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NCP1200D Datasheet(PDF) 10 Page - ON Semiconductor |
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NCP1200D Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 16 page NCP1200A http://onsemi.com 10 Power Dissipation The NCP1200A is directly supplied from the DC rail through the internal DSS circuitry. The average current flowing through the DSS is therefore the direct image of the NCP1200A current consumption. The total power dissipation can be evaluated using: (VHVDC − 11 V) ⋅ ICC2. If we operate the device on a 250 VAC rail, the maximum rectified voltage can go up to 350 VDC. However, as the characterization curves show, the current consumption drops at high junction temperature, which quickly occurs due to the DSS operation. At TJ = 50°C, ICC2 = 1.7 mA for the 61 kHz version over a 1 nF capacitive load. As a result, the NCP1200A will dissipate 350 . 1.7 mA@TJ = 50°C = 595 mW. The SOIC−8 package offers a junction−to−ambient thermal resistance RqJA of 178°C/W. Adding some copper area around the PCB footprint will help decreasing this number: 12 mm x 12 mm to drop RqJA down to 100 °C/W with 35 m copper thickness (1 oz.) or 6.5 mm x 6.5 mm with 70 m copper thickness (2 oz.). With this later number, we can compute the maximum power dissipation the package accepts at an ambient of 50 °C: Pmax + TJmax * TAmax R qJA + 750 mW which is okay with our previous budget. For the DIP8 package, adding a min−pad area of 80 mm @ of 35 m copper (1 oz.), RqJA drops from 100 °C/W to about 75°C/W. In the above calculations, ICC2 is based on a 1 nF output capacitor. As seen before, ICC2 will depend on your MOSFET’s Qg: ICC2 ≈ ICC1 + FSW x Qg. Final calculation shall thus accounts for the total gate−charge Qg your MOSFET will exhibit. The same methodology can be applied for the 100 kHz version but care must be taken to keep TJ below the 125°C limit with the D100 (SOIC) version and activated DSS in high−line conditions. If the power estimation is beyond the limit, other solutions are possible a) add a series diode with pin 8 (as suggested in the above lines) and connect it to the half rectified wave. As a result, it will drop the average input voltage and lower the dissipation to: 350 @ 2 p @ 1.7 m + 380 mW b) put an auxiliary winding to disable the DSS and decrease the power consumption to VCC x ICC2. The auxiliary level should be thus that the rectified auxiliary voltage permanently stays above 10 V (to not re−activate the DSS) and is safely kept below the 16 V maximum rating. Overload Operation In applications where the output current is purposely not controlled (e.g. wall adapters delivering raw DC level), it is interesting to implement a true short−circuit protection. A short−circuit actually forces the output voltage to be at a low level, preventing a bias current to circulate in the optocoupler LED. As a result, the FB pin level is pulled up to 4.2 V, as internally imposed by the IC. The peak current setpoint goes to the maximum and the supply delivers a rather high power with all the associated effects. Please note that this can also happen in case of feedback loss, e.g. a broken optocoupler. To account for this situation, NCP1200A hosts a dedicated overload detection circuitry. Once activated, this circuitry imposes to deliver pulses in a burst manner with a low duty cycle. The system auto−recovers when the fault condition disappears. During the startup phase, the peak current is pushed to the maximum until the output voltage reaches its target and the feedback loop takes over. This period of time depends on normal output load conditions and the maximum peak current allowed by the system. The time−out used by this IC works with the VCC decoupling capacitor: as soon as the VCC decreases from the UVLOH level (typically 12 V) the device internally watches for an overload current situation. If this condition is still present when the UVLOL level is reached, the controller stops the driving pulses, prevents the self−supply current source to restart and puts all the circuitry in standby, consuming as little as 350 mA typical (ICC3 parameter). As a result, the VCC level slowly discharges toward 0. |
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Similar Description - NCP1200D |
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