SSC2005SC datasheet(9/19 Pages) SANKEN

9 / 19 page

SSC2005SC

SSC2005SC-DS Rev.1.1

SANKEN ELECTRIC CO.,LTD.

Feb. 06, 2015

The off duty DOFF of boost converter in CRM mode

have the relation of DOFF(t) = VAC(t)/VOUT and is

proportional to input voltage, where VAC(t) is the input

voltage of AC line as a function of time.

As a result of aforementioned control shown in Figure

8-4, the peak current, ILPEAK, of the inductance current,

IL, becomes sinusoidal. Since the averaged input current

become similar to AC input voltage waveform by a low

pass filter at input stage, high power factor is achieved.

8.2

Startup Operation

Figure 8-5 shows the VCC pin peripheral circuit.

The VCC pin is a control circuit power supply input.

The voltage is supplied by using external power supply.

As shown in Figure 8-6, when VCC pin voltage rises to

the Operation Start Voltage VCC(ON) = 12.0 V, the control

circuit starts operation. When the VCC pin voltage

decreases to VCC(OFF) = 9.5 V, the control circuit stops

operation by Undervoltage Lockout (UVLO) circuit, and

reverts to the state before startup.

Since COMP pin voltage rises from zero during

startup period, the VCOMP signal shown in Figure 8-3

gradually rises

from low

voltage.

The

on-width

gradually increased to restrict the rise of output power

by the Softstart Function. Thus, the stress of the

peripheral component is reduced.

COMP

GND

VCC

External

Power

Supply

Figure 8-5 VCC pin peripheral circuit

ICC

ICC（ON）

VCC（OFF）

VCC（ON）

VCC pin

voltage

Figure 8-6 Relationship between

VCC pin voltage and ICC

8.3

Restart Circuit

The IC is self-oscillation type. The off-time of OUT

pin is set by the zero current detection circuit (refer to

Section 8.5).

When the off-time of OUT pin is maintained for

tRS = 50 μs or more, the restart circuit is activated and

OUT pin turns on.

At intermittent oscillation period in startup and light

load, the restart circuit is activated and the switching

operation is stabilized.

Since tRS = 50 μs corresponds to the operational

frequency of 20 kHz, the minimum frequency should be

set to higher than 20 kHz (above audible frequency) at

the inductance value design.

8.4

Maximum On-time Setting

In order to reduce audible noise of transformer at

transient state, the IC has the Maximum on-time,

tON(MAX). This tON(MAX) is adjusted by the resistance RRT

which is connected to the RT pin.

Figure 8-7 shows the relation between RRT value and

tON(MAX) in IC design. .

The RRT value is set by using the result of tON(MAX)_OP

and Figure 8-7, where tON(MAX)_OP is the maximum

on-time of the peak voltage of the minimum AC input

voltage

● t

ON(SET)MAX Setting

ILP is calculated by Equation (1). ILP is peak current of

the peak voltage of the minimum AC input voltage.

)

MIN

(

ACRMS

OUT









(A)

(1)

where,

POUT : Output power (W)

VACRMS(MIN) : Minimum AC input voltage rms value (V)

η : Efficiency of PFC (About 0.90 to 0.97)

tON(MAX)_OP is calculated by Equation (2) with results

of Equation (1) and Equation (5). tON(MAX)_OP is the

maximum on time of the peak voltage of the

minimum AC input voltage.

)

MIN

(

ACRMS

)

MAX

(





(s)

(2)

where,

LP : Inductance value of the result of Equation (5)

VACRMS(MIN) : Minimum AC input voltage rms value (V)

● R

RT Setting

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