FA5550 Fuji Electric holdings CO.,Ltd, FA5550 Datasheet - Page 15

• Manufacturer Part Number: FA5550
• Description: Power Factor Correction Converter
• Manufacturer: Fuji Electric holdings CO.,Ltd

(For reference)
Rev.1 Feb-2008
Sub-harmonic oscillation and slope compensation
It is shown that ON-period and OFF-period are changing even
though there are no variation in peak values of the inductor
current, the switching time cycle and the current slope
inclinations of ON- and OFF-periods.
Where;
Lu: Slope inclination of inductor current during ON-period
Ld: Slope inclination of inductor current during OFF- period
Kc: Inclination of slope compensation
In case of current mode in peak value control, there is a
possibility to oscillate in a time cycle having integer multiple of
the switching frequency, if the circuit is operated in inductor
current continuous mode and in duty cycles of 50 percent or
more. This is called as sub-harmonic oscillation.
An example of inductor current waveform is shown in Fig.20
when sub-harmonic oscillation is generated.
This sub-harmonic oscillation may cause increases of ripple
voltages contained in output voltages and abnormal noises.
In response, through addition of a certain amount of slope
inclination to threshold value shown in Fig.21, the circuit can be
prevented from the sub-harmonic oscillation. This is called as
slope compensation.
In general, amount of the slope compensation value required to
avoid the sub-harmonic oscillation is shown as following
relational expression:
Kc
>
Ld
−
2
Lu
FA5550 / 51
15
IL
ON
O N
Fig.20 A case without slope compensation
Slope : Lu
Fig.21 A case with slope compensation
T
T
Fuji Electric Device Technology
OFF
O FF ON
Fig.22 Slope compensation
ON
ON
T
T
O FF
OFF
Slope : -Kc
ON
O N
T
T
OFF
Slope : -Ld
OFF O N
OFF
ON
T
T
OFF
OFF
t