TFS757 Datasheet, PDF (8/36 Pages) Power Integrations, Inc. – Combined Two-Switch Forward and Flyback Power Supply Controllers with Integrated High Voltage MOSFETs

TFS757-764HG
Functional Description
The HiperTFS contains two switch-mode power supply
controllers and associated low-side MOSFET’s along with
high-side driver and high-side MOSFET.
Switching
Frequency
f_OSC +
f_OSC -
• The HiperTFS two-switch forward includes a controller along
with low-side power MOSFET, high-side power MOSFET and
high-side driver. This device operates in voltage mode (linear
duty-cycle control) at fixed frequency (exactly half the operat-
ing frequency of the standby controller). The control converts
a current input (FEEDBACK pin), to a duty-cycle at the open
drain MOSFET MAIN DRAIN pin decreasing duty-cycle with
increasing sourced current from the FEEDBACK pin.
• The HiperTFS flyback includes a controller and power MOSFET
which is based on TinySwitch-III. This device operates in
multi-level ON/OFF current limit control mode. The open drain
MOSFET (STANDBY DRAIN pin) is turned on when the sourced
current from the ENABLE pin is below the threshold and
switching is disabled when the ENABLE pin current is above
the threshold.
In addition to the basic features, such as the high-voltage
start-up, the cycle-by-cycle current limiting, loop compensation
circuitry, auto-restart and thermal shutdown, the HiperTFS main
controller incorporates many additional functions that reduce
system cost, increase power supply performance and design
flexibility.
Main Converter General Introduction
The Main converter for the HiperTFS, is a two-switch forward
converter (although the HiperTFS could be used with other
two-switch topologies). This topology involves a low-side and
high-side power MOSFET, both of which are switched at the
same time. In the case of the HiperTFS, the low-side MOSFET
is a 725 V MOSFET (with the substrate connected to the
SOURCE pin). The high-side MOSFET is a 530 V MOSFET
(with the substrate connected to the HIGH-SIDE DRAIN (HD)
pin). As such the substrate of both low-side and high-side
MOSFET’s are tied to quiet circuit nodes (0 V and V_IN
respectively), meaning that both MOSFETs have electrically
quiet substrates – good for EMI.
The low-side MOSFET has a very low C_OSS capacitance and
thus can be hard-switched without performance penalty. Due
to the external clamp configuration it is possible to substantially
soft-switch the high-side MOSFET at high-loads (thus
eliminating a large proportion of high-side capacitive switching
loss) and improving efficiency. The higher breakdown voltage
on the low-side MOSFET allows the transformer reset voltage to
exceed the input voltage, and thus allow operation at duty
cycles greater than 50%. Higher duty cycle operation leads to
lower RMS switch currents and also lower output diode
voltage-rating, both of which contribute to improved efficiency.
4 ms
V_DRAIN
Time
Figure 5. Switching Frequency Jitter (Idealized V_DRAIN Waveforms).
Main Start-Up Operation
Once the flyback (standby) converter is up and running, the
main converter can be enabled by two functions. The first
condition is that the BYPASS pin remote-on current must
exceed the remote-on threshold (I_BP(ON)), provided by an external
remote ON/OFF circuit. This current threshold has a hysteresis
to prevent noise interference. Once the BYPASS remote-on
has been achieved, the HiperTFS also requires that the LINE-
SENSE pin current exceeds the UV Main-on (I_L(MA-UVON)), which
corresponds to approximately 315 VDC input voltage when
using a 4 MW LINE-SENSE pin resistor. Once this LINE-SENSE
pin threshold has been achieved the HiperTFS will enter a 12 ms
pre-charge period (t_D(CH)) to allow the PFC-boost stage to reach
regulation before the main applies a load to the bulk-capacitor.
Also during this pre-charge period the high-side driver is
charged via the boot-strap diode from the low-side auxiliary
voltage, and is charged when the main low-side MOSFET turns
V_IN
385 V
t
Standby
Output
t
Main
Output
Main
Primary
Current
Remote ON
12 ms
12 ms
t
32 ms
115% I_LIM
100% I_LIM
t
The HiperTFS also contains a high-side driver to control the
high-side MOSFET. This internal high-side driver eliminates the
need for a gate-driver transformer, an expensive component
that is required for many other two-switch forward circuits.
t
PI-5619a-102710
Figure 6. Supply Start-Up Sequence by Remote ON.
8
Rev. C 02/11
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