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I'm trying to build a DSP plasma speaker using IR2104 half-bridge to drive a TV transformer. The switching frequency is between 15KHz-100KHz.

  • I chose the topology from an old analog plasma speaker design: enter image description here

enter image description here

  • Instead of analog PWM, I used MCU; Instead of GDT, I used float-channel gate driver

However there are a few problems:

  • I don't know how to add back-EMF snubber to this, because the back-EMF is AC and diodes won't do any good.
  • C3 and C4 overheats, I know they are for DC blocking but I don't know why they need to be 10000uF (I don't have 10000uF in hand so I used 100uF instead). If not the huge capacitance requirement, I would have used MKP capacitor.

Thanks in advance !

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    \$\begingroup\$ Common problem. You need very low tan fi/loss factor capacitors for series connection like this. Have you considered a split rail half-bridge instead? \$\endgroup\$
    – winny
    Commented Jun 14, 2021 at 15:44
  • \$\begingroup\$ Sorry, my bad, you are using a split rail. Tan phi. Look in datasheets. \$\endgroup\$
    – winny
    Commented Jun 14, 2021 at 15:53
  • \$\begingroup\$ ”10000uF (I don't have 10000uF in hand so I used 100uF instead).” Depends on input voltage, switching frequency and current. What’s yours? \$\endgroup\$
    – winny
    Commented Jun 14, 2021 at 16:19
  • \$\begingroup\$ @winny Sry, I forgot to mention. I'd like to use it from 15KHz to 100KHz. The currently configuration is 42KHz. \$\endgroup\$
    – 7E10FC9A
    Commented Jun 14, 2021 at 16:41
  • \$\begingroup\$ Then you need to calculate your max allowable voltage drop from your minimum frequency, minimum input voltage and max current according to Iton=C*deltaU \$\endgroup\$
    – winny
    Commented Jun 14, 2021 at 16:46

1 Answer 1

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C3 and C4 overheats

C3 and C4 overheat because their impedance is too high at the operating frequency, so they dissipate some of the energy passing through them as heat.

You need to use capacitors with lower impedance. Those, incidentally, tend to be physically bigger, and also have higher capacity. There was a reason why the original design used large 10mF capacitors: you can't just use something else "because you don't have them" without understanding why they were needed. Nobody places expensive large capacitors into circuit without there being a good need for it :)

I don't know how to add back-EMF snubber to this, because the back-EMF is AC and diodes won't do any good.

You can use Zener or transient-voltage suppressor diodes. In fact, even for typical use in relays, the back-EMF diode shouldn't be just a rectifier, since that slows the relay opening down, and shortens the contact life (slower opening produces more heat on the contact points, and prolongs the duration of any arc that may be present). Most relay applications driven from DC are best served by a back-EMF Zener set for 1-2x the operating voltage, with the proviso that the driver must be able to survive that.

Alas, a snubber's job is also to dissipate high-frequency transients as the switches open. And for that, you need an RC circuit, perhaps with a ferrite bead, that dissipates energy in frequencies above your application frequency. A high-pass filter plugged in parallel to the primary, set for 1MHz cut-off and, say, 50-100Ohm impedance at that frequency might be a good starting point.

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    \$\begingroup\$ What does impedance have to do with power dissipation? Isn't it entirely based on ESR? \$\endgroup\$ Commented Jun 14, 2021 at 18:26
  • \$\begingroup\$ If that huge Al cap bank is only to provide low impedance, why not use MKP or Al-polymer cap instead ? \$\endgroup\$
    – 7E10FC9A
    Commented Jun 14, 2021 at 22:00
  • \$\begingroup\$ For a flyback transformer to work properly, you need to drive it using a flyback circuit, not push-pull. \$\endgroup\$
    – Kartman
    Commented Jun 15, 2021 at 11:07

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