ST AN833 Application note
AN833
APPLICATION NOTE
FREQUENCY MODULATION ON L4981B
Devices Description
Designing PFC, an important parameter to consider and control is the high frequency noise. This noise is produced by the switching itself and its level depends on several parameters such as high frequency current ripple,
PCB layout, active switch performance and also some particular circuital solutions.
The parameter to keep under control is the electromagnetic interference (E MI) content, v ersus the RFI norms
and design requirements (EN60555 and VDE0871B). To get more information about thi s, please ref er al so t o
the CIRCUIT FOR POWER FACTOR CORRECTION WITH REGARDS TO MAINS F ILTERING (AN510). The
PFC controller L4981B version can be useful to solve the EMI problems using the frequency modulation technique that allows to spread the noise spectrum over a wider range, reducing in this way the peak of the associated energy. This section deals with the way to use this device feature getting the best results. The L4981B
performs the frequency modulation function by using Pin16, that in this version is named FREQ-MOD.
This pin (see fig. 1) is internally connected (with a current mirror) to an input of the multiplier.
The output of the multiplier is used to dynamically change the current forced into the external oscillator capaci-
tance changing in this way the switching frequency. The second terms (input) of the multipl ier is connected to
the divider (1/Vrms) that, through the VRMS (Pin7), s enses the input mains making the fr equency modulation
not affected by the line variations (see also Pin 16 description in AN628).
Figure 1. Frequency Modulation with L4981B.
I
Ifm
1.28V
+
1/V
MRS
17
ROSC
R
OSC
VRMS
1
7
FREQ-MOD
2 POLES
FILTER
10
16
Rfm
· I
COSC
18
200 · I
C
OSC
8.5V
-
+
D94IN065B
November 2003
1/3
AN833 APPLICATION NO TE
Connecting pin16 at the rectified mains through a resistor, it is possible to define the modulation depth using the
formula:
∆f
SW
------------- - K
=
f
SW
⋅
V
IPKROSC
-------------------------------- R
V
RMSRfm
⇒
⋅
fm
V
IPK mains()
---------------------------------------------------------------⋅=
K
V
RMS pin7()
R
⋅⋅
OScfSw
∆f
⋅
sw
where:
R
is the programming current resistor.
fm
K is a constant value = 0.1157
V
is the
V
RMSmAINS
is the voltage at pin 7
V
IPK
RMS
2⋅
Typically a good compromise can be 10% to 20% of the starting frequency. Designing the frequency modulation
it is useful to remind few points :
a) The switching frequency (fsw) is modulated by the mains instantaneo us value and de creases as the
rectified voltage increases, so the minimum fsw occurs at the input peak voltage and current (see fig.
2).
b) The switching losses increase with the frequency (and obviously with the current).
c) The current ripple increases (for the same boost inductor value) as the switching frequency decreases,
the higher current ripple produces an higher EMI.
Figure 2. Modu lation Freque nc y Normalized in a Half Cycle of the Mains Voltage. (eg. RFM = 1100kΩ,
Rosc = 24kΩ, Cosc = 820pF).
fsw
1
0.8
0.4
0.2
0
180
0.8
0.4
0.2
Vl
1
0
0
45 90
135
Electrical degrees
Considering the above mentioned points, to make a reasonable comparison with an equivalent fixed frequency
PFC application in terms of EMI, it is recommended to modify the starting frequency (oscillator).
The suggested criterion for designing a L4981B applicati on i s to fol l ow the same procedure used for t he fixed
frequency version (L4981A) except for the oscillator t h at m u st b e de signed for the desired frequency (fmin) that
occurs at the peak of the current, plus the modulation contribution, that is:
2.44
f
SW
∆fswf
min
-----------------------------------=+=
R
⋅
OSCCOSC
2/3
Loading...