Basic Steps to Design a PSR Flyback Converter Using AP3706/08N

Transcription

1 Application ote 08 Basic teps to Design a R Flyback Converter Using A3706/08 repared by u Qinghua and Dong Wenhui ystem Engineering Department Introduction A3706/08 uses ulse Frequency Modulation (FM) method to realize Discontinuous Conduction Mode (DCM) operation for flyback power supplies; uses rimary ide Regulation (R) to provide Constant Voltage (CV) and Constant Current (CC) regulation. The principle of FM is different from that of ulse Width Modulation (WM), so the design steps of the transformer for FM operation is also different from that for WM operation. In the other side, A3706/08 uses R to provide CV/CC regulation without requiring opto-coupler and loop compensation, which makes the design of transformer more critical than that with WM. The following design steps focus on the transformer design. ome design guides to select diodes and transistor are also included. Figure is a simplified flyback converter controlled by A3706 with a 3-winding transformer: rimary winding ( ), econdary winding ( ) and Auxiliary winding ( ). The A3706 senses the Auxiliary winding feedback voltage at FB pin and obtains power supply at VCC pin. In the circuit: Vdri is a simplified driving signal of primary transistor. I is the primary side current. I is the secondary side current. V is the voltage of secondary side. Figure shows the relatively ideal operation waveforms to illustrate some parameters used in the following design steps. The nomenclature of the parameters in Figure are listed as below: T W is the period of switching frequency Tonp is the time of primary side Tons is the time of secondary side Toff is the discontinuous time is the peak current of primary side s is the peak current of secondary side V equals the sum of V and forward voltage of rectification diode The only difference between A3706 and A3708 is cable compensation. A3708 has built-in it, while A3706 has not. Their design steps are quite similar. V I V Vd - I L M L I V - Vdri V Aux C VCC UT Aux A3706 i BIA GD CM FB VDD R C Figure. implified Flyback Converter Using A3706 ep. 009 Rev.. BCD emiconductor Manufacturing Limited

2 Application ote 08 Figure. Ideal peration Waveforms for a implified Flyback Converter Basic Design tep tep : elect a Reasonable of Flyback Converter Using A3706/08 -. Calculate the Maximum Turn Ratio of the Transformer The maximum turn ration of the transformer should be calculated first to ensure the system working in DCM under all working conditions, especially at the minimum input voltage and full load. If the system meets equation () at minimum input voltage and full load, the circuit can work in DCM in all working conditions. T W Tonp Tons For the primary side current, ep. 009 Rev.....() Tonp where L M is the inductance of primary winding. is the rectified DC voltage of input. When is at the minimum value, the maximum Tonp can be obtained. o, Tonp _ max _ min For the secondary side current, L Tons s V where...()...(3) BCD emiconductor Manufacturing Limited

3 L is the inductance of secondary winding. V V Vd, Vd is the forward voltage of secondary diode. I V I η Application ote 08...(9) For (3), in CV regulation, the V is a constant voltage, so Tons is a constant value with different input voltage. In Flyback converter, the energy stored in the magnetizing inductance L M when the primary transistor turns. o the power transferring from the input to the output is given by: then, I L M f T W f W W...(4) ubstitute T W, Tonp and Tons in equation () with (4), () and (3): L...(5) Because the peak current and inductance of primary side and secondary side have the following relationship: s I L s Vs M I _ min...(6) here, / is the turn ratio of primary to secondary sides, then L L M...(7) where η is the system efficiency. At the full load, the system will work in the boundary of CC regulation. I is determined by: I Then, s can be defined as: s k...(0) I In the design of A3706/08, T k Tons W 3.5 With (8), (9) and (0), it can be obtained: Tons T W s k η _ min ( ) V V...() o, the maximum turn ration of primary and secondary side can be obtained: k η _ MAX _ min ( V V ( k 4)...() Because above calculations are all based on ideal conditions without considering power loss, k is given an approximately value 4 instead of the real value Calculate the eak Current of rimary ide and Current ense Resistor can be calculated by the output current: ) Vd From (5), (6) and (7): I Because V...(8) here, k4. s k I...(3) _ MAX _ MAX In A3706/08, 0.5V is an internal reference voltage. If the sensed voltage V C reaches to 0.5V, the ep. 009 Rev.. BCD emiconductor Manufacturing Limited 3

