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UCY2G150MPD Datasheet(PDF) 5 Page - Diodes Incorporated |
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UCY2G150MPD Datasheet(HTML) 5 Page - Diodes Incorporated |
5 / 12 page AN75 Issue 1 – January 2011 5 www.diodes.com © Diodes Incorporated 2010 Maximum drain-source voltage stress on the power MOSFET for this converter is equal to the input voltage. However, a typical voltage safety margin for the MOSFET defines the maximum reverse voltage as follows, V 485 V 373 3 . 1 V 3 . 1 V (max) IN DSS = × = × = which implies that a common 500V MOSFET is suitable. The power MOSFET losses will be dominated by switching loss. The switching loss depends on the switching time, frequency, MOSFET drain current and drain-source voltage. The switching rise time t RISE and fall time tFALL is a function of the MOSFET’s gate capacitance, the gate driver capability of the AL9910 and layout design. The worse case switching power losses occurs at V LED(min) and VIN(max). The switching loss is approximately, () 455mW 2 63.8kHz 65ns 373V 2 63.8kHz 65ns 88mA 297mA 373V 2 f t I V 2 f t L t V I V P swi(max) FALL pk IN(max) swi(max) RISE BUCK OFF LED(min) pk IN(max) SW = × × + × × − × = × × × + × × ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ − × = where the switching time t RISE and t FALL are measured to be 65ns with the 600V MOSFET SPB03N60S5 as the power MOSFET. As shown in Figure 1, R10 is a series gate resistor that slows down the MOSFET switching and reduces EMI emission. The RMS current through the MOSFET at V LED(min) and VIN(max) is given by, mA 89 12 mH 6 . 6 s μ 9 . 13 V 42 mA 240 V 373 V 42 12 L t V I V V I BUCK OFF (min) LED ) nom ( LED (max) IN (min) LED D(RMS) = ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ × + × = ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ × + × = The power MOSFET conduction loss depends on its static drain-source resistance R DS(ON) at the MOSFET working temperature. It is possible to calculate the continuous conduction loss: () mW 19 5 . 2 mA 89 R I P 2 DS(ON) 2 D(RMS) COND = Ω × = × = The total power MOSFET loss is: mW 474 mW 19 mW 455 P P P COND SW TOT = + = + = Total MOSFET power loss is dissipated from the SMD package into the PC Board. So it is possible to calculate the MOSFET working junction temperature can be calculated if the package junction-to- ambient thermal resistance R thJA is known. The calculated MOSFET junction temperature, TJ, must be lower then the maximum allowable junction temperature T J(MAX): C 4 . 109 C 80 W C 62 mW 474 T θ P T AMB thJA TOT J o o o = + × = + × = The internal ambient temperature within the LED converter, T AMB, is assumed to be 80ºC. θthJA = W C 62 o is the thermal resistance for TO-263 with minimum copper area. For practical design, it is recommended to keep the junction temperature below 110ºC to avoid temperature stress on the device. |
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