Datasheet
SLUS542F − OCTOBER 2003 − REVISED JULY 2009
23
www.ti.com
ADDITIONAL APPLICATION INFORMATION
The UCC2891 family of controllers is dedicated to control current mode active clamp flyback or forward
converters in an isolated power supply. The key advantage of the active clamp topologies is the zero voltage
switching (ZVS) of the primary side semiconductors. This operating mode reduces the switching losses of the
converter, thus facilitates higher switching frequencies or improves efficiency when operated at similar
frequencies as its hard switched designs. The simplified schematics below demonstrate the typical
implementations of these converters.
This active clamp flyback converter shown in Figure 8 highlights a high-side clamp circuit using an N-channel
MOSFET transistor as the auxiliary clamp switch.
UDG−03153
C
BIAS
+V
IN
−V
IN
C
CLAMP
Q
AUX
Q
MAIN
R
CS
9
7
16
14
13
12
FB
VIN
CS
GND
AUX
VDD
OUT
N−Channel
Gate Drive
Secondary−Side
Error Amplifier
and Isolation
Synchronous
Rectifier
Control
6
UCC2893
Bootstrap
Bias
Load
C
IN
R
OT
Figure 8. Zero Voltage Switching Flyback Application
C
BIAS
+V
IN
−V
IN
C
CLAMP
Q
AUX
Q
MAIN
R
CS
C
IN
9
7
14
12
13
FB
VIN
CS
GND
OUT
VDD
AUX
Secondary−Side
Error Amplifier
and Isolation
Synchronous
Rectifier
Control
6
UCC2891
Bootstrap
Bias
Load
P−Channel
Gate Drive
16
UDG−03154
R
OT
Figure 9. Active Clamp Forward Converter