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NCP1205

Single Ended PWM Controller Featuring QR Operation and Soft Frequency Foldback

ON Semiconductor 

NCP1205

APPLICATION INFORMATION

Introduction

By implementing a unique smooth frequency reduction

technique, the NCP1205 represents a major leap toward

low−power Switchmode Power Supply (SMPS) integrated

management. The circuit combines free−running operation

with minimum drain−source switching (so−called valley

switching), which naturally reduces the peak current stress

as well as the ElectroMagnetic Interferences (EMI). At

nominal output power, the circuit implements a traditional

current−mode SMPS whose peak current setpoint is given

by the feedback signal. However, rather than keeping the

switching frequency constant, each cycle is initiated by the

end of the primary demagnetization. The system therefore

operates at the boundary between Discontinuous

Conduction Mode (DCM) and Continuous Conduction

Mode (CCM). Figure 10 details this terminology:

L > Lc

IL

Not 0 at

Turn ON

IP

L = Lc

0

L < Lc

ON

IL(avg)

OFF

Borderline

0 Before

Turn ON

0

D/Fs

Dead−Time

Time

Figure 10. Defining the Conduction Mode, Discontinuous, Continuous and Borderline

When the output power demands decreases, the natural

switching frequency raises. As a natural result, switching

losses also increase and degrade the SMPS efficiency. To

overcome this problem, the maximum switching frequency

of the NCP1205 is clamped to typically 125 kHz. When the

free running mode (also called Borderline Control Mode,

BCM) reaches this clamp value, an internal

Voltage−Controlled Oscillator (VCO) takes over and starts

to decrease the switching frequency: we are in Variable

Frequency Mode (VFM). Please note that during this

transition phase, the peak current is not fixed but is still

decreasing because the output power demand does. At a

given state, the peak current reaches a minimum peak

(typically 250 mV/Rsense), and cannot go further down: the

switching frequency continues its decrease down to a

possible minimum of 0 Hz (the IC simply stops switching).

During normal free−running operation and VFM, the

controller always ensures single or multiple drain−source

valley switching. We will see later on how this is internally

implemented.

The FLYBACK operation is mainly defined through a

simple formula:

Pout

+

1

2

·

Lp · Ip2

·

Fsw

(eq. 1)

With:

Lp the primary transformer inductance (also called the

magnetizing inductance)

Ip the peak current at which the MOSFET is turned off

Fsw the nominal switching frequency

To adjust the transmitted power, the PWM controller can

play on the switching frequency or the peak current setpoint.

To refine the control, the NCP1205 offers the ability to play

on both parameters either altogether on an individual basis.

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