Transistor buffered Voltage regulator question, output ripple.

[David_]

Hello.
Sorry for the vague description but the question is hard to describe, please refer to the picture below.


I have no formal education which shows in many areas.

The circuit is a adjustable regulator intended to be used as a general lab supply(additional circuits needed, not in the schematic), in node A there are some amount of ripple voltage. I don't know how much but it's definitely to high, but the regulator attenuates the ripple considerably.
But how does the amount of ripple in node B impact the amount of ripple in node C?

First I though that the clean voltage in node B would result in a clean voltage in node C, but that might be false...?
Don't mind specific component values.

I'm lost, does anyone have anything to say about this?

Regards

 

[Audioguru]

Didn't you read the datasheet for the TPS7A4701? You might have destroyed it with an input voltage higher than the maximum allowed input of 36V and a maximum input of 35V is recommended.
The datasheet does not show a slow darlington buffer transistor like you have. I think it will oscillate.
Your voltage setting resistors have extremely low values.

 

[David_]

Sorry for being unclear, "Don't mind specific component values." I ment that this schematic is not accurate in regard to values, the resistor divider values are bunk, they should be 160k, 20k and 10k or there about.
And the input voltage is or was restricted to exactly 36V but I have now changed regulator to TPS7A4001. Its a high voltage(10-100Vdc input, 1,2V-90V output) regulator that comes in a much easier IC package, MSOP8 instead of VQFN20.
There are a 48V version as well but I might try to build some high voltage supply later, and no the datasheet does not show any such circuit.
But I hope that I can realize some kind of circuit such as this, as it is know I kind of have to since TPS7A4001 can only handle 50mA instead of 1A as 4701 does.
But it all comes down to if the ripple is being attenuated, as if the regulator output where being given a higher current capability while maintaining the voltage quality.
I have had trouble using a VQFN package properly but know that I have TPS7A4001 in MSOP8 I will finish the board and print out the overhead and etch a prototype, I hope there will be no oscillations due to the circuit but we shall see.

 

[chuckey]

Resistor RR shown as 100 ohms, this is a mistake. I can not see any reason for it. C10 seems to be wrong as its feedinng ripple back from the output of the chip into the FB connection. This is so if the ripple appears on its output pin, this would be fed back so as to reduce it. BUT if the load has a varying current, then the output terminal of the regulator should try and chase it as the Vbes of the darlington change with current, so putting a capacitor here would actually degrade the output impedance as the the darlingtons will not be driven with the varying voltage needed to regulate the voltage if the current is varying. You could try a speedup capacitor across the top half of the voltage sense network.
Frank