rt9591 Richtek Technology Corporation, rt9591 Datasheet - Page 10

• Manufacturer Part Number: rt9591
• Description: Smart Photoflash Capacitor Charger With Igbt Driver
• Manufacturer: Richtek Technology Corporation

RT9591
4. Transformer Secondary Capacitance
Any capacitance on the secondary can severely affect the
efficiency. A small secondary capacitance is multiplied by
N
forms a resonant circuit with the primary leakage
inductance of the transformer. Therefore, both the primary
leakage inductance and secondary side capacitance
should be minimized.
Rectifying Diode
The rectifying diode should be with short reverse recovery
time (small parasitic capacitance). Large parasitic
capacitance increases switching loss and lowers charging
efficiency. In addition, the peak reverse voltage and peak
current of the diode should be sufficient.
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10
(10V/Div)
(20V/Div)
2
(1A/Div)
(1A/Div)
V
V
when reflected to the primary side. This capacitance
I
I
SW
SW
PRI
PRI
Spike
voltage
Swing Current @Switching
Spike Voltage @Switching
Figure 4
Figure 5
Time (250ns/Div)
Time (1us/Div)
Swing Current
Preliminary
Patent Pending
The peak reverse voltage of the diode is approximately:
The peak current of the diode equals primary peak current
divide transformer turn ratio as the following equation:
Note: N is transformer turns ratio.
NMOS
The NMOS is the switching component of the flyback
converter. Select adequate drain to source voltage and
NMOS turn ON drain current is very important. For the
RT9591 typical application circuit,
If V
The NMOS minimum drain to source voltage should be
greater than 26.5V.
In addition, make sure that V
threshold voltage) so as to sufficiently turn on the
MOSFET.
Capacitor
X5R ceramic capacitor ≥ 10uF/10V is recommended for
input capacitor to well decouple the switching current.
Figure 6 and Figure 7 compare the input current waveforms
with different input capacitors.
V
I
V
PK-SEC
OUT
PK-R
DS(MIN)
= 300V, V
≈
= I
Input Average Current & Output Voltage
V
= 300/15 + 6.5 = 26.5V.
OUT
PK-PRI
+ (N x V
BAT
/N
= 6.5V, transformer turn ratio N = 15.
BAT
Figuer 6
)
DD
Time (1s/Div)
is higher than V
DS9591-07 August 2007
V
C
GS(th)
BAT
IN
= 4.7uF
= 3.3V
(Gate