10.2.1 Design Requirements

• VI = 32 V

• VO = 5 V

• IO = 10 A

• fOSC = 20-kHz switching frequency

• VR = 20-mV peak-to-peak (VRIPPLE)

• ΔIL = 1.5-A inductor current change

10.2.2 Detailed Design Procedure

10.2.2.1 Input Power Source

The 32-V dc power source for this supply uses a 120-V input, 24-V output transformer rated at 75 VA. The 24-V

secondary winding feeds a full-wave bridge rectifier, followed by a current-limiting resistor (0.3 Ω) and two filter

capacitors (see Figure 10-2).

120 V

PinName

PinName

Bridge Rectifiers

3 A, 50 V

24 V

3 A

0.3

Ÿ

20 mF

+

20 mF

Figure 10-2. Input Power Source

The output current and voltage are determined by Equation 6 and Equation 7:

RECTIFIER

SECONDARY

V

V

2

24 V

2

34 V

=

´

=

´

=

(6)

O

RECTIFIER(AVG)

O

I

V

5 V

I

I

10 A 1.6 A

V

32 V

»

´

»

´

=

(7)

The 3-A/50-V full-wave bridge rectifier meets these calculated conditions. Figure 10-1 shows the switching and

control sections.

10.2.2.2 Control Circuits

10.2.2.2.1 Oscillator

Connecting an external capacitor and resistor to pins 5 and 6 controls the TL494 oscillator frequency. The

oscillator is set to operate at 20 kHz, using the component values calculated by Equation 8 and Equation 9:

OSC

T

T

1

f

R

C

=

´

(8)

Choose CT = 0.001 μF and calculate RT:

R

T +

1

f

OSC

C

T

+

1

(20

103)

(0.001

10*6)

+ 50 kW

(9)

10.2.2.2.2 Error Amplifier

The error amplifier compares a sample of the 5-V output to the reference and adjusts the PWM to maintain a

constant output current (see Figure 10-3).

TL494

SLVS074I – JANUARY 1983 – REVISED JULY 2022

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