FAN7528 Просмотр технического описания (PDF) - Fairchild Semiconductor

Номер в каталоге

Компоненты Описание

производитель

FAN7528

Dual-Output, Critical Conduction Mode PFC Controller

Fairchild Semiconductor 

Applications Information

1. Error Amplifier Block

The error amplifier block has several functions, such as

dual output function, over-voltage protection function,

and disable function.

1.1 Dual-Output Function

Unlike conventional CRM PFC controllers, the FAN7528

has the dual-output control function according to the AC

line voltage without sensing the rectified AC line voltage.

Because the output voltage of the boost converter is pro-

portional to the peak voltage of the input AC line voltage

before the boost converter starts switching, the INV pin

voltage represents the peak AC line voltage. When the

AC line is connected to the boost converter, VCC voltage

starts to increase from zero voltage. If the VCC voltage

reaches 8.5V, the dual-output reference generator com-

pares the INV pin voltage with 1.3V reference and, if the

INV pin voltage is lower than 1.3V, the dual-output refer-

ence generator sets the reference voltage of the error

amplifier to 1.5V. If the INV pin voltage is higher than

1.3V, the reference voltage is set to 2.5V. That means if

the output voltage of the boost converter is set to 400V at

high line, the output voltage is 240V (400V*1.5/2.5) at

low line. If the output voltage is set to 390V at high line,

the output voltage is 234V at low line. Because this block

does not need the input voltage sensing network, the

power loss and cost related with the sensing network

can be saved. The reference voltage of the error ampli-

fier is not reset until VCC goes below 4.5V.

OVP

2.66V 2.55V

Disable

0.45V 0.35V

1.2 Over-Voltage Protection Function

The control speed of the PFC converter is very slow;

therefore, the over-voltage protection (OVP) of the out-

put voltage is very important. The FAN7528 provides a

precise OVP function that shuts down the drive circuit

when the INV pin voltage exceeds 2.66V and there is

0.11V hysteresis.

1.3 Disable Function

If the INV pin voltage is lower than 0.45V, most of the

internal block is disabled, the operating current is

reduced to be 65µA, and there is 0.1V hysteresis in the

comparator.

1.4 Error Amplifier

The error amplifier is a transconductance type amplifier.

The output current of the amplifier is proportional to the

voltage difference between the inverting input and the

non-inverting input of the amplifier. Some resistors and

capacitors should be connected to the error amplifier

output pin, the COMP pin, for the output voltage loop

compensation.

2. Zero Current Detection Block

The zero current detector (ZCD) generates the turn-on

signal of the MOSFET when the boost inductor current

reaches zero using an auxiliary winding coupled with the

inductor. If the voltage of the ZCD pin goes higher than

1.5V, the ZCD comparator waits until the voltage goes

below 1.4V. If the voltage goes below 1.4V, the zero cur-

rent detector turns on the MOSFET. The ZCD pin is pro-

tected internally by two clamps, 6.7V high clamp and

0.6V low clamp. The 160µs timer generates a MOSFET

turn-on signal if the drive output has been low for more

than 160µs from the falling edge of the drive output.

Dual-Output

Reference

Generator

Error Amp

Gm

1.5V/2.5V

VOUT

INV

1

2

COMP

FAN7528 Rev. 1.00

VIN

ZCD

RZCD

5

6.7V

1.4V

1.5V

Turn-on

160μs Signal

Timer

Zero Current

Detector

S

Q

R

FAN7528 Rev. 1.00

Figure 36. Zero Current Detector Block

Figure 35. Error Amplifier Block

© 2005 Fairchild Semiconductor Corporation

FAN7528 Rev. 1.0.6

13

www.fairchildsemi.com

Share Link: