RCC设计指南

Rev 1.0 September 2005 1/26 26 Introduction This application note is a Ringing Choke Converter (RCC)-based, step-by-step cell phone battery charger design procedure. The RCC is essential to the self-oscillating fly-back converter, and operates within the Discontinuous Conduction Mode (DCM) and Continuous Conduction Mode (CCM) boundaries without noticeable reverse recovery of the output rectifying diodes. RCC control is achieved by using discrete components to control the peak current mode, so the overall RCC cost is relatively low compared to the conventional Pulse Width Modulation (PWM) IC fly-back converter. As a result, RCC is widely used for low power applications in industry and home appliances as a simple and cost-effective solution. Figure 1. STD1LNK60Z-based RCC Printed Circuit Board Bottom ViewTop View AN2228 APPLICATION NOTE STD1LNK60Z-based Cell Phone Battery Charger Design http:/www.st.com AN2228 - APPLICATION NOTE 2/26 Table of Contents 1 Power Transformer Design Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1 Switching Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2 STD1LNK60Z MOSFET Turn Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3 Primary Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.4 Primary Inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.5 Magnetic Core Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.6 Primary Winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.7 Secondary Winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.8 Auxiliary Winding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 1.9 Gap Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2 STD1LNK60Z-based RCC Control Circuit Components . . . . . . . . . . . . . 12 2.1 MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.2 R3 Startup Resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.3 Optocoupler Power Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.4 R7 Sense Resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.5 Constant Power Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.6 Zero Current Sense . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.7 Constant Voltage And Constant Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 3 Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Appendix A: STD1LNK60Z-based RCC Circuit Schematics . . . . . . . . . . 22 Appendix B: STD1LNK60Z-based RCC Circuit Bill of Materials . . . . . . . 23 4 Revision History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 AN2228 - APPLICATION NOTE 3/26 Figures Figure 1. STD1LNK60Z-based RCC Printed Circuit Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. Optocoupler Fly-back Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 3. Optocoupler Forward Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 4. Current Sense Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 5. CV and CC Curve at 110VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 6. CV and CC Curve at 220VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 7. Drain To Source Voltage Operation Waveform, 85VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 8. Drain To Source Voltage Operation Waveform, 110VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 9. Drain To Source Voltage Operation Waveform, 220VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 10. Drain To Source Voltage Operation Waveform, 265VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 11. RCC Control Circuit Components Schematic (see Section on page 1) . . . . . . . . . . . . . . . 22 Figure 12. STD1LNK60Z-based RCC Schematic (full view) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 AN2228 - APPLICATION NOTE 4/26 Tables Table 1. Line and Load Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Table 2. Efficiency Ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Table 3. Standby Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Table 4. BOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 AN2228 - APPLICATION NOTE 1 Power Transformer Design Calculations 5/26 1 Power Transformer Design Calculations ● The specifications: – VAC = 85~265V ● Line frequency: 50~65Hz – VO = 5V – IO = 0.4A Taking transient load into account, the maximum output current is set as 1.1 Switching Frequency The system is a variable switching frequency system (the RCC switching frequency varies with the input voltage and output load), so there is some degree of freedom in switching frequency selection. However, the frequency must be at