NCP1201 datasheet(10/20 Pages) ONSEMI

Part #	NCP1201

Description	PWM Current-Mode Controller for Universal Off-Line Supplies Featuring Low Standby Power with Fault Protection Modes

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NCP1201

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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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