PWM type AC/DC converter IC with Built-in 650V MOSFET BM2P0XX series PWM Flyback converter Technical Design This application note describes the design of the PWM flyback converters using ROHM's AC/DC converter IC BM2Pxxx series devices. It explains the selection of external components and provides PCB layout guidelines. Please note that all performance characteristics have to be verified. They are not guaranteed by the PCB layout shown here. Description The BM2Pxxx series of ICs are AC/DC converters for PWM switching, incorporating a built-in starter circuit having withstanding voltage of 650V and a switching MOSFET having withstanding voltage of 650V. With ROHM's original high-speed switching MOSFET built inside, it is possible to increase the peak current, contributes to miniaturization of the magnetic components. BM2Pxxx supports both isolated and non-isolated circuits, enabling simpler design of various types of low-power converters. Key features - PWM frequency 65kHz (with frequency-hopping function)/ Current mode - Burst-operation and frequency reduction functions when load is light - Built-in 650V starter circuit / Built-in 650V switching MOSFET - VCC pin under-voltage protection/Over-voltage protection - SOURCE pin Open/ Short protection, Leading-Edge-Blanking function - Per-cycle over-current limiter function - Over-current limiter AC correction function - Soft-start function BM2Pxxx Series line-up Function Brownout VCC OVP BM2P051F Latch stop Yes BM2P052F Auto restart 5.5 2.6A 8W BM2P053F Latch stop BM2P054F Auto restart SOP8 BM2P091F Latch stop Yes BM2P092F Auto restart 12 1.3A 5W BM2P093F Latch stop BM2P094F Auto restart BM2P011 Latch stop Yes BM2P012 Auto restart 2.0 10.4A 20W BM2P013 Latch stop BM2P014 Auto restart Latch stop BM2P031 Yes BM2P032 Auto restart 3.6 5.4A 15W BM2P033 Latch stop BM2P034 Auto restart DIP7 BM2P051 Latch stop Yes BM2P052 Auto restart 5.5 2.6A 10W BM2P053 Latch stop BM2P054 Auto restart BM2P091 Latch stop Yes BM2P092 Auto restart 12 1.3A 7W BM2P093 Latch stop BM2P094 Auto restart *1 These are reference values in case of PWM Flyback converter. It is necessary to limit output power depending on power Product Package MOSFET RDS(ON) (max) IDP(max) Max Output Power *1 85-265Vac supply specification. www.rohm.com (c) 2013 ROHM Co., Ltd. All rights reserved. 1/8 Oct. 2013 - Rev.A BM2P0XX series PWM Flyback converter Technical Design Application Note 1. Design Example of Isolated Type Flyback Converter DCM (Discontinuous Conduction Mode) Figure 1-1.Isolated Type Flyback Converter Circuit Example Basic operation of flyback converter (1) When switching is turned ON Np VIN Ns (2) When switching is turned OFF OFF Lp Ls Lp Np VIN Ls Ip ON Ns ON Is OFF When MOSFET is ON, current Ip flows through the When MOSFET is OFF, the accumulated energy is transformer's primary-side winding Lp, and energy is output from the secondary-wide winding Ls, current Is accumulated. flows via the diode. At that time, the diode is off. Ip Np VO Ip toff Ns Ls Ns ton VO VIN Np toff VIN ton Lp www.rohm.com (c) 2013 ROHM Co., Ltd. All rights reserved. Is 2/8 Oct. 2013 - Rev.A BM2P0XX series PWM Flyback converter Technical Design Application Note 1-1. Transformer T1 design 1-1-1. Determination of flyback voltage VOR Flyback voltage VOR is determined along with turns-ratio Np:Ns and duty-ratio. Np ton VIN Ns toff Np VOR Ns VO VOR Duty VIN VOR VOR VO VIN When VIN = 95V (AC 85V x 1.4 x 0.8), VOR = 65V, Vf = 1V: VOR GND Np VOR VOR 65V 5 Ns VO Vout Vf 12V 1V VOR 65V Duty(max) 0.406 VIN(min) VOR 95V 65V Figure 1-2. MOSFET Vds (*) When duty is 0.5 or above, VOR is adjusted to set it below 0.5. 1-1-2. Calculation of secondary-side winding inductance Ls and secondary-side maximum current Ispk For better power efficiency, if Iomax = Io x 1.2 = 1.2A: Ls Vout Vf 1 - Duty2 2 Iomax fswmax 12V 1V 1 - 0.4062 27.3uH 2 1.2A 70kHz Ispk 2 Iomax 2 1.2A 4.04A 1 - Duty(max) 1 - 0.406 Figure 1-3. Primary-side and Secondary-side Current Waveforms 1-1-3. Calculation of primary-side winding inductance Lp and primary-side maximum current Ippk 2 Np 2 Lp Ls 27.3uH 5 683uH Ns Ippk Ispk Ns 1 4.04A 0.81A Np 5 1-1-4. Determination of transformer size Based on Po = 12W, the transformer's core size is EI22. Table 1-1. Output Voltage and Transformer Core Core sectional area Ae (mm2) ~5 EE13 16 ~10 EI19/EE19 23 ~20 EI22/EE22 37 (*) The above are guideline values. For