Part # Application Note Power Supply And Power Management L6565 AN1326 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics

Part Description: L6565 quasi-resonant controller

Part Details:

AN1326 APPLICATION NOTE L6565 QUASI-RESONANT CONTROLLER by Claudio Adragna A variable frequency version of flyback converter, commonly known as Quasi-resonant (QR) ZVS fly-back, is largely used in certain applications, such as SMPS for TV, though it is well suited for other ap-plications too. This peculiar topology features several merits. Besides the others, which will be highlighted in the text,one of them is to be a simple derivative of the standard square-wave flyback, well known to every SMPSdesigner. After deriving the equations governing QR ZVS flyback topology and dealing with the related issues,ST s L6565, a PWM controller specifically designed to fit this particular topology, will be presented andits internal functions discussed in details. Some clues on the design based on this device will be provided and, finally, a design example will begiven that will show how easy and cost-effective such L6565-based systems are. INTRODUCTION Over the past two decades plenty of resonant and quasi-resonant converters have been developed and pro-posed as an answer to the difficulties raised by square-wave converters, especially those related to their para-sitic elements. The basic idea is to put these parasitics in use. The core of both classes of converters is a tank circuit. Unlike resonant converters, where it takes part activelyin the power conversion process, quasi-resonant converters use the tank circuit only to create either a zero-voltage or a zero-current condition for the power switch, to turn on or turn off respectively. The existing types of quasi-resonant converters can be then classified as either Zero-Voltage Switching turn-on(ZVS) or Zero-Current Switching turn-off (ZCS) converters. In ZVS converters the power switch is dynamically connected in parallel to the tank circuit. The superposition ofthe resonant voltage across the tank circuit (and the switch, while it is in off state) on the DC input voltage gen-erates the zero-voltage condition for the switch to turn on. Conversely, in ZCS converters the power switch isconnected in series to the tank circuit. The superposition of the resonant current flowing through the tank circuit(and the switch, while it is in on state) on the normal current flow generates the zero-current condition for theswitch to turn off. An interesting property of quasi-resonant converters is that they will be turned back into a normal square-waveconverter if the tank circuit is removed. Vice versa, a quasi-resonant converter can be obtained starting from anormal square-wave topology. QR ZVS FLYBACK TOPOLOGY In principle there are many ways to make a quasi-resonant (QR) ZVS flyback converter, but most of them arenot suitable for offline applications because of the too high voltages involved. With the above-mentioned prop-erty in mind, it is possible to derive a QR version starting from a standard square-wave flyback power stage andpointing out its major parasitic elements, as illustrated in figure 1. Llk is the leakage inductance, which represents the magnetic flux generated by the primary winding and not cou-pled to the secondary. It stores energy that will not be delivered to the secondary and that needs to be trans-ferred or dissipated elsewhere. Besides, it prevents a portion of the energy stored in the mutual inductance Lm(which is perfectly coupled to the secondary) from being transferred to the secondary and delays the energytransfer process. The energy stored in Llk is the cause of the large overvoltage spike on the MOSFET s drain atturn-off. November 2002 1/34 AN1326 APPLICATION NOTE Figure 1. Flyback power stage with major parasitics (a) and VDS waveform with fixed frequency, DCM (b). Vf Llk & Cd Lp & Cd t = 0 Lm Ls Vout VDS Lp VDSs Cin Llk VR Rp Vin Vin VR VDSmin Cd VDS Tv t TON TFW TOFF T a) b) Cd is the total capacitance of the drain node. It is the sum of the MOSFET s Coss, transformer intrawinding ca-pacitance, stray capacitance due to the layout of the circuit (e.g. a heatsink) as well as other contributions re-flected from the secondary side, such as an R-C damper on the rectifier diode. Actually, Coss is modulated by the drain voltage but the variation becomes significant at very low VDS valuesand the impact is however limited. Therefore, it is possible to assume for Coss the value specified usually atVDS = 25V by manufacturers. Cd is discharged inside the MOSFET as it is turned on, thus causing a current spike. This spike not only givesorigin to additional losses in the MOSFET but may also cause noise problems, especially in case of currentmode control and under light load conditions. Rp is the resistance of the primary side mesh, mostly located in the primary winding. It is important to notice thatthe resistance of the primary winding has to account not only for the ohmic resistance of the wire but also forthe high frequency effects in copper (skin and proximity), the magnetic material losses (hysteresis and eddy cur-rents) and radiation. At least a couple of tank circuits can be then identified in the schematic, whose effect is conspicuous on thedrain voltage waveform in Discontinuous Conduction Mode (DCM) operation (see figure 1b).

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