Component Selection for DC Regulated Power Supplies
Here we will see how to design a silicon regulator regulated power supply when the load voltage UR1 and load current IR1 are known.
(1) Initial selection of regulator D1
In general, according to UD1 = UR2 and ID1 ≈ (IR2) max to the initial selection of regulator D1, if the load may be open-circuit should be selected (ID1) max ≈ (2-3) (IR2) max, this is because when the load when all the current will flow through the D1, so ID1 should be appropriate to choose a larger.
(2) Selection of input voltage
Generally, UI=(2-3)UR2 can be selected.
(3) Select current limiting resistor R1
R1=(UI-UR2)/(ID1+IR2)
However, two limit cases need to be considered:
When UI is maximum and the load is open circuit (i.e. IR2=0), the current flowing through D1 is maximum. In order not to exceed the maximum allowable current (ID1)max of D1, a large enough current resistor is required, otherwise it will burn D1. then R1 needs to satisfy:
R1>((UI)max-UR2)/ ID1)max
When UI is minimum and the load current is maximum, the current flowing through D1 is minimum. In order to ensure that D1 can work in the breakdown zone to play the role of voltage regulation, there should be a certain amount of current flowing through D1, generally take 5mA-10mA. then R1 needs to meet:
R1<((UI)min-UR2)/(ID1+(IR2)max)
The value of the current limiting resistor R1 should be selected within the range of the above two formulas.
(4) Checking circuit stability
Circuit stability needs to be determined according to the requirements of the actual circuit, if the stability is not enough, you can increase R1 and UI appropriately, you can also choose a voltage regulator with a smaller dynamic resistance r.