details, check with the transformer manufacturer, etc. Output voltage Po (W) www.rohm.com (c) 2013 ROHM Co., Ltd. All rights reserved. Core size 3/8 Oct. 2013 - Rev.A BM2P0XX series PWM Flyback converter Technical Design Application Note 1-1-5. Calculation of primary-side turn count Np Np VIN ton Lp Ippk Ae Bsat Ae Bsat Generally, the maximum magnetic flux density B(T) for an ordinary ferrite core is 0.4T @100C, so Bsat = 0.3T. Np Lp Ippk 683uH 0.81A 49.8 turns Ae Bsat 37mm 2 0.3T Np is 50 turns or above Since magnetic saturation does not result from this, Np is set based on the AL-valueNI characteristics. When AL-value = 150 nH/turns2 is set, Np Lp 683uH 67.5turns AL - Value 150nH/turns 2 68 turns NI Np Ippk 68turns 0.81A 55.1Aturns The AL-valueNI characteristics of EI22 are used to confirm that this is within the tolerance range. When it is beyond the tolerance range, Np is adjusted. AL-Value=150nH/turns 2 NI=55.1Aturns Figure 1-4. EI22 AL-value vs. NI Limit Characteristics (Tomita 2G8-EE22) 1-1-6. Calculation of secondary-side turn count Ns Np 5 Ns Ns 68 13.6 turns 5 14 turns 1-1-7. Calculation of VCC turn count Nd When VCC = 15V, Vf_vcc = 1V, Nd Ns VCC Vf_vcc 15V 1V 1turns 17.2turns Vout V 12V 1V 17 turns As a result, the transformer specifications are as follows. Table 1-2. Transformer Specifications Core Tomita 2G8-EI22/EE22 or compatible Lp 683 uH Np 68 turns Ns 14 turns Nd 17 turns www.rohm.com (c) 2013 ROHM Co., Ltd. All rights reserved. 4/8 Oct. 2013 - Rev.A BM2P0XX series PWM Flyback converter Technical Design Application Note 1-2. Selection of main components 1-2-1IC1 Since Pout = 12V x 1A = 12W, BM2P034 is selected. 1-2-2. Input capacitor: C1 Use Table 1-3 to select the capacitance of the input capacitor. Since Pout = 12V x 1A = 12W, C1 = 2 x 12 = 24 33F. Table 1-3. Input Capacitor Selection Table Input voltage (Vac) Cin (F) 85-264 2 X Pout(W) 180-264 1 x Pout(W) (*) The above values are guidelines for full-wave rectification. When selecting, also consider other specifications such as the retention-time. The withstanding voltage of the capacitor becomes, Vac (max) x 1.41. Say for AC 264V, it is 264V x 1.41 = 372V, so this should be 400V or more. 1-2-3. Current-sensing resistor: R1 The current-sensing resistor limits the current that flows on the primary side to provide protection against output overload, and is used for slope compensation of current mode control. Consequently, in some cases limits may be imposed according to the transformer's primary-side inductance and input voltage. In the BM2P0XX Series, an AC voltage correction function is built-in the chip for overload protection. This corrects offsetting of the overload protection point caused by different input voltages (such as AC 100V and AC 200V). Vcs_limit Vcs ton 20mV/us R1 Ippk Ippk Vcs Duty 0.406 20mV/us 0.4V 20mV/us fsw 65kHz 0.64 0.56 Ippk 0.81A Confirm the overload protection point while the resistor is assembled in the product. Sensing resistance loss P_R1: P_R1(peak) Ippk R1 0.812 0.56 0.37W Duty 0.406 R1 0.81 0.56 0.05W P_R1(rms) Iprms R1 Ippk 3 3 Set to 0.5W or above in consideration of pulse resistance. With regard to pulse resistance, the structure of the resistance may vary even with the same power rating. Check with the resistor manufacturers for details. 1-2-4. VCC-diode: D2 A high-speed diode is recommended as the VCC-diode. Reverse voltage applied to the VCC-diode: Vdr VCC(max)VINmax Nd Np When VCC (max) = 29 V, Vdr 29V374V 15 122.5V 60 With a design-margin taken into account, 122.5V / 0.7 = 175V 200V component is selected. (Example: ROHM's RF05VA2S 200V, 0.5A) www.rohm.com (c) 2013 ROHM Co., Ltd. All rights reserved. 5/8 Oct. 2013 - Rev.A BM2P0XX series PWM Flyback converter Technical Design Application Note 1-2-5. VCC capacitor: C2 A VCC capacitor is needed to stabilize the IC's VCC voltage. Capacitance of 2.2F or above is recommended (example: 50V, 10F). Next, determine the startup time of the IC at power-on. Figure 1-5 illustrates VCC capacitor capacitance and startup time characteristics. Figure 1-5. Startup Time (Reference Values) 1-2-6. VCC winding surge-voltage limiting resistor: R2 Based on the transformer's leakage inductance (Lleak), a large surge-voltage (spike noise) may occur during the instant when the MOSFET is switched from ON to OFF. This surge-voltage is induced in the VCC winding, and as the VCC voltage increases the IC's VCC overvoltage protection may be triggered. A limiting resistor R2 (approximately 5 to 22) is inserted to reduce the surge-voltage that is induced in the VCC winding. Confirm the rise in VCC voltage while the resistor is assembled in the product. 