Electronics > Beginners

Linear lab bench power supply - Advise Needed

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mikerj:

--- Quote from: not1xor1 on January 18, 2020, 05:38:47 pm ---there are various problems with that design:

- there is no need for a pre-regulator (L7815cp) as opamp plus power device are enough to provide both current and voltage regulation

--- End quote ---

The regulator is effectively providing the reference voltage.

not1xor1:

--- Quote from: mikerj on January 18, 2020, 07:58:10 pm ---
--- Quote from: not1xor1 on January 18, 2020, 05:38:47 pm ---there are various problems with that design:

- there is no need for a pre-regulator (L7815cp) as opamp plus power device are enough to provide both current and voltage regulation

--- End quote ---

The regulator is effectively providing the reference voltage.

--- End quote ---

that 7815 in the circuit of the original poster works as a preregulator and while it is also used for the reference voltage of the current and voltage regulation circuits, there are plenty of better, smaller and cheaper alternatives than a 1.5A series regulator as a voltage reference.

mikerj:

--- Quote from: not1xor1 on January 19, 2020, 05:45:19 pm ---
--- Quote from: mikerj on January 18, 2020, 07:58:10 pm ---
--- Quote from: not1xor1 on January 18, 2020, 05:38:47 pm ---there are various problems with that design:

- there is no need for a pre-regulator (L7815cp) as opamp plus power device are enough to provide both current and voltage regulation

--- End quote ---

The regulator is effectively providing the reference voltage.

--- End quote ---

that 7815 in the circuit of the original poster works as a preregulator and while it is also used for the reference voltage of the current and voltage regulation circuits, there are plenty of better, smaller and cheaper alternatives than a 1.5A series regulator as a voltage reference.

--- End quote ---

I fully agree, but your first post implies this device could be removed without considering the implications.

David Hess:
At only 15 volts, secondary breakdown of the power MOSFET will not be an issue.

There are a couple of problems:

1. The high output capacitance impedes good current limiting response.  No more than about 22 to 100 microfarads per amp should be necessary.  High performance designs with the best current limiting get by with less than 0.1 microfarads per amp.

2. C5 severely limits bandwidth and should not be necessary.

3. Q2 adds voltage gain within the current control loop and this gain varies with operating point making frequency compensation of the current control loop difficult; replace it with a diode or PNP emitter follower.  If you absolutely want to keep it, then add a resistor in series with the emitter to control and limit transconductance.

4. It is not impossible to make a current control loop work with two cascaded operational amplifiers but there are better ways to do it with only one stage.  Or make the instrumentation amplifier faster.

not1xor1:

--- Quote from: mikerj on January 19, 2020, 05:51:42 pm ---I fully agree, but your first post implies this device could be removed without considering the implications.

--- End quote ---

I've been misunderstood   ;D
as I suggested to trash the whole circuit (and to adopt Jaycee's design instead) I hardly see any implication by 7805 removal  :)

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