fully Disc Required? 12 Mostly Disc? 13 Continuous OK? 14 Calculate D MX from MI, OR 11B et K D (1- D MX )/(067 - D MX ), Recalculate OR 13B et K D 1, Recalculate OR 15 Calculate K R from MI, O, I & D MX OR < B et K R 06, Recalculate D MX 16 K R 06 Back to tep 7 OR < C Recalculate OR from MI, D MX Discontinuous 17 Calculate L Continuous 17B Calculate L To tep 18 To tep 18 I Figure 2B Tinywitch Flyback Design Flowchart teps 8 to 17, B 3

4 -23 From tep 17, B 18 Design Transformer,, L g Discontinuous? 19 Discontinuous Calculate Currents I RM, I RM 19B Continuous Calculate Currents I RM, I RM 20 Determine Output hort Circuit Current I O 21 Determine Output Capacitor C OUT 22 Determine Feedback Circuit and ost Filter Design Complete I Figure 2C Tinywitch Flyback Design Flowchart teps

5 -23 tep by tep Design rocedure ymbols and parameters used in this design procedure are defined in pplication ote TOwitch Flyback Design Methodology (-16) tep 1 Determine system requirements: CMI, CMX, f L, O, O, η Determine input voltage range from Table 1 Input (C) CMI (C) 100/ Universal 85 Table 1 Input oltage Range tep 2 elect output diode Estimate associated efficiency loss The output diode can be selected based on expected power supply efficiency and cost (see Table 2) - Use a chottky diode for highest efficiency for output voltages up to 75 - For output voltages beyond 75 use an Ultra Fast diode - If efficiency is not a concern (or cost is paramount), use a Fast -diode - The chottky and Ultrafast may be used with continuous mode of operation The Fast -diode should be used only with discontinuous mode of operation - Choose output diode type Table 2 shows approximate forward voltage ( D ) 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 chottky Ultrafast- Fast- D () CMX (C) Efficiency Loss (05/ O ) 100% (10/ O ) 100% (10/ O ) 100% Table 2 Diode forward voltage ( D ) 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 R is the diode reverse voltage rating I D is the diode DC current rating The final diode current rating is to be determined in tep 20 to accommodate continuous short circuit current I O Output Diode R () I D () Manufacturer Motorola Motorola chottky MBR Motorola MBR Motorola MBR Motorola UF GI MUR Motorola MUR Motorola UF GI B hilips, GI UF GI UFR UF GI MUR Motorola MUR Motorola MUR Motorola MUR Motorola BW hilips, GI B hilips Table 3 Output diodes tep 3 elect clamp/snubber circuit and determine associated efficiency loss Clamp/snubber circuit is required at DRI to keep DRI voltage below rated B: - snubber alone may be used at low power (< 3 W with Universal input) and will provide lower video noise and superior EMI performance - n 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 Clamp/nubber RC nubber RCD clamp O 0 to 3 W > 3 W Efficiency Loss 20% 15% Table 4 Clamp/nubber efficiency loss tep 4 Estimate power supply efficiency η Total efficiency loss is the sum of the output diode efficiency loss (from tep 2) and the clamp/snubber efficiency loss (from tep 3) 5

6 -23 Calculate overall power supply efficiency as: η 100% - total efficiency loss tep 5 Determine maximum and minimum DC input voltages MX, MI and input storage capacitance C I (see -16 for more detail) Calculate the maximum MX as: MX 2 CMX Choose input storage capacitor, C I per Table 5 et bridge rectifier conduction time t c 3 ms Derive minimum DC input voltage, MI Input oltage 100/ Universal Table 5 C I Range MI 1 2 O 2 2 f ( 2 CMI ) η C where C I : input capacitance f L : line frequency t c : diode conduction time I L tc tep 6 Determine output diode peak inverse voltage I Calculate reflected output voltage OR based on MX, O, D and I Look up output diode reverse voltage R from diode data sheet or Table 3 in tep 2 Calculate maximum peak inverse voltage I The maximum recommended I is 80% of the reverse voltage rating R I 08 R Calculate reflected output voltage OR : OR C I (µf/watt) ( ) + I MX O D O MI () If OR > 150, go back to tep 2 and choose a different diode for higher R Refer to Table 6 for approximate R range for different types of diodes O for Device ingle oltage* T T T Diode Type chottky UltraFast- Fast- R () > 200 Table 6 Diode reverse voltage range tep 7 Choose Tinywitch based on input voltage range and output power O elect appropriate Tinywitch according to Table 7 based on output power O and input voltage range (from tep 1) Output ower Capability (W) O for Universal oltage Table 7 Tinywitch output power ( O ) capability Recommended ower Range for Lowest Cost** (W) O for ingle oltage* O for Universal oltage * ingle oltage 100/115 C with voltage doubler, or ingle oltage 230 C without doubler ** Based on EE16 core transformer For Universal input voltage and an output power range of 1W to 15 W, T254 is usually a better choice than T253 except for applications requiring low video noise For Universal input voltage and an output power range of 35W to 4 W, T255 usually results in smaller transformer size and higher efficiency than T254 tep 8 Determine primary peak current I Calculate maximum duty cycle D MX for discontinuous mode of operation based on MI, O and I rimary peak current is 90% of minimum I LIMIT from the data sheet of the selected Tinywitch 09 is the over temperature derating factor for I LIMIT : 6

