Design guide IDP2303(A)
PFC design
Application Note
21
Revision 2.0
2017-05-03
t
onDesir ed
/ I
in,rms
f
swma x
QR1
Frequency
f
swmin
… ...
f
swburst
Low line
High line
FF
DCM
f
swDCM
Figure 14
Frequency law for selecting operating modes
The real-time operation of QRN is executed by the IDP230x hardware peripheral QR-timer. A new switching
cycle of QRN operation starts with the N-th valley detection of the sensed inductor voltage or with the end of
the maximum switching period, whichever occurs first. As illustrated in Figure 13, occurrence of the N-th
valley is determined by the N-th falling edge of the ZCD comparator plus a propagation delay that is
equivalent to
𝑇
𝑜𝑠𝑐
/4
to reach the valley switching point.
𝑇
𝑜𝑠𝑐
is the resonance period of the oscillation due
to the PFC inductor and the parasitic capacitance across the MOSFET after the end of boost inductor
demagnetization time.
The changing of QRN in multimode control is dependent on the frequency law. When the switching
frequency reaches the min frequency, the frequency law defines that the frequency is too low and the
switching frequency must increase by reducing the QRN. When the switching frequency reaches the
maximum frequency, the frequency law defines that the frequency is too high and switching frequency must
reduce by increasing the QRN. The changing of QRN within each AC half cycle is dependent on both the load
and AC line, and is inherent in multimode PFC control.
3.3.5.3
Setting of
𝒇
𝒎𝒊𝒏
and
𝒇
𝒎𝒂𝒙
The setting of minimum and maximum frequencies of the frequency law needs to meet the application
requirement that in the condition where V
IN
= 90 V
AC
, full load, the PFC should operate with QR1 mode
operation over the whole AC cycle.
From the equation defining the switching frequency with QR1 operation
𝑓 =
𝑣
𝑖𝑛
∗ (𝑣
𝑏𝑢𝑠
− 𝑣
𝑖𝑛
)
𝑣
𝑏𝑢𝑠
∗ 𝐿
𝑃𝐹𝐶
∗ 𝑖
𝑖𝑛
It can be concluded that over the AC cycle, the switching frequency is at a minimum at AC peak and at a
maximum at zero crossing. The same applies to QR2 operation.
Therefore, selection rule for
𝒇
𝒎𝒊𝒏
is : A value above the maximum switching frequency at the peak of the AC
input voltage with QR2 operation and minimum PFC inductance should be selected to force QR1 operation.
Selection rule for
𝒇
𝒎𝒂𝒙
is: A value above the maximum switching frequency near the zero crossing of the AC
input voltage with QR1 operation and minimum PFC inductance should be selected to keep QR1 operation.
