SSC2016S datasheet(11/22 Pages) SANKEN

11 / 22 page

SSC2016S

SSC2016S - DSJ Rev.1.3

SANKEN ELECTRIC CO.,LTD.

Jul. 22, 2016

http://www.sanken-ele.co.jp/en/

DVCC

RCS

RST

VCC

GND

CVCC

CCP

DZVCC

DFW

VOUT

LINE

GND

VAC

VCCP

VIN

Figure 8-7.

VCC Pin Peripheral Circuit

(Power supply from auxiliary winding)

IC startup

VCC

VCC(ON)

VCC(OFF)

Startup failure

Startup success

Target

Operating

Voltage

Time

Increased by output

voltage rising

Figure 8-8.

VCC During Startup Period

8.3

Restart Circuit

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

pin is set by the detecting zero current signal at ZCD

pin.

When the off-time of OUT pin is maintained for

tRS = 220

μs or more, the restart circuit is activated and

OUT pin turns on. The ON time of the OUT pin is

tON(RS) = 1.7

μs in the restart operation.

At intermittent oscillation period in startup and light

load, the restart circuit is activated and the switching

operation is stabilized.

Since tRS = 220

μs corresponds to the operational

frequency of 6.25 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 Maximum on-time, tON(MAX), should

be set. The tON(MAX) depends on the value of C4

connected to CT pin. Section 9.1 shows about tON(MAX)

setting.

8.5

Zero Current Detection and

Bottom-on Timing Setting

Figure 8-9 shows the peripheral circuit of ZCD pin,

Figure 8-10 shows the zero current detection waveform.

The off-time is determined by detecting the zero

current of the boost winding P via the auxiliary winding

D and ZCD pin

The polarity of winding P and winding D of

transformer T1 are shown in Figure 8-9.

When the OUT pin voltage becomes low and the

power MOSFET turns off, the ZCD pin voltage becomes

the voltage of auxiliary winding D as shown in Figure

8-10. After the turning off of the power MOSFET, when

ZCD pin voltage is above VZCD(H) = 1.40 V, OUT pin

voltage is kept to be Low. When ZCD pin voltage

becomes below VZCD(L) = 0.70 V, OUT pin voltage

becomes High and power MOSFET turns on.

After the turning off of the power MOSFET, when the

current in boost winding become zero, VDS waveform

starts free oscillation based on the inductance LP, the

output capacitance of power MOSFET COSS and the

parasitic capacitance. The bottom point of VDS is

calculated as follows:

tHFP≈π�LP×CV(s)

(2)

where

tHFP: Half cycle of free oscillation (s)

LP: Inductance of boost winding (H)

CV: Combined output capacitance of power MOSFET

COSS and parasitic capacitanc (F)

In order to set the timing of turn on to the bottom

point of VDS as shown in Figure 8-11, adjust the turn on

delay time tHFP by using C6 and R1 in actual operation

codition as shown in Figure 8-9. Since R1 have a role as

the current limiting resistor of ZCD pin, adjust the C6

value if R1 value exceeds the range of limiting.

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