STR-A6000_15 Datasheet, PDF (16/28 Pages) Sanken electric – Off-Line PWM Controllers with Integrated Power MOSFET

STR-A6000 Series
 Light load conditions
When load conditions become lighter, the output
voltage, V_OUT, 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, V_SC decreases,
and the peak value of V_ROCP is controlled to be low,
and the peak drain current of I_D 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, V_SC increases and the peak drain current of I_D
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
9-8. 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 V_SC, 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.
Target voltage
without Slope Compensation
t_ON1
T
t_ON2
T
T
Figure 9-8 Drain current, I_D, waveform
in subharmonic oscillation
9.5 Leading Edge Blanking Function
The IC uses the peak-current-mode control method
for the constant voltage control of output.
In peak-current-mode control method, there is a case
that the power MOSFET turns off due to unexpected
response of FB comparator or overcurrent protection
circuit (OCP) to the steep surge current in turning on a
power MOSFET.
In order to prevent this response to the surge voltage
in turning-on the power MOSFET, the Leading Edge
Blanking, t_BW (STR-A60××H for 340 ns, STR-A60××H
and STR-A60××HD for 280 ns) is built-in. During t_BW,
the OCP threshold voltage becomes about 1.7 V which
is higher than the normal OCP threshold voltage (refer
to Section 9.9).
9.6 Random Switching Function
The IC modulates its switching frequency randomly
by superposing the modulating frequency on f_OSC(AVG) in
normal operation. This function reduces the conduction
noise compared to others without this function, and
simplifies noise filtering of the input lines of power
supply.
9.7 Automatic Standby Mode Function
Automatic standby mode is activated automatically
when the drain current, I_D, reduces under light load
conditions, at which I_D is less than 15 % to 20 % of the
maximum drain current (it is in the OCP state). The
operation mode becomes burst oscillation, as shown in
Figure 9-9. Burst oscillation mode reduces switching
losses and improves power supply efficiency because of
periodic non-switching intervals.
Output current,
I_OUT
Burst oscillation
Drain current,
I_D
Normal
operation
Below several kHz
Standby
operation
Normal
operation
Figure 9-9 Auto Standby mode timing
Generally, to improve efficiency under light load
conditions, the frequency of the burst oscillation mode
becomes just a few kilohertz. Because the IC suppresses
the peak drain current well during burst oscillation mode,
audible noises can be reduced.
If the VCC pin voltage decreases to V_CC(BIAS) = 9.5 V
STR-A6000 - DS Rev.4.3
SANKEN ELECTRIC CO.,LTD.
16
Mar. 13, 2015