SSC2005SC datasheet(8/19 Pages) SANKEN

8 / 19 page

SSC2005SC

SSC2005SC-DS Rev.1.1

SANKEN ELECTRIC CO.,LTD.

Feb. 06, 2015

Operational Description

 All of the parameter values used in these descriptions

are typical values, unless they are specified as

minimum or maximum.

 With regard to current direction, "+" indicates sink

current (toward the IC) and "–" indicates source

current (from the IC).

8.1

Critical Conduction Mode: CRM

Figure 8-1 and Figure 8-2 show the PFC circuit and

CRM operation waveform. The IC performs the on/off

operation of switching device Q1 in critical mode (the

inductor current is zero) as shown in Figure 8-1. Thus,

the low drain current variation di/dt of power MOSFET

is accomplished. Also, adjusting the turn-on timing at

the bottom point of VDS free oscillation waveform

(quasi-resonant

operation),

low

noise

and

high

efficiency PFC circuit is realized.

ION

IOFF

RCS

VAC

Figure 8-1 PFC circuit

IL=ION+IOFF

ION

IOFF

Q1 VDS

OFF

Turn on delay time

Bottom on

Free oscillation

ILPEAK

LPEAK

)

AVG

(





Figure 8-2 CRM operation and bottom on operation

Figure 8-3 shows the internal CRM control circuit.

The power MOSFET Q1 starts switching operation by

self-oscillation.

The control of on-time is as follows: the detection

voltage RVS2 is compared with the reference voltage

VFB = 2.50 V by using error amplifier (Error AMP)

connected to FB pin. The output of the Error AMP is

averaged and phase compensated. This signal VCOMP is

compared with the ramp signal VOSC to achieve on-time

control. The ON time becomes almost constant in

commercial cycle by setting VCOMP respond to below

20 Hz (Figure 8-4). This is achieved by tuning the

capacitor connected to the COMP pin.

The off-time and the bottom on timing of VDS are set

by both zero current detection of drain current and the

delay time configured by RDLY pin resistance. Thus,

simple PFC circuit with inductor having no auxiliary

winding is realized.

VOSC

VSET

OSC

Error AMP

PWM COMP

VFB

= 2.5V

VOUT

RVS1

RVS2

GND

COMP

VCS(ZCD)= -10mV

OUT

RCS

RDLY

RRT

RDLY

DZCS

ZCD COMP

VCOMP

Figure 8-3 CRM control circuit

ILPEAK

IL(t)

IAC(t)

tON tOFF

VAC(t)

√2×V

ACRMS

√2×I

ACRMS

VCOMP

VOSC

VSET

OUT pin voltage

VAC(t)

ILPEAK(t)

IL(AVG.)(t)

Figure 8-4 CRM operation waveforms

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