How to match those high power NPN transistors for self resonant circuit driver - Page 1
Forum section: electronics / projects

author: jnk0le
author_rank: Regular Contributor
timestamp: December 15, 2024, 02:55:13 pm
content: Repairing some ultrasonic welding machines. Figured out all the repairs except those transistors for which I need more theoretical insight,as trying to buy transistors that are not in production since at least 20 years is not the solution.The circuit is:- "E+" is 300VDC- "C" is a midpoint- "E-" is GND of the DC bus- "B1"/"B2" is upper/lower transistor base drivetransistor board:base driver:resonant tank (from here):"feedback circuit":The issue is that while the intact transistor modules experience slight frequency downshift (<0.5KHz, gradual after a few seconds),the ones with replaced (all 10 new) transistors (to J6920, 2SC5570, BU2520 etc.) don't, and explode when attempted to run with the horn.Harvested ones from damaged modules (C5047, 2SC5589 etc.) also have that downshift, but due to a shoot-through they heat up until completely dying.e.g: CH1 is base voltage, CH2 is emitter current (on R11), Q1:same on new ones:Is it that those transistors have not enough hfe or situation is much worser requiring a perfect matching of some parameters (that are not even listed in datasheets) in order to work correctly?In former case I could maybe try the swap to darlingtons (with 47-470R of base resistors).Or something else could be wrong?

author: Terry Bites
author_rank: Super Contributor
timestamp: December 15, 2024, 04:01:36 pm
content: Be a practical engineer and cut your losses in time, worry and transistors. It'll only get harder to fix in the future. Buy a ready built modern US drive.They're cheap.egaliexpresseghttps://www.aliexpress.us/item/1005005335695221.html?spm=a2g0o.productlist.main.19.575b3f69ubJhcT&algo_pvid=0b2c69b1-b503-4408-8745-a68803d2a65b&algo_exp_id=0b2c69b1-b503-4408-8745-a68803d2a65b-9&pdp_npi=4%40dis%21GBP%21105.83%2194.19%21%21%21130.17%21115.85%21%40211b815c17342781338012537e3bbc%2112000032653742679%21sea%21UK%213100120370%21X&curPageLogUid=EeN9vFKc4N5Y&utparam-url=scene%3Asearch%7Cquery_from%3A&gatewayAdapt=4itemAdapt

author: moffy
author_rank: Super Contributor
timestamp: December 15, 2024, 09:22:59 pm
content: How much reverse voltage are you putting across the transistor BE junctions?

author: jnk0le
author_rank: Regular Contributor
timestamp: December 15, 2024, 11:21:45 pm
content: it's -15VCH1 is BE voltageCH2 is B1 relative to emitter<1A of base current per transistor (no load)

author: moffy
author_rank: Super Contributor
timestamp: December 15, 2024, 11:42:45 pm
content: Please refer to the datasheet:https://datasheet.octopart.com/2SC5570-Toshiba-datasheet-101341.pdfmaximum reverse voltage for the EB junction is 5V or -5V(BE junction), excessive reverse voltage across the BE junction will break down and destroy a transistor.

author: jnk0le
author_rank: Regular Contributor
timestamp: December 16, 2024, 01:17:18 am
content: So that's how those manufacturer's transistors are slowly killed. Even if that waveform was supposed to be symmetric, that's still -10V on the base.BE breakdown also reduces hfe:https://electronics.stackexchange.com/questions/517109/base-emitter-reverse-breakdown-and-current(does this mean that this frequency shift should happen only on hfe degraded ones?)Not allowing reverse bias will increase the tail current, but adding load by dumb diode protection on bases can mess up with the resonant circuit (and other base drive) though.I'll check it up more.

author: mtwieg
author_rank: Frequent Contributor
timestamp: December 16, 2024, 01:34:55 pm
content: Quote from: jnk0le on December 16, 2024, 01:17:18 amYou are likely correct, simply protecting the BJTs by adding clamping diodes will likely affect the DC bias voltage on C1 and C3 in a way that makes cross conduction more likely.It's strange that diodes D1 and D2 are facing towards the bases, normally I'd expect the opposite in order to discourage cross conduction.Also are you sure that C1 and C3 are really 470u? Not 470nF?