1-2-7. Snubber circuits: C3, D3, R3 Based on the transformer's leakage inductance (Lleak), a large surge-voltage (spike noise) may occur during the instant when the MOSFET is switched from ON to OFF. This surge-voltage is applied between the MOSFET's Drain and Source, so in the worst case damage to MOSFET might occur. RCD snubber circuits are recommended to suppress this surge-voltage. (1) Determination of clamp voltage (Vclamp) and clamp ripple-voltage (Vripple) Consider to take a design-margin based on the MOSFET's withstand voltage, when determining the clamp voltage. Vclamp = 650V x 0.8 = 520V The clamp ripple-voltage (Vripple) is about 50V. (2) Determination of R3 R3 2 Vclamp Vclamp - VOR Lleak Ip 2 fsw(max) When Lleak = Lp x 10% = 683H x 10% = 68H, R3 is derived as: R3 2 520V 520V - 65V 145k 100k 68uH 0.81 2 70kHz R3 loss P_R3 is expressed as P_R3 Vclamp - VIN R3 2 520 - 265V 1.41 100k 2 0.22W A 1W component is determined with consideration for design margin. (3) Determination of C3 Vclamp C3 Vripple fsw(min) R3 520V 1733pF 50V 60kHz 100k 2200pF The voltage applied to C3 is 520V - 264x1.41 = 148V. 300V or above is set with consideration for design margin. (4) Determination of D3 Choose a fast recovery diode as the diode, with a withstanding voltage that is at or above the MOSFET's Vds (max) value. (Example: Rohm RFN1L7S: 200V, 0.8A) The surge-voltage affects not only the transformer's leakage inductance but also the PCB substrate's pattern. Confirm the Vds voltage while assembled in the product, and adjust the snubber circuit as necessary. www.rohm.com (c) 2013 ROHM Co., Ltd. All rights reserved. 6/8 Oct. 2013 - Rev.A BM2P0XX series PWM Flyback converter Technical Design Application Note 1-2-8. Output rectification diode: D4 Choose a high-speed diode (Schottky barrier diode or fast recovery diode) as the output rectification diode. Reverse voltage applied to output diode is Vdr Vout(max)VINmax Ns Np When Vout (max) = 12 V + 5% = 12.6V: Vdr 12.6V372V 12 87V 60 A 87.4V/0.7 = 125V 200V component is determined with consideration for design margin. Also, diode loss (approximate value) becomes Pd = Vf x Iout = 1V x 1A = 1W. (Example: Rohm RF301B2S200V 3A , CPD package) Use of a voltage margin of 70% or less and current of 50% or less is recommended. Check temperature rise while assembled in the product. When necessary, reconsider the component and use a heat sink or similar to dissipate the heat. 1-2-9. Output capacitors: C5 Determine the output capacitors based on the output load`s allowable peak-to-peak ripple voltage (Vpp) and ripple-current. When the MOSFET is ON, the output diode is OFF. At that time, current is supplied to the load from the output capacitors. When the MOSFET is OFF, the output diode is ON. At that time, the output capacitors are charged and a load current is also supplied. When Vpp = 200mV, Vpp 0.2V Z_C5 0.05 Ispk 4.04A at 60kHz (fsw min) With an ordinary switching power supply electrolytic-capacitor (low-impedance component), impedance is rated at 100kHz, so it is converted to 100kHz. Z_C5 0.05 60 0.03 100 at 100kHz Ripple-current Is (rms): 1 - 0.406 1 - Duty 4.04A 1.798A 3 3 The capacitor's withstanding voltage should be set to about twice the output voltage. Is(rms) Ispk Vout x 2 = 12V x 2 = 24V 25V or above Select an electrolytic capacitor that is suitable for these conditions. (Example: low impedance type 35V, 1000 F for switching power supply ) (*) Use the actual equipment to confirm the actual ripple-voltage and ripple-current. 1-3. EMI countermeasures Confirm the following with regard to EMI countermeasures. (*) Constants are reference values. Need to be adjusted based on noise effects. - Addition of filter to input block - Addition of capacitor between primary-side and secondary-side (C7: approximately Y-Cap 2200pF) - Addition of capacitor between MOSFET's drain and source (C8: approximately 1kV, 10 to 100pF) (When a capacitor has been added between the drain and source, loss is increased. Check for temperature rise and adjust accordingly) - Addition of RC snubber to diode (C9: 500V 1000pF, R10: approximately 10, 1W) www.rohm.com (c) 2013 ROHM Co., Ltd. All rights reserved. 