Rev. B 04/15

16

TFS7701-7708

www.power.com

Figure 15. Standard Values for Clamp-to-Rail Scheme. These Work Well for all Power Ranges and both 66 and 132 kHz. Using Standard Recovery Diodes for CR2 and

CR3 Allow for Partial Recycling of Leakage Inductance Energy and Improve Efficiency. R1 and R2 Limit Peak Reverse Current and Provide Some High

Frequency Damping, and R3 Allows VZ1 to Work as a “Bleed” Instead of a Hard Clamp, to Improve Efficiency.

HD

HS

D

G

S

HiperTFS-2 VDDH

R

L

FB

EN

BP

DSB

CONTROL

PI-7019-102513

R1

4.7

Ω

B+

R2

2.2

Ω

C1

2.2 nF

D2

1N4007G

D3

1N4007G

D1

UF4007

R3

100

Ω

VZ1

150 V

Standby

Primary

Main

Primary

B-

Output Choke

The use of iron powder (lower cost) or Sendust (“Kool-Mu”,

higher efficiency) core material is recommended. The

inductance of a powdered iron or Sendust core varies

significantly with load. At light load the inductance is much

higher than at full load, which allows a forward converter to

remain in continuous conduction mode (CCM), down to very

light load.

For multiple outputs the inductor turns ratio should be the same

as the main transformer secondary. Spreading the windings

equally throughout the toroid improves coupling and cross-

regulation.

Output Capacitors

The output capacitors in a forward converter do not see high

AC ripple currents. Very-low ESR capacitors are not necessary.

However the ESR of the capacitors have a direct effect on

output ripple voltage (if no post-filter is used) and on fast load-

transient response. The capacitance has an effect on medium-

speed load-transient response. Polymer (solid electrolyte)

capacitors are not necessary but can be used for output

capacitors if very small size is required. However, their small

capacitance may require an additional, low-cost (moderate

ESR) electrolytic capacitor in parallel, to maintain sufficient

capacitance for loop stabilization and transient response.

Standby Mode Consumption

To improve the standby converter’s light load efficiency, a 7.5 k

W

bias resistor should be connected from V

AUX to the BYPASS pin.

This will turn off the internal high-voltage current source that

otherwise powers the BYPASS pin.

Heat Sinking

The HiperTFS-2 package is eSIP-16F. There is a metal exposed

pad that provides a low thermal resistance path to the heat sink

for the low-side main MOSFET and standby power MOSFET.

There is also an over-molded, electrically isolated section on the

back of the package that provides isolation between the heat

sink and the internal high-side main MOSFET. The heat sink

temperature behind the device should not exceed 95 °C to

provide sufficient thermal margin to avoid activating over-

temperature shutdown. The device does not require Insulation

pad (SIL pad). See Power Integrations’ website “Mounting with

Plastic and Metal Clips”. Clip to provide 20N of clamping force

(15N min, 50N max). Thermal heat sink compound is required

for optimal thermal performance.