least 25kHz to minimize audible noise. Higher switching frequencies will decrease the transformer noise, but will also increase the level of switching power dissipated by the power devices. The minimum switching frequency and maximum duty cycle at full load is expressed as where the minimum input voltage is 50kHz and 0.5, respectively. IO max( ) 1.2IO 4.8A= = fS min( ) 50kHz= Dmax 0.5= 1 Power Transformer Design Calculations AN2228 - APPLICATION NOTE 6/26 1.2 STD1LNK60Z MOSFET Turn Ratio The maximum MOSFET drain voltage must be below its breakdown voltage. The maximum drain voltage is the sum of: ● input bus voltage, ● secondary reflected voltage, and ● voltage spike (caused by the primary parasitic inductance at maximum input voltage). The maximum input bus voltage is 375V and the STD1LNK60Z MOSFET breakdown voltage is 600V. Assuming that the voltage drop of output diode is 0.7V, the voltage spike is 95V, and the margin is at least 50V, the reflected voltage is given as: The Turn Ratio is given as where, Vfl = Secondary reflected voltage V(BR)DSS = MOSFET breakdown voltage Vmargin = Voltage margin VDC(max) = Maximum input bus voltage Vspk = Voltage spike Vf = Voltage drop N = Turn Ratio Np = Primary Winding Turns Ns = Secondary Winding Turns Vfl V BR( )DSS Vm inarg VDC max( ) Vspk––– 600 50– 375– 95 80V=–= = N Np Ns ------- Vfl VOUT VF+ ---------------------------- 80 5 0.7+ ------------------ 14= = = = AN2228 - APPLICATION NOTE 1 Power Transformer Design Calculations 7/26 1.3 Primary Current ● Primary Peak Current is expressed as: ● Primary Root Mean Square (RMS) Current is expressed as where, Ippk = Primary peak current VO = Voltage output IO(max) = Maximum current output η = Efficiency, equal to 0.7 Dmax = Maximum duty cycle VDC(min) = Minimum input bus voltage Iprms = Primary RMS current 1.4 Primary Inductance Primary Inductance is expressed as where, VDC (min) = Minimum Input DC voltage fs (min) = Minimum switching frequency Dmax = Maximum duty cycle fs(min) = Minimum switching frequency Ippk = Primary peak current For example, if Primary Inductance is set to 5.2mH, the minimum switching frequency is: Ippk 2VOIO max( ) ηDmaxVDC min( ) ------------------------------------------ 2 5× 0.48× 0.7 0.5× 90× ----------------------------------- 0.152A= = = Iprms Ippk Dmax 3 -------------- 0.152 0.5 3 -------- 0.062A=×= = Lp VDC min( )Dmax fs min( )Ippk --------------------------------------- 90 0.5× 0.152 50× ---------------------------- 5.92mH= = = fs min( ) VIN DC min( )Dmax LpIppk -------------------------------------------- 90 0.5× 0.152 5.2× ----------------------------- 57kHz= = = 1 Power Transformer Design Calculations AN2228 - APPLICATION NOTE 8/26 1.5 Magnetic Core Size One of the most common ways to choose a core size is based on Area Product (AP), which is the product of the effective core (magnetic) cross-section area times the window area available for the windings. Using a EE16/8 core and standard horizontal bobbin for this particular application, the equation used to estimate the minimum AP (in cm4) is shown as where, Lp = Primary Inductance Iprms = Primary RMS current ku = Window utilization factor, equal to: – 0.4 for margin wound construction, and – 0.7 for triple insulated wire construction Bmax = Saturation magnetic flux density ΔT = Temperature rise in the core 1.6 Primary Winding 1.6.1 Winding Turns The effective area of an EE16 core is 20.1mm2 (in the core’s datasheet). The number of turns of primary winding is calculated as where, Np = Primary Winding Turns VDC (min) = Minimum Input DC voltage Dmax = Maximum duty cycle fs(min) = Minimum switching frequency ΔB = Flux density swing Ae = Effective area of the core AP LpIprms kuBmaxΔT 0.5----------------------------------- 1.316 103×= Np VDC min( )Dmax fs min( )ΔBAe --------------------------------------- 90 0.5× 0.22 20.1× 10 6–× 57× 103× ---------------------------------------------------------------------------- 179= = = AN2228 - APPLICATION NOTE 1 Power Transformer Design Calculations 9/26 1.6.2 Wire Diameter The current density (AJ) allowed to flow through the chosen wire is 4A/mm2. The Copper diameter of primary wire is expressed as where, dp = Diameter of primary winding wire Iprms = Primary RMS current AJ = Current density 1.6.3 Number of Primary Winding Turns per Layer The EE16 bobbin window is about 9mm, so if the enamel wiring chosen has a 0.21mm outer diameter and a 0.17mm Copper diameter, the number of turns per layer is expressed as where, Np1 = Layer 1 Primary Winding Turns Np1 = 42 turns per layer, 4 layers needed Np = 168 (total turns for all 4 layers) 1.6.4 Practical Flux Swing Using the Np = 168 value, the practical flux swing is expressed as where, ΔB = Flux density swing VDC(min) = Minimum input bus voltage Dmax = Maximum duty cycle fs(min) = Minimum switching frequency Ae = Effective area of the core Np = Primary Winding Turns dp 4Iprms AJπ ----------------- 4 0.062× 4 π× ----------------------- 0.142mm= = = Np1 90 0.21 ----------- 43= = ΔB VDC min( )Dmax fs min( )AeNp --------------------------------------- 90 0.5× 168 20.1 10 6– 57 103×××× --------------------------------------------------------------------------- 0.234T= = = 1 Power Transformer Design Calculations AN2228 - APPLICATION NOTE 10/26 1.7 Secondary Winding Using triple insulation wire with a 0.21mm Copper diameter, the number of turns of secondary winding is expressed as where, Ns = Secondary Winding Turns Np = 168 (total turns for all 4 primary winding layers) Np = Primary Winding Turns N = Number of turns per primary winding layer 1.8 Auxiliary Winding 1.8.1 Winding Turns The MOSFET gate voltage at minimum input voltage should be 10V to conduct the MOSFET completely. For this application, the optocoupler is powered by the fly-back method, so the number of auxiliary winding turns of auxiliary winding is calculated as where, Vg = Gate voltage VDC(min) = Minimum input bus voltage Na = Auxiliary Winding Turns Np = Primary Winding Turns Vo = Optocoupler voltage VF = Fly-back voltage Ns = Secondary Winding Turns Ns Np N------- 168 14 ---------- 12= = = Vg VDC min( )Na Np ----------------------------------- Vo VF+( )Na Ns ----------------------------------- 10>+= AN2228 - APPLICATION NOTE 1 Power Transformer Design Calculations 11/26 1.8.2 Wire Diameter With the auxiliary winding turns set to 11 (Na =11), the enamel wire chosen has a 0.21mm outer diameter and a 0.17mm Copper diameter. The Copper diameter of primary wire is expressed as 1.9 Gap Length The gap length setting is based on the number of primary winding turns and primary inductance during the manufacturing process. Note: In practice, the saturation current value must be ensured. If it is not, then the design activity should be restarted. Na 10 VDC min( ) Np -------------------------- Vo VF+ Ns ----------------------+ ------------------------------------------------------- 10 95 168---------- 5.7 12 --------+ --------------------------- 10= => 2 STD1LNK60Z-based RCC Control Circuit Components AN2228 - APPLICATION NOTE 12/26 2 STD1LNK60Z-based RCC Control Circuit Components 2.1 MOSFET The STD1LNK60Z (see Appendix A: STD1LNK60Z-based RCC Circuit Schematics on page 22) has built-in, back-to-back Zener diodes specifically designed to enhance not only the Electrostatic Discharge (ESD) protection capability, but also to allow for possible voltage transients (that may occasionally be applied from gate to source) to be safely absorbed. 2.2 R3 Startup Resistor 2.2.1 Minimum Power Dissipation The startup resistor R3 is limited by its power dissipation because of the high input bus voltage that moves across it at all times. However, the lower the R3 value is, the faster the startup speed is. Its power dissipation should be less than 1% of the converter’s maximum output power. The minimum power dissipation value is expressed as 2.2.2 Maximum Power Dissipation If R3 is set to 4.2MΩ, its max power dissipation is expressed as 2.2.3 Startup Resistors and the Power Margin The power rating for an SMD resistor with a footprint of 0805 is 0.125W. Three resistors (1.2MΩ, 1.2MΩ, and 1.8MΩ, respectively) are placed in series to produce the required startup resistor value and still have enough power margin. VDC max( ) 2 R3 ------------------------------- 1percent VoIo max( ) η ----------------------------×< R3 ηVDC max( ) 2 0.01 VoIo max( )× ------------------------------------------------ 0.7 3752× 0.01 5× 0.48× ------------------------------------------- 4.1 106Ω×= => PR3 max( ) VDC max( ) R3 ---------------------------- 3752 4.2 106× -------------------------- 0.0335W= = = AN2228 - APPLICATION NOTE 2 STD1LNK60Z-based RCC Control Circuit Components 13/26 2.3 Optocoupler Power Methods There are two methods for powering the optocoupler: ● fly-back (see Figure 2), and ● forward (see Figure 3). The fly-back method was chosen for the RCC application because it provides more stable power for the optocoupler. Figure 2. Optocoupler Fly-back Power Figure 3. Optocoupler Forward