7708 datasheet(14/42 Pages) POWERINT

14 / 42 page

Rev. B 04/15

TFS7701-7708

www.power.com

Design, Assembly and Layout Considerations

Power Table

The data sheet power table (Table 1, page 1) represents the

maximum advised continuous power (thermally limited) based

on the following conditions:

1. +12 V output PC main and +12 V standby.

2. A regulated DC input for main with a nominal voltage of

385 VDC and a minimum of 300 VDC.

3. HiperTFS-2 combined main and standby efficiency of 87%

at full load.

4. Schottky high-efficiency output diodes.

5. DC input for Standby 115 VDC to 385 VDC.

6. Sufficient heat sinking and fan cooling to maintain heat sink

temperature below 95 °C.

7. Transformer designed with nominal duty factor of 45%.

HiperTFS-2 Selection

Selecting the optimum HiperTFS-2 depends upon the continuous

output power, peak power, thermal management, (heat sinking)

and maximum ambient operating temperature. OEM applications

typically specify 50 °C max ambient while clone PC supplies

usually specify 25 °C ambient. Higher efficiency can be achieved

by using a larger device and selecting a reduced device current

limit. The maximum output power can be tailored for any given

device by programming the device current limit for main and

standby.

Main Frequency Selection

For single output applications 132 kHz operation is recommended

for the main converter in order to reduce size and cost of the

main transformer and output choke. With optimized magnetics

design, the efficiency can match that of a 66 kHz design. For

multiple output designs, 66 kHz operation is recommended to

provide better voltage centering of the outputs.

Hold-Up Time

The input capacitor is a critical component to ensure meeting

the specified minimum hold-up time. Proper design of the

forward converter’s nominal duty cycle and sufficient primary

winding clamp voltage for sufficient magnetizing reset of the

main transformer are also essential. PIXls (PI Expert Design

Spreadsheet) can compute these values, or refer to formula in

AN-51.

Bias Support for High-Side Driver

The high-side MOSFET driver of HiperTFS-2 is internally biased

by a high-voltage current source. For 66 kHz operation an

external bias circuit is unnecessary. When using the internal

bias, use 4.7

mF for C1. Using external bias support for 66 kHz

operation may in some cases improve very light-load efficiency

of the main converter.

External bias is required for 132 kHz operation. (D1, R1, and C2

in Figure 13) It is sourced from an ultrafast bootstrap diode (D1)

from standby low-side primary bias (V

AUX) which should provide

a typical minimum of 15 V at zero load on the standby converter

to guarantee the 12 V bias required for the high-side driver.

When using external bias, use a 0.1

mF VDDH bypass capacitor

(C1). Note the value of R1 will be different for 66 kHz vs. 132 kHz

operation.

HiperTFS-2 VDDH

DSB

CONTROL

PI-7017-042913

*R1

VIN

VAUX

*C2

*D1

*D1 R1 and C2

optional for 66 kHz

operation

C1 = 0.1 µF if D1, R1, C2

present, 4.7

µF otherwise

Minimum Supply Current to

VDDH = 1.2 mA (66 kHz),

2.3 mA (132 kHz)

Main

Transformer

VHIGH_BIAS

Standby

Transformer

VHIGH_BIAS_(MIN) –12 V

1.2 mA

R1_MAX =

Typical VAUX_(MIN) = 15 V

VHIGH_BIAS_(MIN) = VAUX_(MIN)

VHIGH_BIAS_(MIN) –12 V

2.3 mA

R1_MAX =

66 kHz

132 kHz

Figure 13. Bootstrap Supply for VDDH and Component Calculations. Typical V

AUX_MIN is 17 V. Typical Value is 1 kW for 132 kHz Operation, and 2 kW if the Bias Support

Circuit is used for 66 kHz.

7708 Datasheet(PDF) 14 Page - Power Integrations, Inc.

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