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I don't have much experience with power electronics but I'm trying to design a power supply with AC 220Vrms (311peak) to DC 1500V converter to charge a capacitor in 50 miliseconds (and keep charging it, since it'll discharge as soon as it reaches 1500V). Conventional transformer + bridge rectifier dissipates a lot of power through the resistor needed to limit the current that charges the capacitor. So, I'm thinking about using a bridge rectifier + a boost converter (with IGBT) to do that, like the following schematic:

enter image description here

Note that, with duty cycle of 60%, the voltage reaches 1500V in 50ms:

enter image description here

The problem here is the inductor, that using 5mH the current is too high:

enter image description here

I've found some 5mH inductors that can handle high currents like this but its purpose is for filtering and not store energy for switching. Should I use another topology for the ac/dc converter? Any thoughts?

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    \$\begingroup\$ For that type of a voltage boost, you probably want something like a fly back... Otherwise what is your average current? If it is low you could use something like a voltage multiplier \$\endgroup\$
    – MadHatter
    Apr 30, 2020 at 21:40
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    \$\begingroup\$ I would say your best bet is to generate a 1.5kV supply, then use a constant current source made out of a transistor to feed your capacitor... Deal with the heat via a heatsink on the transistor... This scheme is foolproof and simple. \$\endgroup\$
    – MadHatter
    Apr 30, 2020 at 21:43
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    \$\begingroup\$ @MadHatter I think this is the guy who wants to dump about 5 kW (1500v, 200uF,20Hz) into his capacitor. So, not low current. \$\endgroup\$
    – user16324
    Apr 30, 2020 at 21:44
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    \$\begingroup\$ No offense, but this doesn't sound like a good idea for someone who doesn't have much experience in power electronics. There are safety issues, and lots of subtleties that will make it hard to get working without being experienced in the field. \$\endgroup\$
    – John D
    Apr 30, 2020 at 21:46
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    \$\begingroup\$ Dude... giving your level of knowledge and that you're effectively making a device that's going to be attached to the human body, this is seriously a recipe for criminal negligence. You honestly could kill someone with this project yourself included. I would highly advise you to start with much lower voltages and much lower powers. \$\endgroup\$
    – MadHatter
    Apr 30, 2020 at 22:07

2 Answers 2

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For safety, you should use a flyback transformer as the output, to provide isolation between input and output. However, driving a the flyback primary inductance and driving a boost inductor have essentially the same issues, so I'll concentrate on the boost converter you've illustrated. Note that a boost converter delivers an uncontrolled output current into loads below your input rail voltage, a flyback stays controlled all the way down to zero load volts.

For safety, you must use a mains isolation transformer on the input should you come to work on this. Working on directly connected mains components is a recipe for accidents. At best you toast your oscilloscope, at worst you toast your house, or you and your nearest and dearest.

You can design away the 'current is too high' problem. Put a current sensor resistor into the emitter of the IGBT, and switch it off when the current reaches your target current, rather than using a fixed duty cycle. This will have two profound effects. You control the peak current down, to just below the max of your inductor. You control the trough voltage currents up, to keep charging at all times.

Use a reservoir capacitor on your input bridge, you're wasting significant amounts of time not really charging your load due to low input voltage. A more even charging current means the peaks do not have to be as large.

I recently tore-down a switch-mode LED driver, and it had a (to me) novel capacitor/diode arrangement after the bridge rectifier. This is a win/win/win arrangement that reduces capacitor values and improves power factor, but only for switch mode loads that can use low voltages efficiently. The two capacitors charge in series, at a total of half their capacitance, reducing their peak charging current. However they discharge in parallel, which allows the bridge voltage to fall so that most of the work is done by the mains input, and then hold over at half the input voltage, with twice the capacitance. No 'balancing' is needed, their voltage is equalised every discharge cycle.

The main problem with your choice of inductor is not that it's a 'filter type', but that it's iron cored, so really intended for mains frequencies. You're running it at 1kHz. That's a very unusual frequency to run a boost converter at. Too high for efficient iron magnetics, too low to make use of the speed potential of your IGBT and ferrite magnetics.

