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| Is there any variable DC-DC converter suitable for pre-regulation? |
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| T3sl4co1l:
You may find this helpful for the "computation block": The op-amp could be the internal error amp of the converter, so you can set variable current limiting, and then the postreg only needs to be a C-mult. Tim |
| bloguetronica:
--- Quote from: T3sl4co1l on September 08, 2018, 06:39:32 pm ---You may find this helpful for the "computation block": The op-amp could be the internal error amp of the converter, so you can set variable current limiting, and then the postreg only needs to be a C-mult. Tim --- End quote --- I'm not sure if that was what I had in mind. How does that circuit work? I see some kind of current mirror to limit the current (transistor at the bottom), then the input stage of an op-amp, and then a transistor pulling up the negative input of the amp-op, which I presume is to implement CC limiting. Kind regards, Samuel Lourenço |
| T3sl4co1l:
Right, the transistor stuff wraps around the existing error amp, say of a CV power supply to add CC operation. The transistors have limited gain, a necessity to deal with the already high loop gain of the op-amp without introducing too much slop themselves (it will transition from CC to CV mode in the span of maybe 100mV at the output pin). This is better than two opamps wired-OR with diodes, because there is no integrator windup. It acts ~instantly, going from CC to CV mode. Tim |
| Wolfgang:
Hi, a design I saw was from Jim Williams with an LT1074 preregulating an LT1083. ChangPuak also used this idea in a wide range lab PSU: https://www.changpuak.ch/electronics/PETH-581.php |
| bloguetronica:
Hi Wolfgang, I just don't want to copy a design. I want to build a design that I can understand. That is one of the points of the project. Anytwy, I appreciate your suggestion. --- Quote from: T3sl4co1l on September 08, 2018, 10:48:11 pm ---Right, the transistor stuff wraps around the existing error amp, say of a CV power supply to add CC operation. The transistors have limited gain, a necessity to deal with the already high loop gain of the op-amp without introducing too much slop themselves (it will transition from CC to CV mode in the span of maybe 100mV at the output pin). This is better than two opamps wired-OR with diodes, because there is no integrator windup. It acts ~instantly, going from CC to CV mode. Tim --- End quote --- Thanks Tim! Anyway, I've decided to ditch the whole computational block idea, since it wont calculate the voltage well. So, I will base the DC-DC pre-regulator feedback voltage on the output voltage via the PNP transistor. I decided to implement constant current on another way, using a transistor to starve the base of the pass transistor (perhaps using a MOSFET instead of a BJT to starve the base of the main transistor is better?). That transistor (BJT or MOSFET?), in turn, will have its base connected to the output of a op-amp wired as a comparator. The reason I'm using an op-amp here instead of a comparator is because I want the op-amp to operate in its linear region. At the inputs of the op-amp, is connected the output of an INA180 current sensing amplifier and the output of a DAC that sets the current. I've also decided to ditch the voltage sensing. After all, I don't need the extra precision. I'll post the drawn schematic later. Kind regards, Samuel Lourenço |
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