CM6903A

2005/01/20

Champion Microelectronic Corporation

Page 10

fpfc/6, one sixth of the PFC switching frequency. Therefore,

the boost inductor can be reduced 6 times without

and light load, and Startup current

There are 4 purposes for IAC pin:

1.) For the leading edge modulation, when the duty

cycle is less than 50%, it requires the similar slope

compensation, as the duty cycle of the trailing

edge modulation is greater than 50%. In the

CM6903A, it is a relatively easy thing to design.

connect IAC pin and the rectified line voltage. It

will do the automatic slope compensation. If the

need to be reduced more.

2.) During the startup period, Rac also provides the

initial startup current, 100uA;therefore, the bleed

resistor is not needed.

signal, it also enhances the signal to noise ratio

and the THD of the input current.

4.) It also will try to keep the maximum input power to

be constant. However, the maximum input power

will still go up when the input line voltage goes up.

Start Up of the system, UVLO, and VREFOK

up current~100uA from the rectified line voltage to IAC pin.

Inside of CM6903A during the start-up period, IAC is

connected to VCC until the VCC reaches UVLO voltage

which is 13V and internal reference voltage is stable, it will

disconnect itself from VCC.

PFC section wakes up after Start up period

After Start up period, PFC section will softly start since

VEAO is zero before the start-up period. Since VEAO is a

slew rate enhanced transconductance amplifier (see figure

3), VEAO has a high impedance output like a current

source and it will slowly charge the compensation net work

which needs to be designed by using the voltage loop gain

equation.

Before PFC boost output reaches its design voltage, it is

around 380V and VFB reaches 2.5V, PWM section is off.

PWM section wakes up after PFC reaches steady state

PWM section is off all the time before PFC VFB reaches

2.45V. Then internal 10mS digital PWM soft start circuit

slowly ramps up the soft-start voltage.

PFC OVP Comparator

PFC OVP Comparator sense VFB pin which is the same the

voltage loop input. The good thing is the compensation

network is connected to VEAO. The PFC OVP function is a

relative fast OVP. It is not like the conventional error amplifier

which is an operational amplifier and it requires a local

feedback and it make the OVP action becomes very slow.

The threshold of the PFC OVP is 2.5V+10% =2.75V with

250mV hysteresis.

Tri-Fault Detect Comparator

To improve power supply reliability, reduce system

component count, and simplify compliance to UL1950 safety

standards, the CM6903A includes Tri-Fault Detect. This

feature monitors VFB (Pin 8) for certain PFC fault conditions.

In case of a feedback path failure, the output of the PFC

could go out of safe operating limits. With such a failure, VFB

will go outside of its normal operating area. Should VFB go

too low, too high, or open, Tri-Fault Detect senses the error

and terminates the PFC output drive.

Tri-Fault detect is an entirely internal circuit. It requires no

external components to serve its protective function.

VCC OVP and generate VCC

For the CM6903A system, if VCC is generated from a source

that is proportional to the PFC output voltage and once that

source reaches 17.9V, PFCOUT, PFC driver will be off.

The VCC OVP resets once the VCC discharges below

16.4V, PFC output driver is enabled. It serves as redundant

PFC OVP function.

Typically, there is a bootstrap winding off the boost inductor.

The VCC OVP comparator senses when this voltage

exceeds 17.9V, and terminates the PFC output drive. Once

the VCC rail has decreased to below 16.4V the PFC output

drive be enabled. Given that 16V on VCC corresponds to

380V on the PFC output, 17.9V on VCC corresponds to an

OVP level of 460V.

It is a necessary to put RC filter between bootstrap winding

and VCC. For VCC=13V, it is sufficient to drive either a

power MOSFET or a IGBT.