Welcome to Regulator Design School
This page will take you through from the basics of linear series regulator design, to the final circuit showing the Jung Super-Regulator topology. Along the way we will analyse the various circuit elements contribution to performance and look at ways of addressing them. The Sulzer circuit will also be presented as a stop along the road to perfection (or at least somewhere quite close!).
Basic Block Diagram
The circuit above is the basic block diagram of a linear series regulator.
A series control element (imagine it as a variable resistor) is controlled by an error amplifier (X1). This error amp compares a fixed stable voltage from a reference to the regulator's output. If a load is placed at the regulator output the output voltage tends to drop (imagine a potential divider formed by the series control element and the load). This also causes the voltage at the inverting input of the amp to drop (via R1 / R2) which in turn causes X1's output voltage to rise, turning on the series control element harder (imagine it's resistance dropping) causing the voltage at the output to rise - we have regulation!
A simple practical example of the basic linear regulator topology is shown above. The incoming raw supply (V1, 12V) is used to feed the pass device (Q1) and to provide a bias current to D1, a 4.7V zener diode, along with X1 the error amplifier. R1 / D1 in fact form a simple shunt regulator giving a reasonably stable reference voltage to the input of X1, the error amplifier. Since the input impedance of X1 is very high, it does not load the reference voltage and hence we have a stable voltage against which we can compare the regulator's output. This function is provided by R2 / R3 which take a fraction (in this case half) of the output voltage to feed to X1's inverting input. The output voltage in this case is twice the reference voltage, or about 9.4V due to the equal values for R2 / R3.
What are the limitations of the above circuit?
Well there are several, the following sections will analyse these and offer possible solutions.
The reference voltage produced by R1 / D1 has a number of limitations, the primary ones detailed below: -
The details of the above solutions are left as an exercise for the reader, for reasons that will become clear later. An elegant solution is always a nice thing to have, so analysis of the other circuit contributions is worthwhile first - sometimes a single cure exists form multiple problems, preventing a lot of additional complexity and cost.
Series Control Element
Q1 has a number of parameters that require careful selection for the regulator to work as intended.
It's gain (hFE), in terms of absolute and dynamic variation, affects the amount of current required from X1 in order to control it, if too low X1 cannot provide enough base drive and the regulator's performance will drop. It has a direct effect on regulator output impedance. It should be noted that FET's of any description are a fundamentally bad choice here, since gm is much lower than bipolar parts, by almost an order of magnitude.
The device's AC characteristics impact numerous, sometimes subtle parameters - junction capacitances affect frequency response of the regulator as a whole, line rejection at high frequencies and even stability of the error amp if care is not taken to ensure phase shifts introduced do not affect stability margins.
Sometimes a darlington pair is used here, but ac performance can be severely affected by such choices, so choose wisely.
Probably the part that has the single most important effect on performance. If using an op-amp there are a number of parameters that affect performance, some obvious, some more subtle, but all equally important: -
With all the above parameters you MUST look beyond the numbers, to the AC response graphs in the data sheets. The manufacturers only give you the headline best figures, look deeper and things tend to look much worse.
One thing that should not be overlooked here is potentially we could be powering thousands of pounds worth of precious investment, from a few pennies or pounds worth of components.
What would happen in the above circuit if we drew too much current from the regulator, or one of it's components failed? The answer is it could get nasty, there is no over voltage or over current protection in the above circuit.
Adding these elements is not difficult, but it does raise complexity, and care has to be taken to ensure performance does not suffer. Suffice to say an elegant, simple and reasonably safe solution is available, which we will see, as soon as the schematic for the Jung regulator is presented.
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