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Voltage regulator details

Posted by John Swenson on May 14, 2008 at 13:59:17:
The voltage regulator circuit shown is a series pass regulator, not a shunt. Its based on the Gary Pimm CCS with the adition of one extra MOSFET. First a little about how the CCS works and how this makes a good starting point for a voltage regulator.

This type of CCS consists of a voltage regulator developing a contant voltage across a fixed resistor, by ohms law a constant voltage across a fixed resistor means there is a constant current through the resistor. So basically everything in the design is already a voltage regulator! As a matter of fact you CAN use the SBCCS as a regulator exactly as is. The voltage is developed across R1 in Gary's schematic. So take out R1 and put the load there. It works.

As a general purpose regulator the CCS has some problems, primarily because its basically a source follower without any feedback, the output impedance is not very low, it doesn't regulate all that well with changing load currents. What I have done is add Q5 which acts as an error amplifier, it compares the output voltage to the reference voltage and drives the source follower such as to maintain the output voltage matching the reference. This radically loweres the output impedance and due to the high bandwidth of Gary's CCS design the impedance stays low over a wide frequency range.

Next I'll go over some details of how the circuit works and why its a fairly neat implementation. The following is refering to my schematic posted above, not to Gary's circuit, the part identifiers are completely different, although the arrangement is the same except for the one new transistor.

On the right side is a cascoded source follower. The voltage on the bottom (source) of Q1 follows the voltage on its gate minus about 4 volts or so. (if the gate is at 100V the source will be at 96V). The exact difference between the gate and the source will vary somewhat depending on the current flowing through it and the voltage across the transistor. The second is handled by Q2. Its called a cascode, it maintains a constant voltage across Q1, any difference in the input voltage is developed across Q2, thus preventing it from changing the gate to source voltage of Q1.

The left side is in charge of producing the voltage references to drive the gates of Q1 and Q2. This is done by Q3 and Q4 which form their own high quality low current (0.33mA) CCS. This current flows through R2, R3 and R8 developing constant voltages across them. (R1 is part of the CCS) The voltages can be changed by changing the resistor values.

R3 and R8 comprise the main voltage reference. They are filtered by C1 to make a low noise reference. R2 is the cascode bias. It determines the voltage difference between Q1 and Q2. As described so far this is the original Gary design. The Q3/Q4 CCS performs two functions, main voltage reference and cascode bias.

The modification to a feedback regulator is done by adding Q5. This is basically a comparitor, one input is the reference voltage underneath it, the other is the output voltage. The "output" is the drain of the MOSFET which drives the gate of Q1. The beauty here is that the same CCS that provides the consant current for the references is also the CCS load for the error amplifier! The same CCS is being used for three different tasks. Most error amplifiers just use a resistor load for the amp, but they work even better with a CCS load, but that is rearely done because of the extra complexity of the CCS load. Because one was already in Gary's design it was a piece of cake to have a CCS loaded error amp.

This error amp takes care of the changes in gate to source voltage caused by current fluctuations from the load. The result is very good regulation no matter what the load is doing.

Why didn't Gary do this in the original CCS design? Because its not really needed in a CCS. The problem is caused by changing current through Q1, but in a CCS the current through Q1 is constant so it doesn't matter. When using it as a general purpose regulator it DOES matter, so I added the error amp.

If anyone wants to use this as a voltage regulator for other purposes make sure you include C6 and R7. Without them the regulator will oscilate up in the MHz region somewhere. Especially if you convert an existing board of Gary's make sure you add C6 (R7 goes in place of Gary's R1).

Well thats it, I hope thats not TOO much detail on how this works. I've tried this in many different places and its the best sounding regulator I've ever tried, including a bunch of different shunt regulator designs.

John S.