4 power transistor (AT3003 for this application) will be shut down and Tonp will be ended. o, R C can be calculated as: 0. 5V R C Then, select R C with a real value from the standard resistor series. After R C determined, should be modified according to the selected R C. tep : Design Transformer -. Calculate the Inductance of rimary ide- --L M The primary side inductance L M is relative with the stored energy. L M should be big enough to store enough energy, so that _max can be obtained from this system. From equation (9), the output power can be obtained from: fw η Then, L M is:...(4)...(5) The turns of primary winding is: Ae B The turns of secondary winding is: The turns of Auxiliary winding is: Application ote (8)...(9) V...(0) V here, V can be set to its typical value 5V. Vs is equal to V Vd. Ae can be obtained automatically after core-type is selected. tep 3: elect Diode and rimary Transistor 3-. elect Diodes of econdary and Auxiliary ides Maximum reverse voltage of secondary side is: f W η...(6) Vdr V _ max...() To achieve good overall system performance, the optimum switching frequency f W is recommended to be 50 to 60kHz under full load. -. Re-calculate the Turn Ratio of rimary to econdary ide--- From formula (3), the turn ratio of primary and secondary side should be re-calculated. k I (k 4)...(7) -3. Calculate the Turns of rimary, econdary and Auxiliary Windings First, select the reasonable core-type and B. Then, decide the turns of 3-winding transformer respectively. ep. 009 Rev.. Maximum reverse voltage of Auxiliary side is: Vdar V _ max In () and (), the maximum DC input voltage should be used. 3-. elect the rimary ide Transistor It should be noted that the value of Vdc_spike will be different in circuits with different snubber. V Vdc _ max Vdc _ spike _ max...() BCD emiconductor Manufacturing Limited...(3) 4

5 Design Example Design pecification Input Voltage utput Voltage utput Current ettings elected by Users Design tep 85 to 65 VAC V 5.5V I 0.5A Efficiency 75% witching Frequency Forward Voltage of econdary Diode Forward Voltage of Auxiliary Diode Feedback Voltage of Auxiliary Winding Core_type B Vdc_spike f W 55kHz Vd0.4V VdaV V 5V EE6 (Ae9.mm ) B850G 00V tep : elect a Reasonable for the Flyback Converter Using A V R C R C Application ote 08...(4). Ω Re-calculate peak current of primary side, 38mA tep : Design Transformer -. Calculate the Inductance of rimary ide- --L M f L M. 35mH W η...(6) -. Re-calculate the Turn Ratio of rimary to econdary ide--- k I (k 4)...(7) Calculate the Turns of rimary, econdary and Auxiliary Windings -. Calculate the Maximum Turn Ratio of the Transformer 8 0 Ae B...(8) ( k 4) k η _ min ( ) Vo Vo Vd _ MAX _ min V I _ MAX 8.59 _ min 40...() -. Calculate the eak Current of rimary ide and Current ense Resistor s k I...(3) _ MAX _ MAX 0T...(9) T V V 3T...(0) 4mA ep. 009 Rev.. BCD emiconductor Manufacturing Limited 5

6 tep 3: elect Diode and rimary Transistor 3-. elect Diodes of econdary and Auxiliary ides Maximum reverse voltage of secondary side. Application ote 08 Maximum reverse voltage of secondary side. Vdar V Vdar 9V _ max...() _ max Vdr V _ max V _ max Vdr 50V I...() 3-. elect the rimary ide Transistor Vdc V _ max Vdc _ spike _ max...(3) Vdc _ max 65V Design Results ummary. Calculate the Maximum eak Current of rimary ide and R C eak Current of rimary ide Current ense Resistor 38mA R C.Ω. Design Transformer Inductance of rimary ide L M.35mH(/-8%) Turn Ratio of rimary to econdary Windings 8.4 Turns of rimary ide 0T Turns of econdary ide T Turns of Auxiliary ide 3T 3. elect Diode and rimary Transistor Maximum Reverse Voltage of econdary Diode Maximum Reverse Voltage of Auxiliary Diode Voltage tress of rimary Transistor Vdr50V Vdar9V Vdc_max65V ep. 009 Rev.. BCD emiconductor Manufacturing Limited 6