TFS757 Datasheet, PDF (12/36 Pages) Power Integrations, Inc. – Combined Two-Switch Forward and Flyback Power Supply Controllers with Integrated High Voltage MOSFETs

TFS757-764HG
are slightly different for the ENABLE pin versus the FEEDBACK
pin. The ENABLE pin internal current selection is chosen
according to the above table.
The current limit selection for both FEEDBACK pin and ENABLE
pin takes place when the BYPASS pin first reaches 4.7 V. Once
the short detection period is complete, the BYPASS pin is
ramped on up to 5.7 V, and the FEEDBACK pin is allowed to
float to it’s nominal voltage of 3.5 V.
Standby Line Compensated Current Limit
to Flatten Output Overload
For many power supplies, the power output capability of the
power supply increases dramatically as the input voltage
increases. This means that most power supplies are able to
deliver much more power (up to 30-40% more power), into a
fault overload when operating at higher input voltage (versus
operating at lower input voltage). This can cause a problem
since many specifications require that the output overload
power capability of the device is more tightly managed.
In the case of the HiperTFS, the standby current limit is adjusted
as a function of line (input voltage), in such a ways as to always
provide substantially the same maximum overload power
capability. The input voltage is detected via the LINE-SENSE
pin current and the internal standby current limit of the device is
adjusted accordingly on a cycle-by-cycle basis. This means that
the HiperTFS standby will only deliver approximately 5% more
overload power at high-line as it did at low-line. This feature
provides a much safer design.
150
140
Not Compensated
130
120
110
Compensated
100
90
80
50 100 150 200 250 300 350 400 450
V_IN DC (V)
Figure 12. Shows Output Overload Power for Both Compensated and
Uncompensated Standby Current Limits.
Standby Line Undervoltage Detection (UV)
The LINE-SENSE pin resistor is connected to V_IN and generates
a current signal proportional to V_IN. The LINE-SENSE pin
voltage is held by the device at 2.35 V. The LINE-SENSE pin
current signal is used to trigger under/overvoltage thresholds for
both the standby and main converters. Assuming a LINE-SENSE
pin resistor of 4 MW, the standby will begin operating at
approximately 100 V (as defined by I_{L(SB_UVON)}). The standby will
shutdown if regulation is lost when input voltage is below 100 V.
However the standby will be forced to shutdown if this input
voltage drops below approximately 40 V (as defined by I_L(SB-UVOFF)).
Main and Standby Oscillator and Switching Frequency
The standby converter operates at a frequency of 132 kHz. The
main converter operates at exactly half that frequency at 66 kHz.
The two converters both include a common frequency jitter
profile that varies the switching frequency ±4 kHz for the main
(twice the jitter frequency range ±8 kHz for the standby), during
a 4 ms jitter period. The frequency jitter helps reduce quasi-
peak and average EMI emissions.
It should be noted that the HiperTFS has a collision avoidance
scheme in which the main converter is the master and the
standby is the slave, which avoids the main and standby switching
at exactly simultaneous moments. The most common
condition would be close to 50% duty cycle, if the main (master)
is about to switch (turn-off), then the standby (slave), waits for
short instant (200 ns) before starting it’s next cycle. The
standby is used as the slave, since the ON/OFF control of the
HiperTFS standby is less easily disrupted by sudden delays in
switching, versus the linear control loop of the main converter.
Standby and Main Thermal Shutdown
The HiperTFS provides a thermal shutdown function, (OTP) that
protects the HiperTFS. This hysteretic thermal shutdown allows
the device to automatically recover from any thermal fault event.
The thermal shutdown is triggered at a die-temperature of
approximately 118 °C and has a high hysteresis to ensure the
average device temperature is within safe levels. In a well
designed system the HiperTFS thermal shutdown is not
triggered during any normal operation and is only present as a
safety feature to protect against abnormal or fault conditions.
BYPASS (BP) Pin Operation
The BYPASS (BP) pin is the supply pin for the entire HiperTFS
device. The BYPASS pin is internally connected to a high-voltage
current source via the STANDBY DRAIN power MOSFET. This
high-voltage source will charge the BYPASS pin to 4.7 V during
initial power up. Once the BYPASS pin reaches 4.7 V, the
BYPASS pin will check the main and standby current limit
selection (FEEDBACK pin and ENABLE pin resistors respectively).
This selection takes a very short period, thereafter the BYPASS
pin continues being charged until it reaches 5.7 V, at which
point the standby power supply is ready to begin operation.
Like the TinySwitch-III the high-voltage current source will
continue to charge the BYPASS pin if it droops below 5.7 V.
However in most typical applications, a resistor (typically 7.5 kW)
is connected from primary bias (12 V) to the BYPASS pin. This
resistor provides the operating current to the BYPASS pin,
preventing the need to draw power from the high-voltage
current source. Like the TinySwitch-III, the BYPASS pin contains
a shunt regulator, which will be enabled if the BYPASS pin
voltage is externally driven above 5.7 V. The BYPASS pin shunt
current is used for two functions:
1. First, for a 4 mA threshold (I_BP(ON)) for main remote-on. When
the BYPASS pin current exceeds this threshold, the main is
enabled.
12
Rev. C 02/11
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