TFS757 Datasheet, PDF (9/36 Pages) Power Integrations, Inc. – Combined Two-Switch Forward and Flyback Power Supply Controllers with Integrated High Voltage MOSFETs

TFS757-764HG
on, while the main high-side MOSFET is held off. By the end of
the pre-charge period, the PFC-boost voltage should be at or
above the nominal boost voltage. The HiperTFS begins
switching, going through the soft-start period (t_SS). During the
soft-start period the maximum duty cycle starts at 30% and is
ramped during a 12 ms period to the maximum. The ramped
duty cycle controls the rise slew rate of the output during
start-up, allowing well controlled start-up and also facilitates a
smooth transition when the control loop takes over regulation
towards the end of soft-start. Also during a 32 ms period
(starting at the beginning of soft-start), the main current limit is
boosted to 115% of the nominal selected Main current. This
allows the main to start-up within the required period for the
application (typically < 20 ms for PC main applications), when
there is a substantial capacitive load on the output. After the
soft-start period, the current limit returns to 100% of the
nominal selected current limit.
Main Converter Control FEEDBACK (FB) Pin Operation
The FEEDBACK pin is the input for control loop feedback from
the main control loop. During normal operation the FEEDBACK
pin is used to provide duty cycle control for the main converter.
The system output voltage is detected and converted into a
feedback current. The main converter duty cycle will reduce as
more current is sourced from the FEEDBACK pin, reaching zero
duty cycle at approximately 2.1 mA. The nominal voltage of the
FEEDBACK pin is maintained at approximately 3.5 V. An
internal pole on the FEEDBACK pin is set to approximately
12 kHz, in order to facilitate optimal control loop response.
The maximum duty cycle of the main converter is defined by the
LINE-SENSE pin and RESET pin behavior and is a dynamically
calculated value according to cycle-by-cycle conditions on the
LINE-SENSE pin and RESET pin.
Duty (D)
78%
63%
Limited by L & R
pin duty limit
I_L = 60 µA
I_R = 170 µA
Typical I_L and I_R
currents at V_MIN
FEEDBACK Pin
0%
Current I_FB
1 mA
2.1 mA
PI-5885-082610
Figure 7. PWM Duty Cycle vs. Control Current.
Main High-Side Driver
The high-side driver is a device that is electrically floating at the
potential of the HIGH-SIDE MOSFET SOURCE (HS) pin. This
device provides gate-drive for the high-side Main MOSFET. The
low-side main and high-side main MOSFET’s switch simul-
taneously. The high-side driver has a HIGH-SIDE OPERATING
VOLTAGE supply pin. External circuitry provides a current
source into this HIGH-SIDE OPERATING VOLTAGE pin. The
high-side operating voltage has an internal 12 V shunt-regulator.
The device consumes approximately 2 mA when driving the
high-side MOSFET.
The HIGH-SIDE OPERATING VOLTAGE pin has an undervoltage
lock-out threshold, to prevent gate-drive when the supply voltage
drops below a safe threshold. At power-up the high-side driver
remains in the off-state, until the HIGH-SIDE OPERATING
VOLTAGE pin is charged above 10.5 V, at which point the
high-side driver becomes active. The high-side driver is initially
charged via a boot-strap diode connected via a diode to the
HIGH-SIDE OPERATING VOLTAGE pin from the low-side
standby auxiliary supply (approximately 12 V). During start-up
the high-side MOSFET remains off, but the low-side MOSFET is
turned on for a period of 14 ms to allow pre-charge of the
high-side operating voltage to 12 V. After this period, the high-
side operating voltage is supplied by a forward-winding coupled
to the main transformer. This floating winding provides energy
every time the main converter switches one cycle. The
operating power for high-side operating voltage can also be
provided from a floating winding on the standby supply.
However this would continue delivering power even when the
main converter is in remote-off, and thus is considered
undesirable from a standby light-load efficiency point of view.
Once the high-side driver is operating it receives level-shifted
drive commands from the low-side device. These drive
commands cause both turn-on and turn-off drive of the
high-side main MOSFET simultaneously with that of the
low-side main MOSFET.
The high-side driver also contains a thermal shutdown on-chip,
but this is set to a temperature above the thermal shutdown
temperature of the low-side device. Thus the low-side will
always shutdown first.
Main Converter Maximum Duty Cycle
The LINE-SENSE pin resistor converts the input voltage into an
LINE-SENSE pin current signal. The RESET pin resistor
converts the reset voltage into an RESET pin current signal.
The LINE-SENSE pin and RESET pin currents allow the
HiperTFS to determine a maximum duty cycle envelope on a
cycle-by-cycle basis. This feature ensures sufficient time for
transformer reset on a cycle-by-cycle basis and also protects
against single-cycle transformer saturation and at high-input
voltage by limiting the maximum duty cycle to prevent the
transformer from reaching an unsafe flux density during the
on-time period. Both of these features allow the optimal
performance to be obtained from the main transformer. The
duty cycle limit is trimmed during production.
The LINE-SENSE pin and RESET pin are sampled just before
the turn-on of the next main cycle. This is done to sample at a
point when there is minimal noise in the system. Due to the low
current signal input to the LINE-SENSE pin and RESET pin,
care should be taken to prevent noise injection on these pins
(see Applications section layout guidelines for details).
Main On-Chip Current Limit with External Selection
During start-up, the FEEDBACK pin and ENABLE pin are both
used to select internal current limits for the main and standby
converters respectively. The detection period occurs at the
initial start-up of the device, and before the main or standby
MOSFETs start switching. This is done to minimize noise
interference.
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Rev. C 02/11