Your IGBT, with a high current base drive (use a gate driver IC) could manage switching frequencies an order of magnitude higher, or more, allowing the use of a far smaller inductor, which would need to be ferrite cored. Think in the 20 kHz to 100 kHz region, still a managable speed for low switching and magnetic losses, and high enough for reasonable size magnetics.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I don't think he's going to get 5 kW out of a flyback. \$\endgroup\$
    – MadHatter
    May 1, 2020 at 13:58
  • \$\begingroup\$ @MadHatter It depends what you mean by 'a flyback'. I'm assuming a 'big enough' flyback. The lack of isolation and the shoot through direct from input problems with a basic boost converter are quite serious. With a current output, they're easy to put in parallel. With a modest step-up winding, the IGBT voltage can be reduced. You could handle the boost shoot through with an inverting output, with a few hundred volts more on the output device. Do you like the nifty reservoir capacitor arrangement? I was quite taken with those when I saw them for the first time. \$\endgroup\$
    – Neil_UK
    May 1, 2020 at 14:04
  • \$\begingroup\$ If his average power really is 5 kilowatts, he needs something like a forward converter, half bridge or full bridge converter etc. building a fly back to handle 5 kilowatts without saturating the core will be a challenge otherwise. All of this stuff is also probably way beyond his ability right now... \$\endgroup\$
    – MadHatter
    May 1, 2020 at 16:38
  • \$\begingroup\$ @MadHatter pf course it's way beyond his capability right now. I didn't get where I am now without making some big explosions, and some very expensive (fortunately for me, for my employers) very bad decisions. But most importantly, do you like the nifty reservoir capacitor arrangement? I was quite taken with those when I saw them for the first time. \$\endgroup\$
    – Neil_UK
    May 1, 2020 at 18:03
  • \$\begingroup\$ @Neil_UK thank you very much! This helped me a lot. \$\endgroup\$ May 3, 2020 at 4:20
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This is not an answer to how to make your supply.

This is an answer on safety, if you intend to continue down your path.

  1. I highly suggest you learn about high power and high voltage safety more. You are even approaching topics like Arc Flash safety, and with out a doubt lethal electrocution voltages.
  2. I highly suggest you learn about electrical safety from the context of "machine guarding and safety". Topics like E-Stop circuit design, safe machine maintenance procedures etc.

Before you even work on your contraption I would adopt the following:

  1. A multi pole contractor appropriately rated running both your Line and Neutral wires through it Normally Open. The coil should be driven by Normally Closed (NC) switches etc. so if a wire comes loose your device stops. Never use something like a micro controller for primary safety. If you want to have an over current system as part of your circuit, put it in series with an E-Stop, and use deterministic parts like a comparator. Ensure your E-Stop is visable and within reach at all times. enter image description here
  2. Plug your system into a GFCI outlet. This way if you somehow come in contact with High Voltage, the outlet should trip out, saving your life.
  3. When dealing with the electrical system with pulsed power like this, test it at a physical distance. For example, use a power strip with a switch from 10ft away to turn it on the first time or after changes.
  4. Do not wear flammable clothing or synthetics. Stick to pure cotton shirts and pants.
  5. Wear safety glasses, always.
  6. Buy a fire extinguisher.

Please note this just touches the surface, and some of this is just common sense. Also the safety circuit posted is minimally acceptable. In an industrial setting, what I posted would not meet minimum requirements like “A single fault does not lead to the loss of safety function.”. That is actually a good motto, wether it is a safty circuit, wire insulation. With this type of power and voltage, you should design so a single fault should not kill you.

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  • \$\begingroup\$ "Buy a fire extinguisher" Better buy one to be used at electrical circuits with 1500 V. \$\endgroup\$
    – Uwe
    May 1, 2020 at 10:19
  • \$\begingroup\$ Thanks man, you literally saved my life. \$\endgroup\$ May 3, 2020 at 4:17

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