EEVblog #221 - Lab Power Supply Design - Part 1

[joniengr081]

I was watching this video about lab power supply design.

EEVblog #221 - Lab Power Supply Design - Part 1

At time 30:00, a power supply circuit is shown for constant voltage and constant current.

The right side is a circuit of constant voltage.
The left side is the circuit of constant current.

Both of these circuits uses LM3080 in which the output voltage is the same as applied at the adjust pin.

If the input to the constant voltage is higher then 10 V then we can control the output through adjust pin. The output varies between 0 V to 10 V as the adjust pin varies from 0 V to 10 V. Provided the input to that section is higher than 10 V.

Now the left side is not clear.

How do we assume that there is 10 V at one end of the 1R through which we have voltage divide connected and also the same is going to LT3080.

 

[AnalogTodd]

What happens in this circuit is that the right side wants to deliver 0-10V at currents up to the current limit of the LT3080 (typically 1.4A). This is driven by the voltage follower op amp based on the VSET voltage.

On the left side, what you need to look at are the adjust pin and output pin of the LT3080. The LT3080 wants to drive the output to match the adjust pin. If there is 1V at the ISET point, that puts the output of the voltage follower op amp at 1V. We now have one op amp that is left which is not a voltage follower. With no current through the 1R resistor (and no voltage across it), you end up with the inverting input of this op amp below the non-inverting input and it rails to its maximum voltage. The LT3080 now drives as high as it can, going into dropout.

When you get 1A through the 1R resistor, you now get a 1V drop across it and the op amp balances, driving the LT3080 in a feedback loop to control the current. The output voltage will vary, but that is fine. The right side LT3080 is no longer in regulation and drives as hard as it can, going into its dropout.

I know the LT3080 quite well. I designed it.

[joniengr081]

Thank you for reply. I also have watched Part 2 of the "Lab Power Supply Design" series and found a circuit at time 21:07 of 38:23. The screenshot of the circuit is attached.

This circuit involve only one LT3080.

The right part of is constant voltage part. The set voltage (Vset) comes from an OpAmp that feeds in to the Adj pin of LT3080. There are two 1k ohm resistors in the way to Adj pin. The output of the LT3080 is controlled by the Vset.

The input to the LT3080 has to be at least equal or more the output voltage plus the voltage drop across LT3080.

The left part of is constant current part.

The current measurement is done using the resistor R and differential amplifier. The value of resistor is 1 ohm. This makes the math simple. Let's do the calculation for setting 1 A current limit.

In order to set the current limit to 1A, we need to have 1 V at Iset pin. As long as the voltage across the resistor R is less then or equal to 1 V, the compactor OpAmp output will be low, which will keep the transistor OFF all the time and power supply works normal. The maximum allowable voltage across R is 1 V which equals to the current 1 A through the resistor. As the voltage across the resistor R increase beyond 1 V which corresponds to more then 1 A current through it, the compactor OpAmp output will turn High that reduces the voltage at Adj pin of the LT3080. The output of LT3080 follows Adj pin.

This means that the Iset pin is responsible to set the current limit. The Iset pin 0 to 1 V corresponds to 0 to 1 A.

 

[Kleinstein]

That series, trying to build a lab supply from a LT3080 voltage regulator is kind of the usual beginners way. On may thing the voltage regulator already does half the job and it is only adding the current limit / regulation, that is often not considered that critical (no need to be super accurate).
However there are several things wrong with this:
1) the voltage regulator is made for a reasonable load, not a variable load impedance
2) the power stage and reference / precision amplifier in the same case is not good as the chip does get quite hot
3) There is no separate sense pin on the LT3080 (and most other voltage regulator chips) - so the cabeling to the output and the bond wire add to the output resistance (in part this helps with stability and makes the 1st point no longer that bad ad least).
4) Adding the current loop is not that easy.

It can be OK to watch and learn from the mistakes, but it is not a sensible way to build a lab supply.