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AN4130 Datasheet(PDF) 7 Page - STMicroelectronics |
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AN4130 Datasheet(HTML) 7 Page - STMicroelectronics |
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7 / 41 page  AN4130 Theory of operation Doc ID 023315 Rev 1 7/41 2.2 PFC-flyback In the PFC-flyback converter the input voltage is the rectified line voltage, with almost no filtering. Converter input voltage goes to zero when the line voltage crosses zero. It's common practice to use the rectified line voltage as a reference for the peak current in the flyback converter switch. This does not result in sinusoidal input current, but it's close enough. The duty cycle change with input voltage still distorts the waveform. This is discussed in detail in ST's AN1059 application note. Figure 4. Distortion of input current with sinewave reference 2.3 Non-isolated flyback In this circuit the transformer secondary shares part of the primary winding. The lack of isolation means that the burden of electrical safety rests on the LED thermal mounting. The LEDs must be isolated from human contact. Electrical stress on the power conversion components is lower than a corresponding isolated flyback converter. Leakage inductance in the transformer is lower, because the secondary occupies the same space as part of the primary, and the remaining primary is very close to the secondary - interwinding insulation is not required. Reduced leakage inductance results in less overshoot in the FET drain voltage, so a lower voltage FET can be used safely. The HVELD815PF used here has an 800 V FET - too much for the US 120 V application but ideal for European line voltage. 2.4 Primary-side control A PFC-flyback converter usually uses a PFC controller chip such as ST's L6562AT with an external FET and a feedback loop. The secondary side voltage and/or current are monitored, compared to a reference on the secondary side, and a control signal sent to the primary side with an opto-isolator. This signal is multiplied by a reference waveform (the rectified line voltage) and used to control peak switch current. ST has developed a primary-side control circuit that eliminates the need for the secondary- side components. Voltage is monitored on the housekeeping winding at the end of the flyback converter discharge cycle, just as the secondary current reaches zero. Secondary current is set by measuring duty cycle and adjusting peak primary current, to provide a calculated secondary average current. But the circuit cannot work with a multiplier, so another method of shaping the peak switch current waveform must be found. |
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