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1、Design of a flyback switching power suddI、with the LM3488 low-side N-channel current mode controllerWritten by Michele SclocchiApplication Engineer, National SemiconductorThe Flyback topology is the most common topology used for the design of multi output isolated switching power supplies and in 48V
2、 input telecom applications.This paper describes the design of a low cost discontinuous mode flyback with the LM3488, a high efficiency low side N-channel controller for switching regulators.The reader will be guided from the theoretical approach to a complete practical design solution.To fully unde
3、rstand a flyback power supply design, it is useful to review the theory of a flyback and of the general aspects of a switching power supply such as continuous and discontinuous operation mode, currcnt-voltagc mode, and all the related problems of a high switching frequency transformer design.The fly
4、back converter is based on the buck-boost converter.The single inductor of figure 1-a can be split in two inductors connected in parallel, with a 1:1 turn ratio (figure 1-b). The basic function of the inductor is unchanged , the parallel windings made on the same core are equivalent to the single wi
5、nding. If the connection between the two windings is opened, the winding LI is used while the transistor conducts, and the other winding L2 is used when the diode conducts.(a)(b)(d)Figure 1: Derivation of flyback converterA flyback topology can be immediately recognized from the opposite dots on the
6、 transformer primary and secondary. The primary difficulty associated with is topology is the design of the inductor-transformer, which now assumes the role of both an energy storage device and a transformer element. Unlike the ideal transformer, current docs not flow simultaneously in both windings
7、.The magnetizing primary inductance works in the same way as the inductor of the original buck-boost converter. When the transistor conducts, the energy from the de input source is stored on L1. When the diode is in conduction, the energy is transferred to the output capacitor and to the load.Discon
8、tinuous mode versus continuous mode:A flyback converter has two different modes of operation, discontinuous mode and continuous mode. Both modes have the identical circuit diagram, the waveforms of the primary and secondary currents on the transformer are shown in figure 2-a and 2-b.A circuit that h
9、as been designed for discontinuous mode will move into continuous mode when the output current is increased beyond a certain value.In the discontinuous mode all the energy stored in the primary during the on time is completely delivered to the secondary and to the load before the next cycle, and the
10、re is also a dead time between the instant the secondary current reaches zero and the start of the next cycle.In the continuous mode there is still some energy left in the secondary at the begin of the next cycle.The flyback can operate in both modes, but it has different characteristics.The discont
11、inuous mode has higher peak currents, and therefore it has higher output voltage spikes during the turn-off. On the other hand it has faster load transient response, lower primary inductance and therefore the transformer can be smaller in size. The reverse recovery time of the diode is not critical
12、because the forward cun ent is zero before the reverse voltage is applied. Conducted EMI noise is reduced in discontinuous mode because transistor turn on occurs with zero collector current.The continuous mode, even if it has lower peak currents, and therefore lower output voltage spikes, is seldom
13、used for low power applications because the converter transfer function has a right half plane zero, therefore the bandwidth must be drastically reduced to stabilize the feedback loop.primary currentFigure 2-a: Primary and secondary currents in discontinuous modecontinuous modeFigure 2-b: Primary an
14、d secondary currents in continuous modeVoltage mode control versus current mode control:The voltage mode control has a single feedback loop. The PWM circuit is shown in figure 3-a.A constant sawtooth triangle waveform Vst is generated by an oscillator and a constant current charge of an external cap
15、acitor. An error amplifier compares the feedback of the output voltage to a voltage reference, and produces an error voltage Ve; Ve is compared to Vst by a voltage comparator. The PWM output is high when the Vst is higher than the error voltage Ve.A voltage mode control does not control the output c
16、urrent, therefore a load transient has to be sensed through an output voltage change, and then coiTected by the feedback loop. Current mode control has the advantage of controlling the output current and the output voltage within the same circuit (figure 3-b). Line and current transient response is done by variation of the duty cycle of the power transistor. Duty cycle is determined by both the output voltage error Ve and th
