MC34262 데이터 시트보기 (PDF) - Motorola => Freescale

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MC34262

Power Factor Controllers

Motorola => Freescale 

MC34262 MC33262

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

voltage–to–current gain. The amplifier features a typical gm

of 100 µmhos (Figure 5). 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 µA, 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 R2. 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 amplifier’s 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 µA 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 Vref. In order to prevent false tripping during

normal operation, the value of the output filter capacitor C3

must be large enough to keep the peak–to–peak ripple less

than 16% of the average dc output. The Overvoltage

Comparator input to Drive Output turn–off propagation delay

is typically 400 ns. A comparison of startup overshoot without

and with the Overvoltage Comparator circuit is shown in

Figure 23.

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, Figure 1. 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 on–time

to track the input line voltage, resulting in a fixed Drive Output

on–time, 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.

VCS, Pin 4 Threshold ≈ 0.65 (VPin 2 – Vth(M)) VPin 3

A significant reduction in line current distortion can be

attained by forcing the preconverter to switch as the ac line

voltage crosses through zero. The forced switching is

achieved by adding a controlled amount of offset to the

Multiplier and Current Sense Comparator circuits. The

equation shown below accounts for the built–in offsets and is

accurate to within ten percent. Let Vth(M) = 1.991 V

VCS, Pin 4 Threshold = 0.544 (VPin 2 – Vth(M)) VPin 3

+ 0.0417 (VPin 2 – Vth(M))

Zero Current Detector

The MC34262 operates as a critical conduction current

mode controller, whereby output switch conduction is initiated

by the Zero Current Detector and terminated when the peak

inductor current reaches the threshold level established by

the Multiplier output. The Zero Current Detector initiates the

next on–time by setting the RS Latch at the instant the

inductor current reaches zero. This critical conduction mode

of operation has two significant benefits. First, since the

MOSFET cannot turn–on until the inductor current reaches

zero, the output rectifier reverse recovery time becomes less

critical, allowing the use of an inexpensive rectifier. Second,

since there are no deadtime gaps between cycles, the ac line

current is continuous, thus limiting the peak switch to twice

the average input current.

The Zero Current Detector indirectly senses the inductor

current by monitoring when the auxiliary winding voltage falls

below 1.4 V. To prevent false tripping, 200 mV of hysteresis is

provided. Figure 9 shows that the thresholds are

well–defined over temperature. The Zero Current Detector

input is internally protected by two clamps. The upper 6.7 V

clamp prevents input overvoltage breakdown while the lower

0.7 V clamp prevents substrate injection. Current limit

protection of the lower clamp transistor is provided in the

event that the input pin is accidentally shorted to ground. The

Zero Current Detector input to Drive Output turn–on

propagation delay is typically 320 ns.

MOTOROLA ANALOG IC DEVICE DATA

7

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