LT1248 データシートの表示（PDF） - Linear Technology

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LT1248

Power Factor Controller

Linear Technology 

LT1248

APPLICATI S I FOR ATIO

VOUT/(VCC – 2V) = NP/NS.

For 382V VOUT and 18V VCC, 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 (VOUT – VZ)(C)(f), where VZ is Zener voltage and f is

switching frequency. For VOUT = 382V, VZ = 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,

VOUT ramps up slower during start-up. Then C4 has to

hold VCC 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:

VP-P = (2) (ILOAD(DC))(Z)

where ILOAD(DC): DC load current.

Z: capacitor impedance at 120Hz.

For 180µF at 300W load, ILOAD(DC) = 300W/385V = 0.78A,

VP-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:

I1RMS ≈ 0.71 • ILOAD(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.

I2RMS = 0.82A at 120VAC, 200W

The third component is the switching ripple from the load,

if the load is a switching regulator.

I3RMS ≈ ILOAD(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:

√ IRMS = (I1RMS)2 + (I2RMS/1.43)2 + (I3RMS/1.43)2

For a typical system that runs at an average load of 200W

and 385V output:

ILOAD(DC) = 0.52A

I1RMS ≈ 0.71 • 0.52A = 0.37A

I2RMS ≈ 0.82A at 120VAC

I3RMS ≈ ILOAD(DC) = 0.52A

√ IRMS = (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:

(105°C+∆TK) – (TA+∆TO)

L = LO • 2

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where:

L: expected life time

LO: hours of load life at rated ripple current and rated

ambient temperature.

∆TK: Capacitor internal temperature rise at rated condi-

tion. ∆TK = (I2R)/(KA). Where I is the rated current,

R is capacitor ESR, and KA is a volume constant.

TA: Operating ambient temperature.

∆TO: Capacitor internal temperature rise at operating

condition.

In our example LO = 2000 hours and ∆TK = 10°C at rated

0.95A. ∆TO can then be calculated from:

∆TK = (IRMS/0.95A)2 • ∆TK = (0.77A/0.95A)2 • 10°C = 6.6°C

Assuming the operating ambient temperature is 60°C, the

approximate life time is:

(105°C +10°C) – (60°+ 6.6°C)

LO ≈ 2000 • 2

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≈ 57,000 hours

For longer life, a capacitor with a higher ripple current

rating or parallel capacitors should be used.

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