TK75003 Ver la hoja de datos (PDF) - Toko America Inc

Número de pieza

componentes Descripción

Fabricante

TK75003

PWM CONTROLLER

Toko America Inc  

TK75003

APPLICATION INFORMATION

SELF-BIASED POWER SUPPLY WITH CONSTANT-

FREQUENCY CURRENT-MODE CONTROL

Figure 3(a) shows the TK75003 IC in the typical application:

a flyback converter with self-bias and constant-frequency

current- mode control. Figure 3(b) shows the feedback 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

feedback pin: 1) the feedback voltage that develops across

the resistor R1, 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.

VIN

RST

VAUX

D3

n3

D2

+

n2

CAUX

VCC

CT

CT

DRV

OC

FB

GND

RS

+

VOUT

-

STABILIZING

RAMP

SWITCH

CURRENT

SIGNAL

0.98 V

FEEDBACK

VOLTAGE

0

R1

TL431

(a)

(b)

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 Q1, turns on and pulls up the

feedback pin when the switch current times the resistance

of the sense RS reaches the threshold set by the resistive

divider R2 and R3 and the base-emitter voltage of Q1.

VIN

VAUX

+

OC

VCC

R2

CT

DRV

FB

R3

GND

OC

Q1

RS

R1

TL431

PWM

RAMP

FEEDBACK

VOLTAGE

0.98 V

0

(a)

(b)

FIGURE 4: TK75003 IN A VOLTAGE-MODE-

CONTROLLED CONVERTER WITH ADDITIONAL

CYCLE-BY-CYCLE CURRENT LIMIT

(a) SCHEMATIC (b) VOLTAGE AT FEEDBACK PIN

FIGURE 3: TK75003 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 TK75003 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 R1, that terminates the feedback pin. In current-

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 Q1

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

pin and feedback pin voltages for the TK75003. The

transistor Q1 keeps the timing pin at ground potential

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

drive output returns to low and Q1 is turned off. The off-time

for typical charge and discharge currents and peak and

valley voltages is:

Page 10

January 1999 TOKO, Inc.

Share Link: