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I have been troubleshooting a clicking noise on my Tesla coil driver that causes the system to stop working if my AC line voltage got to high (80V when system is designed for 120V). Suddenly one of the main rectification capacitors (1000uF 250V) puffed smoke violently... I'm guessing the clicking sound was the capacitor breaking down and arcing internally... I have multiple small ceramic capacitors also on the circuit to bypass any RF from the main supply line.

  • 1000uF 250V mfg p/n:381LR102M250K052
  • 2.2uF 630V are generic film caps
  • 4.7nF (Schematic 0.1uF) 1kV mfg p/n:S472M39Z5UN63J5R

I'm trying to figure out if it is my design that caused the failure, or just an unlucky capacitor being ran near its rating.

Does anyone see any issue, such as an RF path through the bulk capacitance (the 1000uF caps)?

enter image description here

enter image description here

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  • \$\begingroup\$ What's the ripple current rating on the capacitors? How much ripple are you subjecting them to? \$\endgroup\$ – winny Aug 29 '17 at 6:26
  • \$\begingroup\$ @winny The ripple current spec is 4A @ 20kHz. I'm not sure how much current they are subject to, I guess one way to calculate it is with a average power dissipation of 250W, and 170V stored, there should be something like 1.5A of ripple? But I'm not sure if that is a valid way to look at it... \$\endgroup\$ – MadHatter Aug 30 '17 at 0:38
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Design fault

Your Cap ripple current exceeding spec. on primary side by a wide margin.

Based on typical caps of that size, Imax @120Hz is rated at a few amps.

Meanwhile using half rectified AC @60Hz Zc (1000uF)= 3 ohms so ripple current starts at 80V/3 Ohms *50% df= 13.3 A then reduces as it charges up , then controlled by PWM load.

There is no provision for current limiting with ICL's on primary side or regulation with current sensing on secondary side.

Meanwhile a random power-on at 80V or even 120V into an ESR of about 0.1 Ohms can lead to >80A surge currents with a time constant depending on ESR*C = T, time constant. I might expect 100us

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  • \$\begingroup\$ So the AC cycle inrush is killing the capacitor because I am discharging it to much during each cycle? Or are you referring to the current I am pulling from it at ~250kHz via PWM? \$\endgroup\$ – MadHatter Aug 29 '17 at 4:10
  • \$\begingroup\$ And a Fix would be a better rated capacitor or multiple in parallel? \$\endgroup\$ – MadHatter Aug 29 '17 at 4:13
  • \$\begingroup\$ reminds me of the sound of a hot dog in a microwave oven with popcorn. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 29 '17 at 4:46
  • \$\begingroup\$ fix is in my answer about current protection. Are you expecting to generate >100W? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 29 '17 at 4:47
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    \$\begingroup\$ wrong you want an NTC for inrush \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 29 '17 at 4:48
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120 VAC is the RMS (root mean square), its 169 V peak, and 338 V peak to peak. So at 80 V RMS (what your meter reads) its really ~226 V peak. I suspect it is blowing at slightly over 80 volts (closer to 88).

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  • \$\begingroup\$ Interesting thought, although 169V should not be killing a 250V capacitor. Unless the system is very out of balance... \$\endgroup\$ – MadHatter Aug 29 '17 at 4:02

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