TFS7705H datasheet(8/42 Pages) POWERINT

8 / 42 page

Rev. B 04/15

TFS7701-7708

www.power.com

Functional Description

The HiperTFS-2 contains two switch-mode power supply

controllers and associated low-side MOSFET’s along with

high-side driver and high-side MOSFET.

•

The HiperTFS-2 two-switch forward includes a controller along

with low-side power MOSFET, high-side power MOSFET,

high-side driver and selectable main switching frequency

(synchronous with standby) of 66/132 kHz. The main converter

operates in voltage-mode (linear duty cycle control) at fixed

frequency (exactly half the operating frequency of the standby

controller when is 66 kHz mode). 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-2 fixed frequency (132 kHz) standby flyback

includes a controller and power MOSFET which is based on

TinySwitch-4. This device operates in multi-level ON/OFF

current limit control mode. The open drain MOSFET (STAND-

BY 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-2

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-2, is a two-switch forward

converter (although the HiperTFS-2 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-2, 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-2 also contains a high-side driver to control the

high-side MOSFET. This external bootstrap diode (or internally

self-biased) 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-2 also requires that the LINE-

SENSE pin current exceeds the UV main-on (I

L(MA-UVON)), which

corresponds to approximately 336 VDC input voltage when

using a 4 M

W LINE-SENSE pin resistor. Once this LINE-SENSE

pin threshold has been achieved the HiperTFS-2 will enter a 60 ms

pre-charge period (t

D(CH)) to allow the PFC-boost stage to reach

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

60 ms

385 V

Main

Primary

Current

Remote ON

PI-6775-022113

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