TL494_15 Datasheet, PDF (14/33 Pages) Unisonic Technologies – VOLTAGE MODE PWM CONTROL CIRCUIT

TL494
SLVS074G – JANUARY 1983 – REVISED JANUARY 2015
www.ti.com
Typical Application (continued)
10.2.1 Design Requirements
• V_I = 32 V
• V_O = 5 V
• I_O = 10 A
• f_OSC = 20-kHz switching frequency
• V_R = 20-mV peak-to-peak (V_RIPPLE)
• ΔI_L = 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).
Bridge
Rectifiers
3 A/50 V
120 V
24 V
3A
0.3 W
+
20,000 μF
+32 V
+
20,000 μF
Figure 10. Input Power Source
The output current and voltage are determined by Equation 6 and Equation 7:
V_RECTIFIER = V_SECONDARY ´ 2 = 24 V ´ 2 = 34 V
(6)
I_{RECTIFIER( AVG)}
»
V_O
V_I
´ I_O
»
5V
32 V
´10 A = 1.6 A
(7)
The 3-A/50-V full-wave bridge rectifier meets these calculated conditions. Figure 9 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:
1
f_OSC = _{RT ´ CT}
(8)
Choose C_T = 0.001 μF and calculate R_T:
R_T
+
1
f_OSC
C_T ⁺ (20
10³)
1
(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 11).
14
Submit Documentation Feedback
Product Folder Links: TL494
Copyright © 1983–2015, Texas Instruments Incorporated