SSC1S311A 查看數據表（PDF） - Sanken Electric co.,ltd.
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SSC1S311A Datasheet PDF : 28 Pages
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SSC1S311A
voltage is VFB(STBOP) = 0.8V or less), when the VCC pin
voltage falls to the Startup Current Supply Threshold
Voltage, VCC(BIAS) = 11.0V, the Bias Assist function is
activated. While the Bias Assist function is operating,
the decrease of the VCC pin voltage is suppressed by
providing the startup current, ICC(STARTUP), from the
Startup circuit. By the Bias Assist function, the use of a
small value C2 capacitor is allowed, resulting in
shortening startup time. Also, because the increase of
VCC pin voltage becomes faster when the output runs
with excess voltage, the response time of the OVP
function can also be shortened. It is required to check
and adjust the process so that poor starting conditions
may be avoided.
Figure 8-3. VCC during Startup Period
8.1.4. Auxiliary Winding
In actual switch-mode power supply (SMPS) circuits,
there are cases in which the VCC pin voltage fluctuates in
proportion to the output of the SMPS (see Figure 8-4),
and the Overvoltage Protection (OVP) on VCC pin may
be activated.
This happens because C2 is charged to a peak voltage
on the auxiliary winding D, which is caused by the
transient surge voltage coupled from the primary
winding when the power MOSFET turns off.
For alleviating C2 peak charging, it is effective to add
some value R2, of several tenths of ohms to several
ohms, in series with D2 (see Figure 8-5). The optimal
value of R2 should be determined using a transformer
matching what will be used in the actual application,
because the variation of the auxiliary winding voltage is
affected by the transformer structural design.
7
VCC
U1
GND
8
D2 R2
Added
D
C2
Figure 8-5. VCC pin peripheral circuit with R2
The variation of VCC pin voltage becomes worse if:
● The coupling between the primary and secondary
windings of the transformer gets worse and the surge
voltage increases (low output voltage, large current
load specification, for example).
● The coupling of the auxiliary winding, D, and the
secondary side stabilization output winding (winding
of the output line which is controlling constant
voltage) gets worse and it is subject to surge voltage.
In order to reduce the influence of surge voltages on
the VCC pin, alternative structures of the auxiliary
winding, D, can be used as examples of transformer
structural designs see Figure 8-6.
● Winding structural example (a)
- Separating the auxiliary winding D from the primary
side windings P1 and P2.
- The primary side winding is divided into two
windings, P1 and P2.
● Winding structural example (b)
- Placing the auxiliary winding D within the secondary
winding S1 in order to improve the coupling of those
windings.
- The output winding S1 is a stabilized output winding
controlled to constant voltage.
Figure 8-4. VCC versus IOUT with/without resistor R2
SSC1S311A-DSE Rev.1.3
SANKEN ELECTRIC CO., LTD.
10
Dec. 22, 2017
http://www.sanken-ele.co.jp/en
© SANKEN ELECTRIC CO., LTD. 2012
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