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NCP1201 Datasheet(PDF) 10 Page - ON Semiconductor |
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NCP1201 Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 20 page NCP1201 http://onsemi.com 10 DETAILED OPERATING DESCRIPTION Introduction The NCP1201 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 criteria, particularly in low−cost AC−DC adapters, auxiliary supplies etc. Due to its high−performance High−Voltage technology, the NCP1201 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 NCP1201 does NOT need an auxiliary winding to operate: the device is self supplied from the high−voltage rail and delivers a VCC to the IC. This system is named 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 following simple logic equations: POWER−ON: IF VCC < VCCOFF THEN Current Source is ON, no output pulses IF VCC decreasing > VCCON THEN Current Source is OFF, output is pulsing IF VCC increasing < VCCOFF THEN Current Source is ON, output is pulsing Typical values are: VCCOFF = 12.5 V, VCCON = 10.5 V To better understand the operation principle, Figure 27 sketch offers the necessary explanation, Figure 27. The Charge/Discharge Cycle Over a 10 mF VCC Capacitor 10 mS 30 mS 50 mS 70 mS 90 mS Current Source OFF VCC Output Pulses Vripple = 2 V VCCOFF = 12.5 V VCCON = 10.5 V ON 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 70 kHz for the oscillator 60 kHz, the average power necessary to drive the MOSFET (excluding the driver efficiency and neglecting various voltage drops) is: Pdriver + Fsw(max) Qg VCC (eq. 1) Where, Pdriver = Average Power to drive the MOSFET Fsw(max) = Maximum switching frequency Qg = MOSFET’s gate charge VCC = VGS level applied to the gate of the MOSFET To obtain an estimation of the driving current, simply divide Pdriver by VCC, Idriver + Fsw(max) Qg + 1.54 mA (eq. 2) The total standby power consumption at no−load will therefore heavily rely on the internal IC current consumption plus the 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 NCP1201 current consumption (neglecting the switching losses of the HV current source). If ICC2 equals 2.1 mA @ TA = 25°C, then the power dissipated (lost) by the IC is simply: 350 V x 2.1 mA = 735 mW. For design and reliability reasons, it would be interesting to reduce this source of wasted power. In order to achieve that, different methods can be used. 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: VmainsPEAK 2 p (eq. 3) |
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