A704
In this design, the Vbridge is 425 V and IFSM is 30A. So the ESR of the chock should > (425/30)=14.1Ω. If the
power loss of such high ESR chock is too high, the designer can replace the high ESR chock with a low ESR chock
and add a negative-temperature-coefficient (NTC) resistor to limit the inrush current. As long as the (NTC (hot)
resistance + low ESR Chock resistance) is lower than the resistance of the high ESR chock, the efficiency of the A704
DC-DC can be improved.
The hold-up and input filter capacitor required at the diode bridge output have to be calculated at the minimum
AC input voltage. The minimum capacitor value can be calculated as:
Vo,max × Io,max ×(1− 2× freq×tc )
(2×Vm2in,ac −Vm2in,dc )×η × freq
C1≥
In this design,
freq
freq
Where,
is the AC input frequency, as a rule,
is 50 ~ 60Hz.
is the conduction angle of the AC input, use 45°
tc
η is efficiency of the system , and
conduction angle if unknown.
12×0.35×(1− 0.5)
≥ 5.4uF
C1≥
(2×902 −902 )×0.8×60
The voltage rating of the capacitor should be at least 1.15 times greater than the peak input voltage for a safety
margin. For example, the input 220Vac input, the input capacitor voltage should exceed than as:
V
≥ 1.15 × 2 ×V
⇒ V
≥ 360V
max,ac
max,cap
max,ac
Choose a 400V/ 10µF electrolytic capacitor in the example.
Such electrolytic capacitor has sizable ESR component. The large ESR of these capacitors makes it inappropriate
to absorb the high frequency ripple current generated by the buck converter. Thus, adding a small MLCC capacitor in
parallel with the electrolytic capacitor is recommended.
4. Choose the power choke
The inductor selection should make the Buck converter work in CCM; the inductor value depends on the ripple
current in the LEDs. For example, assume a +/- 30% ripple current in the LEDs. Then, the inductor can be calculated
as:
V (1− D)T
o
s
L =
0.6I ×η
o
In this design example, Ts is 16.6µ s, D=0.12, Vo=12Vdc , η =0.8, Io=0.35
12× (1− 0.12)×16.6µ
L =
= 1.05mH
0.6× 0.35× 0.8
Choose the power choke inductance is 1mH.
Contack Info:Samsun Zhang 13556850583 IC9898@163.com
Copyright © 2008 ADDtek Corp.
9
A704_V0.6 -- AUGUST 2008
ADDTEK [ ADDTEK CORP ]
