TK83854 データシートの表示（PDF） - Toko America Inc

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TK83854

HIGH POWER FACTOR PREREGULATOR

Toko America Inc  

TK83854

APPLICATION INFORMATION (CONT.)

used to calculate these resistors:

RIAC = VPK(MAX) / 400 E - 6

= (260 VAC x 2 ) / 400 µA

= 910 k

RREF = RIAC / 4 = 220 k

where VPK is the peak line voltage.

Line Current (ISENSE/MULTOUT )

The voltage drop across the 0.25 Ω current-sense resistor

is applied to Pins 4 and 5 as shown. The current-sense

amplifier also operates with high low-frequency gain, but

unlike the voltage amplifier, it is set up to give the current-

control loop a very wide bandwidth. This enables the line

current to follow the line voltage as closely as possible. In

the present example, this amplifier has a zero at about

500 Hz, and a gain of about 18 dB thereafter.

RMS Line Voltage (VRMS)

An important feature of the TK83854 preregulator is that it

can operate with a three-to-one range of input line voltages,

covering everything from low line in Japan (85 VAC) to

high line in Europe (255 VAC). This is done using line feed-

forward, which keeps the input power constant with varying

input voltage (assuming constant load power). To do this,

the multiplier divides the line current by the square of the

rms value of the line voltage. The voltage applied to Pin 8,

proportional to the average of the rectified line voltage (and

proportional to the RMS value), is squared in the TK83854,

and then used as a divisor by the multiplier block. The

multiplier output, at Pin 5, is a current that increases with

the current at Pin 6 and the voltage at Pin 7, and decreases

with the square of the voltage at pin 8.

With RSET = 15 k:

IOM(MAX) = -3.75 V / 15 k = -250 µA

It is also important to note that the multiplier output current

will never exceed twice IAC.

With the 3.9 k resistor from MULTOUT to the 0.25 Ω current

sense resistor, the maximum current in the current sense

resistor will be:

IRCS(MAX) = (-IOM(MAX) x 3.9 k) / 0.25 Ω = -3.9 A

Having selected RSET, the current sense resistor, and the

resistor from MULTOUT to the current sense resistor,

calculate CT for the desired PWM oscillator frequency from

the equation:

CT = 1.25 / (f OSC x RSET)

PWM Frequency

The PWM oscillator frequency in Figure 1 is 100 kHz. This

value is determined by CT at Pin 14 and RSET at Pin 12.

RSET should be chosen first because it affects the maximum

value of IOM according to the equation:

IOM(MAX) = -3.75 V / RSET

This effectively sets a maximum PWM-controlled current.

Page 10

January 1999 TOKO, Inc.

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