I met a big problem that the DC-DC converter circuit failure happend frequently
I use UC3843B to control the Planar TRf 101 transformer to provide multiply DC output;
and almost every failure has the same feature:
The failure on the low-ESR electrolytic capacitor at the second wiring of the transformer;
it's kind of Low-ESR electrolytic capacitor, 1000uf/10V (ESR=0.07 ohm).
The UC3843B working at 100KHZ.
Is there anybody met that problem before?
What's the possible reason to lead to the failure?
Some body may say because of the high temperature, but it happends on the product even working in less than 30 cent degree ambient operation temperature.
"Simon" skrev i en meddelelse news: snipped-for-privacy@i12g2000prf.googlegroups.com...
1) Cooking it - either by wave soldering or by overload. There is an RMS ripple current rating for these things - and at the max rated operating temperature, normally specified as 80 deg.C., the expected lifetime will be something like 2000 hours so maybe they are performing as designed!!
2) Overvoltage/Cooking due to resonance between the capacitor and the wiring/transformer; low ESR is sometimes bad. I have blown some solid polymer to hell on that.
you didn't say what type of failure?, So I'll assume a short.
Is it possible you're having periodic over voltage spikes on the secondary side? This could happen for example if the cap was already at a full charge near it's 10 volt limit then for what ever reason, the switcher got unstable and caused a much higher voltage to appear on the output? Because the cap is already saturated, there isn't much to hold back that short pulse. I suppose a cap being new might be able to hold off on this for a bit. Maybe you should try some TVS diodes closely matched to the voltage of your cap on the output.
I've seen many low voltage caps that can handle only and no more than what it states for the package. Could it be you have some closely rated caps? Have you performed a leakage test on the caps you're using?
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i heard that story before, i hope we are not the victim of that reason.
Otherwise, it will be a disaster for us. Although i gradually got so many bad news from different customer ,
with the same failure features.
The capacitor swollen, and other related diode around the PWM controller failed due to the bigger ESR just 1 to 2 month later.
I hope it is due to the customer wrong application, or even our design. Since the customer will not accept the answer that we buy poor quality capacitor
"Simon" skrev i en meddelelse news: snipped-for-privacy@e10g2000prf.googlegroups.com...
It's cooked - the electrolyte vaporises and the pressure flexes the can which breaks the connection inside so it does not blow entirely apart. Measure how hot it actually gets in the application. There are temperature sensitive labels for that purpose.
Check what the "105 Centi degree" rating actually means in Hours. It may well be dying within the specified lifetime (because it is s**te to begin with) in which case it's your fault and not the manufacturers.
Like Phil says: Panasonic (or Nichion) are probably better. They *also* tend to be specced at *85 centigrade* for a mere *2000-6000* Hours of lifetime!!! The high temp rating of the Hitano component is suspekt IMO.
"Simon" skrev i en meddelelse news: snipped-for-privacy@e6g2000prf.googlegroups.com...
If they can't hande truth then lie!
What I also did in my younger years as part of design verification was to cook the prototypes in an oven for a month or two at 105% rated load and above rated max ambient temperature to see if I could force a failure and how long it takes before the device failed - i.e. if it is consistent with the reliability statistics, in which case we are good or Before which would be a problem. Saves later grief and suffering!!
Every 10 degrees rise in operating temperature should halve the lifetime of electrolytics f.ex. Which is why I go on about it - 'lytics are a chemically programmed failure mechanism.
You could have bad caps as others have described, but unlikely if they have a recent date code. Of course if they were purchased surplus or from a dubious dealer, it's more than likely the cause. Hitano does not have a history of producing bad caps AFAICS.
What is your actual RMS ripple current?
That's the temperature inside, near the capacitor, including all the self-heating of the electronics?
The Hitano EXR series caps are rated for 5,000 hours at 105°C so there should not be failures as you describe unless you have a bad batch or you are actually exceeding the ratings.
Keep in mind that 24/7 industrial operation will quickly exceed 5,000 hours (7 months) and you need to heavily derate the capacitors to get reasonable life. If you are actually exceeding the RMS (I can't emphasize the RMS enough) ripple current by, say, double or triple, the electronics is running hot in a warm environment, and the customer is running three shifts, this might be normal.
Best regards, Spehro Pefhany
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What's your Vcc voltage and load current? What's your measured output-voltage ripple on the 1000uF caps? What's the uc3843 output-switching frequency, 50kHz? Can you tell us the cap's manufacturer and part number? Wait, I see, HITANO EXR Low-ESR 1000uf, 10V (0.08ohm, 1040 mA ripple current 105 Centi degree, 100KHZ).
The ripple current in each 1000uF capacitor is equal to half your load current; compare this to the datasheet's ratings. (If the rating is for 120Hz, you can multiply this value by about 1.3, for use at 50 to 100kHz.)
A ripple-current rating for a good 1000uF 10V cap, like nichicon's PW series, is 1.0A. This would limit your supply to well under 2 amps, say 0.75 to 1.0A max. Note, a 50kHz smps running at 0.5A per 1000uF cap, would have 10mV of 50kHz dV/dt = I/C ripple voltage, plus more from the cap's esr, 32mV across 65 milliohms for the PW part, or 42mV total, which is a lot. Your L-C output filter makes me wonder if your capacitor's voltage ripple isn't rather high? Hmm, the HITANO EXR cap has 76m esr compared to 65m for the nichicon part.
If your ripple voltage is too high, your best choice is a larger capacitor: higher value or voltage rating, or both, to lower the esr and increase the ripple-current rating. If this isn't possible, you can try increasing the 3843's operating frequency to reduce the capacitor's dV/dt ripple voltage, which may help reduce its stress.
Phil's suggestion of a bad electrolyte is sadly a real possibility. But this scene has been most common with inexpensive substitutes for premium-grade conductive polymer or organic-semiconductor high-conductivity- electrolyte capacitors, like Sanyo's OS-CON types. It's more likely you're just over-stressing your caps.
To test this idea, remove some "good" capacitors from power supplies that have not yet failed, but that have experienced a known stressful operating life so far. Measure the capacitor's esr at 100kHz, etc., looking for a significant increase compared to new capacitors. If you have a supply with one failed capacitor and can remove and test the other, that'd be an opportunity to find one going bad before it self destructs. I measure capacitors with an hp 4192A impedance meter, purchased on eBay for $3k. That's what capacitor manufacturer's use. But you may be able to get by with a Dick Smith Electronics ESR meter. I bought mine from Canada, let us know if you need help finding one.
If these things aren't burned in or the product of carefully evaluated and tested design, the caps could be inserted reverse polarity. Even the artwork could be wrong, so everything looks ok 'per dwg'.
Electrolytics don't always fail immediately or catastrophically when mis-applied in low voltage circuitry. With your double section filter, there need be no immediate indication of misbehavior on the outputs, if you're measuring ripple/regulation at final test. Cross-regulation might show an exageration or shifted centering.
I've seen batches of capacitors fail because they were mislabelled. There's no defence against this other than burn-in and supplier vetting
On multiple output flybacks, you are at the mercy of the end-user when it comes to operating stress - end-use loading can grossly exceed the (rapparent) 2A ouput rating of this circuit. Assuming a flyback as it's a capacitive input filter and a single ended switch controller.
I'm surprised to see the relative distribution of the components - the bulk of the capacitance budget should be in the first stage of a flyback output.
1) VCC is around 5 V 2) According to the transformer requirment, the curren will be less than 1.7ADC (here, we depart to 0.3A + 1.5A (1.5 A is the trip value of fuse, not the practical load) ) 3) I think the circuit edited is right 4) UC383B working frequency 100KHZ
5) 1040 mA is the ripple current rating of that EXR capacitor, not the real ripple current, i need to measure that
6) I need to measure the voltage ripple on the capacitor.
7) How to calcurate the needed ripple current rating, if say the voltage ripple is some value, say, 60mV?
I had stated, "This would limit your supply to well under 2 amps, say 0.75 to 1.0A max." in which I used a 2x safety margin. But I didn't know about your use, and I failed to take into account the greater safety margin needed for an extended operating lifetime, as Spehro Pefhany pointed out. It appears your failures are due to pushing the capacitors beyond their ability, rather than bad parts. You need more serious caps.
The ripple current for a 1A7 output flyback will depend on duty cycle and secondary inductance. If you're quoting measured secondary rms current, however, then the capacitor rms ripple current should equal:
Icaprms = ( Isecrms^2 Ioutdc^2 )^0.5
..............and the actual output current will be less than 1A7 under those conditions.
Note that the ripple current in the capacitor is not greater than the output current, but the ripple current in the transformer winding is. Perhaps this is where some of the other responders are confused.
You should make the measurements at high line, full load, as this gives the worst case output capacitor ripple in mixed-mode circuits, with the output current being more discontinuous in nature.
If the circuit is unstable, or cycle-skips, the output ripple current will also increase. It doesn't look like you're worried about cryogenic conditions or stability.
I think most of this is right - I tend to have to look it up anyways nowadays.
What happens in a no-load condition? Does the voltage across the capacitor go above 10V? In the company where I worked some years back there was a design of a flyback converter that did just this, increased the voltage with no load connected.
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