10.2.2.3 Inductor Calculations

The switching circuit used is shown in Figure 39.

R1

C1

D1

S1

L

VO

VI

Figure 10-6. Switching Circuit

The size of the inductor (L) required is:

d

= duty cycle = VO/VI = 5 V/32 V = 0.156

f

= 20 kHz (design objective)

ton

= time on (S1 closed) = (1/f) × d = 7.8 μs

toff

= time off (S1 open) = (1/f) – ton = 42.2 μs

L

≉ (VI – VO ) × ton/ΔIL

≉ [(32 V – 5 V) × 7.8 μs]/1.5 A

≉ 140.4 μH

10.2.2.4 Output Capacitance Calculations

Once the filter inductor has been calculated, the value of the output filter capacitor is calculated to meet the

output ripple requirements. An electrolytic capacitor can be modeled as a series connection of an inductance, a

resistance, and a capacitance. To provide good filtering, the ripple frequency must be far below the frequencies

at which the series inductance becomes important. So, the two components of interest are the capacitance and

the effective series resistance (ESR). The maximum ESR is calculated according to the relation between the

specified peak-to-peak ripple voltage and the peak-to-peak ripple current.

O(ripple)

L

V

V

ESR(max)

0.067

I

1.5 A

D

=

=

»

W

D

(14)

The minimum capacitance of C3 necessary to maintain the VO ripple voltage at less than the 100-mV design

objective is calculated according to Equation 15:

L

3

O

I

1.5 A

C3

94 F

8f

V

8

20

10

0.1 V

D

=

=

=

m

D

´

´

´

(15)

A 220-mF, 60-V capacitor is selected because it has a maximum ESR of 0.074 Ω and a maximum ripple current

of 2.8 A.

10.2.2.5 Transistor Power-Switch Calculations

The transistor power switch was constructed with an NTE153 pnp drive transistor and an NTE331 npn output

transistor. These two power devices were connected in a pnp hybrid Darlington circuit configuration (see Figure

10-7).

www.ti.com

TL494

SLVS074I – JANUARY 1983 – REVISED JULY 2022

Copyright © 2022 Texas Instruments Incorporated

Submit Document Feedback

17

Product Folder Links: TL494