author: jnk0le
author_rank: Regular Contributor
timestamp: December 16, 2024, 07:38:14 pm
content: tried clamping it with shottky diode with 4.7 ohm resistor across B1/C, B2/E-- maximum -6V on BE junction- positive voltage swing also lowered (meaning less current drive)- no more excessive load on R13-R16 and related high "horn match"- waveform similar to startup, and some harvests- high power load from transistors (5570 died quicker than harvested ones)voltages on base and B1 (relative to emitter)voltages on base and current (relative to emitter):looks like operation in linear region.Quote from: mtwieg on December 16, 2024, 01:34:55 pmC1,C3 are electrolytic. 470uF. positive terminal going to transformer.Both measure >470uF, not spotted any faulty ones unlike C7,C8 or the HV ladder.D1,D2 are probably for discharging those electrolytics (and some forward boost)?

author: jnk0le
author_rank: Regular Contributor
timestamp: December 16, 2024, 07:53:39 pm
content: now the transistor openingJ6920 - smells chinesium, isn't it?c5047 (as used by manufacturer)2sc5570

author: jnk0le
author_rank: Regular Contributor
timestamp: December 17, 2024, 12:32:20 am
content: Quote from: jnk0le on December 16, 2024, 07:38:14 pmAcoording to rough simulation in falstad, increasing C1/C3 gives sharper leading slope and reducing R1/R3, more overall drive current.However this transformer is not a perfect sinewave source as in simulation.Will try protection with more components and cutting traces.

author: mtwieg
author_rank: Frequent Contributor
timestamp: December 17, 2024, 01:16:16 pm
content: QuoteIs that not expected?I tried making an LTspice simulation of the circuit, but it's not very convincing because I don't think I have accurate models of any of the BJTs you're testing, and I don't know the properties of the load (guessing it's close to a parallel resonant tank). I'm guessing the output voltage (node C) is supposed to be a sinusoid with amplitude slightly less than the DC bus voltage so the BJTs never saturate.QuoteFrom my simulations, D1 and D2 are forward biased all the time, and carry most of the average base current (much more than the 6.8K resistors). So basically the combination of R1+C1+D1 and R3+C3+D2 is to just the DC bias voltage at B1/B2 down slightly (about -2V in my sim). That bias helps avoid cross conduction, but this circuit is a strange way of going about it.On the other hand, if I add clamping circuits like you described, then the bias changes to about +3.6V, leading to some cross conduction and dissipation in the BJTs roughly doubling.

author: jnk0le
author_rank: Regular Contributor
timestamp: December 17, 2024, 10:38:23 pm
content: Tried clamping with this circuit:-1 fuseCH1 is B1_out, CH2 is B1_in (tuned down to 15KHz, still overload):That's a miller, and it keeps oscillation after relay closes (at more manageable power draw)added a pmos to clamp the B1_out rail while the B1_in is negative (had -2.5V threshold ones)CH1 is B1_out, CH2 is B1_in:CH1 is base V and emitter current on CH2:Miller effect keeps it oscillating even after relay closes (load is even more manageable before and after "turning it off")Turns out that this large negative swing is supposed to counter the miller effect, clamping B1/B2 to C/E- is not enough.Putting 10x small pmos directly on the transistor legs should clamp it for good, I think?Quote from: mtwieg on December 17, 2024, 01:16:16 pmDon't have those "intact" modules anymore, but current waveforms (+ deadtimes from offset, and overall design) are not looking like it was supposed to generate sinewave.I think the C5,C6 in series with transformer (measured ~2.35mH on input and ~5.90mH output) should handle that. Rest of the anomalies are taken by "feedback circuit" (r13-r16 are 10W ones, burnt laminate around them is common, always >=1.5V RMS).Or I'm just coping, as adjusting this circuit for a perfect near rail to rail sinewave would be a nightmare.Quote from: mtwieg on December 17, 2024, 01:16:16 pmcurrently testing on 2sc3998 (probably the most recent used by manufactures, 1 replacee from unknown source, behaved similarly to "new" 5570,6920), still have 5570 and buv48a (ST)transformer (between transistor board and IN1,IN2) is ~2.35mH on input and ~5.90mH output."pinking coil" (IN3, IN4) is ~1,2mH(device is probably 18kHz)Testing it unloaded, even if I get the horn, BE breakdown and miller has to be solved. (internet says it's ~11nF piezo)Quote from: mtwieg on December 17, 2024, 01:16:16 pmWould a swap of D1,D2 to shottky or reduction of R1,R3 affect that DC bias?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: December 18, 2024, 07:40:57 pm
content: Quote from: jnk0le on December 15, 2024, 02:55:13 pmDo you have a known part code that (almost?) never fails ?Or is the design simply marginal, so all parts eventually fail ?Those old self-oscillating amplifiers were very much controlled chaos, they 'worked', but everything was a compromise.This design looks to set an average startup base current via high value resistors (6k8 x 2) and the positive feedback then imposes switching on that average.Positive drive is set by Transformer parasitics plus D2 R3 C3 and C3 also gives some DC restoration.

