ML4841 Просмотр технического описания (PDF) - Micro Linear Corporation

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ML4841

Variable Feedforward PFC/PWM Controller Combo

Micro Linear Corporation 

ML4841

FUNCTIONAL DESCRIPTION (Continued)

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

discharge 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

=

1× 10−5

(0.463)(390 × 10−12)

=

56.2kΩ

(10)

PWM SECTION

Pulse Width Modulator

The PWM section of the ML4841 is straightforward, but

there are several points which should be noted. Foremost

among these is its inherent synchronization to the PFC

section of the device, to which it also provides its basic

timing. The PWM operates in current-mode. In

applications utilizing current mode control, the PWM

ramp (RAMP 2) is usually derived directly from a current

sensing resistor or current transformer in the primary of

the output stage, and is thereby representative of the

current flowing in the converter’s output stage. The DC

ILIMIT comparator provides cycle-by-cycle current limiting

and is connected to RAMP 2 internally. If the current sense

signal exceeds the 1V threshold, the PWM switch is

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

pulse at the start of the next PWM power cycle.

PWM Current Limit

The DC ILIMIT comparator is a cycle-by-cycle current

limiter for the PWM section. Should the input voltage at

this pin ever exceed 1V, the output of the PWM will be

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

pulse at the start of the next PWM power cycle.

VIN OK Comparator

The VIN OK comparator monitors the DC output of the

PFC and inhibits the PWM if this voltage on VFB is less

than its nominal 2.5V. Once this voltage reaches 2.5V,

which corresponds to the PFC output capacitor being

charged to its rated boost voltage, the soft-start

commences.

PWM Control (RAMP 2)

The PWM section utilizes current mode control. RAMP 2

is generally used as the sampling point for a voltage

representing the current in the primary of the PWM’s

output transformer, derived either by a current sensing

resistor or a current transformer.

Soft Start

Start-up of the PWM is controlled by the selection of the

external capacitor at SS. A current source of 50µA supplies

the charging current for the capacitor, and start-up of the

PWM begins at 1.25V. Start-up delay can be programmed

by the following equation:

CSS

=

tDELAY

×

50µA

1.25V

(11)

where CSS is the required soft start capacitance, and

tDELAY is the desired start-up delay.

It is important that the time constant of the PWM soft-start

allows the PFC time to generate sufficient output power

for the PWM section. The PWM start-up delay should be

at least 5ms.

Solving for the minimum value of CSS:

CSS

=

5ms

×

50µA

1.25V

=

200nF

VBIAS

VCC

ML4841

GND

10nF

ceramic

1µF

ceramic

Figure 3. External Component Connections to VCC

10

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