Batch of General Purpose H-Bridge PCBs Now on Sale!

I’m working on a new CM600 DRSSTC project, so in order to stir up some funding, I’ve finally decided to sell off some PCBs I made for my smaller TO247 IGBT coil last year. They are now available on eBay here.

Here’s an example of a DRSSTC I built with this board (but you can use this PCB for basically any purpose):

DSC_0217

And here’s a very simple high voltage SMPS with the H-bridge board driving a homemade ferrite transformer. The driver is a TL494 and some gate drive chips deadbugged onto some blank copper clad board in an afternoon:

The boards are a derivative of the general purpose H-bridge design that I posted about in 2013, which I’ve had many requests to sell over the years. There are a few notable improvements that I’ll document here along with the rest of the design.

Key Features:

  • H-Bridge with four spots for TO247 IGBTs or MOSFETs
  • Gate protection network on each gate
  • 0805 and 2010 dual package landing pattern for gate resistors
  • 1.5KExxx series TVS across each TO247 device
  • CDE940 series snubber capacitor
  • Mounting space for two 1″-diameter GDTs
  • SMA connectors for GDT drive signals
  • Voltage doubler input with two TO220 diodes with heatsinks, two 10.16mm grid, 35mm diameter bus capacitors, and bleeder resistors
  • 5mm x 20mm fuseholder
  • 10AWG holes for external DC bus input
  • 4oz copper on both sides with low inductance laminated bus structure

Digikey Bill of Materials (Excel file hosted in Dropbox)

Schematic:Sendout_Bridge_Rev2_Schematic

Layout:Sendout_Bridge_Rev2_Layout

Top side scan:PCB_Hbridge_FT14_Top_0001

Bottom side scan:PCB_Hbridge_FT14_Bottom_0001

CAD files are available upon request. Please send in any feedback you have so I can improve future versions of this board.

Currently, there is one known issue with the board; the labels on the mains input connector are wrong. They read “GND (GRN), NTRL (WHT), HOT (BLK)”, but they should read “GND (GRN), NTRL (BLK), HOT (WHT)”. Basically, the ground, neutral, and hot connections are labeled correctly, but their standard wire colors are not. Neutral is black and hot is white, not the other way around.

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IAP Project: 3.6kW Spark Gap Tesla Coil

Sparks 6 (PDU Unballasted, 5.5' Target, No Discharge Rod)

As per my usual protocol for these sorts of things, I decided to build a large spark gap Tesla coil this IAP (read: winter term) after I finally collected all of the necessary components through a couple years of hoarding and one lucky find from a dorm basement cleanout early in January. It turns out that many years ago, someone in East Campus had attempted to build a fairly large SGTC (it appears in some old i3 videos), gotten to the point where it made some amount of sparks, and then moved the pile of parts to the server room in the basement, which may or may not have contained an EE bench at the time. Fast forward to 2015 and the server room was getting cleaned out and refurbished, and there was a pile of Tesla coil parts sitting in the hallway waiting to be picked up for disposal. I took the old power supply, secondary coil, topload, and asynchronous rotary spark gap and promptly got to work after redesigning the entire system in JavaTC (see linked files at the end).

The first order of business was getting the power supply to turn on. It consisted of a 14.4kV potential transformer and a PDU (Power Distribution Unit) with a pair of variacs (one for the transformer, one for the rotary gap) and had certainly seen better days. After a day of reverse engineering the wiring and fixing broken connections inside the PDU, the electronics were restored to working order, and I could even turn the power on after the keyswitch was bypassed! (It didn’t come with a key…)

Front of PDU

Back Side of PDU

Next up was the rotary spark gap. Everything appeared to be in working order so I slowly and carefully powered it up. It promptly self-destructed, and I spent the next few days waiting for some tungsten welding rods to arrive from eBay (5/32″ x 7″ Lanthanated TIG rods) and then cutting them down on an angle grinder and got to work rebuilding the mechanism. It turns out that the bolts holding the motor to the faceplate of the rotary gap were loose and allowed the slight, inevitable imbalance in the rotor to develop into a serious problem. Lessons learned: use lock washers, and make sure the fasteners in your high RPM mechanical assembly are tightened!

Rotary Spark Gap

I decided to try my hand at some basic woodworking, so I routed two 32″-diameter circles of wood for the base of the coil, drilled mounting holes after mocking everything up on a computer, and then sanded, stained, and varnished the parts:

Routing Coil Bases Using Jig

The primary supports were laser cut from 0.25″ thick acrylic sheets and the primary was wound with 0.25″ diameter flexible copper tubing ziptied to the supports.

