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7. Technical Study (SMPS Operation)
7-1. V
terminal(pin 4) : Start-up circuit
IN
The start-up circuit starts and stops the operation
of the control IC by detecting the voltage appearing
at the V
terminal (pin 4).
IN
The power supply (V
IC uses a circuit as shown in Fig. 1.
At start-up of the power supply, C2 is charged
through the start-up resistor Rs.
When the V
terminal voltage reaches 16V
IN
(typical), the control circuit starts operation by the
function of the start-up circuit.
As shown is Fig.2, since the circuit current is
suppressed to 100 Amax.(at V
until the control circuit starts its operation, Rs can
be of high resistance.
Taking into account the slight increase of the
holding current (400 Amax.) of the latch circuit,
the Rs value shall be determined in a manner
that 500 A or higher current can flow even at the
lowest AC input voltage.
However, if the Rs value is too large, the current
charging C2 after switching on the AC input is
decreased to require longer time for reaching the
operation start voltage.
Therefore, the Rs value shall be examined
together with the C2 value which will be discussed
later.
The V
terminal voltage decreases after the control
IN
circuit starts operation.
Therefore, if the C2 value is larger, the voltage drop ratio if the V
Consequently, even if the drive winding voltage is delayed in rising, the V
not drop to the shutdown voltage to maintain start-up function.
If the capacitance of C2 is too large, time from switch-on of AC input to operation start by C2
charging becomes longer.
In the typical power supply specifications, C2 may be 47 ~ 100 F for start-up.
terminal input) of the control
IN
=141V, Tc=25 )
IN
Fig. 1 start-up circuit
Fig. 2 VIN terminal voltage circuit
current IIN
terminal voltage is reduced.
IN
29
terminal voltage does
IN
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