MC33262DG【PDF 7/19 页】IC PFC CONTROLLER CRM 8SOIC

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MC34262, MC33262

http://onsemi.com

Operating Description

The MC34262, MC33262 contain many of the building

blocks and protection features that are employed in modern

high performance current mode power supply controllers.

There are, however, two areas where there is a major

difference when compared to popular devices such as the

UC3842 series. Referring to the block diagrams in

Figures 20, 21, and 22 note that a multiplier has been added

to the current sense loop and that this device does not

contain an oscillator. The reasons for these differences will

become apparent in the following discussion. A description

of each of the functional blocks is given below.

Figure 18. Active Power Factor Correction Preconverter

Rectifiers

PFC Preconverter

High

Frequency

Bypass

Capacitor

Converter

Bulk

Storage

Capacitor

Load

MC34362

Line

Error Amplifier

An Error Amplifier with access to the inverting input and

output is provided. The amplifier is a transconductance

type, meaning that it has high output impedance with

controlled voltagetocurrent gain. The amplifier features

a typical gm of 100 mmhos (Figure 6). The noninverting

input is internally biased at 2.5 V ?2.0% and is not pinned

out. The output voltage of the power factor converter is

typically divided down and monitored by the inverting

input. The maximum input bias current is 0.5 mA, which

can cause an output voltage error that is equal to the product

of the input bias current and the value of the upper divider

resistor R

. The Error Amp output is internally connected

to the Multiplier and is pinned out (Pin 2) for external loop

compensation. Typically, the bandwidth is set below 20 Hz,

so that the amplifiers output voltage is relatively constant

over a given ac line cycle. In effect, the error amp monitors

the average output voltage of the converter over several

line cycles. The Error Amp output stage was designed to

have a relatively constant transconductance over

temperature. This allows the designer to define the

compensated bandwidth over the intended operating

temperature range. The output stage can sink and source

10 mA of current and is capable of swinging from 1.7 V to

6.4 V, assuring that the Multiplier can be driven over its

entire dynamic range.

A key feature to using a transconductance type amplifier,

is that the input is allowed to move independently with

respect to the output, since the compensation capacitor is

connected to ground. This allows dual usage of of the

Voltage Feedback Input pin by the Error Amplifier and by

the Overvoltage Comparator.

Overvoltage Comparator

An Overvoltage Comparator is incorporated to eliminate

the possibility of runaway output voltage. This condition

can occur during initial startup, sudden load removal, or

during output arcing and is the result of the low bandwidth

that must be used in the Error Amplifier control loop. The

Overvoltage Comparator monitors the peak output voltage

of the converter, and when exceeded, immediately

terminates MOSFET switching. The comparator threshold

is internally set to 1.08 V

ref

. In order to prevent false

tripping during normal operation, the value of the output

filter capacitor C

must be large enough to keep the

peaktopeak ripple less than 16% of the average dc

output. The Overvoltage Comparator input to Drive Output

turnoff propagation delay is typically 400 ns. A

comparison of startup overshoot without and with the

Overvoltage Comparator circuit is shown in Figure 24.

Multiplier

A single quadrant, two input multiplier is the critical

element that enables this device to control power factor.

The ac full wave rectified haversines are monitored at Pin 3

with respect to ground while the Error Amp output at Pin 2

is monitored with respect to the Voltage Feedback Input

threshold. The Multiplier is designed to have an extremely

linear transfer curve over a wide dynamic range, 0 V to

3.2 V for Pin 3, and 2.0 V to 3.75 V for Pin 2, Figures 2 and

3. The Multiplier output controls the Current Sense

Comparator threshold as the ac voltage traverses

sinusoidally from zero to peak line, Figure 18. This has the

effect of forcing the MOSFET ontime to track the input

line voltage, resulting in a fixed Drive Output ontime, thus

making the preconverter load appear to be resistive to the

ac line. An approximation of the Current Sense

Comparator threshold can be calculated from the following

equation. This equation is accurate only under the given

test condition stated in the electrical table.

, Pin 4 Threshold H 0.65 (V

th(M)

) V

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