NTE331

TL494

Control

Q2

11

10

9

Q1

NTE153

32 V

8

R10

270 W

R12

30 W

R11

100 W

I

O

L

I

10.8 A

2

D

+

=

Figure 10-7. Power-Switch Section

The hybrid Darlington circuit must be saturated at a maximum output current of IO + ΔIL/2 or 10.8 A. The

Darlington hFE at 10.8 A must be high enough not to exceed the 250-mA maximum output collector current of the

TL494. Based on published NTE153 and NTE331 specifications, the required power-switch minimum drive was

calculated by Equation 16 through Equation 18 to be 144 mA:

FE

C

h

(Q1) at I of 3 A

15

=

(16)

FE

C

h

(Q2) at I of 10.0 A

5

=

(17)

L

O

B

FE

FE

I

I

2

i

144mA

h

(Q2) h

(Q1)

+

³

³

´

(18)

The value of R10 was calculated by:

[

]

I

BE

CE

B

V

V

(Q1)

V

(TL494)

32 (1.5

0.7)

R10

i

0.144

R10

207

-

+

-

+

£

=

£

W

(19)

Based on these calculations, the nearest standard resistor value of 220 Ω was selected for R10. Resistors R11

and R12 permit the discharge of carriers in switching transistors when they are turned off.

The power supply described demonstrates the flexibility of the TL494 PWM control circuit. This power-supply

design demonstrates many of the power-supply control methods provided by the TL494, as well as the versatility

of the control circuit.

TL494

SLVS074I – JANUARY 1983 – REVISED JULY 2022

www.ti.com

18

Submit Document Feedback

Copyright © 2022 Texas Instruments Incorporated

Product Folder Links: TL494