AN4149 Datasheet, PDF (6/19 Pages) STMicroelectronics – Designing a CCM PFC pre-regulator based on the L4984D

AN4149
APPLICATION NOTE
V_cc
12V
11V
Normal mode
V_sync
Standby mode
4.6V
2.6V
3.6V
1.3V
Figure 9. Burst operation in standby mode
C_DC
AC line
I_sup
R_str
Vcc
R_a
D_a
KA5Q-series
C_a
Figure. 10 Startup Resistor and Vcc Auxiliary Circuit
In general, switched mode power supply employs an error
amplifier and an opto-coupler to regulate the output voltage.
However, Primary Side Regulation (PSR) can be used for a
low cost design if output regulation requirements are not
very tight. PSR scheme regulates the output voltage
indirectly by controlling the Vcc voltage without an opto-
coupler. KA5Q-series has an internal error amplifier with a
fixed reference voltage of 32.5V for PSR applications. If
PSR is used, set Vcc to 32.5V.
After determining V_cc voltage in normal operation, the
number of turns for the Vcc auxiliary winding (N_a) is
obtained as
N_a
=
-V----^c---^c----+-----V----^F---^a-
V_o1 + V_F1
⋅
N_s1
(turns)
( 16 )
where V_Fa is the forward voltage drop of D_a as defined in
Figure 8.
- Startup resistor (R_str) : The average of the minimum
current supplied through the startup resistor is given by
Isupavg
=
⎜⎜⎛^{-----2-----⋅---V----l-}_π^{-i-n----e----m-----i--n-- –}
⎝
V-----s---2t--a----r---t_{⎟⎟⎞}
⎠
⋅
-----1------
Rstr
( 17 )
where V_linemin is the minimum input voltage, V_start is the
start voltage (15V) of FPS and R_str is the startup resistor. The
startup resistor should be chosen so that I_supavg is larger than
the maximum startup current (200uA). If not, Vcc can not be
charged up to the start voltage and FPS will fail to start up.
The maximum startup time is determined as
T_st
m
r
ax
=
C_a
⋅
-------------------V----^s---^t--^a---^r-^t-------------------
(I_supavg
–
I_st
ma
art
x
)
(18)
Where C_a is the Vcc capacitor and I_startmax is the maximum
startup current (200uA) of FPS.
[STEP-8] Determine the startup resistor
Figure 10 shows the typical startup circuit for KA5Q-series.
Because some protections are implemented as latch mode,
AC startup is typically used to provide a fast reset. Initially,
FPS consumes only startup current (max 200uA) before it
begins switching. Therefore, the current supplied through the
startup resistor (R_str) can charge the capacitors C_a1 and C_a2
while supplying startup current to FPS. When Vcc reaches a
start voltage of 15V (V_START), FPS begins switching, and the
current consumed by FPS increases. Then, the current
required by FPS is supplied from the transformer’s auxiliary
winding.
Once the startup resistor (R_str) is determined, the maximum
approximate power dissipation in R_str is obtained as
P_str
=
-----1------
Rstr
⋅
⎛
⎜
⎜
⎜
⎝
⎝⎛ ^Vl i n em ^a
x_⎞
⎠
2
+
Vs
t
a
r
2
t
---------------------------------------------------------------
2
– ²--------²-----^â‹---^V-----^s---^t--^a----^r--_π^t----^â‹---^V----^l--ⁱ-ⁿ----^e----^m-----^a----^x-_{⎟⎟⎟⎞}
⎠
(19)
where V_linemax is the maximum input voltage, which is
specified in STEP-1. The startup resistor should have a
proper dissipation rating based on the value of P_str.
©2005 Fairchild Semiconductor Corporation
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