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L6565D 数据表(PDF) 9 Page - STMicroelectronics |
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L6565D 数据表(HTML) 9 Page - STMicroelectronics |
9 / 17 page 9/17 L6565 the L6565 puts a limit on the minimum OFF-time of the switch. This is done by blanking the triggering block of the ZCD circuit as mentioned before. The duration of the blanking time (3.5µs min.) is a function of the error amplifier output VCOMP, as shown in the diagram of figure 6. If the load current and the input voltage are such that the switch OFF-time falls below the minimum blanking time of 3.5µs, the system will enter the "Frequency Foldback" mode, a sort of "ringing cycle skipping" illustrated schematically in figure 14. Figure 14. Frequency foldback: ringing cycle skipping as the load is progressively reduced In this mode, uneven switching cycles may be observed under some line/load conditions, due to the fact that the OFF-time of the MOSFET is allowed to change with discrete steps (2·Tv), while the OFF-time needed for cycle-by-cycle energy balance may fall in between. Thus one or more longer switching cycles will be compen- sated by one or more shorter ones and vice versa. However, this mechanism is absolutely normal and there is no appreciable effect on the performance of the converter or on its output voltage. t VDS TFW TBLANKmin TV t VDS TBLANK Pin = Pin' (limit condition) Pin = Pin'' < Pin' Pin = Pin''' < Pin'' t VDS TBLANK Figure 15. Frequency Foldback: qualitative frequency dependence on power throughput Further load reductions involve lower values for VCOMP, which increases the blanking time. There- fore, more and more ringing cycles will be skipped. When the load is low enough, so many ringing cycles need to be skipped that their amplitude becomes very small and they can no longer trigger the ZCD cir- cuit. In that case the internal starter of the IC will be activated, resulting in burst-mode operation: a series of few switching cycles spaced out by long periods where the MOSFET is in OFF state. Voltage Feedforward block (see fig. 17b): The power that QR flyback converters with a fixed overcurrent setpoint (like fixed-frequency systems) are able to deliver changes with the input voltage considerably. With wide-range mains, at maximum line it can be more than twice the value at minimum line, as shown by the upper curve in the diagram of figure 16. The L6565 has the Line Feedforward func- tion available to solve this issue. Figure 16. Typical power capability change vs. input voltage in ZVS QR flyback converters fsw Pin without frequency foldback with frequency foldback Vin fixed Vin Vinmin 11.5 22.533.5 4 0.5 1 1.5 2 2.5 system optimally compensated system not compensated |
类似零件编号 - L6565D |
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类似说明 - L6565D |
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