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Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Interesting about pure copper vs. treated copper. I will see what I can find and how much the difference is at the frequencies we work at. It could be it is just worth mentioning, but the effect is so little that its not worth worrying about.
True. The real life difference is quite minor. For my induction heating experiments I was looking for copper tubing with higher conductivity, but this turns out to be almost impossible to get, so there is no easy way to avoid this minor problem either.
In the DC bus capacitor chapter, you write "Voltage sharing between the two capacitors depends on their mutual ESR and ESL values." I would say that static voltage sharing only depends on the relative leakage currents of the capacitors. Voltage sharing when current is flowing also depends on the ratio of the capacitances. The voltage appearing across each capacitor in the dynamic case will also depend somewhat on ESR and ESL, but these parameters are more tightly matched between capacitors.
When the capacitance is large, it can sometimes be an advantage to have separate resistors for bleeding and voltage sharing, to minimize dissipation while still having a fast discharge time when the coil is powered down. The discharge resistor can also serve as a precharge resistor if you use a relay to switch it between the two different functions.
It's good that you include considerations about the RMS current rating of the capacitor, this is often overlooked.
A single phase active PFC can be placed after a three phase rectifier, and this will result in excellent power factor (theoretically up to 0.95 IIRC), as well as full compatibility with single phase input. I was researching this for a car charger project, and I was surprised at how good this solution was.
Do you know if anybody has tried RC snubbers in DRSSTCs? In other converters, I've had good success with these. By just adding capacitance across the bus, you're effectively lowering the characteristic impedance (and resonance frequency) of the bus, effectively making the stray resistance larger in comparison to get the required damping. Adding intentional resistance will often result in the same goal with much less capacitance.
Excellent stuff in general, and a lot of information I've not seen elsewhere. Looking forward to the next chapter.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Wolfram wrote ...
In the DC bus capacitor chapter, you write "Voltage sharing between the two capacitors depends on their mutual ESR and ESL values."
Do you know if anybody has tried RC snubbers in DRSSTCs? In other converters, I've had good success with these. By just adding capacitance across the bus, you're effectively lowering the characteristic impedance (and resonance frequency) of the bus, effectively making the stray resistance larger in comparison to get the required damping. Adding intentional resistance will often result in the same goal with much less capacitance.
Excellent stuff in general, and a lot of information I've not seen elsewhere. Looking forward to the next chapter.
That sentence was quite contradictory to what I write just below about balancing resistors, so I changed it to only be about leakage current.
The R in RC snubbers is there to dampen the ringing, thus lowering the overall power loss in the capacitor, it is not really a big problem with our short average duty cycle. It would also have to be a carefully selected low inductance resistor, else we would risk just adding the inductance that we were fighting against in the first place. This is however properly only a problem with low power inverters, inductance of a resistor should not pose a problem in a huge bridge construction :)
The next chapter is ready, it is not that very long. It is about sizing the snubber capacitor according to bridge inductance or rules of thumb on selecting against primary peak current.:
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
kilovolt wrote ...
Very nice work, Mad, thanks for your effort. Just a detail: I don't think the CM600 IGBT-modules are cheaper than the CM300, are they?
Best regards kilovolt
This is based entirely on my own purchases, I got a good deal on the CM600s and properly standard price on the CM300s, I added a little paragraph about it, it should only be an example and not a valuation list :) Always fight for those price knock-offs!
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Chapters Secondary coil (in review) and Topload (in review) are now in review and I welcome all feedback, criticism and correction of any mistakes, errors or mistyping I have made :)
Mads, thank you for collecting all this useful information. Some comments:
The Qsec = 1/k rule maximises the power transfer to the secondary for a given primary current in a steady state situation. Deviations from this rule will lead to larger primary currents as power is fed from the bridge into the primary. In other words, arc loading will have less effect on limiting the primary current. This situation is roughly similar to either a low coupling or bad tuning.
This has an effect on the choice of the primary tanks Z. Generally one would choose a high Z tank for a bridge with high voltage (e.g. full bridge) and low current capabilities and a low Z tank for the opposite. Low Z tanks are generally preferable, since they need less cycles to ramp up and the number of primary turns is lower, thus reducing resistance losses. The drawback of a low Z tank is, that the current limit of the bridge will be reached sooner unless it is limited by arc loading. This is where the proper choice of Qsec comes in. It will keep the current lower and allows for a lower Z primary.
In the table 2 listing of the pros and cons of high and low coupling, I think, that tuning is less critical with higher coupling. The poles, where the coils are run, are spread further apart and are less sensitive to the tuning.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
It has been a very long time under way with the next chapter to go in review and it is the widely sought for: MMC design and calculations
You can read the article at: and to see it, the password is "test".
The reason for the password is to avoid google indexing and random visitors reading it before it has been reviewed by people with experience in the field.
This is where I need your help, there is most likely some spelling errors, bad sentences, does-not-make-any-sense chapters or calculations errors. Please reply to this thread about such findings for a discussion about it.
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DRSSTC design guide
