PFS704 datasheet(14/30 Pages) POWERINT

14 / 30 page

Rev. D 12/11

PFS704-729EG

www.powerint.com

PI-6228-111110

VCC

CONTROL

HiperPFS

B+

VCC

R7 and capacitor C2 is the loop compensation network which

introduces a low frequency zero required to tailor the loop

response to ensure low cross-over frequency and sufficient

phase margin. Resistor R6 isolates the fast portion (resistor

voltage divider network comprising of resistors R1 to R5) and

the slow feedback loop compensator circuit (resistor R7 and

capacitor C2). Transistors Q1 and Q2, biased with resistors R3

and R4 respectively, detect output voltage transient conditions

and provide the FEEDBACK pin with “fast” information to

increase the loop response of the system. Diode D1 is included

to cover a single point fault condition wherein capacitor C2 is

shorted. In the event C2 is short-circuited, the FEEDBACK pin

is forced below the FB

OFF threshold through diode D1 and

subsequently turns the HiperPFS off. Only a standard recovery

diode should be used for D1. Use of ultrafast or fast recovery

diode is not recommended including small signal diodes (e.g.

1N4148) which are typically also fast recovery.

The recommended values for the components used are as

follows:

R5 = 57.6 kW

R3, R4 = 2.2 kW

R2 = 732 kW

C1 = 0.1 mF, 100 V X7R/NPO

R6 = 160 kW

R7 = 3 kW

R8 = 2 kW

C2 = 4.7 mF

C3 = 10 nF (For layouts that result in excessive noise on the

feedback signal, a 20 nF capacitor may be used).

D1 = BAV116 W or 1N4007 (A general purpose standard

recovery diode should only be used).

Q1, Q2 = Small signal transistors equivalent to 2N4401 and

2N4403.

Figure 14. Recommended Feedback Circuit.

When the above component values are used, the value of

resistor R1 can be calculated using the equation below:

10010

Since the total voltage across resistor R1 is approximately

301 V, resistor R1 may have to be divided into two or more

resistors to distribute the voltage stress below the voltage

ratings of the resistor used.

The value of resistor R7 will have to be adjusted in some

designs and as a guideline the value from the following

calculation can be used:

Maximum continuous output power in watts

Nominal PFC output voltage in volts

PFC output capacitance in farads

Improvement in low frequency phase margin can be achieved by

increasing the value of the capacitor C2 however increase in value

of capacitor C2 will result in some increase in overshoot at the

output of the PFC during transient loading and should be verified.

Diode D2 connected in series with the collector of the NPN

transistor Q1 is to prevent loading of the feedback circuit when

the V

CC is absent. Presence of this diode ensures that there is

no start-up delay when the V

CC is applied to the HiperPFS, the

feedback circuit, and transistor.

Heat Sinking and Thermal Design

The exposed pad on the HiperPFS eSIP package is internally

connected to the drain of the MOSFET. Due to the significant

amount of power dissipated in the part, the HiperPFS should be

mounted on a rectangular heat spreader made of thermally

conductive material such as Aluminum or Copper. Figure 15

shows an example of the recommended assembly for the

HiperPFS. In this assembly shown, a 0.76 mm thick aluminum

heat spreader is used. A thermally conductive sil pad should be

used to separate the heat spreader from the heat sink. A thin

film of thermally conductive silicone grease should be applied to

the rear surface of the HiperPFS to ensure low thermal

resistance contact between the package of the HiperPFS and

the heat spreader.

For universal input applications up to 150 W and 230 VAC only

applications up to 300 W, the heat spreader is not essential.

Use of heat spreader in these applications will help reduce

temperature of the part and heat spreaders can be used if

necessary. Figure 17 shows an example of the recommended

assembly for lower power designs that do not need a heat

spreader.

The HiperPFS is electrically connected to the heat spreader and

the heat sink is required to be connected to the source in order

to reduce EMI. The voltage between the heat spreader and

heat sink can easily exceed 400 V during transient conditions.

Attention should be placed on creepage and clearance based

on applicable safety specification.

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