TFS757 Datasheet, PDF (15/36 Pages) Power Integrations, Inc. – Combined Two-Switch Forward and Flyback Power Supply Controllers with Integrated High Voltage MOSFETs

TFS757-764HG
HiperTFS VDDH
CONTROL
HD
R
HS
L
D
FB
~50 Newtons
EN
DSB
BP
To Bulk
Capacitor
G
S
PI-5883-032410
Minimum Clearance
is 0.078 inches
PI-5882-111710
Figure 16. HiperTFS Layout Considerations.
Figure 17. HiperTFS Heat Sink Mounting.
EMI
The frequency jitter feature modulates the switching frequency
over a narrow band as a means to reduce conducted EMI
average and quasi-peaks associated with the harmonics of the
fundamental switching frequency. This is particularly beneficial
for average conduction mode where the sampling bandwidth is
narrow. The modulation rate is nominally 250 Hz which is high
enough to reduce EMI but low enough to have negligible effect
on output ripple (rejected by control loop).
Transformer Design
It is recommended that the transformer be designed for a
maximum flux density of 3000 Gauss during continuous
maximum output power and a maximum peak transient flux
density no greater than 4000 Gauss. The turns ratio should be
chosen for a nominal duty factor of 45% at 385 VDC input to
guarantee transformer reset with typical primary winding
clamp-to-rail (Figure 15). For nominal duty factor of higher value
it is recommend to refer to AN-51 and use PIXLS spreadsheet
for optimal transformer design. Typically the transformer should
have foil secondary windings for outputs above 10 amps. The
primary winding should be split primary type to keep leakage
inductance low.
Standby Mode Consumption
The HiperTFS standby converter is essentially a TinySwitch-III
controller which uses whole-cycle ON/OFF control. This has the
benefit of operating at a low average frequency at lighter loads
which increases efficiency and reducers no-load consumption.
Heat Sinking
The HiperTFS package is eSIP-16/12. There is a metal exposed
pad that provides a low thermal path to the heat sink for the
low-side power device and standby power device. There is also
an over-molded, electrically isolated section of the package
backside that provides isolation between the heat sink and the
internal high-side switch. Thermal heat sink compound, and a
mounting clip providing a minimum torque of 50 Newtons, are
required for good thermal performance. The heat sink
temperature behind device should not exceed 95 °C to avoid
activating the over-temperature shutdown of HiperTFS. Since
some of the HiperTFS pins are bent towards the heat sink, there
needs to be a minimum of 0.078 inches clearance between
heat sink and PC board.
Layout Considerations
Use a single point connection between, SOURCE pin, GROUND
pin and bypass capacitor. Typically the bypass capacitor is a
surface mount type and is located directly under the HiperTFS
package between the GROUND pin and the BYPASS pin.
The FEEDBACK pin and ENABLE pin along with the LINE-
SENSE and RESET pins should be kept away from noisy, high
voltage switching areas. If it is unavoidable to have long traces
connecting to FEEDBACK pins then route these traces close to
quiet, low impedance traces, that act as a Faraday shield. The
LINE-SENSE and RESET pins are associated with multiple
series resistor sections due to the high-voltage sensing. Make
sure the last resistor in series chain is SMD type and place it
very close to the pin. This will minimize the pick-up of noise.
The primary auxiliary bias output rectifier and filter should be
star referenced to bulk capacitor. Any Y capacitors referenced
to DC primary should also be tied to quiet nodes of bulk
capacitor negative or positive terminal.
www.powerint.com
15
Rev. C 02/11