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CS5308 データシート(PDF) 21 Page - ON Semiconductor |
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CS5308 データシート(HTML) 21 Page - ON Semiconductor |
21 / 31 page CS5308 http://onsemi.com 21 QG is the MOSFET total gate charge to obtain RDS(on). Commonly specified in the data sheet. Vg is the gate drive voltage. QRR is the reverse recovery charge of the lower MOSFET. Qoss is the MOSFET output charge specified in the data sheet. For the lower or synchronous MOSFET, the power dissipation can be approximated from: PD,SYNCH + (IRMS,SYNCH2 @ RDS(on)) ) (Vfdiode @ IO,MAX 2 @ t_nonoverlap @ fSW) (26) The first term represents the conduction or IR losses when the MOSFET is ON and the second term represents the diode losses that occur during the gate non−overlap time. All terms were defined in the previous discussion for the control MOSFET with the exception of: (27) IRMS,SYNCH + 1 * D @ [(ILo,MAX2 ) ILo,MAX @ ILo,MIN ) ILo,MIN2) 3]1 2 where: Vfdiode is the forward voltage of the MOSFET’s intrinsic diode at the converter output current. t_nonoverlap is the non−overlap time between the upper and lower gate drivers to prevent cross conduction. This time is usually specified in the data sheet for the control IC. + − w RCS1 CS1 CCS1 L1 0 A GVDRP + − RCS2 CS2 CCS2 L2 0 A GVDRP CSREF COMP Error Amp VID Setting IBIASVFB RDRP RVFBK VDRP = VID VFB = VID VCORE IDRP = 0 IFBK = IBIASVFB VCORE = VID + IBIASVFB w RVFBk Figure 17. AVP Circuitry at No−Load + − When the MOSFET power dissipations are known, the designer can calculate the required thermal impedance to maintain a specified junction temperature at the worst case ambient operating temperature qT t (TJ * TA) PD (28) where: qT is the total thermal impedance (qJC + qSA); qJC is the junction−to−case thermal impedance of the MOSFET; qSA is the sink−to−ambient thermal impedance of the heatsink assuming direct mounting of the MOSFET (no thermal “pad” is used); TJ is the specified maximum allowed junction temperature; TA is the worst case ambient operating temperature. For TO−220 and TO−263 packages, standard FR−4 copper clad circuit boards will have approximate thermal resistances ( qSA) as shown below: Pad Size (in2/mm2) Single−Sided 1 oz. Copper 0.50/323 60−65°C/W 0.75/484 55−60°C/W 1.00/645 50−55°C/W 1.50/968 45−50°C/W As with any power design, proper laboratory testing should be performed to insure the design will dissipate the required power under worst case operating conditions. Variables considered during testing should include maximum ambient temperature, minimum airflow, maximum input voltage, maximum loading, and component variations (i.e., worst case MOSFET RDS(on)). Also, the inductors and capacitors share the MOSFET’s heatsinks and will add heat and raise the temperature of the circuit board and MOSFET. For any new design, its advisable to have as |
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同様の説明 - CS5308 |
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