Part # Application Note Power Supply And Power Management L6561 AN1089 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics

Part Description: Control loop modeling of L6561-based TM PFC

Part Details:

AN1089 ® APPLICATION NOTE CONTROL LOOP MODELING OF L6561-BASED TM PFC by Claudio Adragna This paper provides a model and a tool for evaluating and improving the control loop characteristicsof L6561-based PFC preregulators in boost topology and operated in Transition Mode (TM).Such a subject is now becoming topical since TM PFC preregulators are more and more used in sys-tems other than electronic lamp ballast where the input voltage range is limited and the load currentis almost constant.The ability to operate under large variations of both input voltage and load current, as well as the useof TM PFC systems as preregulators for switching converters, requires a more accurate design of thecontrol loop. The goal will be not only to ensure a narrow bandwidth in order to achieve a high PowerFactor, but also to have enough phase margin so as to make sure the system is stable over a largerange of operating conditions. INTRODUCTION PFC preregulators based on the boost topology working in Transition Mode (TM, see fig. 1) have beenwidespread in electronic lamp ballast systems. This kind of equipment almost always works under a sin-gle mains supply (110 or 220 VAC, with some tolerance) and the use of a PFC preregulator is mainlyaimed at optimising the downstream half-bridge lamp driver and improving their inherent extremely poorPF.The PFC preregulator sees the downstream stage as a constant load, so it is requested to work under alimited range of operating conditions. From the control loop standpoint, this means that the frequencycompensation of the error amplifier can be very simple, typically just a feedback capacitor. Its capaci-tance will be high enough to ensure the crossover frequency of the open loop gain is low, so as toachieve a high PF (see Ref. [1]). Figure 1. Typical L6561-based TM PFC preregulator R7 Vcc Compensation Network + R9 5 2 1 Vo 8 7 - L6561 Co Vac 3 6 4 R10 Rs R8 March 2000 1/12 AN1089 APPLICATION NOTE Things get more complicated when an electronic ballast can supply two lamps and is required to workeven if one lamp is not used or is exhausted, so that it is expected to work at half load as well.The L6561, thanks to its highly linear, wide dynamics multiplier, extends the use of TM PFC boost pre-regulators to applications that experience a wide range of operating conditions, both in terms of inputvoltage variations and load change. High power (60 to70 W) AC-DC adapters for portable equipmentand computer monitor SMPS are the most noticeable examples.This, however, calls for a more accurate design of the control loop than the one illustrated in Ref.[1]. Thecontrol goal will no longer be to achieve only a low crossover frequency but also an adequate phasemargin. Besides ensuring stability over a large variety of operating conditions, this is necessary to pre-vent dangerous oscillations of the output voltage as a result of load changes. PFC Boost Preregulator Control LoopTo the aim of finding a compensation network able to achieve the above mentioned control goal, it isnecessary to get an insight into the control loop of such systems. This can be synthesised as shown inthe block diagram of fig. 2. Figure 2. Control loop of a PFC Preregulator: Block Diagram Virms ZCD G3 ERROR AMPLIFIER MULTIPLIER PWM MODULATOR POWER STAGE Vref VCOMP Vcspk ILpk Vo + + G1(s) G2 G4(s) -

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