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Help understanding and deciding on DRSSTC

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Ken M.

Sat Apr 12 2008, 01:36AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Hello everyone,

I realive In the past I often refered a SSTC as a DRSSTC, but this time I really Mean DRSSTC. I have been doing some research on it threw both Steves, Dan's, Richies, and Wiki. I'm still sifting through Wikis info and doing the calculations and trying to figure out some stuff.

Anyways I decided I'd go with Ward's Design for his .5 coil

, and while reading his design guide and checking through the schem for his 3rd coil

(only cuz he suggested its driver electronics) and I'm unsure of D20 and 21, Could they be 4007's or what should\could they be?

Also since most people use a variac, and I have non nor the desire to purchase one, Could a DRSSTC theoreticly run off a 24V 2A Xformer and a Voltage doubler?

The last question I have ATM is, For the CT's Could I use a Triad 200T with CT

, using the whole Coil for the driver and the CT and one outer pins for the OCD?

Also Heres my Schematic representation Of steves with my choice of Bridge, and otehr parts (excluding teh Interrupter).

HV Enthusiast

Sat Apr 12 2008, 03:14AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

1. D20 and D21 on your schematic are 33V zeners.
2. The D29's are in the wrong direction on your schematic.
3. You need balancing resistors on your DC doubler capacitors
4. The CT can work for both, but you have to make sure you have it connected right so the impedance of one circuit doesn't load down the other.
5. You need a variac. Makes everything easier and safer. Also, without a variac, you'll have to worry about inrush current.

Ken M.

Sat Apr 12 2008, 06:49AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

I mean D1 and D4 on mine, and D21 and 20 on Steves DRSSTC 3, I dunno what they are # wise, so I just wanna kow.

Is it just the D29's or is it 27 28 and 29's?

By balancing resistors what do you mean?

If I have to worry about impeadance matching on a CT then I'll just go with individual ones per section.

I realize I variac WOULD make life sooo much easier but I don't exactly have the funding for one of those nor do I wanna get one even if it would be convient. I was just more concerned if a 24v 2A Xformer would work if doubled for some reasons,

1) it allows low Vin meanin (hopefully) if I use 300-600V TO-247 IGbt (samples) they'd have a lower chance of getting blasted.

2) Even if Peak current is 1KA it sure wouldn't be pulling that from the wall since If your circuit breaker box is the same as mine the largest one for the WHOle house is only 100A so thered be no way to draw 1KA thrrough that thing let alone the power wires, so I was thinking I could allow 50W in power and just pull off the cap discharge power for more bang energy.

You say I'd have to worry about inrush current..What would that be exactly? Something like reverse current pumping power out the primary of the Xformer?

Josh Johnson

Sat Apr 12 2008, 07:42AM

Registered Member #793 Joined: Sun May 20 2007, 06:50PM
Location: Minneapolis, MN
Posts: 35

I am a little confused by the "cap discharge power" but I believe I know what your are thinking. I think you are missing the time aspect. It takes a certain amount of time to charge the caps in a voltage multiplier circuit (any caps for that matter). The amount of time it takes is directly related to the current being applied. If the input current is limited to a small value the capacitor will take a long time to charge, maybe several seconds or minutes. A DRSSTC is definitely not as fun when it only fires once a minute. You'd have to do the math to see what your planned setup is capable of.

Some research to help you out:

Capacitor charging:

Voltage Multipliers and Balancing resistors

Inrush Current

Hope this helps a bit,
Josh

Ken M.

Sat Apr 12 2008, 03:52PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Yea I understand the concept of RC charging and discharge.

By "cap discharging" I mean I understand how, you can get a lot of current from a cap discharging in a short amount of time, E.I. you have a 10A power source with a large C in parralel, in most cases the the C would only help to maintain 10A @ xV, but if the demand for current spikes then the cap can discharge providing that extra boost without taxing the source too much.