Primary Construction

For the primary capacitor, I used 8 parallel Ion Physics model 4006-311 low-inductance, high-voltage polypropylene film capacitors rated for 100kV at DC and several nanofarads each. The total capacitance of the bank was 54.98nF. It’s definitely overkill (and probably not quite enough capacitance), but it’s what I had on hand. I’m planning to replace it with an 0.7uF, 50kV Maxwell pulsecap from eBay, but that hasn’t arrived yet. These capacitors are gigantic, by the way:

Ion Physics Co. Capacitor (1 of 8)

The fully-charged bank (at 100kV) can store in excess of 250 Joules (this is not a totally inconceivable scenario if you consider resonant charging!); charged to the peak of a 14.4kV RMS sine wave, the bang energy is closer to 12J, which is reasonable for a coil of this size, but the physical size of the capacitors is mostly wasted.

The secondary coil was re-varnished, laser cut end caps were installed, and I was faced with an interesting problem. After assembling the coil in my dorm room, I realized it was far too big to fit through the door, and I didn’t have anywhere near the amount of power required to turn it on, even if I took it outside. Luckily, though the base it didn’t fit through my room’s door, it did fit through most other doors in the building and I rolled everything over to Walker Gym in building 50, which is right next to the amateur radio club and has 15A and 20A single phase and 40A three phase outlets.

Operating without a ballast (this is the maximum performance mode, as the rotary spark gap quenches very effectively), the sparks are 6 feet to a grounded target and break out in multiple directions, which suggests that the performance could be improved further with a larger topload. As I am near the maximum primary tap with my current primary capacitor, I have not yet tried this.

Setting up the coil and target:

Measuring Perpendicular Distance to Target

6 foot ground strike:

Sparks 5 (PDU Unballasted, 5.5' Target)

Additional documentation:

YouTube video

Photo Album

JavaTC Data File

Laser Cut Parts Inventor and DXF Files

If you are in the area and would like a demonstration some weekend evening, shoot me an email and perhaps something can be arranged.

General Purpose H-Bridge Inverter

Obligatory picture at the beginning of my post so it doesn’t make my entire front page look like a wall of text:

A quick skim through my site will reveal that I haven’t been very diligent about posting new projects or filling in the many blank pages that are supposed to document my old ones. Sadly, I’ll probably be too busy with schoolwork until the summer to fix that, but in the meantime, I’ve decided to try something new; this page is going to have short weekly/monthly documentation of what I’m building, rather than some sort of weird chronological list of the projects I finally get around to posting about. In other words, this is finally going to be a real blog. Here it goes!

After about a year of playing around with all kinds of different H-bridge designs for my DRSSTC projects, I decided to put together a finalized general purpose H-bridge board that would work well for all kinds of small to medium-sized high voltage projects (read: those that don’t use bricks). Basically, any time I need an inverter for something like a Tesla coil or ferrite transformer, I want to be able to grab one of these boards, add a driver, and gogogo!

The first obvious design choice is mini brick (SOT-227), or TO-247? Seeing as mini bricks are about 5 to 10 times more expensive ($20 – $30 a pop, as opposed to $2 – $5 for the average TO-247), I’ve only ever used the latter in my projects. It’s been experimentally determined that it’s best to replace all the silicon on your bridge every time something fails (there’s an old 4hv thread on this out there somewhere), that means replacing an entire bridge on a TO-247-based coil after a device failure costs about as much as a single lower-end mini brick transistor. If that alone isn’t enough to convince you, here are some other reasons TO-247s are better at this scale:

1. Mini bricks have isolated metal backplates, while TO247s do not. At first glance, this might seem like a good thing because it means you can bolt all your mini bricks to the same heatsink without worrying about creating short circuits. The downside is that internally, the thermal contact between the die and the mounting plate is much poorer than what you get with a TO-247. A bridge of TO-247s with individual heatsinks is the ideal route to take, although silpads or the occasional surplus Beryllium Oxide heatsink insulators found on eBay will allow you to put the entire bridge on the same heatsink.

2. On average, mini bricks have much more gate charge and switch slower than TO-247 IGBTs. Beefy gate drivers with gate drive chips driving discrete FETs and phase lead networks (for feedback-tuned systems) can turn minibricks  into a more viable option, but it’s nice not to have to go through all of that for a board that’s supposed to be easy to use.

3. Modern TO-247 IGBTs such as the Fairchild FGH60N60SMD can reliably handled peak currents as high as 300A (and probably more) in conventional DRSSTC use up to hundreds of kHz, which is good enough. While it’s possible to push some mini bricks up to insane currents as high as 1000A or more to get giant sparks out of little coils due to the mini bricks’ larger dies, it limits their average operating lifetimes down to 5 to 10 minutes due to rapid degradation of the silicon. The “old way” of making a DRSSTC was to push the primary current as high as possible and regularly replace the transistors (this practice was born back when mini bricks could be had as free samples by the box!), but that’s not generally considered to be a good idea anymore. If you need that much current, you can use a ferrite transformer to enforce current sharing between multiple bridges (see Steve Ward’s new phase-ramping QCW coil, it’s really clever!).

tl;dr: I used TO-247s.

