NCP1200AP60 데이터 시트보기 (PDF) - ON Semiconductor

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NCP1200AP60

PWM Current-Mode Controller for Universal Off-Line Supplies Featuring Low Standby Power

ON Semiconductor 

NCP1200A

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:

VCC

Vripple = 2 V

UVLOH = 12 V

UVLOL = 10 V

ON

Current

Source

OFF

OUTPUT PULSES

The DSS behavior actually depends on the internal IC

consumption and the MOSFETs 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 = MOSFETs 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

VMAINS(peak) @ 2

becomes

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 startup.

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.

10.0 M 30.0 M 50.0 M 70.0 M 90.0 M

Figure 15. The charge/discharge cycle over a

10 mF VCC capacitor

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