R8

5.1 kW

VO

VREF

+

−

1

14

13

Error

Amplifier

2

3

TL494

R9

5.1 kW

R5

510 W

R7

51 kW

R3

5.1 kW

R4

5.1 kW

TL494

Figure 10-3. Error-Amplifier Section

The TL494 internal 5-V reference is divided to 2.5 V by R3 and R4. The output-voltage error signal also is

divided to 2.5 V by R8 and R9. If the output must be regulated to exactly 5.0 V, a 10-kΩ potentiometer can be

used in place of R8 to provide an adjustment.

To increase the stability of the error-amplifier circuit, the output of the error amplifier is fed back to the inverting

input through RT, reducing the gain to 101.

10.2.2.2.3 Current-Limiting Amplifier

The power supply was designed for a 10-A load current and an IL swing of 1.5 A, therefore, the short-circuit

current should be:

L

SC

O

I

I

I

10.75 A

2

=

+

=

(10)

The current-limiting circuit is shown in Figure 10-4.

15

16

R1

1 NŸ

R2

4 NŸ

Load

+

TL494

14

VREF

TL494

R13

0.1 Ÿ

VO

Figure 10-4. Current-Limiting Circuit

Resistors R1 and R2 set the reference of approximately 1 V on the inverting input of the current-limiting amplifier.

Resistor R13, in series with the load, applies 1 V to the non-inverting terminal of the current-limiting amplifier

when the load current reaches 10 A. The output pulse width reduces accordingly. The value of R13 is calculated

in Equation 11.

1V

R13

0.1

10 A

=

=

W

(11)

10.2.2.2.4 Soft Start and Dead Time

To reduce stress on the switching transistors at the start-up time, the start-up surge that occurs as the output

filter capacitor charges must be reduced. The availability of the dead-time control makes implementation of a

soft-start circuit relatively simple (see Figure 10-5).

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TL494

SLVS074I – JANUARY 1983 – REVISED JULY 2022

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