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I am looking to reduce the voltage ripple on a DC/DC converter operating at 5kV.

I have looked extensively on the internet and have found multiple ways to reduce output voltage ripple, an interesting reference is by EEVBlog which uses capacitance multipliers with BJT/MOSFET/Darlington transistors.

Multiple other sources state to use multi-stage LC filters, and high value capacitors, etc. However, when the output voltage enters the kV range, the use of inductors is avoided as much as possible, if possible. Furthermore the size of capacitors with large values rated for kV values are very bulky and again, would be avoided if possible. This brings limitations in the amount of voltage ripple that can be attenuated in this range - which makes designing low output voltage ripple DC/DC converters in ripple sensitive applications (such as travelling wave tubes) very difficult. Splitting a transformer secondary into multiple windings reduces the ratings of inductors/capacitors but to what extent does this beat having a single inductor?

Does anyone have any tips on how the voltage ripple can be minimzed in such cases?

I am planning on using HV rated silicon carbide devices in the isolated power supply so possibly something that utilizes a MOSFET in the rectifier can reduce the ripple.

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    \$\begingroup\$ An old Jim Williams technique for making quiet HV rails was to use RC filtering, but that was for relatively low current consumption. \$\endgroup\$ – W5VO Jul 16 at 14:45
  • \$\begingroup\$ Can you tolerate a small amount of loss? \$\endgroup\$ – Voltage Spike Jul 16 at 15:09
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    \$\begingroup\$ How about a shunt regulator? \$\endgroup\$ – analogsystemsrf Jul 17 at 4:50
  • \$\begingroup\$ @analogsystemsrf A shunt regulator with a MOSFET? I'm not aware of any zener diodes that have voltage ratings in the kV range? \$\endgroup\$ – jvnlendm Jul 17 at 10:47
  • \$\begingroup\$ @laptop2d, Yes, I can tolerate some loss. The voltage ripple is the most important factor for output stability in my application so if that comes at the expense of loss then I suppose that's the cost! \$\endgroup\$ – jvnlendm Jul 17 at 10:48
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There are 100 V 5 W Zener diodes on market. You can construct a voltage limiter circuit. All you need to do is building the limiter circuit with less voltage level. For example consider a 5 kV DC with +250 V peak to peak ripples, a voltage limiter with 4.9 kV will kill all the ripples. This solution is likely cost you more because larger diodes that on market is around 15$.

Another solution is dividing the output voltage and filtering them seperately with simple RC networks. But it is a trivial solution. Because this method will likely increase the power loss.

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You did not state the power or current output nor the frequency so it is difficult to understand best approach. However, one approach is to have 2 converter stages operating 180 degrees out of phase to help cancel the ripple.
If you can't use an inductor in your output how did generate the 5kV - without an inductor? A common output filter is an LCL filter with a resistor in series with the capacitor. The capacitors at that voltage are larger but you should not need a very large value. The resistor is for damping. Another approach is to consider a feed forward control.

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  • \$\begingroup\$ Hi Rob, power is 600W. I use an inductor to supply current feed to the my main bridge, through a buck converter. An inductor need not exist in the output in this configuration. Therefore it can be eliminated although of course then the ripple suffers. \$\endgroup\$ – jvnlendm Aug 27 at 10:51
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You should consider higher frequency switching operations and a means to minimize filtering needs. Look into interleaving options (running n converters shifted by 360/n electrical degrees for their PWM periods) as well - this will have a similar effect as a single converter running at n*f frequency and can provide a modular/scalable solution. Rob B.'s suggestion was for a 2x interleaved system, but it's extendable to many stages/systems.

Depending on the application... an important point to consider for high-voltage supplies is their performance under arcing conditions. You may particularly want to minimize filter energy for this purpose as much as size/cost/weight reasons.

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