Ok I read th article for Voltage Multiplier balancing resistors, "R1 and R2 are equal value, balancing resistors that stabilize the charges of the two capacitors." But what I'm wondering is what wattage would these have to be to NOT blow up from power usage?

Hmm..So then for inrush current, you saying I'd have to worry about the initail powering on of teh voltage multiplier sending out 50-70V @ a high current initialy causing some serious damage to teh fets and other items?

If thats the case perhaps insert a switch between F1 and the voltage doubler?

Another question I was thinking of, From those who have build DRSSTC's what is the measured Current draw from the line, Example, like how much current is being drawn from the house to the Bridge?

HV Enthusiast

Sat Apr 12 2008, 05:16PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Regarding capacitor power . . .

The DC bus capacitors are essentially your energy source for the DRSSTC. NOT the wall. If you do the analysis, you'll find that during each pulse of the DRSSTC, almost all of the energy comes from the capacitors, not the wall.

A capacitor can release enormous amounts of peak current in a very short period of time.

For a DRSSTC, your typical ON time is about 5% while OFF time is 95%.

During the ON time (5%), all the energy is being fed from the DC capacitors.

During the OFF time (95%), the wall is charging those caps back up.

With really high power DRSSTCs, you should have a choke between the wall and DC bus capacitors. This smooths out the current and makes the wall see an average current, rather than big spikes.

The balancing resistors should be at least sized for 10 times the leakage current (from datasheet) of each capacitor. Power rating would be based on this resistor value and the voltage on the caps (max)

Inrush current is if you don't slowly increase voltage into DC supply circuit, those caps are just going make a bang and suck everything they can from the wall all at once. This could lead to very high peak currents, tripping circuit breakers, etc...

My miniBrute coils draw maybe 5-10A from the wall depending on power. It can approach 20A, though when i'm pushing long pulsewidths and high PRFs (high duty cycles)

Ken M.

Sat Apr 12 2008, 05:46PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

For the surge impedance, What inductor is that the primary or the secondary, I presume its the Primary since we're figuring Z for the tank circuit.

With teh info that wiki provided and a bit of digging threw Ward's site, I came up with...

I'm aiming for a Zsurge of 12.5 (~middle of range)

If (L) refers to primary then C=21.11nF
If (L) refers to Secondary then C=139.136uF (which seems Extremely high)

I'm aiming for Pk currents of 200-400A.
At those current ratings I'm looking at ~50A RMS.

With the Calculated currents and Zsurge aim of 12.5 I came up with a MMC of 2.5-5KV.

After going threw all of Wiki on the DRSSTC subject and doing all the available calculations, I'm now trying to decide which caps to get..I realize wiki suggests teh CDE And Ge ones but Having no luck finding low cost CDE's and the GE's are a bit pricey, So what other caps could be used?

HV Enthusiast

Sat Apr 12 2008, 06:39PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

wrote ...

I'm aiming for Pk currents of 200-400A.
At those current ratings I'm looking at ~50A RMS

Its not that simple actually. The peak of only one cycle is going to be 200-400A. The actual cycle is going to start from zero and increase cycle by cycle until its reaches it peak. So the RMS current is going to be much much smaller than simply doing a duty cycle current calculation based on peak current.

Ken M.

Sat Apr 12 2008, 07:14PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Well then how would you suggest I calculate those Currents?

Besides I keep saying "aiming" only because I have yet to pick out what IGBTs I'm going to use and Wiki says that the TO-247's Have handled 400A's so I was gonna try and make that be the max that the IGBT's Might have to handle. I'm not sure of which IGBT's I'm going to use only because I'm gonna try and get the IGBT's As samples but Not sure from which company or what modle I'm going to get, Although I know I'm going to try and get ones that can handle 400KHz if possible.

HV Enthusiast

Sat Apr 12 2008, 08:43PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

If you use TO-247 based IGBTs, you want to keep your frequency of operation to about 100kHz or less. The losses at 400kHz are too great with those types of peak currents and switching losses.

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