Page 10

January 1999 TOKO, Inc.

TK75001

APPLICATION INFORMATION

SELF-BIASED POWER SUPPLY WITH CONSTANT-

FREQUENCY CURRENT-MODE CONTROL

Figure 3(a) shows the TK75001 IC in the typical application:

a flyback converter with self-bias and constant-frequency

current-mode control. Figure 3(b) shows the FB Pin voltage.

In the converter, the voltage-error amplifier (a TL431 shunt

regulator IC) is located at the output side and the error

signal is transmitted to the input side through the opto-

coupler OC. Three signals are added together at the FB

Pin: 1)the feedback voltage that develops across the

resistor R

1

, 2) the switch current signal, and 3) the stabilizing

ramp. In each cycle, the MOSFET switch is turned off when

the sum of those three signals reaches 0.98 V.

FIGURE 3: TK75001 IN A SELF-BIASED FLYBACK

CONVERTER WITH CONSTANT-FREQUENCY

VOLTAGE-MODE CONTROL

(a) SCHEMATIC

(b) VOLTAGE AT FEEDBACK PIN

POWER SUPPLY WITH CONSTANT-FREQUENCY

VOLTAGE-MODE CONTROL AND CYCLE-BY-CYCLE

CURRENT LIMIT

Voltage-mode control is free from some of the

disadvantages (e.g., subharmonic instability and noise

sensitivity) of current-mode control. It is very easy to

implement that control method with the TK75001 IC.

Figure 4(a) shows the IC in a voltage-mode-controlled

flyback converter. Figure 4(b) shows the feedback pin

voltage. The only circuit difference between current-mode

control and voltage-mode control is in the connection of

the resistor R

mode control, that resistor is connected to the current-

sense resistor of the converter. In voltage-mode control,

that resistor is connected to ground.

In voltage-mode control, overload protection can be realized

by adding a simple circuit to the control IC, as shown in the

figure. The PNP transistor Q

1

, turns on and pulls up the

feedback pin when the switch current times the resistance

of the sense R

S

reaches the threshold set by the resistive

divider R

2

and R

FIGURE 4: TK75001 IN A VOLTAGE-MODE-

CONTROLLED CONVERTER WITH ADDITIONAL

CYCLE-BY-CYCLE CURRENT LIMIT

(a) SCHEMATIC

(b) VOLTAGE AT FEEDBACK PIN

POWER SUPPLY WITH CONSTANT OFF-TIME

CURRENT-MODE CONTROL

The advantages of constant off-time current-mode control

over constant-frequency current-mode control are: 1) there

is no need for a stabilizing ramp, 2) the converter is free

from subharmonic instability (i.e., there is no need for

slope compensation), and 3) the line voltage variation is

automatically canceled in buck-derived converters (e.g.,

the forward converter). Figure 5 shows the implementation

of that control method. As can be seen, a transistor Q

1

must be added to the controller. Figure 6 shows the timing-

pin and feedback pin voltages for the TK75001. The

transistor Q

1

keeps the timing pin at ground potential

during the on-time of the switch. Timing begins when the

drive output returns to low and Q

1

is turned off. The off-time

for typical charge and discharge currents and peak and

valley voltages is:

t

OFF

= C

T

x 14 k

Ω.

GND

VCC

CT

FB

DRV

CAUX

R1

VAUX

RST

VIN

n3

n2

D3

RS

OC

TL431

+

+

VOUT

D2

FEEDBACK

VOLTAGE

SWITCH

CURRENT

SIGNAL

STABILIZING

RAMP

0.98 V

0

-

CT

GND

VCC

CT

FB

DRV

VAUX

VIN

RS

TL431

+

R3

Q1

R1

R2

OC

OC

FEEDBACK

VOLTAGE

PWM

RAMP

0

0.98 V

(b)

(a)

(b)

(a)