STR3A455 datasheet(13/26 Pages) SANKEN

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STR3A400 Series

STR3A400-DSE Rev.2.1

SANKEN ELECTRIC CO.,LTD.

Sept. 18, 2015

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

providing the startup current, ICC(ST), from the startup

circuit. Thus, the VCC pin voltage is kept more than

VCC(OFF).

Since the startup failure is prevented by the Bias

Assist Function, the value of C2 connected to the VCC

pin can be small. Thus, the startup time and the response

time of the Overvoltage Protection (OVP) become

shorter.

The operation of the Bias Assist Function in startup is

as follows. It is necessary to check and adjust the startup

process based on actual operation in the application, so

that poor starting conditions may be avoided.

Figure 9-3 shows the VCC pin voltage behavior

during the startup period. After the VCC pin voltage

increases to VCC(ON) = 15.0 V at startup, the IC starts the

operation. Then circuit current increases and the VCC

pin voltage decreases. At the same time, the auxiliary

winding voltage, VD, increases in proportion to output

voltage. These are all balanced to produce the VCC pin

voltage.

When

the

VCC

voltage

decrease

VCC(OFF) = 8.5 V in startup operation, the IC stops

switching operation and a startup failure occurs. When

the output load is light at startup, the output voltage may

become more than the target voltage due to the delay of

feedback circuit. In this case, the FB pin voltage is

decreased by the feedback control. When the FB pin

voltage decreases to VFB(OFF) or less, the IC stops

switching operation and the VCC pin voltage decreases.

When the VCC pin voltage decreases to VCC(BIAS), the

Bias Assist Function is activated and the startup failure

is prevented.

IC starts operation

VCC pin

voltage

VCC(ON)

VCC(BIAS)

VCC(OFF)

Startup failure

Startup success

Target operating

voltage

Time

Bias assist period

Increase with rising of

output voltage

Figure 9-3 VCC pin voltage during startup period

9.4

Soft Start Function

Figure 9-4 shows the behavior of VCC pin voltage

and drain current during the startup period.

The IC activates the soft start circuitry during the

startup period. Soft start time is fixed to around 8.75 ms.

during the soft start period, overcurrent threshold is

increased step-wisely (7 steps). This function reduces

the voltage and the current stress of a power MOSFET

and the secondary side rectifier diode.

Since the Leading Edge Blanking Function (refer to

Section 9.6) is deactivated during the soft start period,

there is the case that ON time is less than the leading

edge blanking time, tBW = 330 ns.

After the soft start period, D/ST pin current, ID, is

limited by the Overcurrent Protection (OCP), until the

output voltage increases to the target operating voltage.

This period is given as tLIM.

In case tLIM is longer than the OLP Delay Time, tOLP,

the output power is limited by the Overload Protection

(OLP) operation.

Thus, it is necessary to adjust the value of output

capacitor and the turn ratio of auxiliary winding D so

that the tLIM is less than tOLP = 55 ms (min.).

VCC(ON)

VCC(OFF)

Time

VCC pin

voltage

Startup of SMPS

Normal opertion

D/ST pin

current, ID

tLIM < tOLP (min.)

Soft start period

approximately 8.75 ms (fixed)

Time

Startup of IC

tSTART

Limited by OCP operation

Figure 9-4 VCC and ID behavior during startup

9.5

Constant Output Voltage Control

The IC achieves the constant voltage control of the

power supply output by using the current-mode control

method, which enhances the response speed and

provides the stable operation. The FB/OLP pin voltage

is internally added the slope compensation at the

feedback control (refer to Section 4. Block Diagram),

and the target voltage, VSC, is generated. The IC

compares the voltage, VROCP, of a current detection

resistor with the target voltage, VSC, by the internal FB

comparator, and controls the peak value of VROCP so that

it gets close to VSC, as shown in Figure 9-5 and Figure

9-6.

● Light load conditions

When load conditions become lighter, the output

voltage, VOUT, increases. Thus, the feedback current

from the error amplifier on the secondary-side also

increases. The feedback current is sunk at the FB/OLP

pin, transferred through a photo-coupler, PC1, and the

FB/OLP pin voltage decreases. Thus, VSC decreases,

and the peak value of VROCP is controlled to be low,

and the peak drain current of ID decreases.

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