Infineon XDPL8218, Руководство по конструкции

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Design Guide 27 of 48 V 1.0
2018-06-06
XDPL8218 design guide
For high power factor flyback converter with constant voltage output
Regulated mode parameters
In QRM1 and DCM, the minimum on-time variable t
on,min
(V
in
) is based on the t
on,min
parameter or t
on,min,V,out,sense
(V
in
)
variable, whichever is higher. The t
on,min,V,out,sense
(V
in
) variable denotes the minimum on-time required for V
outOV
sensing, which is dependent on V
in
measurement and the t
min,demag
parameter (see the equation in
Figure 17
).
The recommended QRM1 and DCM on-time limit related parameter configuration from
Table 6 is selected in
this design example.
Table 6 QRM1 and DCM on-time limit parameter configuration
Parameter name Recommended value Unit
t
on,max,at,V,in,low
Refer to calculation in Section 12 µs
t
on,min
1.38 µs
t
min,demag
4 µs
P
foldback,gain
1 –
In QRM1, the maximum switching frequency parameter f
sw,max
affects the number of first valley switching pulses
over every AC input half sine wave period. To lower the input current total harmonic distortion (iTHD) at high
input voltage and high output power, the number of first valley switching pulses can be reduced by lowering
f
sw,max
. However, please note that the efficiency could be reduced if the number of first valley switching pulses is
reduced too much. As a start, it is recommended to use f
sw,max
= 136 kHz and fine-tune it later if necessary.
The minimum switching frequency parameter f
sw,min
is not configurable and is fixed as 20 kHz.
14.3 ABM FB voltage sensing and control
In ABM, the switching pulse on-time t
on
and burst pulse number N
ABM
are controlled based on V
FB,filtered
taken at
the last pulse of the previous burst cycle, as shown in Figure 18 . The ABM minimum on-time and minimum
pulse number per burst are based on the t
on,min,ABM
and n
ABM,min
parameters, respectively.
During ABM burst pause, the controller enters sleep mode with the FB pin internal pull-up disabled, to reduce
the power consumption. Before the next ABM burst pulse starts, the controller wakes up with the FB pin
internal pull-up re-enabled. To avoid measuring the FB pin voltage spikes, which could present initially when
the internal pull-up is re-enabled, the start of both ABM burst pulsing and FB pin voltage sampling is delayed
upon the controller wake-up, based on the n
wakeup
parameter.
The ABM switching period is based on 1/f
sw,min
(50 µs typ.), while the the burst cycle period t
ABM
is based on
1/f
burst
, as shown in Figure 18 .
V
GD
V
FB
n
wakeup
1/f
burst
1/f
burst
1/f
burst
n
ABM
= 4
V
FB,filtered
n
wakeup
n
wakeup
n
wakeup
x 19 µs x 19 µs x 19 µs x 19 µs
n
ABM
= 4
t
on
= α
Approx. Approx. Approx. Approx.
1/f
sw,min
1/f
sw,min
1/f
sw,min
1/f
sw,min
t
on
= α
n
ABM
= 3
t
on
= β
n
ABM
= 2
t
on
= γ
n
ABM
= 3
t
on
= β
n
ABM
= 2
t
on
= γ
t
t
wake
up
sleep
wake
up
sleep sleep
wake
up
wake
up
sleep
Figure 18 ABM t
on
and n
ABM
control based on V
FB,filtered
taken at the last pulse of the previous burst cycle