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NCP1015ST65T3G Datasheet(PDF) 10 Page - ON Semiconductor |
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NCP1015ST65T3G Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 22 page NCP1015 http://onsemi.com 10 Plugging Equation 7 and Equation 8 into Equation 6 leads to <Vds(t)> = Vin and thus: PDSS + Vin @ ICC1 (eq. 9) The worse case occurs at high line, when Vin equals 370 Vdc. With ICC1 = 1.2 mA (65 kHz version), we can expect a DSS dissipation around 440 mW. If you select a higher switching frequency version, the ICC1 increases and it is likely that the DSS consumption exceeds 500 mW. In that case, we recommend adding an auxiliary winding in order to offer more dissipation room to the power MOSFET. Please read application note AND8125/D “Evaluating the power capability of the NCP101X members” to help selecting the right part / configuration for your application. Lowering the Standby Power with an Auxiliary Winding The DSS operation can bother the designer when a) its dissipation is too high b) extremely low standby power is a must. In both cases, one can connect an auxiliary winding to disable the self−supply. The current source then ensures the startup sequence only and stays in the off state as long as VCC does not drop below VCC(on) or 7.5 V. Figure 17 shows that the insertion of a resistor (Rlimit) between the auxiliary dc level and the VCC pin is mandatory a) not to damage the internal 8.7 V zener diode during an overshoot for instance (absolute maximum current is 15 mA) b) to implement the fail−safe optocoupler protection as offered by the active clamp. Please note that there cannot be bad interaction between the clamping voltage of the internal zener and VCC(off) since this clamping voltage is actually built on top of VCC(off) with a fixed amount of offset (200 mV typical). Self−supplying controllers in extremely low standby applications often puzzles the designer. Actually, if a SMPS operated at nominal load can deliver an auxiliary voltage of an arbitrary 16 V (Vnom), this voltage can drop to below 10 V (Vstby) when entering standby. This is because the recurrence of the switching pulses expands so much that the low frequency re−fueling rate of the VCC capacitor is not enough to keep a proper auxiliary voltage. Figure 18 shows a typical scope shot of a SMPS entering deep standby (output un−loaded). So care must be taken when calculating Rlimit 1) to not excess the maximum pin current in normal operation but 2) not to drop too much voltage over Rlimit when entering standby. Otherwise the DSS could reactivate and the standby performance would degrade. We are thus able to bound Rlimit between two equations: Vnom * Vclamp Itrip v Rlim v Vstby * VCC(on) ICC1 (eq. 10) Where: Vnom is the auxiliary voltage at nominal load Vstdby is the auxiliary voltage when standby is entered Itrip is the current corresponding to the nominal operation. It thus must be selected to avoid false tripping in overshoot conditions. ICC1 is the controller consumption. This number slightly decreases compared to ICC1 from the spec since the part in standby does almost not switch. VCC(on) is the level above which Vaux must be maintained to keep the DSS in the OFF mode. It is good to shoot around 8 V in order to offer an adequate design margin, e.g. to not re−activate the startup source (which is not a problem in itself if low standby power does not matter) Since Rlimit shall not bother the controller in standby, e.g. keep Vaux to around 8 V (as selected above), we purposely select a Vnom well above this value. As explained before, experience shows that a 40% decrease can be seen on auxiliary windings from nominal operation down to standby mode. Let’s select a nominal auxiliary winding of 20 V to offer sufficient margin regarding 8 V when in standby (Rlimit also drops voltage in standby). Plugging the values in Equation 10 gives the limits within which Rlimit shall be selected: 20 * 8.7 6.3 m v Rlimit v 12 * 8 1.1 m (eq. 11) that is to say: 1.8 k W < Rlimit < 3.6 kW. If we are designing a power supply delivering 12 V, then the ratio auxiliary/power must be: 12 / 20 = 0.6. The ICC current has to not exceed 6.4 mA. This will occur when Vaux grows−up to: 8.7 V + 1.8 k x (6.4 m + 1.1 m) = 22.2 V for the first boundary or 8.7 V + 3.6 k x (6.4 m +1.1 m) = 35.7 V for second boundary. On the power output, it will respectively give 22.6 x 0.6 = 13.3 V and 35.7 x 0.6 = 21.4 V. As one can see, tweaking the Rlimit value will allow the selection of a given overvoltage output level. Theoretically predicting the auxiliary drop from nominal to standby is an almost impossible exercise since many parameters are involved, including the converter time constants. Fine tuning of Rlimit thus requires a few iterations and experiments on a breadboard to check Vaux variations but also output voltage excursion in fault. Once properly adjusted, the fail−safe protection will preclude any lethal voltage runaways in case a problem would occur in the feedback loop. |
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