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| Lab Power Supply - The Lost Current |
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| xavier60:
The 2 diodes would clamp the Gate at -1.4 with respect to the Source. The Gate needs to be reduced quickly to below about 4v for the MOSFET I'm using. |
| C:
You have to think of a load as being dynamic. At some point in time you will try to use your lab power supply to power almost anything You should have no problem seeing a load switch from no current to max current and back. Think of what this is doing to your current op amp. The higher freq op amp change hides some of this problem. Think of the states of your power supply. You are designing for two states CC & VC. You have a third state where output is not at set voltage or set current due to a load change. The time it takes for output to get to a set state is this third state. Most circuits handle less then set a lot better then greater then set. You need to think about when you connect a circuit you are designing in the future. That circuit could have over voltage protect and/or over current protect circuit that is faster then this power supply. It could be more sensitive. What will happen when the load makes a massive change on just a little overshoot? Protection will most likely go open circuit or dead short. Remember that this lab power supply has to protect it's self & the load. So the higher speed op amp makes circuit respond faster. This also increases freq response range of power supply output. This adds greater chance for circuit to oscillate. Try testing you circuit with smaller value for C6. C6 is there to prevent the fast current changes from causing your power supply to oscillate. T Here smaller is better( less un-controlled dump into load). Find where circuit starts oscillating and try to make osc freq higher in rest of circuit first. This will reduce output overshoot some. Then make circuit stable with min parts change. While testing, if you are not using a lab power supply to supply your 30 Volt supply you might want to add some protection to Q2 to prevent magic smoke. A temporary resistor between Q2 & 1000uf could do this. R1 should be the lowest value possible. Need to remember that active circuits take time to change. A good test point is to look at anode of D! & D3 and look at what is happening on cathodes. |
| xavier60:
--- Quote from: radoczi94 on February 23, 2018, 12:31:31 pm --- No, I just tought about diodes, 2 or 3, they only open if there is a radical intervention by the CC opamp. Something like the on the schematic below. The 2 diodes on the bottom are the analog OR gate. --- End quote --- I understand your idea now. After the MOSFET is turned off, the CC op-amp will help pull the output down, It is only 30ma though. Actually the op-amps can't pull the PSU output down directly because they are referenced to the the output rail. Current needs to be sunk to the negative output rail. |
| xavier60:
It did oscillate when I reduced C6 to 10uF. I found a choice of 2 possible fixes. One is by simply reducing the Proportional response of the CV op-amp by reducing R3 to 33k and increasing C2. The other is by increasing the frequency response of the MOSFET by reducing the Gate resistor to 100 \$\Omega\$ and also making the response of the CV op-amp only Integral by putting only a 100pF capacitor in its feedback path. With C6 changed back to 100uF, the voltage dips by 0.25V with a 3A load and recovers in 5uS. |
| C:
The lowest possible gate resistor is best here. Remember that connecting a scope to gate changes things. It is only here to prevent ringing on the gate. You are trying to charge/discharge a gate cap and need a lot of peek current. Digital gate drivers peek current is in the amp's to get fast change. Keep in mind that C6 is like a CV hack and CC harm. It is there to handle very high freq CV & CC changes. Smaller is better for CC while larger is better for CV. C6 also hides change. Your CV circuit has two modes. One is changing between two currents in CV mode. Second is changing to/from CV to CC Your CC circuit also has two modes. You op amp datasheet should list specifications for large and small output changes. Also of note is your op amp has a limited amount of output current. In your testing look for differences. In CV mode 1. lower current to higher current. 2. higher current to lower current. Here you are looking at voltage change, rate of change and overshoot. 3. CV to CC where CC mode is small or large currents. 4. CC to CV where CC mode is small or large currents. again for CC mode and many more. Good testing is a must. DC up to freq's hidden by C6 and all possible dynamic load changes. Think of each part and what would happen if you larger or smaller and all the effects on the circuit. For example it C6 was huge output changes are slower & at same time change is slowed so sense is slower.. At same time it's different for larger or smaller currents. Your circuit can not compensate for what it can not see. Huge also turns a short in to a spot welder. And you have no perfect(matched) parts for a copy. Think of using changed values or added parts to assist in making other parts of circuit function better. For example changing R5 makes current sense larger or smaller & also effects CV mode with more or less change needed. Some cap's are speed up and some are slow down, use with a lot of care. Change I would thing about is using a quad comparator to drive CC & CV leds. This should leave more current for driving Q2 and provide output test points that reduce effect on control circuits. One problem area is mode shift. In CV mode, CC wants more current. You could have one op amp at rail before change and require a huge output change for transition. |
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