IEEE Power Electronics Magazine - December 2020 - 39
ACF flyback converter are significantly lower than 200 V,
often practically achieving near-ZVS operation, reducing
switching losses and the EMI. In summary, ACF flyback
has several advantages over conventional RCD snubberbased flyback converters, including efficiency improvement and EMI reduction, both of these are considered as
the enabling technologies for high density power adapters.
In an ACF converter, the leakage energy stored in the
transformer is recycled at the output, improving the power
processing efficiency. Furthermore, the peak voltage across
the primary switch during the turn off events is significantly reduced (~80%), allowing a more aggressive turns
ratio between the primary and secondary winding of the
transformer, which in return allows utilizing a lower voltage rating synchronous rectifier switch on the secondary side. Lower voltage rating of the switch translates
into both lower cost and/or lower on-resistance, reducing conduction losses. In addition, the QR valleys in the
Power Adapter Applications
Modern electronic devices are designed with larger capacity
batteries and faster charging needs, requiring more power
Vbulk + 2 ∗nVout
Vds
Vbulk + nVout
High Voltage
Spike
Vbulk - nVout
High QR Valley
t
FIG 4 Drawback with a conventional RCD snubber in a conventional flyback converter. (Source: Silanna Semiconductor;
used with permission.)
Lk
Lk
+
ip
Cclamp
1
Lm
SWC
Vbulk
Off
ic
Gac
ip
+
N:1
-
Vc
+
Cclamp
ilm
3 V
bulk
On
N:1
-
Vc
+
Lm
On
ic
SWC
Gac
G1 SW
pri
-
-
G1 SW
pri
Off
Current Changes Direction and Stored Energy Is
Recycled to the Output
Lk and Lm Are Energized
Lk
Cclamp
2
-
Vc
+
Vbulk
+
N:1
Lm
SWC
Cclamp
ilm
ic
Off
Gac
-
Lk
ip
+
ilm
G1 SW
pri
Off
Current Commutes Through SWC Body Diode
and Lk Energy Is Stored in Cclamp
4
Vbulk
ip
-
Lm
Vc
+
SWC
Off
Gac
-
N:1
ilm
ic
G1 SW
pri
Coss
Off
Current Discharges Coss of SWpri To
Achieve Near ZVS
FIG 5 Fundamental active clamp flyback operation. (Source: Silanna Semiconductor; used with permission.)
December 2020
z IEEE POWER ELECTRONICS MAGAZINE
39
IEEE Power Electronics Magazine - December 2020
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