TL494_15 Datasheet, PDF (18/33 Pages) Unisonic Technologies – VOLTAGE MODE PWM CONTROL CIRCUIT

TL494
SLVS074G – JANUARY 1983 – REVISED JANUARY 2015
32 V
R11
100 W
11 10
9
NTE331
Q2
R12
DI_L
30 W ^{IO +} ₂ ^{= 10.8 A}
Q1
NTE153
R10
270 W
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Control
TL494
8
Figure 15. Power-Switch Section
The hybrid Darlington circuit must be saturated at a maximum output current of I_O + ΔI_L/2 or 10.8 A. The
Darlington h_FE 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:
h_FE (Q1) at I_C of 3 A = 15
(16)
h_FE (Q2) at I_C of 10.0 A = 5
(17)
i_B
³
I_O
+
I_L
2
h_FE (Q2) ´ h_FE (Q1)
³
144 mA
(18)
The value of R10 was calculated by:
R10 £ ^{VI - [VBE(Q1) + VCE(TL494)]} = ^{32 - (1.5 + 0.7)}
i_B
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.
18
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