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Registered Member #267
Joined: Mon Feb 27 2006, 09:44PM
Location:
Posts: 46
I've built a small SGTC, so I have some experience with building Tesla coils, but I want to build something better. I've seen several DRSSTC designs on the web that I could follow, but I would rather design everything for the coil (controller, H-Bridge/F-Bridge, interrupter, etc) myself so I can fully understand what each component on the DRSSTC is doing.
I've got a BS in EE, focusing on power electronics, so I understand basic power converters. However, I have little experience with actually building circuits on PCB's. I am also not sure what is completely required for DRSSTC operation (specifically with the controls). Are there any resources available that explain more about the control of the DRSSTC? I read the HVwiki article, but that doesn't go into detail much on the control circuits.
I am looking to build a DRSSTC similar in size and power level to the minibrute, but I don't want to exactly follow that design, or use the pre-made boards. I will learn much more by designing everything myself. Is Daniel McCauley's "DRSSTC : Building the Modern Day Tesla Coil" book a good resource for understanding the operation of a DRSSTC? Are there other resources I can look at to understand the control of the DRSSTC better?
EDIT: Also, what is the cheapest oscilloscope I could use to test/fine-tune the circuits of a DRSSTC?
Registered Member #1535
Joined: Wed Jun 11 2008, 11:37PM
Location: Northeastern Pennsylvania - USA
Posts: 117
I bought that book to learn about his design. It is far more detailed than any website I've ever come across. It is more than just a how to build a specific coil, it goes into the detail of DRSSTC concept also. It was worth reading. Personally, I took a different approach to building my coil, but I still referenced the book from time to time to see what approach he took. There's always more than one way to skin a cat. I read and re-read dozens of websites. The book was very detailed. I thought it was worth buying....
Registered Member #267
Joined: Mon Feb 27 2006, 09:44PM
Location:
Posts: 46
TheBoozer wrote ...
I bought that book to learn about his design. It is far more detailed than any website I've ever come across. It is more than just a how to build a specific coil, it goes into the detail of DRSSTC concept also. It was worth reading. Personally, I took a different approach to building my coil, but I still referenced the book from time to time to see what approach he took. There's always more than one way to skin a cat. I read and re-read dozens of websites. The book was very detailed. I thought it was worth buying....
Did your DRSSTC use an H-bridge or a F-Bridge?
Other than the higher parts count of the F-bridge, does it run cooler than the H-bridge for a given power level due to the higher output voltage, and therefore, lower primary current? Would there be any reliability issues with using the F-Bridge instead of the H-bridge?
Registered Member #1535
Joined: Wed Jun 11 2008, 11:37PM
Location: Northeastern Pennsylvania - USA
Posts: 117
The advantage to the full bridge is that full bus comes out of the bridge. In a half bridge only half of the bus comes out due to the bus capacitors being in series. I built a full bridge because I wanted to run at a higher voltage. I don't believe there is any reliability issues between the two. If I only had enough parts to build a half bridge, then I would have used a voltage doubler to supply the bus. I'm not into doublers because each of the caps in series only get replenished every 1/2 of the incoming ac cycle. If the doubler, or better yet, boost converter was able to maintain a constant bus, the IGBT's would see the same current flow as a full bridge.
I'm not sure how to answer the cooling question. Due to the higher output voltage of my bridge, I have higher primary currents.
Registered Member #267
Joined: Mon Feb 27 2006, 09:44PM
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Posts: 46
Ok, so basically you would design the voltage output of the HB or FB to be the same, only the HB would require a voltage doubler at the input in order to have the same output voltage as the FB.
While we are discussing this topic, how do you go about choosing the DC input voltage of the FB? Obviously the IGBT's ratings will limit the voltage, but how high should the DC voltage go based on the input power to the coil?
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
magnetomotive wrote ...
Ok, so basically you would design the voltage output of the HB or FB to be the same, only the HB would require a voltage doubler at the input in order to have the same output voltage as the FB.
While we are discussing this topic, how do you go about choosing the DC input voltage of the FB? Obviously the IGBT's ratings will limit the voltage, but how high should the DC voltage go based on the input power to the coil?
With a voltage doubler on a halfbridge you would also need higher ratings than what is needed in a fullbridge at same voltage.
I think the general idea is to run rectified 230Vac ~ 330VDC on 600V IGBTs and a doubler on 1200V IGBTs, this is with a driver like steve wards.
The limiting factor would be shootthrough and Finn Hammers prediktor driver should help you towards running higher bus voltages without riscing a breakdown in the IGBT.
Registered Member #267
Joined: Mon Feb 27 2006, 09:44PM
Location:
Posts: 46
MadsKaizer wrote ...
magnetomotive wrote ...
Ok, so basically you would design the voltage output of the HB or FB to be the same, only the HB would require a voltage doubler at the input in order to have the same output voltage as the FB.
While we are discussing this topic, how do you go about choosing the DC input voltage of the FB? Obviously the IGBT's ratings will limit the voltage, but how high should the DC voltage go based on the input power to the coil?
With a voltage doubler on a halfbridge you would also need higher ratings than what is needed in a fullbridge at same voltage.