Power C6 C5 + C7 Q2 3904 U1B Q1 STD1LNK60Z R3 R7 R9 R10 R11 R11a R12 AI11829 C6 C5 + C4C7 Q2 3904 U1B Q1 STD1LNK60Z R3 R7 R9 R10 R11 R11a R12 AI11830 2 STD1LNK60Z-based RCC Control Circuit Components AN2228 - APPLICATION NOTE 14/26 2.4 R7 Sense Resistor 2.4.1 Minimum Power Dissipation Sense resistor R7 is used to detect primary peak current. It is limited by its maximum power dissipation, which is set to 0.1% of the maximum power. The minimum power dissipation is expressed as 2.4.2 Maximum Power Dissipation If R7 is set to 3.4Ω, its maximum power dissipation is expressed as 2.4.3 Sense Resistors and the Power Margin Two resistors (6.8Ω, and 6.8Ω, respectively) are placed in parallel to produce the required sense resistor value and still have enough power margin. Ramp-up voltage (via R7 x Ippk), when added to the DC voltage [(I1+Ie)(R7+R9)] achieves good output voltage and current regulation (see Figure 4). Note: The R9 value should be much greater than the R7 value. The minimum primary current, Ippk, and the maximum current, I2, are in a stead state at the minimum load, while the maximum Ippk and the minimum I2 are in a stead state at the maximum load. The cathode current, Ik, of TL431 is limited to 1mA< Ik <100mA, and the maximum diode current of optocoupler PC817 is 50mA. In order to decrease quiescent power dissipation, the maximum operation diode current, IF, of PC817 can be set to 10mA. The Current Transfer Ratio (CTR) of PC817 is about 1:0 at the stead state. As a result, the maximum operation transistor current Ie of PC817 is also set to 10mA. Initially the effect of I1 is neglected. At minimum load, At maximum load, where, VQbe = Cut off voltage; when the voltage between the base and the emitter of transistor Q2 reaches this value, MOSFET Q1 is turned off. For the purposes of this application design: R9 = 360Ω, and C6 = 2.2nF; the role of C6 is to accelerate the MOSFET’s turning OFF. R7 0.01 VoIo max( )× ηIprms 2 ------------------------------------------------ 0.01 5× 0.48× 0.7 0.0622× --------------------------------------- 8.9Ω= =< PR7 max( ) Iprms 2R7 0.062 2 3.4× 0.013W= = = R7IF min( ) R7 R9+( )Ie max( )+ R7 R9+( )Ie max( ) R9Ie max( ) VQbe<≈ ≈ R7Ippk R7 R9+( )Ie min( )+ R7Ippk R9Ie min( ) VQbe>+≈ AN2228 - APPLICATION NOTE 2 STD1LNK60Z-based RCC Control Circuit Components 15/26 Figure 4. Current Sense Circuit 2.5 Constant Power Control The pole of capacitor C7 can filter the leading edge current spike and avoid a Q2 switch malfunction. However, it will also lead to delays in primary peak transfer as well as the turning on of Q2. As a result, different power inputs are produced at different input voltages. Z1, R11, and R11a provide constant current, which is proportional to the input voltage. This way, power inputs are basically the same at different input voltages. Note: They must be carefully selected and adjusted to achieve basically constant power input at different input voltages. The basic selection process is expressed as where, ΔI = Current change VDC = Input bus voltage Lp = Primary Inductance Td = Transfer delay In relation to the present RCC application, where, Na = Auxiliary Winding Turns Np = Primary Winding Turns Vo = Optocoupler voltage VF = Fly-back voltage Ns = Secondary Winding Turns Vz1 = Zener diode 1 voltage C6 C7 Q2 3904 U1B Z1 R7 R9 R11 R11a R12 Ippk I1 Ie AI11831 ΔI VDC Lp -----------Td= ΔIR7 R7 VDC Lp --------------Td NaVDC Np ----------------------- Na Vo VF+( ) Ns -------------------------------------- Vz1–+ R7 R9 R11+ + --------------------------------------------------------------------------------------- R9 R7+( )= = 2 STD1LNK60Z-based RCC Control Circuit Components AN2228 - APPLICATION NOTE 16/26 Note: R11>> R9 >> R7, so in this case, only R11 is used: Note: Constant control accuracy is not as good if Z1 is not used, and applying it is very simple. For the purposes of this application design: C7 = 4.7nF, and R11 = 36KΩ. 2.6 Zero Current Sense C5 blocks DC current during starting up and allow charge to be delivered from the input voltage through starting up resistor until MOSFET turns on for the first time. The MOSFET C5 and input capacitor Ciss form a voltage divider at the MOSFET gate, so C5 value should be ten times more than that of Ciss. This decreases the MOSFET (full) turn-on delay. In this case, C5 = 6.8nF. R10 limits power dissipation of zener diode inside the MOSFET. The selection process is expressed as where, VDC(max) = Maximum input bus voltage Na = Auxiliary Winding Tur