10
LT1248
The third component is the switching ripple from the load,
if the load is a switching regulator.
I
3RMS
≈ I
LOAD(DC)
For the United Chemicon KMH 400V capacitor series,
ripple current multiplier for currents at 100kHz is 1.43. The
equivalent 120Hz ripple current can be then found:
I
RMS
= (I
1RMS
)
2
+ (I
2RMS
/1.43)
2
+ (I
3RMS
/1.43)
2
For a typical system that runs at an average load of 200W
and 385V output:
I
LOAD(DC)
= 0.52A
I
1RMS
≈ 0.71 • 0.52A = 0.37A
I
2RMS
≈ 0.82A at 120VAC
I
3RMS
≈ I
LOAD(DC)
= 0.52A
I
RMS
= (0.37A)
2
+(0.82A/1.43)
2
+(0.52A/1.43)
2
= 0.77A
The 120Hz ripple current rating at 105°C ambient is 0.95A
for the 180µF KMH 400V capacitor. The expected life of the
output capacitor may be calculated from the thermal
stress analysis:
where:
L: expected life time
L
O
: hours of load life at rated ripple current and rated
ambient temperature.
∆T
K
: Capacitor internal temperature rise at rated condi-
tion. ∆T
K
= (I
2
R)/(KA). Where I is the rated current,
R is capacitor ESR, and KA is a volume constant.
T
A
: Operating ambient temperature.
∆T
O
: Capacitor internal temperature rise at operating
condition.
In our example L
O
= 2000 hours and ∆T
K
= 10°C at rated
0.95A. ∆T
O
can then be
calculated from:
∆T
K
= (I
RMS
/0.95A)
2
•
∆T
K
=
(0.77A/0.95A)
2
•
10°C = 6.6°C
Assuming the operating ambient temperature is 60°C, the
approximate life time is:
For longer life, a capacitor with a higher ripple current
rating or parallel capacitors should be used.
(105°C+∆T
K
) – (T
A
+∆T
O
)
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
V
OUT
/(V
CC
– 2V) = N
P
/N
S
.
For 382V V
OUT
and 18V V
CC
, Np/Ns ≈ 19.
In Figure 6, a new technique for supply voltage eliminates
the need for an extra inductor winding. It uses capacitor
charge transfer to generate a constant current source
which feeds a Zener diode. Current to the Zener is equal
to (V
OUT
– V
Z
)(C)(f), where V
Z
is Zener voltage and f is
switching frequency. For V
OUT
= 382V, V
Z
= 18V, C =
1000pF, and f = 100kHz, Zener current will be 36mA. This
is enough to operate the LT1248, including the FET gate
drive. Normally soft-start is not needed because the
LT1248 has overcurrent limit and overvoltage protection.
If soft-start is used with a 0.01µF capacitor on SS pin,
V
OUT
ramps up slower during start-up. Then C4 has to
hold V
CC
longer, and the circuit may not start. Increasing
C4 to 100µF ensures start-up, but start-up time will be
extended if the same 90k trickle charge resistor is used.
Output Capacitor
The peak-to-peak 120Hz output ripple is determined by:
V
P-P
= (2) (I
LOAD(DC)
)(Z)
where I
LOAD(DC)
: DC load current.
Z: capacitor impedance at 120Hz.
For 180µF at 300W load, I
LOAD(DC)
= 300W/385V = 0.78A,
V
P-P
= 2 • 0.78A • 7.4Ω = 11.5V. If less ripple is desired,
higher capacitance should be used. The selection of the
output capacitor should also be based on the operating
ripple current through the capacitor. The ripple current
can be divided into three major components. The first is at
120Hz; it’s RMS value is related to the DC load current as
follows:
I
1RMS
≈ 0.71 • I
LOAD(DC)
The second component contains the PF switching fre-
quency ripple current and its harmonics. Analysis of the
ripple is complicated because it is modulated with a 120Hz
signal. However computer numerical integration and Fou-
rier analysis approximate the RMS value reasonably close
to the bench measurements. The RMS value is about 0.82A
at a typical condition of 120VAC, 200W load. This ripple is
line-voltage dependent, and the worst case is at low line.
I
2RMS
= 0.82A at 120VAC, 200W
√
√
L = L
O
•
2
L
O
≈ 2000 • 2
≈ 57,000 hours
10
(105°C+10°C) – (60°+6.6°C)
10
