ACT412
Rev 1, 30-Oct-13
TYPICAL APPLICATION CONT’D
An EE16 core is selected for the transformer. From
the manufacture’s catalogue recommendation, the
core with an effective area AE is 19.2mm2 .The turn
of the primary winding is:
down the turn ratio and inductance.
(21)
Np = 68T ,Ns = 12T ,NA = 17T ,Lp = 0.5mH
PCB Layout Guideline
Ilim × Lp _ max
AE × Bmax
0.75 × 0.00073
NP ≥
=
= 95T
Good PCB layout is critical to have optimal
performance. Decoupling capacitor (C4) and
feedback resistor (R5/R6) should be placed close to
VDD and FB pin respectively. There are two main
power path loops. One is formed by C1/C2, primary
winding, Mosfet transistor and current sense
resistor (R9). The other is secondary winding,
rectifier D8 and output capacitors (C5/C6). Keep
these loop areas as small as possible. Connecting
high current ground returns, the input capacitor
ground lead, and the ACT412 GND pin to a single
point (star ground configuration).
19.2 × 3000 ×10−10
(13)
(14)
Ilim = 2 × Ip _ FL = 2 × 375 mA ≈ 750 mA
The turns of secondary and auxiliary winding
can be derived accordingly:
N p
N s = N p
/
= 95 / 5.53 ≈ 17 T
N S
(15)
N A
N A
=
× N s = 1.4 × 17 ≈ 24 T
N S
Determining the value of the current sense resistor
(R9) uses the peak current in the design. Since the
ACT412 internal current limit is set to 1V, the
design of the current sense resistor is given by:
VCS
Ilim
1
RCS
=
=
≈ 1 .3 .Ω
(16)
0 .75
The voltage feedback resistors are selected
according to frequency at full load. The design
frequency at full load is given by:
N p
N s
Rfb1 × Rfb 2
Rfb1 + Rfb 2
VO + VD
fs
=
×
×
Vcs
(17)
Lp ×
× K f
_ sw
Rcs
The design Vo is given by:
Rfb1
Rfb2
Ns
Na
Vo = (1 +
)×
×VFB −VD
(18)
Where k is IC constant and K=0.0000065, then we
can get the value:
Rfb1 = 45 .3 K ,Rfb 2 = 6.9K
(19)
When selecting the output capacitor, a low
ESR electrolytic capacitor is recommended to
minimize ripple from the current ripple. The
approximate equation for the output capacitance
value is given by:
IOUT
1.8
COUT
=
=
= 327μF
fsw ×V
110k ×50mV
(20)
RIPPLE
Two 330µF electrolytic capacitors are used to keep
the ripple small.
In fact, consider of transformer production, we scale
Innovative PowerTM
- 11 -
www.active-semi.com
Active-Semi Proprietary―For Authorized Recipients and Customers
Copyright © 2013 Active-Semi, Inc.
ActivePSRTM is a trademark of Active-Semi.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
