Constant power buck converter - Please Review - Page 1
Forum section: electronics / beginners

author: liviu.gelea
author_rank: Contributor
timestamp: June 03, 2022, 11:06:38 am
content: Hello. I'm working on a draft for a constant power buck converter for use in science experiments involving heat and insulation.https://oshwlab.com/liviu.gelea/constant-power(Includes both Circuit diagram and pcb layout)My draft pcb is with through hole components so it's both bad topologically and from a trace-widthI'm building the protopype soon on perf board as soon as the mosfet drivers arrive from shipping.Meanwhile, anyone interested in the project willing to give a few tips? I'm worried about trace lengths, loop sizes as well as potential fails that may make the project completely unfeasable.Thanks.EDIT: Corrected screenshots after realizing impropper inductor placement

author: liviu.gelea
author_rank: Contributor
timestamp: June 03, 2022, 12:19:50 pm
content: I'll be the firstI placed the large cap before the inductor by mistake. the order should be reverse. The Coil should "push" voltage unto the cap.Unfortunately that means I have to Move the large coil even closer to the other traces

author: mariush
author_rank: Super Contributor
timestamp: June 03, 2022, 12:52:35 pm
content: So what exactly does this do?What's the load, a big resistor, some heating element?When I read the text, my mind went to soldering stations and something basic like hakko 936 soldering station schematic, which uses a triac to turn on and off the heating element that runs on 24v AC ... and uses an optocoupler with zero crossing detection to keep things simple.

author: liviu.gelea
author_rank: Contributor
timestamp: June 03, 2022, 01:06:45 pm
content: The Load is a 1ohm 100W resistor. Its purpose is to heat a small enclosure with constant power no matter the temperature of the resistor. Then, by measuring the temperature difference between inside and outside the enclosure i can infer the quality of the walls as insulators.Also, another project I want to do is a counter-current heat recovery system and I need this device in order to determine the efficiency of the heat recovery.The reason I went for this system is because:1. I want to control the power via a potentiometer2. I want said power displayed on an lcd screen3. I want to get 10-20watts of powerMeasuring temperature of the heating element is NOT useful. I need real real RMS power. The temperature is only proportional to the power if the ambient temperature is constant and in my case they're not. In my case, the enclosure temperature is dictated by the heat resistence of the enclosure. If the insulation is 3*C / W, having a 10watt heating element will make the enclosure 30C hottern than outside. This will also mean the temperature of the heating element will be 30C higher, to allow the same power draw.

author: liviu.gelea
author_rank: Contributor
timestamp: June 03, 2022, 01:23:18 pm
content: I agree I could go with your idea and instead of keeping power constant I would keep the temperature constant while simultaneously measuring the power consumed. However I would require more math and effort to do the calculations I want and measuring the heat that was put into a system by a heating element that turns on and off at regular intervals is itself not trivial. I would need to heat until equilibrium and then zero the measurement and follow it up for 30 minutes to see how much power was drawn on average after equilibrium

author: Mechatrommer
author_rank: Super Contributor
timestamp: June 03, 2022, 02:00:51 pm
content: 1) shorting all opamp 2 pins is generally a bad idea, at least make it as gnd (ideally Vcc/2) voltage follower with floating output.2) why do you need N-FET-L in buck convertor? doesnt look like a buck converter, or at least looks like a complicated or upside down buck converter to me...

author: liviu.gelea
author_rank: Contributor
timestamp: June 03, 2022, 02:24:00 pm
content: Quote from: Mechatrommer on June 03, 2022, 02:00:51 pmHello. And thenks for your answer and suggestions1. I got the idea from the datasheet. Not sure I understood it right, but it said:"Unused amplifiers should be connected as grounded unity-gain followers to avoid possible oscillation."https://www.ti.com/lit/ds/symlink/tlc272.pdfAnd it gave the following image:Not sure if I got it wrong.....2. It's a synchronous design. The second mosfet replaces the diode but with a smaller voltage drop The mosfet driver has an internal "dead-time" to prevent shoot-through.