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NCP1200

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

ON Semiconductor 

NCP1200

APPLICATIONS INFORMATION

INTRODUCTION

The NCP1200 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 NCP1200 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 NCP1200 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 < 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 = 11.4 V, VCCON = 9.8 V

To better understand the operational principle, Figure 15’s

sketch offers the necessary light:

VCCOFF = 11.4 V

10.6 V Avg.

VCC

VCCON = 9.8 V

ON

OFF

Current

Source

Output Pulses

10.00M 30.00M 50.00M 70.00M 90.00M

Figure 15. The Charge/Discharge Cycle

Over a 10 mF VCC Capacitor

The DSS behavior actually depends on the internal IC

consumption and the MOSFET’s gate charge, Qg. If we

select a MOSFET like the MTD1N60E, Qg equals 11 nC

(max). With a maximum switching frequency of 48 kHz (for

the P40 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 driver contribution to the IC

consumption, simply divide this result by VCC: Idriver =

Fsw @ Qg = 530 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

400 V DC line. To fully supply the integrated circuit, let’s

imagine the 4 mA source is ON during 8 ms and OFF during

50 ms. The IC power contribution is therefore: 400 V . 4 mA

. 0.16 = 256 mW. If for design reasons this contribution is

still too high, several solutions exist to diminish it:

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

2

*

Vmains

p

PEAK.

Our

power

contribution

example drops to: 160 mW.

Dstart

1N4007

EMI

Filter

C3 +

4.7 mF

400 V

NCP1200

1 Adj HV 8

2 FB NC 7

3 CS VCC 6

4 GND Drv 5

Figure 16. A simple diode naturally reduces the

average voltage on pin 8

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