Datasheet

 
 

SLUS542F − OCTOBER 2003 − REVISED JULY 2009
19
www.ti.com
APPLICATION INFORMATION: SETUP GUIDE
The UCC2891 family offers a highly integrated feature set and excellent accuracy to control an active clamp
forward or active clamp flyback power converter. In order to take advantage of all the benefits integrated in these
controllers, the following procedure can simplify the setup and avoid unnecessary iterations in the design
procedure. Refer to Figure 7 setup diagrams for component names.
Before the controller design begins, the power stage design must be completed. From the power stage design
the following operating parameters are needed to complete the setup procedure of the controller:
D Switching frequency (f
SW
)
D Maximum operating duty cycle (D
MAX
)
D Soft start duration (t
SS
)
D Gate drive power requirements of the external power MOSFETs (Q
G(main)
, Q
G(aux)
)
D Bias method and voltage for steady state operation (bootstrap or bias supply)
D Gate drive turn-on delay (t
DEL
)
D Turn−on input voltage threshold (V
ON
)
D Minimum operating input voltage (V
OFF
) where V
IN (off)
< V
IN(on)
D Maximum operating input voltage (V
OVP
)
D overvoltage protection hysteresis (V
OVH
)
D The down slope of the output inductor current waveform reflected across the primary side current sense
resistor
ǒ
dV
L
ńdt
Ǔ
Step 1. Oscillator
The two timing elements of the oscillator can be calculated from f
SW
and D
MAX
by the following two equations:
R
ON
+
t
ON
37.33 10
*12
ǒ
W
s
Ǔ
+
D
MAX
f
SW
37.33 10
*12
ǒ
W
s
Ǔ
R
OFF
+
t
OFF
16 10
*12
ǒ
W
s
Ǔ
+
1 * D
MAX
f
SW
16 10
*12
ǒ
W
s
Ǔ
where D
MAX
is a dimensionless number between 0 and 1.
Step 2. Soft Start
Once R
ON
is defined, the charge current of the soft-start capacitor can be calculated as:
I
SS
+ 0.43
V
REF
2
1
R
ON
During soft start, C
SS
is being charged from 0 V to 5 V by the calculated I
SS
current. The actual control range
of the soft-start capacitor voltage is between 1.25 V and 4.5 V. Therefore, the soft-start capacitor value must
be based on this narrower control range and the required start up time (t
SS
) according to:
C
SS
+
I
SS
t
SS
4.5 V * 1.25 V
(9)
(10)
(11)
(12)