Page 6

January 1999 TOKO, Inc.

TK75002

Figure 3 shows the magnitude and phase diagram of the

transfer function. As can be seen, the feed-forward path

adds a left half-plane zero (sometimes called a "parasitic"

zero) to the transfer function of the ideal integrator. The

parasitic zero is undesirable because it increases the high

frequency loop gain of the system, potentially leading to

subharmonic instability or chaotic behavior. To avoid the

effect of the zero, either a low pass (decoupling) filter must

be added to the supply rail of the opto-coupler, or an

additional pole must be introduced around the frequency

of the zero, or the loop gain must be reduced. (Note that

reducing the loop gain slows down the transient response

of the system.)

The TK75002 eliminates the parasitic zero by changing

the reference point of the error signal from the GND pin to

the V

circuit between the output of the error amplifier and the

buffer of the pulse-amplitude modulator (see Figure 4).

The output voltage of the level shifter is proportional to the

error signal (i.e., the voltage of the COMP pin) and is

conveyed relative to the supply voltage V

is that the transfer function becomes:

H(s) = Z

(4)

In the case of a capacitive feedback impedance and

resistive input impedance, the transfer function will be that

of an ideal integrator, without the parasitic zero.

FIGURE 3: MAGNITUDE AND PHASE VS.

FREQUENCY OF AN ERROR AMPLIFIER / OPTO-

COUPLER COMBINATION

H (dB)

-20 dB / dec

0

f

1 / (2

-90

°

0

°

f

φ

APPLICATION INFORMATION

THE PARASITIC ZERO AND HOW IT IS ELIMINATED IN

THE TK75002

In a feedback-regulated isolated power supply, isolation is

usually provided by an opto-coupler whose photodiode

section is connected between the output of the power

supply and the output of the error amplifier (in series with

a resistor that converts the voltage difference between the

two outputs into a current). Due to the feed-forward path

established by that connection, the transfer function of the

error amplifier is significantly modified (the modification in

the transfer function appears also if the opto-coupler is

replaced by the combination of a transformer and peak

detector driven by a pulse amplitude modulator).

Figure 2 shows the error amplifier/opto-coupler

combination. The output voltage V

measured across the photodiode and the series resistor)

can be written as:

V

(1)

From (1), the transfer function of the error amplifier (from

the supply voltage V

H(s) = V

(2)

In the case when the feedback impedance is a capacitor C

and the input impedance is a resistor R, the transfer

function becomes:

H(s) = (1 + SRC) / SRC

(3)

FIGURE 2: ERROR AMPLIFIER / OPTO-COUPLER

COMBINATION

V 2

Vref

V 1

R B

ZFB

ZIN