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NCP1200AP60 Datasheet(PDF) 7 Page - ON Semiconductor |
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NCP1200AP60 Datasheet(HTML) 7 Page - ON Semiconductor |
7 / 16 page NCP1200A http://onsemi.com 7 APPLICATION INFORMATION Introduction The NCP1200A implements a standard current mode architecture where the switch−off time is dictated by the peak current setpoint. This component represents the ideal candidate where low part−count is the key parameter, particularly in low−cost AC−DC adapters, auxiliary supplies, etc. Due to its high−performance High−Voltage technology, the NCP1200A incorporates all the necessary components normally needed in UC384X based supplies: timing components, feedback devices, low−pass filter and self−supply. This later point emphasizes the fact that ON Semiconductor’s NCP1200A does NOT need an auxiliary winding to operate: the product is naturally supplied from the high−voltage rail and delivers a VCC to the IC. This system is called the Dynamic Self−Supply (DSS). Dynamic Self−Supply The DSS principle is based on the charge/discharge of the VCC bulk capacitor from a low level up to a higher level. We can easily describe the current source operation with a bunch of simple logical equations: POWER−ON: IF VCC < VCCH THEN Current Source is ON, no output pulses IF VCC decreasing > VCCL THEN Current Source is OFF, output is pulsing IF VCC increasing < VCCH THEN Current Source is ON, output is pulsing Typical values are: VCCH = 12 V, VCCL = 10 V To better understand the operational principle, Figure 15’s sketch offers the necessary light: Figure 15. The charge/discharge cycle over a 10 mF VCC capacitor 10.0 M 30.0 M 50.0 M 70.0 M 90.0 M VCC Current Source OFF ON OUTPUT PULSES Vripple = 2 V UVLOH = 12 V UVLOL = 10 V The DSS behavior actually depends on the internal IC consumption and the MOSFET’s gate charge Qg. If we select a MOSFET like the MTP2N60E, Qg max equals 22 nC. With a maximum switching frequency of 68 kHz for the P60 version, the average power necessary to drive the MOSFET (excluding the driver efficiency and neglecting various voltage drops) is: FSW ⋅ Qg ⋅ VCC with FSW = maximum switching frequency Qg = MOSFET’s gate charge VCC = VGS level applied to the gate To obtain the final IC current, simply divide this result by VCC: Idriver = FSW ⋅ Qg = 1.5 mA. The total standby power consumption at no−load will therefore heavily rely on the internal IC consumption plus the above driving current (altered by the driver’s efficiency). Suppose that the IC is supplied from a 350 VDC line. The current flowing through pin 8 is a direct image of the NCP1200A consumption (neglecting the switching losses of the HV current source). If ICC2 equals 2.3 mA @ TJ = 25°C, then the power dissipated (lost) by the IC is simply: 350 x 2.3 m = 805 mW. For design and reliability reasons, it would be interesting to reduce this source of wasted power which increases the die temperature. This can be achieved by using different methods: 1. Use a MOSFET with lower gate charge Qg 2. Connect pin through a diode (1N4007 typically) to one of the mains input. The average value on pin 8 becomes VMAINS(peak) @ 2 p . Our power contribution example drops to: 223 x 2.3 m = 512 mW. If a resistor is installed between the mains and the diode, you further force the dissipation to migrate from the package to the resistor. The resistor value should account for low−line startups. 3. Permanently force the VCC level above VCCH with an auxiliary winding. It will automatically disconnect the internal startup source and the IC will be fully self−supplied from this winding. Again, the total power drawn from the mains will significantly decrease. Make sure the auxiliary voltage never exceeds the 16 V limit. |
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