TFS757 datasheet(8/36 Pages) POWERINT

8 / 36 page

Rev. E 04/15

TFS757-764HG

www.power.com

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.

•

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

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.

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.

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 M

W 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

Figure 5.

Switching Frequency Jitter (Idealized V

DRAIN Waveforms).

OSC -

4 ms

Time

Switching

Frequency

VDRAIN

OSC +

Figure 6.

Supply Start-Up Sequence by Remote ON.

VIN

Standby

Output

Main

Output

12 ms

385 V

Main

Primary

Current

Remote ON

12 ms

32 ms

PI-5619a-102710

100% ILIM

115% ILIM

TFS757 Datasheet(PDF) 8 Page - Power Integrations, Inc.

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