AN833 Datasheet, PDF (2/3 Pages) STMicroelectronics – FREQUENCY MODULATION ON L4981B

AN833 APPLICATION NOTE
Connecting pin16 at the rectified mains through a resistor, it is possible to define the modulation depth using the
formula:
⇒ ^∆---_f-_S-^f--^S-_W--^W--- = K -^V-_V---^I-_RP----_MK----_S^â‹---^R--_â‹---_R^O----_fS--_m--^C--
R_fm = K ⋅ ^V-----^I-^P---_V-^K---_R⁽--^m--_M---^a-_S-ⁱ--ⁿ-₍-_p-^s--⁾_i--_n--^â‹-₇--^R-₎---_â‹^O---_∆-^S---_f-^c-_s---_w^â‹---^f-^S----^w--
where:
R_fm is the programming current resistor.
K is a constant value = 0.1157
V_IPK is the V_{RM SmAINS} ⋅ 2
V_RMS is the voltage at pin 7
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 instantaneous value and decreases 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. Modulation Frequency Normalized in a Half Cycle of the Mains Voltage. (eg. RFM = 1100kΩ,
Rosc = 24kΩ, Cosc = 820pF).
1 ^Vl
fsw
1
0.8
0.8
0.4
0.4
0.2
0.2
0
0
0
45
90
135
180
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 application is to follow the same procedure used for the fixed
frequency version (L4981A) except for the oscillator that must be designed for the desired frequency (fmin) that
occurs at the peak of the current, plus the modulation contribution, that is:
f_SW = ∆f_sw + f_min = _R-----_O----_S---²-_C---^.-⁴_â‹----_C4----_O----_S---_C--
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