L4984D STMicroelectronics, L4984D Datasheet - Page 28

• Manufacturer Part Number: L4984D
• Description: Power Factor Correction ICs Current Mode PFC CCM Line Modulated Bi
• Manufacturer: STMicroelectronics

Specifications of L4984D

• Rohs: yes

THD optimizer circuit
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THD optimizer circuit
The L4984 is provided with a special circuit that reduces the conduction dead-angle
occurring at the AC input current near the zero-crossings of the line voltage (crossover
distortion). In this way the THD (total harmonic distortion) of the current is considerably
reduced.
A major cause of this distortion is the inability of the system to transfer energy effectively
when the instantaneous line voltage is very low. This effect is magnified by the high-
frequency filter capacitor placed after the bridge rectifier, which retains some residual
voltage that causes the diodes of the bridge rectifier to be reverse-biased and the input
current flow to temporarily stop.
To overcome this issue the device forces the PFC pre-regulator to process more energy
near the line voltage zero-crossings as compared to that commanded by the control loop.
This results in both minimizing the time interval where energy transfer is lacking and fully
discharging the high-frequency filter capacitor after the bridge.
Figure 35. THD optimizer circuit
@ Vac1
@ Vac2 > Vac1
MULT
t
t
VFF
Doc ID 023489 Rev 3
1 / V
GENERATOR
2
MULTIPLIER
OFFSET
COMP
t
+
+
t
comparator
to PWM
t
t
AM13250v1
L4984