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CCPS (Capacitor Charging Power Supply)

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Daniel Uhrenholt

Sun Sept 02 2007, 12:56AM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Hi Finn.

I think itâ€™s time for me to say a few things about our project .

I like your gate driver, and after an afternoon in your workshop this weekend, with blown optocouplers and a headache, I am pretty satisfied with the results. Too bad we didnâ€™t get a chance to test it on our CCPSâ€¦

About the transformer, I must agree with you!

I think we should do it the â€œWardâ€ way, and place the primary under the secondary winding, and make an extra series inductor to adjust from. If you agree with me Iâ€™ll order two of these from RS (Part number) 213-4470, and use the cores we have in parallel.

We need more space in our HP214A cabinet

Cheers Daniel.

Steve Conner

Sun Sept 02 2007, 10:40AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yay, this is looking great

About the matching inductor, I think Steve used an air cored one. You don't need very much inductance if you're transferring a lot of power, as you figured out. If you use ferrite, you'll need a large air gap to avoid saturation. You certainly wouldn't want to use that RS part you mentioned without any airgap!

Marko

Sun Sept 02 2007, 10:59AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi Steve, Daniel, Finn

You need Only few tens of uH at most, and I think material #26 (yellow-white toroids) would be more than fine for you at these currents.
With rough calculation I think even the size from PC power supply would suffice.

About the matching inductor, I think Steve used an air cored one. You don't need very much inductance if you're transferring a lot of power, as you figured out. If you use ferrite, you'll need a large air gap to avoid saturation. You certainly wouldn't want to use that RS part you mentioned without any airgap!

Steve, I don't think airgap actually does any good with inductors like these.

Inductance will reduce proportionally with permeability and we really get nothing except need to wind more copper onto the core. (OK, with rod cores, this may save some ferrite from other side).

Material #26 has saturation B of about afaik 1T while #77 ferrite is 0.45T max according to CWS table. Most use ferrite to maybe 0.3T.

In any wa ferrite core would need to be larger and more expensive for this application.

The great thing about air cored coil is it's absolute stability, but it is a bad thing when we need 15amps continuously through it at 50kHz.

Daniel Uhrenholt

Sun Sept 02 2007, 11:56AM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Hi Steve and Marko

I did some calculations today on how many Amps we need on the primary to get about 15kVA, and it looks like we need a major rebuild of the supply

I think we have to get a Fo about 50kHz and a C about 1uF

So if P= F*E

E=0.5*560 2* 0.000002 = 0.3136J

P= 50000Hz*0.3136J =15680W

And with a L of 5.07uH we get 248Amps on the primaryâ€¦ Thatâ€™s a lot of heat we need to get rid off!!

Well I will buy the cores anyway, I want to build a smaller but powerful CCPS when we are done with this coil.

Cheers, Daniel

Steve Conner

Sun Sept 02 2007, 12:15PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Are you sure about that?

The peak current in the resonant circuit should only be something like 3 times what it would be in a hard-switched converter. That gives a loaded Q of 3 even at full output voltage.

For 15kW on a 560V DC bus, a hard-switched converter would run about 30A, so you should have something like 90A peak current. Say 100 since you have an IGBT brick handy

Try designing your circuit such that the leakage inductance and capacitor end up with a surge impedance of (560/100) ohms and a resonant frequency of say 100kHz, or whatever frequency you decide gives acceptable losses. (Switching losses should be low because you have zero current at turn-on and turn-off, so you might be able to aim higher in frequency than the IGBT datasheet would suggest. However, you have to bear in mind that if the loaded Q is too low, you will lose your zero-current turn-off at higher output voltages. This is a trade-off.)

Daniel Uhrenholt

Sun Sept 02 2007, 12:49PM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Hi Steve,

Iâ€™m almost sure that my calculations are correctâ€¦ The I peak of 248A is in the resonance circuit, and looking at other peoples work with high power CCPS`s, their I peak resonant current is in the 100-1100A area..

1120 Amps for their 42kW supply

152A for their 32kHz 15kW supply.

Cheers, Daniel

Marko

Sun Sept 02 2007, 12:55PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

For 15kW on a 560V DC bus, a hard-switched converter would run about 30A, so you should have something like 90A peak current. Say 100 since you have an IGBT brick handy

Right now he has impedance of 2,23 ohms which is quite way far. As far as I understood it, he's limited by total charge he can push through the cap in one cycle, and making the cap big enough makes characteristic Z way too low. If I understood anything of this?

Steve: You didn't answer the thing about cores.. I actually wonder if I'm right or wrong

Steve Conner

Sun Sept 02 2007, 04:09PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Re the thing about cores, it's a known fact that the energy storage before saturation is proportional to the size of the airgap. An ungapped ferrite core can store practically no energy. I've not investigated it myself, but I hear that gapped ferrite isn't nearly so good as iron powder toroids.

Daniel: It always seemed to me that the CCPS tank circuit impedance was a tradeoff between conduction losses caused by higher peak currents, and switching losses caused by it falling out of ZCS at certain output voltages due to not having enough peak current. I think Marco Denicolai explored this with PSpice in a lot of detail. Running at a lower frequency than f0/2 might help too.

Finn Hammer

Sun Sept 02 2007, 05:13PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Thanks for the input,

These numbers are for a single transformer, topping out at 4kV.

At present:
Z= 25.44, 92kHz
68nF, 44uH, Ipeak, start 25A Ipeak, end 50A , 4kW (1uF to 4kV, 8J, in 2mS)

So we we missed the goal by 50%.....

With a Z of 5, 100kHz,
315nF, 8.04uH, Ipeak, start 100A Ipeak, end 200A (1uF to 4kV, 8J, in 0.4mS = 20kW)

Yes, I get the picture.

The transformer probably has to leave the cabinet, and sure is up for a rewind. The winding window will get *optimally filled* by pie shaped secondary windings.

Waw, this sure is fun stuff.

I feel kind of tempted to skip the negative gate drive on the IGBT`s and adopt Terry`s source follower supplies, from his DRSSTC, but that may have to wait untill the next iteration.

Cheers, Finn Hammer

Daniel Uhrenholt

Sun Sept 02 2007, 07:51PM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Finn,

It looks great, I canâ€™t wait to make an 20kW transformer

Cheers, Daniel

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