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ML4841

Variable Feedforward PFC/PWM Controller Combo

Fairchild Semiconductor 

PRODUCT SPECIFICATION

ML4841

There are two major concerns when compensating the

voltage loop error amplifier; stability and transient response.

Optimizing interaction between transient response and

stability requires that the error amplifier’s open-loop cross-

over frequency should be 1/2 that of the line frequency, or

23Hz for a 47Hz line (lowest anticipated international power

frequency). The gain vs. input voltage of the ML4841’s

voltage error amplifier has a specially shaped nonlinearity

such that under steady-state operating conditions the

transconductance of the error amplifier is at a local

minimum. Rapid perturbations in line or load conditions

will cause the input to the voltage error amplifier (VFB) to

deviate from its 2.5V (nominal) value. If this happens, the

transconductance of the voltage error amplifier will increase

significantly, as shown in the Typical Performance Charac-

teristics. This increases the gain-bandwidth product of the

voltage loop, resulting in a much more rapid voltage loop

response to such perturbations than would occur with a

conventional linear gain characteristic.

The current amplifier compensation is similar to that of the

voltage error amplifier with the exception of the choice of

crossover frequency. The crossover frequency of the current

amplifier should be at least 10 times that of the voltage

amplifier, to prevent interaction with the voltage loop. It

should also be limited to less than 1/6th that of the switching

frequency, e.g. 16.7kHz for a 100kHz switching frequency.

For more information on compensating the current and volt-

age control loops, see Application Notes 33 and 34. Appli-

cation Note 16 also contains valuable information for the

design of this class of PFC.

Oscillator (RT/CT)

The oscillator frequency is determined by the values Of RT

and CT, which determine the ramp and off-time of the

oscillator output clock:

fOSC = t--R----A----M----P----+-----t--D--1--I--S---C----H----A---R----G----E-

(2)

The ramp-charge time of the oscillator is derived from the

following equation:

tRAMP

=

CT

×

RT

×

I

n





V-V----RR---EE----FF----––-----13---..--27---55--

(3)

at VREF = 7.5V:

tRAMP = CT × RT × 0.51

The discharge time of the oscillator may be determined

using:

tDISCHARGE = 5---2-.--1-.-5-m---V---A-- × CT = 490 × CT

(4)

The deadtime is so small (tRAMP >> tDEADTIME) that the

operating frequency can typically be approximated by:

fOSC = -t-R----A-1---M----P-

(5)

EXAMPLE:

For the application circuit shown in the data sheet, with the

oscillator running at:

fOSC = 200kHz = -t-R----A-1---M----P-

tRAMP = 0.51 × RT × CT = 5 × 10–6

Solving for RT x CT yields 1 x 10-5. Selecting standard com-

ponents values, CT = 390pF, and RT = 24.9kΩ.

RAMP 1

The ramp voltage on this pin serves as a reference to which

the PFC’s current error amp output is compared in order to

set the duty cycle of the PFC switch. The external ramp volt-

age is derived from a RC network similar to the oscillator’s.

The PWM’s oscillator sends a synchronous pulse every other

cycle to reset this ramp.

The ramp voltage should be limited to no more than the out-

put high voltage (6V) of the current error amplifier. The tim-

ing resistor value should be selected such that the capacitor

will not charge past this point before being reset. In order to

ensure the linearity of the PFC loop’s transfer function and

improve noise immunity, the charging resistor should be

connected to the 13.5V VCC rather than the 7.5V reference.

This will keep the charging voltage across the timing cap in

the "linear" region of the charging curve.

The component value selection is similar to oscillator RC

component selection.

fOSC = t--C----H----A---R----G----E-----+----1-t-D----I--S---C----H----A---R----G----E--

(6)

The charge time of Ramp 1 is derived from the following

equations:

tCHARGE = -f-O---2-S----C--

(7)

tCHARGE = CT × RT × In-V-V---C-C--C--C---–--–---R-R---a--a--m-m----p-p----V-P---a-e--l-al--e--k-y-

(8)

At VCC = 13.5V and assuming Ramp Peak = 5V to allow for

component tolerances:

tCHARGE = 0.463 × RT × CT

(9)

The capacitor value should remain small to keep the dis-

charge energy and the resulting discharge current through the

part small. A good value to use is the same value used in the

PWM timing circuit (CT).

For the application circuit shown in the data sheet, using a

200kHz PWM and 390pF timing cap yields RT:

RT = (---0---.--4---6----3---1)---(--×3----91---0-0---–-×--5---1---0---–---1---2---) = 56.2kΩ

(10)

REV. 1.0.3 6/13/01

9

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