I'm puzzled - Page 1
Forum section: electronics / beginners

author: uliano
author_rank: Regular Contributor
timestamp: June 24, 2023, 07:40:04 am
content: In the long therm goal of making my own version ofPSL3604I started building a prototype of the synchronous rectifier used as a bridge see (schematic.pdf) but I got strange oscillations on the gate signals (sorry can't find old screenshot), so I went full monty for a proper PCB and started with the gates of the mosfet grounded relying on the intrinsic diode and I checked everthing it came to my mind (see pictures of scope screenshots) and got those (reasonable) valuesV1-AC1 13.3VV2-AC2 13.4VV3-(V-) 16.4VV+ 8.5vV- 8.5VAC1 6.6VAC2 6.6VThen I switched the solder bridges to connect MOSFET gates to comparators outputs and found the following (also very reasonable) valuesV1-AC1 13.55VV2-AC2 13.57VV3-(V-) 17.46VV+ 9.11VV- 9.06VAC1 6.6VAC2 6.6VBut when I connected the scope I was really puzzled, at first by the roundish shape of 3 signals, but then I thought that when the MOS channel start conducting some voltages and among them the comparators inputs may change... or some charging of the gate capacitor or whatever. What puzzles me more now is the Q4 gate (the magenta trace) which is unchanged since when they were all unconnected.I even replaced the MOS (and its zener) to no avail, the waveform is still the same.

author: uliano
author_rank: Regular Contributor
timestamp: June 24, 2023, 08:09:05 am
content: I forgot the most important one

author: uliano
author_rank: Regular Contributor
timestamp: June 24, 2023, 08:28:48 am
content: I noticed that one of my crappy chinese differential probe has a slight offset, so this is more realistic

author: gnuarm
author_rank: Super Contributor
timestamp: June 24, 2023, 08:44:02 am
content: If you can come up with a schematic, maybe some help can be provided.

author: uliano
author_rank: Regular Contributor
timestamp: June 24, 2023, 09:21:36 am
content: its' attached in the OPEDITOh, I see it' doesn't workEDIT2here it is

author: gnuarm
author_rank: Super Contributor
timestamp: June 24, 2023, 09:44:35 am
content: Quote from: uliano on June 24, 2023, 08:28:48 amOk, so now I have a schematic, but I don't know what I'm looking at on the scope displays. Did I miss where you explain that?

author: uliano
author_rank: Regular Contributor
timestamp: June 24, 2023, 09:55:26 am
content: picture's filename

author: uliano
author_rank: Regular Contributor
timestamp: June 24, 2023, 11:41:48 am
content: Tried simulating, and gate signals are rectangular (as I originally expected).

author: gnuarm
author_rank: Super Contributor
timestamp: June 24, 2023, 08:35:54 pm
content: Quote from: uliano on June 24, 2023, 09:55:26 amSorry, I have no idea what you mean. I don't know which trace is which signal. How does the file name indicate that?

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 25, 2023, 12:09:24 am
content: RE: "Gate_Signal_with_MOSFET_enabled.jpg"How are you measuring the 4 gate signals independently? I can see measuring only Q2 & Q4 gates simultaneously, because their source terminals are common, but how do you also measure Q1 & Q3 gates at the same time? ( Q1's & Q3's source are not connected together nor are either connect to Q2 & Q4 source) Are you using differential probes? If not, that would explain the weirdness.EDIT: I see now you are using "crappy chinese differential probe" to measure the Q1 & Q3 gates. As a sanity check, what happens if you take a single probe (Nothing else connected to the "bridge") and probe each gate--source independently? Do you still get the wonky waves on the Q1 & Q3 gate--source?

author: uliano
author_rank: Regular Contributor
timestamp: June 25, 2023, 05:53:02 am
content: Are you trolling me?Besides the fact that Kim doesn't seem to have had major deciphering problems, let's see how to unveil the conundrum:"Gate_Signal_with_MOSFET_enabled.jpg"It should be related to gate's signals, let's see we have 4 gates and four traces this seems more than a clue. Does it really matter which is which? Anyway only one is different and in the text it's mentioned as Q4: the magenta trace."Low_Side_Gates_MOSFET_enabled_vs_+V_minus_AC.jpg :Well low side MOSFET are Q2 and Q4, from the clue above it's not difficult to understand which is which (albeit not so important really) they are compared with V+ (the output rectified voltage as clearly indicated on the schematic) minus (the arithmetic operator - ) AC, yes we have 2 possibility here, each subtraction is respectively carried out with the relevant AC voltage to give meaningful insight on what's happening at the comparators input

author: uliano
author_rank: Regular Contributor
timestamp: June 25, 2023, 05:54:53 am
content: Quote from: uliano on June 25, 2023, 05:53:02 amTrying this afternoon, however on the low side I have one funky trace with regular probe.

author: gnuarm
author_rank: Super Contributor
timestamp: June 25, 2023, 06:33:00 am
content: Quote from: uliano on June 25, 2023, 05:53:02 amOk, why didn't you just say, "They are the signals on the four gates", rather than being pissy with me? No, don't answer the question. Thanks anyway.

author: Kim Christensen
author_rank: Super Contributor
timestamp: June 25, 2023, 10:13:25 pm
content: Quote from: uliano on June 25, 2023, 05:54:53 amWhere does the AC voltage come from? old_proto_and_new_PCB.jpg shows 4 wires (Two yellow and two Blue) connected to the AC input. Sounds like things might not be properly isolated from the common ground.

author: uliano
author_rank: Regular Contributor
timestamp: June 26, 2023, 03:55:58 am
content: Here I'mI got hold of a dummy load and (most of) the mystery has been unveiled. Still It remained that funky yellow trace, so I replaced Q2 but no, here the understanding still misses me. Maybe the tolerances in the 10k resistor cascade? Funny thing: Q2 replacement also modified slightly the corresponding (in time) cyan trace (Q3).In conclusion I'm still little puzzled by Q2 trace, nevertheless I'm now convinced that Liv's design is solid and I should probably proceed with other PSU parts. Also I may want to increase C4 capacitance as, under load the PW3 line seems to sag a bit.@Kim at this stage it comes from a tiny 3VA transformer (as per picture), further on it will be replaced by a much larger toroidal one.