7 -23 I 09 minimum I LIMIT Calculate maximum duty cycle D MX for discontinuous mode of operation as: D MX 2 O η I MI tep 9 Calculate K D from MI, OR and D MX KD is the ratio between the off-time of the switch and the reset time of the core: K D OR ( 1 DMX ) D MI MX tep 10, 11, 11B, 12, 13, 13B Check K D to ensure discontinuous mode of operation Raise OR 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 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 junction diode as output rectifier also demand fully discontinuous mode of operation This can be accomplished by raising OR higher if necessary until K D (1- D MX )/(067 - D MX ) To keep the worst case DRI voltage below the recommended level of 650, OR should be kept below 150 Mostly discontinuous mode of operation (K D 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 (K D < 1 ) provides higher output power In this mode a chottky output diode should be used to prevent long diode reverse recovery times that could exceed leading edge blanking period (t LEB ) tep 10 Check for fully discontinuous operation K D (1- D MX )/(067 - D MX ): Fully discontinuous Go to tep 17 K D < (1- D MX )/(067 - D MX ): Go to tep is the reciprocal of the percentage of duty cycle relaxation caused by various parameters such as the tolerance in Tinywitch current limit and frequency tep 11, B Determine if fully discontinuous is necessary If yes, set K D (1- D MX )/(067 - D MX ) Recalculate OR as OR K D 1 D D MI MX MX - If OR < 150, go to tep 17 - If OR > 150, go back to tep 7 and select higher current Tinywitch If not, go to tep 12 tep 12 Check for mostly discontinuous K D 1 Operation is mostly discontinuous Go to tep 17 K D < 1 Go to tep 13 tep 13, B Determine if continuous is acceptable for the application If yes, go to tep 14 If not, set K D 1 Recalculate OR as: OR K D 1 D D MI MX MX - If OR < 150, go to tep 17 - If OR > 150, go back to tep 7 and select higher current Tinywitch 7

8 -23 tep 14 Recalculate D MX for continuous mode of operation from MI and OR tart continuous mode design Recalculate D MX as: D MX OR OR + MI Continuous mode: 6 10 O L KR 1 2 KR ( 1 ) I 2 09 Z ( 1 η) + η η f tep 15 Calculate K R from MI, O, η, I, and D MX K R is the ratio between the primary ripple current I R and primary peak current I nd I is 90% of minimum I LIMIT From -16, I and I IG KR ( 1 ) D 2 G O η MI MX By combining the above equations, K R can be expressed as: I is 90% of minimum I LIMIT from Tinywitch data sheet as previously defined in tep 8 f is minimum switching frequency from Tinywitch data sheet lease note the cancellation effect between the over temperature variations of I and f resulting in the additional 1/09 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 ince output diode loss and clamp/snubber loss are both secondary losses, Z 1 is a reasonable starting point tep 18 Design Transformer Calculate turns ratio / : K R 2 I DMX MI O ( η ) I D η MX MI OR + O D tep 16, B, C Check K R against 06 K R 06, go to tep 17B K R < 06, set K R 06 - Recalculate D MX using tep 15 equation - Recalculate OR using tep 14 equation - If OR < 150, go to tep 17B - If OR > 150, go back to tep 7and select higher current Tinywitch tep 17, B Calculate primary inductance L Discontinuous mode: electing core and bobbin - With triple insulated secondary wire and no margin winding, EE16 core is suitable for most Tinywitch 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 (B ) not to exceed 3000 gauss Limit B to 2500 gauss for low audio noise designs Use the lowest practical value of B for the greatest reduction in auido noise ee -24 for additional information Calculate primary number of turns ( ) L 6 10 O I f 2 09 Z ( 1 η) + η η 100 I L B e where I equals to maximum I LIMIT 8

9 -23 Calculate secondary number of turns : - Calculate secondary RM current I RM ( O + D) OR 2 KR IRM I ( 1 DMX ) KR Calculate gap length (L g ) Gap length should be larger than 01 mm to ensure manufacturability L g 2 40 π e 1000 L 1 lease refer to ower Integrations Web site wwwpowerintcom for audio noise suppression techniques applicable to transformer design tep 19, B Calculate primary RM current I and secondary RM current I RM Discontinuous mode: L RM where I I and I equals to maximum I LIMIT Choose wire gauge for primary and secondary windings based on I RM and I RM 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 WG to prevent excessive winding capacitance and to improve manufacturability tep 20 Determine output short circuit current I O - Calculate primary RM current I RM I RM I DMX 2 3 where I equals to maximum I LIMIT - Calculate secondary RM current I RM Calculate maximum output short circuit current I O from I and /, where I is the maximum I from Tinywitch LIMIT data sheet and / is the turns ratio from tep 18: I O I k where k is the peak to RM current conversion factor where I I RM I I 1 D 3 K MX D The value of k is determined based on empirical measurements: k 09 for chottky diode and k 08 for junction diode Check I O against diode DC current rating I D If necessary, choose higher current diode (see Table 3) tep 21 Determine Output Capacitor C OUT and I equals to maximum I LIMIT Continuous mode: - Calculate primary RM current I RM 2 KR IRM I DMX KR where I equals to maximum I LIMIT Calculate output ripple current: I I I 2 2 RILE RM O Choose output capacitor with RM current rating equal to or larger than output ripple current Use low ER electrolytic capacitor rated for switching power supply use Examples are LXF series from UCC, L series from ichicon, and HFQ series from anasonic 9

10 -23 tep 22 Determine feedback circuit and output post filter The output voltage of the Tinywitch 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 Output voltage O is determined by where LED 1 O Z + LED Replace the Zener with a TL431 for better output accuracy In non-isolated design, use a bipolar transistor in place of the optocoupler Replace the LED with the base emitter junction and connect the collector to the EBLE pin of the Tinywitch 10

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