Datasheet L6565 (STMicroelectronics) - 9
| Fabricante | STMicroelectronics |
| Descripción | Quasi-Resonant SMPS Controller |
| Páginas / Página | 17 / 9 — L6565. Figure 14. Frequency foldback: ringing cycle skipping as the load … |
| Formato / tamaño de archivo | PDF / 256 Kb |
| Idioma del documento | Inglés |
L6565. Figure 14. Frequency foldback: ringing cycle skipping as the load is progressively reduced
Línea de modelo para esta hoja de datos
Versión de texto del documento
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
VDS VDS VDS t t t TFW TV TBLANKmin TBLANK TBLANK Pin = Pin' (limit condition) Pin = Pin' < Pin' Pin = Pin' ' < Pin' 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.
Figure 15. Frequency Foldback: qualitative
Voltage Feedforward block (see fig. 17b):
frequency dependence on power
The power that QR flyback converters with a fixed
throughput
overcurrent setpoint (like fixed-frequency systems) are able to deliver changes with the input voltage fsw 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 without frequency foldback figure 16. The L6565 has the Line Feedforward func- 00000000 Vin fixed tion available to solve this issue. 00000000
Figure 16. Typical power capability change vs.
00000000DE
input voltage in ZVS QR flyback
00000000 00000000
converters
with frequency foldback 00000000 00000000 2.5 BURST MO system not Pin compensated 2 Further load reductions involve lower values for in min in VCOMP, which increases the blanking time. There- @ V 1.5 fore, more and more ringing cycles will be skipped. @ V lim in lim When the load is low enough, so many ringing cycles P in P need to be skipped that their amplitude becomes 1 very small and they can no longer trigger the ZCD cir- system optimally compensated cuit. In that case the internal starter of the IC will be activated, resulting in burst-mode operation: a series 0.5 1 1.5 2 2.5 3 3.5 4 of few switching cycles spaced out by long periods Vin where the MOSFET is in OFF state. Vinmin 9/17
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
VDS VDS VDS t t t TFW TV TBLANKmin TBLANK TBLANK Pin = Pin' (limit condition) Pin = Pin' < Pin' Pin = Pin' ' < Pin' 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.
Figure 15. Frequency Foldback: qualitative
Voltage Feedforward block (see fig. 17b):
frequency dependence on power
The power that QR flyback converters with a fixed
throughput
overcurrent setpoint (like fixed-frequency systems) are able to deliver changes with the input voltage fsw 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 without frequency foldback figure 16. The L6565 has the Line Feedforward func- 00000000 Vin fixed tion available to solve this issue. 00000000
Figure 16. Typical power capability change vs.
00000000DE
input voltage in ZVS QR flyback
00000000 00000000
converters
with frequency foldback 00000000 00000000 2.5 BURST MO system not Pin compensated 2 Further load reductions involve lower values for in min in VCOMP, which increases the blanking time. There- @ V 1.5 fore, more and more ringing cycles will be skipped. @ V lim in lim When the load is low enough, so many ringing cycles P in P need to be skipped that their amplitude becomes 1 very small and they can no longer trigger the ZCD cir- system optimally compensated cuit. In that case the internal starter of the IC will be activated, resulting in burst-mode operation: a series 0.5 1 1.5 2 2.5 3 3.5 4 of few switching cycles spaced out by long periods Vin where the MOSFET is in OFF state. Vinmin 9/17