Theory Of Transistor Power Switch Calculations

The transistor power switch was constructed with a TIP30 pnp drive transistor and a TIP73 npn output transistor. These two power devices were connected in a pnp hybrid Darlington circuit configuration as shown in Figure 6-25.

Figure 6-25. Power Switch Section

The hybrid Darlington 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 TL594. Based on published TIP30 and TIP73 hFE

specifications, the required power switch minimum drive was calculated by the following equations to be 108 mA.

Used on these calculations, the nearest standard resistor value of 270 Ω was selected for R10. Resistors R11 and R12 permit the discharge of carriers in the switching transistors when they are turned off.

The power supply described demonstrates the flexibility of the TL594 pulse-width-modulation control circuit. This power supply design demonstrates many of the power supply control methods provided by the TL594 as well as the versatility of the control circuit.