STR-A6000MZ datasheet(13/26 Pages) SANKEN

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STR-A6000MZ/HZ Series

STR-A6000MZ/HZ - DS Rev.1.2

SANKEN ELECTRIC CO.,LTD.

Mar.13, 2015

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).

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.).

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.

FB/OLP pin voltage is internally added the slope

compensation at the feedback control (refer to Section

4.Functionnal 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 9-7 and Figure 9-8.

PC1

FB/OLP

S/OCP

IFB

GND

ROCP

VROCP

Figure 9-7 FB/OLP pin peripheral circuit

VSC

FB Comparator

Drain current,

Voltage on both

sides of ROCP

VROCP

Target voltage including

Slope Compensation

Figure 9-8 Drain current, ID, and FB comparator

operation in steady operation

 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.

This

control

prevents

the

output

voltage

from

increasing.

 Heavy load conditions

When load conditions become greater, the IC performs

the inverse operation to that described above. Thus,

VSC increases and the peak drain current of ID

increases.

This

control

prevents

the

output

voltage

from

decreasing.

In the current mode control method, when the drain

current waveform becomes trapezoidal in continuous

operating mode, even if the peak current level set by the

target voltage is constant, the on-time fluctuates based on

the initial value of the drain current.

This results in the on-time fluctuating in multiples of

the fundamental operating frequency as shown in Figure

9-9. This is called the subharmonics phenomenon.

In order to avoid this, the IC incorporates the Slope

Compensation Function. Because the target voltage is

added a down-slope compensation signal, which reduces

the peak drain current as the on-duty gets wider relative

to the FB/OLP pin signal to compensate VSC, the

subharmonics phenomenon is suppressed.

Even if subharmonic oscillations occur when the IC

has some excess supply being out of feedback control,

such as during startup and load shorted, this does not

affect performance of normal operation.

tON1

Target voltage

without Slope Compensation

tON2

Figure 9-9 Drain current, ID, waveform

in subharmonic oscillation

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