7/8 Oct. 2013 - Rev.A BM2P0XX series PWM Flyback converter Technical Design Application Note 1-4. Output noise countermeasures As an output noise countermeasure, add an LC filter (L:10H, C10: approximately 10F to 100F) to the output. (*) Constants are reference values. Need to be adjusted based on noise effects. Figure 1-6. LC Filter Circuit 1-5. Proposed PCB layout A proposed layout (example) for these circuits is shown in Figure 1-7. Single-sided board, lead component view Components in red are surface-mounted components Figure 1-7. Proposed PCB Layout (Example) www.rohm.com (c) 2013 ROHM Co., Ltd. All rights reserved. 8/8 Oct. 2013 - Rev.A Notice Notes 1) The information contained herein is subject to change without notice. 2) Before you use our Products, please contact our sales representative and verify the latest specifications : 3) Although ROHM is continuously working to improve product reliability and quality, semiconductors can break down and malfunction due to various factors. Therefore, in order to prevent personal injury or fire arising from failure, please take safety measures such as complying with the derating characteristics, implementing redundant and fire prevention designs, and utilizing backups and fail-safe procedures. ROHM shall have no responsibility for any damages arising out of the use of our Poducts beyond the rating specified by ROHM. 4) Examples of application circuits, circuit constants and any other information contained herein are provided only to illustrate the standard usage and operations of the Products. The peripheral conditions must be taken into account when designing circuits for mass production. 5) The technical information specified herein is intended only to show the typical functions of and examples of application circuits for the Products. ROHM does not grant you, explicitly or implicitly, any license to use or exercise intellectual property or other rights held by ROHM or any other parties. ROHM shall have no responsibility whatsoever for any dispute arising out of the use of such technical information. 6) The Products are intended for use in general electronic equipment (i.e. AV/OA devices, communication, consumer systems, gaming/entertainment sets) as well as the applications indicated in this document. 7) The Products specified in this document are not designed to be radiation tolerant. 8) For use of our Products in applications requiring a high degree of reliability (as exemplified below), please contact and consult with a ROHM representative : transportation equipment (i.e. cars, ships, trains), primary communication equipment, traffic lights, fire/crime prevention, safety equipment, medical systems, servers, solar cells, and power transmission systems. 9) Do not use our Products in applications requiring extremely high reliability, such as aerospace equipment, nuclear power control systems, and submarine repeaters. 10) ROHM shall have no responsibility for any damages or injury arising from non-compliance with the recommended usage conditions and specifications contained herein. 11) ROHM has used reasonable care to ensur the accuracy of the information contained in this document. However, ROHM does not warrants that such information is error-free, and ROHM shall have no responsibility for any damages arising from any inaccuracy or misprint of such information. 12) Please use the Products in accordance with any applicable environmental laws and regulations, such as the RoHS Directive. For more details, including RoHS compatibility, please contact a ROHM sales office. ROHM shall have no responsibility for any damages or losses resulting non-compliance with any applicable laws or regulations. 13) When providing our Products and technologies contained in this document to other countries, you must abide by the procedures and provisions stipulated in all applicable export laws and regulations, including without limitation the US Export Administration Regulations and the Foreign Exchange and Foreign Trade Act. 14) This document, in part or in whole, may not be reprinted or reproduced without prior consent of ROHM. Thank you for your accessing to ROHM product informations. More detail product informations and catalogs are available, please contact us. ROHM Customer Support System http://www.rohm.com/contact/ www.rohm.com (c) 2013 ROHM Co., Ltd. All rights reserved. R1102A