Okay, now here’s the actual design of the board. Note that the IGBTs are meant to be mounted on the heatsink under the board:

Board layout

Board layout

Key features:

– Laminated DC bus with a snubber very close to the transistors: The reason the positive and negative rails are right on top of each other is to keep the inductance between the bus capacitors and the transistors as low as possible. The stray capacitance of this arrangement lumps into the snubber capacitor, which also works to our benefit. The idea behind all of this is to minimize the bus inductance that gets excited during switching transients.

– Current transformer (compatible with standard Triad Magnetics CTs from Digikey) and coax cable connector on the board: this leaves one less unsecured or semi-secured thing flopping around the DRSSTC. The SMA cable connector is also much more reliable than a twisted pair of wires, which has actually been a failure point in one of my coils.

– GDT (gate drive transformer) mounting pads: yet another thing that would probably be better off not floating around between the driver board and the bridge!

– FUSE! A fast-acting fuse saves transistors (sometimes) and board trace explosions/things catching on fire (always).

– External connector pads: let you connect an external DC supply for testing, or a larger set of buscaps for a coil that runs long bursts (like a QCW).

Here’s the schematic (click to view – wordpress derped the preview):

Eagle Schematic

Eagle Schematic

You’ll note that I didn’t include a spot for TVS (transient voltage suppressors). You can deadbug them onto the IGBT pads, but given the laminated bus and the position of the snubber capacitor, I’m not sure it’s necessary. I haven’t had any problems with excessive buss ringing, even while hard-switching in my attempts to build a QCW:

Sort of…

Here’s a picture of the actual board once it’s populated. This was done using a temperature-controlled iron and some regular solder, no fancy surface mount equipment or supplies required. If the IGBT leads are left uncut, they make nice scope probe test points:

The snubber looks sad because it had a few angry encounters with a soldering iron when I was installing the IGBTs!

The snubber looks sad because it had a few angry encounters with a soldering iron when I was installing the IGBTs!

I will try to get some pictures of the bare boards soon. This was a test run from PCBFabExpress, which made 5 of these with a 2-day lead time and 2-day shipping for a net cost of something like $20 per board. Quite good! A future sendout would probably be done using MyroPCB, which makes higher-quality boards in a variety of colors. Their black boards have looked nice in the past (look up oneTesla and RageBridge to see what they might look like).

If you’re interested in participating in a group sendout, let me know! Comments, PMs, emails, etc. are all welcome. If there’s interest, I’ll write a assembly guide with some more documentation. In the meantime, I’ll post a parts list soon.

Here are the Eagle files.

And here’s a shot of what an earlier prototype of this board did with when I put it in my DRSSTC (something like 250 – 300A peak primary current):

3'+ from a full bridge of TO247s.

3’+ from a full bridge of TO247s.

Demo-Coil! (A New SGTC)

Pushing 48"+ streamers and ground strikes.

Pushing 48″+ streamers and ground strikes.

This coil was built as a general demonstration coil for MITERS after a tour for some high school students involved in FIRST robotics exposed that we had no working projects to show! Although oneTesla is now is a state where it’s completely “plug and play”, I’d rather have that somebody doing the plugging be myself or Bayley, and we’re not always around to give tours. The idea behind this coil is that it’s as simple as a Tesla coil gets (so anyone demoing it can explain it, hardly the case with a DRSSTC!), and that it’s bulletproof: anyone can plug it in without tuning or dabbling with software; it just works. The main components of the system are similar to my previous SGTC (“2-Day Coil“), but a 240V step-up variac, new secondary coil, and rolling base have been added.

Maximum spark length achieved was 4-feet; shoot me an email if you want to see it in action and are in the Boston area!

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2 Day Coil

Outdoor ground strike.

Outdoor ground strike.

A spark gap coil that was built over a weekend at MITERS using some spare parts from my DRSSTC, 3 (overkill, overkill, overkill!) Maxwell pulsecaps, and a Raytheon high voltage radar magnetron transformer.

It produced 2 – 3 foot streamers and ground strikes, and was later revamped to become Demo coil.

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Dual 811A Vacuum Tube Tesla Coil

VTTC spitting out 3" sword sparks in its original configuration.

VTTC spitting out 3″ sword sparks in its original configuration.

This was my first major electronics project! After putzing around for a few months in 8th grade with a flyback transformer-powered spark gap coil, I decided that I wanted to take a step up and build something a little more interesting, so I selected one of Steve Ward’s early VTTC schematics and got to work. The performance of first version of the coil (pictured above) was somewhat underwhelming, but it worked! Later improvements more than doubled the spark length.

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