R&S ZVR VNA power supply +15v wide ripple - Page 1
Forum section: electronics / repair

author: mrf245
author_rank: Regular Contributor
timestamp: April 22, 2019, 02:02:27 pm
content: Hello all,recently I got a bad R&S ZVR nice and clear, there are some problems in it.At first I checked power supply, it was made by other manufacturer not R&S. I surprised found there are very wide and stable ripple in +15V output line, about 3VP-P, 244Hz, whether in empty load or normal. Please see picture below.Another my friend has the same power supply and also have this problem, the same in 15V.In ZVR service manual there is few words describe it, I can only understand 15V is output from secondary DCDC after main transformer.Anyone can give me some advice? Thanks.

author: PKTKS
author_rank: Super Contributor
timestamp: April 22, 2019, 03:55:30 pm
content: Impossible to guess without the schematic or even a clueabout what topology and rated power the PSUUsually a long shot guess would say that the regulator justhave gone wild for a walk.Even faulty output capacitors would kindly be compensatedby a good linear regulator working feedback loop - when unloadedThings will go really bad when loading the rated power.Put the regulator CHIP ID or datasheet to at least havepins to do some checksPaul

author: mrf245
author_rank: Regular Contributor
timestamp: April 24, 2019, 02:21:10 pm
content: Its difficult to describe details of schematic because I don't have that.Only block diagram.15V is output from DCDC, input is a common 28V, other DCDCs also supplied by it.Output filter capacitor is 2200uF/25V, its OK, low ESR.I check PCB find these DCDCs all use UC3845, LM358 and N-MOSFET.By connect "failed" UC3845 all pins out to check it by oscilloscope, I found some stranges:UC3845 pin2(feedback) is not continue waveform, its a continue pulse, frequency is the same as output ripple;UC3845 pin6(driver output to MOSFET) is also pulse, it contain driver PWM waveform;So it can explan why output still fluctuant.But why fluctuant regular, no explan.

author: PKTKS
author_rank: Super Contributor
timestamp: April 24, 2019, 02:50:56 pm
content: Several possible issuesTop list being a faulty IC - the 38xx family is widely usedin several topologies, looks like you a have a simple buck.Remove that IC and do the OPEN LOOP test from the manufct.If the oscillator and the internal VREF are still ok you can goto the next items ...Being that the feedback loop error parts (if the OPAMP is part of it...)a bad error amp or current sense error will mess any SMPSSMPS can go wacky for dozens of reasons.The FB margin is narrow and if you have protection (OVP/OCP)you should care about they are working as wellTEST pdf attached - 38xx family members differ in just somefeatures being the test gig practically the same

author: PKTKS
author_rank: Super Contributor
timestamp: April 24, 2019, 03:11:13 pm
content: I would change the note about being a simple BUCK...Once these ICs are most widely used in forward converters....**IF** your NMOS is driving a transformer... e.g. you havea forward converter and the uc38xx works in classic"CURRENT SENSE" forward modeotherwise check how your mosfet is being driven e.g.a buck would be a synchronous mosfet.In forward mode your optocoupler should be checkedas if it fails you have no feedback loop at all(if you have an optocoupler.. some cheap designsignore the fact that a forward should be isolated.)2 very different scenarios - being the CURRENT MODE forward converterthe most widely used for these controllersPaul

author: PKTKS
author_rank: Super Contributor
timestamp: April 24, 2019, 03:25:00 pm
content: Typical LOW POWER BUCK for uc38xx series..not a very common found topology as the current senseforward mode is the killer topology in these chips2 cents to help decide how the OPAMP and MOSFET fit

author: mrf245
author_rank: Regular Contributor
timestamp: April 25, 2019, 02:00:32 am
content: Thanks Paul,As you mentioned feedback loop issue, I check waveform again by combine them together, that is answer......See below.Yellow is output voltage, blue is UC3845 pin2(feedback), purple is UC3845 pin6(PWM output), green is UC3845 pin1(compare output)output voltage ripple is from 13V to 16.5V, when voltage rise up, feedback is no change along with output. But when PWM output stop, feedback start to rise up to as much as 6.5V........then drop down.I geuss some wrong in feedback path, and other protect circuit force output off when detect voltage up to 16.5V, if output drop to 13V, it will work again...So I need check feedback path. It a big project.

author: PKTKS
author_rank: Super Contributor
timestamp: April 25, 2019, 10:36:23 am
content: You may test the chip itself outside the loop...If the chip failed there is no chance to succeed.But even failing and putting a new in.. you may have otherculprits that caused the failure.I would not skip the chip OPEN LOOP test in a simple breadboard.These chips design date back from the 80s.The "thing" with these series is the "CURRENT MODE" operationIn that mode you have stability direct proportional to the "inductor"current which MUST BE in CC mode for proper operationthe other error amps just care about putting the loop inproper output voltage being the "SENSE" pin the realloop regulator.That idea indeed works being a current driven regulatorthe inductor (e.g the transformer) should operate in linearand CC mode all the time...so that SENSE input working directly driven the MOSFETis subject to some spikes.. that pin may indeed fail and theregulator goes dead wackyThat is the thing with the uc38xxPaul

author: mrf245
author_rank: Regular Contributor
timestamp: April 28, 2019, 01:42:42 am
content: Paul,Thanks for your guide, finally I finished repair working by find out 2 capacitors lost their value.These are beside DCDC main inductor but not final output stage filter, there is a 2200uF capacitor with very good status.A current feedback loop is connect beside main inductor so that if too much ripple there, it will lead to unstable loop detect so you can see waveform before.After replace them, everything goes great again.You are right, that's I forgot some unimpressive place.

author: PKTKS
author_rank: Super Contributor
timestamp: May 01, 2019, 01:48:17 pm
content: BAD CAPS are common place in this SMPS things..They just can not stand the heat inside or cumulative outside themTake a closer look in the current "sense" pin of ucxx chipsbecause all regulation in these chips are based on that current senseThey (UCs) are good even being designed in the 80s ..clones exist till todayPaul