0090-B1771:Since the net difference between the two voltage inputs to the comparison amplifier is kept at zero by feedback action, the voltage across resistor RR is also held equal to the reference voltage ER. Thus the programming current IP flowing through RR is constant and equal to ER/RR. The input impedance of the comparison amplifier is very high, so essentially all of the current IP flowing through RR also flows through RP. Because this programming current IP is constant, EP (and hence the output voltage) is variable and directly proportional to RP. Thus, the output voltage becomes zero if RP is reduced to zero ohms. Of course, not all series regulated supplies are continuously adjustable down to zero volts.
Those of the OEM modular type are used in system applications and, thus, their output voltage adjustment is restricted to a narrow range or slot. However, operation of these supplies is virtually identical to that just described.
Pros and Cons of Series Regulated Supplies
Series regulated supplies have many advantages and usually provide the simplest most effective means of satisfying high performance and low power requirements.
In terms of performance, series regulated supplies have very precise regulation properties and respond quickly to variations of the line and load. Hence, their line and load regulation and transient recovery time are superior to supplies using any of the other regulation techniques. These supplies also exhibit the lowest ripple and noise, are tolerant of ambient temperature changes, and with their circuit simplicity, have a high reliability.
Power supply performance specifications are described in the Definitions section of this handbook.
The major drawback of the series regulated supply is its relatively low efficiency. This is caused mainly by the series transistor which, operating in the linear mode, continuously dissipates power in carrying out its regulation function. Efficiency (defined as the percentage of a supply's input power that it can deliver as useful output: POUT/PIN), ranges typically between 30 and 45% for series regulated supplies. With the present need for energy conservation, this inefficiency is being scrutinized very closely.
For many of today's applications, the size and weight of linear supplies constitute another disadvantage. The power transformer, inductors, and filter capacitors necessary for operation at the 60Hz line frequency tend to be large and heavy, and the heat sink required for the inherently dissipative series regulator increases the overall size.
