CM6800AGIP datasheet(9/18 Pages) CHAMP

9 / 18 page

CM6800A

LOW START-UP CURRENT PFC/PWM CONTROLLER COMBO

2009/08/10 Rev. 1.6

Champion Microelectronic Corporation

Page 9

The output of the gain modulator is a current signal, in the

form of a full wave rectified sinusoid at twice the line

frequency. This current is applied to the virtual-ground

(negative) input of the current error amplifier. In this way the

gain modulator forms the reference for the current error

loop, and ultimately controls the instantaneous current draw

of the PFC form the power line. The general for of the

output of the gain modulator is:

IGAINMOD =

RMS

VEAO

x 1V

(1)

More exactly, the output current of the gain modulator is

given by:

IGAINMOD = K x (VEAO – 0.625V) x IAC

Where K is in units of V

-1

Note that the output current of the gain modulator is limited

around 228.47

μ A and the maximum output voltage of the

gain modulator is limited to 228.47uA x 3.5K=0.8V. This

0.8V also will determine the maximum input power.

However, IGAINMOD cannot be measured directly from ISENSE.

ISENSE = IGAINMOD-IOFFSET and IOFFSET can only be measured

when VEAO is less than 0.5V and IGAINMOD is 0A. Typical

IOFFSET is around 60uA.

Selecting RAC for IAC pin

IAC pin is the input of the gain modulator. IAC also is a

current mirror input and it requires current input. By

selecting a proper resistor RAC, it will provide a good sine

wave current derived from the line voltage and it also helps

program the maximum input power and minimum input line

voltage.

RAC=Vin peak x 7.9K. For example, if the minimum line

voltage is 80VAC, the RAC=80 x 1.414 x 7.9K=894Kohm.

Current Error Amplifier, IEAO

The current error amplifier’s output controls the PFC duty

cycle to keep the average current through the boost

inductor a linear function of the line voltage. At the inverting

input to the current error amplifier, the output current of the

gain modulator is summed with a current which results from

a negative voltage being impressed upon the ISENSE pin.

The negative voltage on ISENSE represents the sum of all

currents flowing in the PFC circuit, and is typically derived

from a current sense resistor in series with the negative

terminal of the input bridge rectifier.

In higher power applications, two current transformers are

sometimes used, one to monitor the IF of the boost diode. As

stated above, the inverting input of the current error amplifier

is a virtual ground. Given this fact, and the arrangement of

the duty cycle modulator polarities internal to the PFC, an

increase in positive current from the gain modulator will

cause the output stage to increase its duty cycle until the

voltage on ISENSE is adequately negative to cancel this

increased current. Similarly, if the gain modulator’s output

decreases, the output duty cycle will decrease, to achieve a

less negative voltage on the ISENSE pin.

Cycle-By-Cycle Current Limiter and Selecting RS

The ISENSE pin, as well as being a part of the current feedback

loop, is a direct input to the cycle-by-cycle current limiter for

the PFC section. Should the input voltage at this pin ever be

more negative than –1V, the output of the PFC will be

disabled until the protection flip-flop is reset by the clock

pulse at the start of the next PFC power cycle.

RS is the sensing resistor of the PFC boost converter. During

the steady state, line input current x RS = IGAINMOD x 3.5K.

Since the maximum output voltage of the gain modulator is

IGAINMOD max x 3.5K= 0.8V during the steady state, RS x line

input current will be limited below 0.8V as well. Therefore, to

choose RS, we use the following equation:

RS =0.8V x Vinpeak/(2x Line Input power)

For example, if the minimum input voltage is 80VAC, and the

maximum input rms power is 200Watt, RS = (0.8V x 80V x

1.414)/(2 x 200) = 0.226 ohm.

PFC OVP

In the CM6800A, PFC OVP comparator serves to protect the

power circuit from being subjected to excessive voltages if

the load should suddenly change. A resistor divider from the

high voltage DC output of the PFC is fed to VFB. When the

voltage on VFB exceeds 2.75V, the PFC output driver is shut

down. The PWM section will continue to operate. The OVP

comparator has 250mV of hysteresis, and the PFC will not

restart until the voltage at VFB drops below 2.50V. The VFB

power components and the CM6800A are within their safe

operating voltages, but not so low as to interfere with the

boost voltage regulation loop. Also, VCC OVP can be served

as a redundant PFCOVP protection. VCC OVP threshold is

17.9V with 1.5V hysteresis.

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