Electronic Components Datasheet Search |
|
LT1248C Datasheet(PDF) 10 Page - Linear Technology |
|
LT1248C Datasheet(HTML) 10 Page - Linear Technology |
10 / 12 page 10 LT1248 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 180 µFKMH400Vcapacitor.Theexpectedlifeofthe output capacitor may be calculated from the thermal stress analysis: 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: For longer life, a capacitor with a higher ripple current rating or parallel capacitors should be used. (105 °C+∆TK) – (TA+∆TO) S APPLICATI 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 0.01 µF 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 100 µF 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 180 µF 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 √ √ L = LO • 2 LO ≈ 2000 • 2 ≈ 57,000 hours 10 (105 °C+10°C) – (60°+ 6.6°C) 10 |
Similar Part No. - LT1248C |
|
Similar Description - LT1248C |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.COM |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Datasheet Upload | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |