Finding low-ESR 400V capacitors

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  • PeteS in CA
    Badcaps Legend
    • Aug 2005
    • 3579
    • USA, Unsure of Planet

    #21
    Re: Finding low-ESR 400V capacitors

    I can't give anything like an evolutionary history of electrolytic capacitors, as I'm too young (I don't say that often!). I read some late 50s TV repair magazines, in which they were regarded as royal PITAs (Pains In The Ass), due to inconsistent quality and frequent-friers being semi-custom multi-section, multi-value parts. The latter aspect meant difficulty keeping suitable replacement parts on hand.

    Fast-forward to my days at DeVry (early-mid 70s),and about the farthest profs went beyond, "Assume an ideal capacitor ..." was to mention that electrolytics and tantalums are polarized.

    By the time I started at Boschert, my first job working with switching power supplies, in 1980, low ESR capacitors and "Computer Grade" (beer can sized screw terminal) capacitors had been on the market for quite a while. Switching power supplies need output capacitors the can handle high ripple (AC) current without overheating, thus, low ESR as it was then called. United Chemicon RX and RZ series, Nichicon PA and PB series, and Sprague 672D and 674D series were the best available at the time (I didn't see Marcon, Rubycon, or Panasonic at that time).

    Since then "low impedance" became the common usage, and impedances of series have fallen steadily. Going back to the, "Assume an ideal capacitor ..." line, the impedance of a part at a given frequency is the vector sum of the part's capacitive reactance, inductive reactance, and resistance (reactance is for AC what resistance is for DC, but changes with frequency). In the case of a real-world capacitor, it's the vector some of the capacitive reactance, and the parasitic inductive reactance (ESL) and resistance (ESR) due to the wire leads, swaging the leads onto the foils, the foils, and the electrolyte (in an electrolytic capacitor, the dielectric is an aluminum oxide layer on the anode foil; the electrolyte is a very very low resistance conductor).

    As frequency increases, inductive reactance increases, and capacitive reactance decreases. At 100KHZ, the capacitive reactance of a 2200uF capacitor is ~.72 milliohms. In the vector sum, capacitive and inductive reactance subtract. The inductive reactance at 100KHz is probably higher that than the capacitive reactance, so the impedance of the 2200uF capacitor is close to the vector sum of the ESR and the inductive reactance of the ESL.

    Back in the late 70s and early 80, switch frequencies were in the 20KHz-30KHz. Since inductive reactance is proportional to the frequency, multiplying the frequency by 5 or 10 multiplied the inductive reactance by the same factor. So ESL was less relevant back 1980ish.

    Some the others here can be more precise about the time-frame that I am, but somewhere around 2000 computer MB started including VRMs that stepped the 12V or 5V from the PSU down to the voltage needed by the processor or memory. These started as little plug-in boards, but soon were integrated into the MBs. Nowadays there are server MBs that just take 12V power, and have all the integrated DC-DC converters necessary to generate the various voltages needed by the MB.
    PeteS in CA

    Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
    ****************************
    To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
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    • PeteS in CA
      Badcaps Legend
      • Aug 2005
      • 3579
      • USA, Unsure of Planet

      #22
      Re: Finding low-ESR 400V capacitors

      Originally posted by Behemot
      There is the typical 60-100 kHz in active (boost) PFC.

      Most of the better HV caps are of long-life type, so the ESR is lower than of standard but still higher than of low-ESR. Also putting too low ESR caps into such old device is usually worse than better.
      ...
      The snap-in types used for 50/60 Hz filtering and as output caps for PFC stages are not designed for low impedance. But they are designed to handle high ripple current, which requires being fairly low impedance (so they don't fry themselves dry). I think many/most PFC stages also have a film capacitor in parallel with the big snap-in electrolytic.
      PeteS in CA

      Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
      ****************************
      To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
      ****************************

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      • Behemot
        Badcaps Legend
        • Dec 2009
        • 4845
        • CZ

        #23
        Re: Finding low-ESR 400V capacitors

        I meant more like the D16-D18 leaded, not snap-in. The leaded ones go all the way up to 12000 hours @temperature and ripple so I think they are lower ESR than comparable snap-in caps.
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        • PeteS in CA
          Badcaps Legend
          • Aug 2005
          • 3579
          • USA, Unsure of Planet

          #24
          Re: Finding low-ESR 400V capacitors

          Originally posted by Behemot
          I meant more like the D16-D18 leaded, not snap-in. The leaded ones go all the way up to 12000 hours @temperature and ripple so I think they are lower ESR than comparable snap-in caps.
          Could be. The PSUs I dig into these days may be too high power to use 16 mm and 18 mm diameter leaded parts. I see lots of snap-ins at PFC outputs.

          Those super long life parts may also be optimized for long life in CFLs and ballasts, where cooling may be poor.
          PeteS in CA

          Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
          ****************************
          To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
          ****************************

          Comment

          • Stefan Payne
            Badcaps Legend
            • Dec 2009
            • 1267
            • Germany

            #25
            Re: Finding low-ESR 400V capacitors

            Originally posted by PeteS in CA
            but somewhere around 2000 computer MB started including VRMs that stepped the 12V or 5V from the PSU down to the voltage needed by the processor or memory.
            That started with the Pentium Processors, the P54C. First ones had linear regulators...
            But then there was Cyrix who had a higher power consumption and thus linar regulators wasn't enough.

            And by the time the Triton II Boards came around (and also the P55C), switch mode VRMs became more and more common. Because those CPUs were the first with a split I/O and Core Voltage...


            The older Triton I Boards (i430FX) had the VRM connector you mentioned...

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