Hi,
I found in my bin with transformers a type that is 20V at 3.5-Ampere.
And I thought this transformer could become the basis of some explanation of what is all important for a good LAB Power Supply.
A lot of information that I find important in Power Supply's is often not found in designs on the Internet or only partially.
And I would like to share this information with you.
There are many similarities with other power supply's, even from a totaly different setup, such as the Harrison and HP schematics that I prefer.
So this schematic / project is mainly meant to explain what is involved in developing a good LAB Power Supply.
If i'am wrong about my explanation or i have to do it better, please tel me!
First I want to say this, this is a project in development, and meanwhile I have done already a lot of measurements on the schematic below.
This Power Supply will not be a "One Size Fits All" Power Supply.
So don't ask me if is is posible to change it to 30V 5-Ampere, or 60V at 2-Ampere... you are on your own then.
Processor controlled, please don't ask, just two 10 turn Model 534 potmeters for setting U and I. and a switch for enable.
The basis of this Power Supply is a standard LM723 circuit with parts around it, that make's it a good Power Supply. (my opinion)
Preliminary SpecificationsThe Output current wil be 2-Ampere Max.
Max output voltage 20V.
Current control from say 50mA to peak 2.2-Ampere, no sharp current limiting knee! this is not posible with one base emitor voltage.
DC Ri < 0.002 Ohm, at the moment around 0.001 Ohm.
Small output capacitor 100uF preferred or 150uF, will depend on the final measurement and the capacitor quality.
Dynamic properties, at the moment between 55 and 65 phase margin, have tot do more testing on that later.
Picture of the dyamic behaviour will come later.
KIND POWER SUPPLY!What this means? there are so many professional Power Supply's that have Power On and Power Off abbarations and i downt like that!
I have done my best so that the output of this Power Supply never gives more than 0.2V abberation at power on/off or the enable switch.
This only happens just before the power supply is coming on and when the Power Supply is already off for about five seconds.
I will do my best to make these abberations even smaller, that's where most of the work is done.
It is a combination of the lowest possible noise behaviour on the output and the timing to keep the abberations as clean as possible.
At the moment it is almost perfect, and i mean enable/off is perfect for small and large output currents.
Switching to power supply off with al kind of loads, is perfect.
There is a very small abberation when powering on, about 0.25V, that is no problem, but i will do work some more on that part.
The explanation of the schematic belowKeep in mind that the schematic is not yet finished and will certainly change.
Let's start with the transformer, so this is sucandair 20V at 3.5-Ampere.
This gives at double-phase rectification a little more than 2-Ampere to output current without overloading the transformer.
Usually you take about 60 to 65% of the current that the transformer can supply to a resistor or lamp.
By rectifying, together with the buffer capacitor, the maximum current without overloading the transformer in continuous operation is about 60 to 65%.
This is because the transformer has to supply large peak currents to charge the capacitor as well as the current that draws the load.
These peak currents make the i2r losses in the transformer higher than with a load such as a lamp or an ordinary resistor without bridge rectifier and capacitor.
R1 and C2 is a Snubber circuit, these components remove the ringing of the transformer when the diodes come out of conductance below say 0.5V.
The transformer sees suddenly no load anymore and will ring, (The induction and the winding capacity makes a small resonator circuit)
When you wil pull the power plug or switch the power supply off, the transformer will also "ring" R1 and C2 wik kill this.
I used a standard component of the brand WIMA.
The transformer also is used to make aditional voltages.
D4 and C7 makes a "clean" Power supply for the LM723 and the current source around Q4 that is used for the current limiting.
The ripple on C7 is far smaller than the ripple on C4 and also the DC level is higer as say 2-Ampere load,
so if de mains voltage is not to low, i can still have a clean 20V output at 2-Ampere current.
R5 help's with keeping fast pulses from the mains out of this power rail, Ri of the tranformer + R5 and C7 makes a 30Hz Low Pass filter.
The bridgerectifier is a 15Amp Low VF Glass Passivated type.
At 7,5-Ampere peak current the Vf is 0.9V, and this is low, it kan even be better with Schottky types, but for me this is good eneugh.
C1, C3, D1 and D2 are making a negative supply rail, also R2 is here used for some filtering, the real value of it comes later, for now it is 1 Ohm.
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End 6-1-2019
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The time is up for today, tomorrow I will make some mistakes in the text, English is not my mother tongue.
I will keep updating this first post as I continue to explain this schematic.
I would like to hear your comments.
Kind regards,
Bram