TFS757 Datasheet, PDF (20/36 Pages) Power Integrations, Inc. – Combined Two-Switch Forward and Flyback Power Supply Controllers with Integrated High Voltage MOSFETs

TFS757-764HG
Duty
Factor
L and R Pin Transformer Reset and
Forward Duty Clamp Protection
Regulation (FB)
duty cycle
Hard
limit
60%
Available
duty cycle range
Reset duty
clamp
To V_IN
To Clamp
Reset Circuit
R_L
R_R
45%
This region for
transient response
75 µA
(300 V)
Forward duty
clamp
100 µA L Pin Current
(385 V) (V_IN with R_L = 4 MΩ)
R
L
PI-5880-111710
Figure 26. L and R Pin Duty Limit With R_L = 4 MW and R_R = 4 MW.
Applications Example
High Efficiency +12 V, 25 A Main Output and +5 V 2.5 A
Standby Power Supply
The circuit in Figure 26 is an example of a design using HiperTFS
providing a 300 W +12 V output forward derived Main converter
and a 12 W +5 V Standby output from the flyback controller of
HiperTFS. The very high integration of two full converters within
a single package immediately shows the result of very low
external parts count for the entire design. Both the main
converter and the flyback section of HiperTFS are designed to
give very high-efficiency. The main converter takes advantage
of the ability to operate above 50% duty factor which lowers
RMS switch currents and allows using lower voltage more
efficient Schottky diodes on the output. The flyback section
uses Power Integrations TinySwitch technology which is often
used in designs that demand high-efficiency and low no-load
input power consumption.
The design in Figure 27 is intended to work with a PFC boost
front end that nominally provides a 385 VDC input. The main
converter will regulate to full load between 300 VDC and 385
VDC. This voltage range guarantees greater than 20 ms hold-up
time with C1 (270 mF).
The standby section is designed to operate whether the boost
PFC stage is on or off. The standby therefore is designed to
operate from 100 VDC to 385 VDC which covers the normal
universal input of 90 VAC to 265 VAC.
The start-up sequence is initiated with HiperTFS charging the
BYPASS pin capacitor via internal high-voltage current source.
Current limit selection then follows via FEEDBACK pin and
ENABLE pin resistors. The HiperTFS then senses the input
voltage via the LINE-SENSE pin resistor series chain R12, R13,
R35. When the input voltage reaches 100 V VDC the LINE-
SENSE pin UV standby threshold is reached and the standby
converter turns on. After several milliseconds the standby
output will reach regulation and the primary V_ON +12 V bias will
be stable. When the input bulk voltage reaches 315 VDC
which is the UV threshold for the main converter, the main
converter will initiate a turn on sequence once the remote-on
command from secondary is activated. The remote-on switch
(SW1) on the secondary-side for this particular design allows
the user to manually activate that main converter by turning on
the remote-on optocoupler. In actual PC designs the remote-
on would be controlled by a computer start-up command. This
optocoupler sources 5 mA into the BYPASS pin of the HiperTFS
which is the threshold current to start the turn on sequence for
20
Rev. C 02/11
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