NCP1207A(2004) 查看數據表（PDF） - ON Semiconductor

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NCP1207A
(Rev.:2004)

PWM Current-Mode Controller for Free Running Quasi-Resonant Operation

ON Semiconductor 

NCP1207A

APPLICATION INFORMATION

Introduction

The NCP1207A implements a standard current mode

architecture where the switch−off time is dictated by the

peak current setpoint whereas the core reset detection

triggers the turn−on event. This component represents the

ideal candidate where low part−count is the key parameter,

particularly in low−cost AC/DC adapters, consumer

electronics, auxiliary supplies, etc. Thanks to its

high−performance High−Voltage technology, the

NCP1207A incorporates all the necessary components /

features needed to build a rugged and reliable Switch−Mode

Power Supply (SMPS):

• Transformer core reset detection: borderline / critical

operation is ensured whatever the operating conditions

are. As a result, there are virtually no primary switch

turn−on losses and no secondary diode recovery losses.

The converter also stays a first−order system and

accordingly eases the feedback loop design.

• Quasi−resonant operation: by delaying the turn−on

event, it is possible to re−start the MOSFET in the

minimum of the drain−source wave, ensuring reduced

EMI / video noise perturbations. In nominal power

conditions, the NCP1207A operates in Borderline

Conduction Mode (BCM) also called Critical

Conduction Mode.

• Dynamic Self−Supply (DSS): due to its Very High

Voltage Integrated Circuit (VHVIC) technology,

ON Semiconductor’s NCP1207A allows for a direct pin

connection to the high−voltage DC rail. A dynamic

current source charges up a capacitor and thus provides

a fully independent VCC level to the NCP1207A. As a

result, there is no need for an auxiliary winding whose

management is always a problem in variable output

voltage designs (e.g. battery chargers).

• Overvoltage Protection (OVP): by sampling the plateau

voltage on the demagnetization winding, the

NCP1207A goes into latched fault condition whenever

an over−voltage condition is detected. The controller

stays fully latched in this position until the VCC is

cycled down 4.0 V, e.g. when the user un−plugs the

power supply from the mains outlet and re−plugs it.

• External latch trip point: by externally forcing a level

on the OVP greater than the internal setpoint, it is

possible to latchoff the IC, e.g. with a signal coming

from a temperature sensor.

• Adjustable skip cycle level: by offering the ability to

tailor the level at which the skip cycle takes place, the

designer can make sure that the skip operation only

occurs at low peak current. This point guarantees a

noise−free operation with cheap transformer. This

option also offers the ability to fix the maximum

switching frequency when entering light load

conditions.

• Overcurrent Protection (OCP): by continuously

monitoring the FB line activity, NCP1207A enters burst

mode as soon as the power supply undergoes an

overload. The device enters a safe low power operation

which prevents from any lethal thermal runaway. As

soon as the default disappears, the power supply

resumes operation. Unlike other controllers, overload

detection is performed independently of any auxiliary

winding level. In presence of a bad coupling between

both power and auxiliary windings, the short circuit

detection can be severely affected. The DSS naturally

shields you against these troubles.

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 some

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 = 12 V, VCCON = 10 V

To better understand the operational principle, Figure 12’s

sketch offers the necessary light.

VRIPPLE = 2 V

VCCOFF = 12 V

VCCON = 10 V

ON

OFF

Output Pulses

Figure 12. The Charge/Discharge Cycle Over a 10 mF

VCC Capacitor

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