7708 datasheet(23/42 Pages) POWERINT

23 / 42 page

Rev. B 04/15

TFS7701-7708

www.power.com

2. Efficiency of 80% at full load, minimum nominal input.

3. Minimum data sheet value of I2f.

4. Transformer primary inductance tolerance of ±10%.

5. Reflected output voltage (V

OR) of 100 V.

6. Voltage only output of 5 V with a Schottky diode.

7. Continuous conduction mode operation with transient

P* value of 0.25.

8. Highest standby current limit selection.

9. Heat sink max temperature is 95 °C.

*Below a value of 1, K

P is the ratio of ripple to peak primary

current. A transient K

P limit of ≥0.25 is recommended to

prevent reduced power capability due to premature termination

of switching cycles. Due to the initial current limit (I

INIT) being

exceeded at MOSFET turn-on.

Reducing No-Load Consumption

The BYPASS pin can be powered from the internal high-voltage

current source from the HIGH-SIDE DRAIN pin, but R16 (7.5 k

in Figure 30 will reduce no-load consumption by providing the

BYPASS pin current from a lower voltage and inhibiting the

internal high-voltage current source.

Audible Noise

Standard dip varnishing on the standby transformer will prevent

the possibility of audible noise in the standby converter.

Additionally, the peak core flux density should be kept below

3000 Gauss (300 mT). Vacuum impregnation of the transformer

is not recommended because it will increase standby no-load

losses due to the increased primary capacitance. Higher flux

densities are possible, however careful evaluation of the audible

noise performance should be made using production transformer

samples before approving the design. Ceramic capacitors that

use dielectrics such as Z5U, when used in clamp circuits with

high ripple voltage, may also generate audio noise. If this is the

case, try replacing them with a capacitor having a different

dielectric or construction, for example a film type.

Recommended First-Time Power-Up Procedure

Place a small, fast-blow low-capacity fuse between the bulk

capacitor and the HiperTFS-2 circuitry. Use a current-limited

bench power supply to power the HiperTFS-2 converter instead

of using the PFC or the AC mains. Be careful using a

programmable bench AC source in DC mode, because when

they are loaded with a large bulk capacitor and their output is

turned off, the output of the AC source can undershoot to a

negative voltage and damage the HiperTFS-2. If a remote-on

circuit is present, keep it OFF so that only the standby will run.

Place a voltage and current probe on the STANDBY DRAIN pin.

Raise the bulk capacitor voltage slowly until the standby turns

on. Check for proper waveforms (peak voltage, and check for

core saturation) and for output regulation. Check the VAUX

voltage. Check for over-heating components. Slowly increase

the load and the input voltage. Check for over-heating

components again.

Place voltage probes on DRAIN and HIGH-SIDE SOURCE pins.

Place current probe in DRAIN pin. If a remote-on circuit is

present turn remote to ON. Keep input voltage below UV start

threshold (typically 330 V). Slowly increase input voltage until

main converter starts. Check for proper waveforms and for

output regulation. Check for over-heating components,

especially the Drain clamp diode, and associated snubber

components. Slowly increase input voltage and load. Check

for over-heating components again.

Quick Design Checklist

Flyback

1. Maximum standby drain voltage – Verify that DSB voltage

does not exceed 675 V at highest input voltage and peak

(overload) output power. This 50 V margin to the 725 V

BVDSS specification gives margin for unit-to-unit variation.

2. Maximum DSB current – At maximum ambient temperature,

maximum input voltage and peak output (overload) power,

verify Standby Drain current waveforms for any signs of

transformer saturation and excessive leading edge current

spikes at start-up. Repeat under steady-state conditions

and verify that the leading edge current spike event is below

LIMIT(N)(MIN) at the end of the tLEB(MIN). Under all conditions, the

maximum Standby Drain current should be below the

specified absolute maximum ratings.

3. Thermal check – With main converter off (and any system

fans are also off), at specified maximum output power,

minimum input voltage and maximum ambient temperature,

verify that the temperature specifications are not exceeded

for the HiperTFS-2, transformer, output diode, and output

capacitors.

Main (Forward) Converter

Examine the voltage and current at 20% load and nominal input

voltage. Measure and check the following:

•

Switching frequency

•

Duty cycle

•

Peak voltage

Repeat the measurements at full load. Be cautious of over-

heating the power supply and use a strong fan. Measure the

source voltage (HS) of the high-side MOSFET at turn-on for

each steady-state switch cycle (Figure 28). It should be < 40%

of the bulk voltage. Calculate and verify the K

P from the current

waveform and verify with the spreadsheet. Also check the peak

current at peak load – do not dwell at peak load for more than

several seconds to prevent over-heating.

Check for any oscillation visible in the Drain current envelope.

Start-Up

Examine the start-up voltage and current. The peak start-up

current should be close to the I

LIMIT of the device. Examine the

output voltage monotonicity. Check for the high-side misfiring

during start-up. If the bootstrap diode is omitted (66 kHz only),

the VDDH capacitor should be ≥4.7

mF to prevent misfiring.

Start-up should be acceptable with either the remote-on/off

switch, or by bringing up the HVDC supply with remote-on already

asserted. Check startup at maximum expected input voltage.

7708 Datasheet(PDF) 23 Page - Power Integrations, Inc.

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