I think the general idea is to run rectified 230Vac ~ 330VDC on 600V IGBTs and a doubler on 1200V IGBTs, this is with a driver like steve wards.
The limiting factor would be shootthrough and Finn Hammers prediktor driver should help you towards running higher bus voltages without riscing a breakdown in the IGBT.
I will probably be running my coil from 120 VAC. This would only give me about 110-160 VDC depending on the size of my smoothing capacitor. Is it common practice with DRSSTCs to use an AC transformer to increase the AC input voltage to say 240 VAC? That seems much simpler than using a voltage doubler.
It was my understanding that shoot-through was only a problem for larger coils. I am wanting to build something similar in size to the minibrute. I was only going to use snubbers on the IGBTs unless the extra protection of the prediktor is needed for a coil of this size.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
I will probably be running my coil from 120 VAC. This would only give me about 110-160 VDC depending on the size of my smoothing capacitor. Is it common practice with DRSSTCs to use an AC transformer to increase the AC input voltage to say 240 VAC? That seems much simpler than using a voltage doubler.
A doubler will weigh a lot less compared to a step up transformer... despite this some of my small DRs make use of a 240V variac so i can run 240VAC or boost 120VAC up to the right level. Either solution works fine.
It was my understanding that shoot-through was only a problem for larger coils.
Care to explain why you thought this? The last thing we need is some more spread misconceptions. Shoot-through is mostly a function of the gate drive and the IGBT and the switching conditions (read the datasheet??). Its possibly worse with high frequency setups, not just because of the repetition rate of the shoot through causing heating, but because builders are more likely to reduce gate resistances to gain faster switching times, which can inevitably run into shoot-through, and you may never even know it. Shoot-through does not mean instant destruction, it depends on the severity of the condition. These are things you should learn as you progress into the field of power electronics.
I am wanting to build something similar in size to the minibrute. I was only going to use snubbers on the IGBTs unless the extra protection of the prediktor is needed for a coil of this size.
I havent seen short duration voltage spikes cause problems, honestly. You can add snubbers as you wish, but try and understand what problem you are addressing, rather than add "protection" for something you dont understand (it makes the design process a lot easier for you to understand the problem). I personally just use a low inductance de-coupling capacitor on my bridges, no snubbers or TVS diodes. Keep the bus inductance low to the lytic to avoid low frequency (at Fo or less of the coil) voltage swinging across the bus L from di/dt. Keep the de-coupling (or snubbering if you will) inductance low to maximize its effectiveness.
The best approach is to make things as good as you can from the start, only after that should you address it with bandaids like snubbers or TVS diodes. Ive built DRSSTC bridges that didnt require anything but the IGBTs and electrolytic capacitors (no film caps or snubbers) because the bus was of excellent design.
Registered Member #267
Joined: Mon Feb 27 2006, 09:44PM
Location:
Posts: 46
wrote ... A doubler will weigh a lot less compared to a step up transformer... despite this some of my small DRs make use of a 240V variac so i can run 240VAC or boost 120VAC up to the right level. Either solution works fine.
Would a single transformer be cheaper than a doubler? The doubler will be larger in volume with those huge electrolytic caps, correct?
BTW, do you know the peak power level of the minibrute?
wrote ... Care to explain why you thought this? The last thing we need is some more spread misconceptions.
It's not that I thought the shoot-through spikes wouldn't be there. I simply thought that with higher power coils you would have more heat dissipation to deal with, so more elaborate measures (i.e. the predikter) were preferred to eliminate the spikes.
wrote ... I havent seen short duration voltage spikes cause problems, honestly. You can add snubbers as you wish, but try and understand what problem you are addressing, rather than add "protection" for something you dont understand (it makes the design process a lot easier for you to understand the problem). I personally just use a low inductance de-coupling capacitor on my bridges, no snubbers or TVS diodes. Keep the bus inductance low to the lytic to avoid low frequency (at Fo or less of the coil) voltage swinging across the bus L from di/dt. Keep the de-coupling (or snubbering if you will) inductance low to maximize its effectiveness.
I understand what causes the voltage spikes. I haven't built any power circuits before, but to me it is good practice to have the added protection just in case. I'd prefer to have as few IGBT failures as possible.
Registered Member #1535
Joined: Wed Jun 11 2008, 11:37PM
Location: Northeastern Pennsylvania - USA
Posts: 117
Check out Steve's simplest of doublers at Overall bus capacitance is only 500 µF because the capacitors are in series.
Are you asking about the peak in the tank circuit of the minibrute or peak it pulls from the wall?
I have minor spikes in my first coil. It looks like ringing at switch time. This is why I stay within 2/3 of the IGBT's voltage rating. I've been focusing on a stronger gate drive and better matched gate resistor. I also believe latency between my primary CT and the actual output of the GDT is the culprit. A GDT driver with faster response time would probably make things smoother.
Snubbers and would be fine for a finished coil in my opinion. My coil is in development. A properly made RC snubber (wastes power) would probably eliminate my spikes, but as I work on increasing the input supply voltage, and also redesigning my primary for higher peak currents, I would expect the spikes to resurface.
When it comes to switching, the Prediktor is a classic example of how to approach perfection without using protection...
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Designing a DRSSTC from scratch