author: jnk0le
author_rank: Regular Contributor
timestamp: December 19, 2024, 01:18:30 am
content: Quote from: PCB.Wiz on December 18, 2024, 07:40:57 pmDidn't read the transistor models from "intact" ones, but I think that everything will die eventually.Properties under BE breakdown are highly undefined and will definitely vary across manufacturers, models and lots.The only list of "recommended" transistors I could find is this:https://www.youtube.com/watch?v=VHamHJ3C960.Those machines are often operated in short bursts, so they should work even with somewhat degraded transistors, until eventually failing.From the 3 units I initially thought were repaired, 2 blown up immediately and 1 "worked for 30 minutes then died". (no info which one)About the rest of failures: it's mostly the "feedback circuit" then 2kV 104 caps in HV ladder, DC bus rectifier, burns in "pinking coil", glue failure in adjustment coil, and cracked transformer core (the way it's assembled guarantees the cracking).

author: jnk0le
author_rank: Regular Contributor
timestamp: December 20, 2024, 12:39:18 am
content: Tried clamping individual transistors and the issue remained, even after moving to buv48a.I guess that this actually needs to run on the BE breakdown.I'll give up on clamping to -5V.transistor opening 2:2SC5570 from "different brand"2SC3998 (as used by manufacturer)Will check how long the buv48a survives with load in "normal" configuration.

author: jnk0le
author_rank: Regular Contributor
timestamp: December 21, 2024, 01:17:18 pm
content: Got that frequency shift on buv48a, sounded exactly like the first "intact" one.After that shift, reverse voltage on the base went down to reasonable 5V. (somehow lost the screensot)Then killed 3 of them with a horn(resonance frequency of 18KHz horn is of course 17,1kHz and it overloads when tuned to just slightly above that)buv48a opening:here is C3998 (1 replaced by yellow die 5570) running with the horn:

author: jnk0le
author_rank: Regular Contributor
timestamp: January 03, 2025, 01:06:59 am
content: That c3998 module died in like 20 minutes.Now new buv48a doesn't want to "shift" (on different unit), but harvested ones behave as usual.observation of that "shift" on harvested ones:- emitter current goes down proportionally to base voltage- current through D3/D4 does as well (conducting during dead time, 6A -> 2A)- those "sidelobes" is jumping frequency, more subtle on harvested onesAttached to C line through resistor divider:It remains rectangular even after that frequency shift on harvested ones.Considering the device nominal power and snubber values, I don't think that the transistors are supposed to run in linear region.Still no idea what causes that "shift" (or lack of it).

author: jnk0le
author_rank: Regular Contributor
timestamp: January 14, 2025, 01:30:26 am
content: Got the whole circuit into simulation (pardon the falstad)- hfe <= 13 can't sustain oscillation with 11nf load- increasing hfe slightly adds some frequeny and increases circulating currents (even after readjusting freq, diminishing so "infinite" hfe still works similarly to 50)Thought about a brute force approacgh, by the IGBT swap. That would require designing a new transistor board, which is the easy part.It should be possible to proxy loaded B1/B2 through uC, bu how do I start this?Single pulse is not enough. Alternate is forced frequency (subharmonics on adjustment leg, waveforms are not looking good) or do some zero voltage/current sensing, then switch to B1/B2 proxy.Is that even possible?