SG6203DZ Fairchild Semiconductor, SG6203DZ Datasheet - Page 8

• Manufacturer Part Number: SG6203DZ
• Description: IC SYNCHRONOUS FRWRD CTRLR 8DIP
• Manufacturer: Fairchild Semiconductor

Specifications of SG6203DZ

• Applications: Synchronous Forward Controller
• Voltage - Supply: 4.8 V ~ 20 V
• Current - Supply: 1.7mA
• Operating Temperature: -30°C ~ 85°C
• Mounting Type: Through Hole
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Input: -

Synchronous Rectifier Controller for Flyback Converter
OPERATION DESCRIPTION
Synchronization
The DET pin provides on/off information of the primary
switch. As shown in the following reference circuits, one
diode connected between the DET pin and the secondary
winding of the transformer is used to get this information.
Low DET voltage level indicates the secondary rectifier is
conducted; therefore, the synchronous MOSFET should
be turned on. The voltage on DET pin stays high when the
primary switch is conducting and the synchronous
MOSFET should be turned off. This innovative feature
requires no auxiliary winding and the circuit complexity
is greatly reduced.
Anticipation and Phase Lock Loop
A resistor from the RT pin to ground is used to determine
the internal current source reference. When the primary
switch is turned on with DET high, an internal one-shot
timer outputs high for a duration proportional to the R
resistor. The duration of this one-shot signal can be
expressed as:
The on-time of the synchronous MOSFET is obtained by
subtracting the on-time of the primary switch from the
one-shot signal duration. If the resistor is too small, the
synchronous MOSFET may be turned off when the
secondary rectifier is still conducting, which decreases
system efficiency. If the resistor is too large, the output is
© System General Corp.
Version 1.0.1 (IAO33.0010.B1)
Primary Switch PWM
T
RT
=
One-Shot Signal
15
×
R
24
T
( )
K
Output
Ω
(
µs
)
T
(1)
T
- 8 -
shut off immediately once DET gets high. In SG6203, an
internal phase locked loop modulates the duration of the
internal one-shot signal to maintain a suitable dead time
between the primary switch and the synchronous
MOSFET. The duration of the one-shot signal can be
extended 80% compared with the original value decided
by R
Current Sense
In discontinuous conduction mode, the secondary rectifier
turns off before the next switching cycle of the primary
switch. Once the secondary rectifier is off, the
synchronous MOSFET must be turned off to prevent the
reverse energy flow. Therefore, a zero current crossing
detector is needed when the converter is operated under
DCM. The SG6203 provides two configurations to
achieve this: output capacitor ESR method (reference
Circuit-A) and current-sensing resistor method (reference
Circuit-B). When the secondary rectifier is turned off, the
voltage on IN+ is lower than that of IN-. Once this
happens, the driving signal for the synchronous MOSFET
is turned off.
Under-Voltage Lockout (UVLO)
The SG6203 has an internal UVLO circuit with hysteresis.
The IC is turned on if V
on, SG6203 is turned off if V
the flyback output voltage is higher than 18V, the VZ pin
can be used to provide a stable 18V to VCC pin.
T
resistor.
www.sg.com.tw • www.fairchildsemi.com
CC
is higher than 6V. Once turned
CC
Product Specification
is lower than 5.1V. When
September 24, 2007
SG6203