SSTC GDT - Page 1
Forum section: electronics / projects

author: WZOLL
author_rank: Regular Contributor
timestamp: May 21, 2015, 03:14:59 am
content: I'm building a SSTC based on Steve Ward's designhttp://www.stevehv.4hv.org/SSTC5.htm. I'm having troubles with the gate drive transformer. I've tried homemade GDT's but none worked and the transformer I'm using now is the HA3858-ALhttp://www.coilcraft.com/ha3858.cfm#HA3858. The SSTC schematic shows a 0.1uf capacitor but I'm using 0.47uf. There is a lot of ringing on the output and I'm pulling my hair out trying to figure it out. Anyone have ideas or do I even need the transformer. Thanks.

author: Dago
author_rank: Frequent Contributor
timestamp: May 21, 2015, 04:22:03 am
content: Quote from: WZOLL on May 21, 2015, 03:14:59 amRinging is caused by parasitic inductance (=leakage inductance in your GDT and inductance from layout) and/or poor decoupling. Would need to see your layout to be able to say more. I haven't measured the leakage inductance of the GDTs I've wound but the figure for the transformer you linked sounds quite high to me. So probably not the best choice. Just get a suitable ferrite toroid and wind your own (remember to twist all the primaries/secondaries together to reduce the leakage inductance!).You can always just use a high-side driver like IR2110 but for those you need an isolated power supply (and they also have larger delays).

author: mzzj
author_rank: Super Contributor
timestamp: May 21, 2015, 06:38:17 am
content: Quote from: Dago on May 21, 2015, 04:22:03 amThose coilcraft GDT's have HORRIBLE leakage inductance. Been there done that, tried to use some of them and end up winding my own. AFAIK 200nH was pretty typical and easy to reach value in leakage inductance in a GDT suitable for >50khz operation.WZOLL, whats wrong with your homemade GDT's other than "none worked"?

author: johansen
author_rank: Super Contributor
timestamp: May 21, 2015, 07:01:22 am
content: try using a common mode choke from the nearest scrap computer power supply for a gdt.using a standard 50 ohm output impedance signal generator you should be able to get 20 volts peak to peak square wave through any common mode choke into an irfz48 fet at 50khz or more....with sufficient rise time to enable reasonably low switching losses, even in a hard switched buck or boost application.. you can reduce the frequency and still keep the voltage up but you'll need a lower impedance supply...i would use at least 1 uF but theoretically that is more than enough capacitance. you can add more turns if you really need it.ringing is caused by too low a resistance, not too much leakage inductance.some ringing is ok, but you Do Not want your mosfet to turn on, then turn off, then turn on again for every transition.. in a teslacoil circuit.. usually these circuits are on the edge already.also usually the problem with most folks telsa coil experiments is they neglect the low inductance need for the input.. the loop formed by the input capacitor and the fets or igbts.. so if you do blow up your IR2110 or similar driver, its probably because the switching node dropped hundreds of volts below ground for those dozens of nanoseconds while the current found its way through the diodes... and the half bridge driver simply can't handle that. usually they can only handle a dozen volts or so below ground... read the datasheet

author: WZOLL
author_rank: Regular Contributor
timestamp: May 21, 2015, 07:20:51 am
content: I wound my own GDT's with 3 twisted strands of cat 5 cable around a toroid that I salvaged. I may need a bigger toroid as I don't think I wound enough turns (around 8 ).

author: Dago
author_rank: Frequent Contributor
timestamp: May 21, 2015, 08:04:26 am
content: Quote from: johansen on May 21, 2015, 07:01:22 amA common-mode choke is made out of lossy iron powder; no good for a GDT. You want high permeability ferrite.Quote from: WZOLL on May 21, 2015, 07:20:51 amWhat type of toroid? There are hundreds of different types and many are not suitable for a GDT.

author: Psi
author_rank: Super Contributor
timestamp: May 21, 2015, 08:11:46 am
content: would i be correct in saying 'you want the same core material you would use for a dc/dc psu if running at the same switching freq as your gate driver'

author: WZOLL
author_rank: Regular Contributor
timestamp: May 21, 2015, 08:27:15 am
content: I used a black ferrite core like in computer psu transformers except toroidal instead of E shaped. I got them out of a printer with with some wires wrapped around them. My SSTC will probably run ~200 kHz

author: mzzj
author_rank: Super Contributor
timestamp: May 21, 2015, 09:45:25 am
content: Quote from: Dago on May 21, 2015, 08:04:26 amIn my experience toroid cores from common mode chokes are usually pretty good for GDT. Differential mode chokes are definitely not good, maybe you mix these?common mode filters: usually high permeability ferrite like Mag-Inc J or W type. Somewhat higher losses than "power transformer" type ferrites but usually acceptable.Sometimes you can find real sweet amorphous nanocrystalline toroid cores used as common mode chokes like VAC (Vacuumschmelze) vitroperm series!http://www.coilws.com/index.php?main_page=page&id=128I want one of thesehttp://fi.mouser.com/ProductDetail/Vacuumschmelze/T60004-L2130-W630/?qs=sGAEpiMZZMs2JV%252bnT%2fvX8PvC43ppqs%252bkXhmJEE%252buXOU%3dProbably enough for one dude called Kizmo also...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 21, 2015, 12:14:00 pm
content: Quote from: Dago on May 21, 2015, 08:04:26 amNo, it's high-mu ferrite, or even amorphous strip.However, CMCs are almost always wound formaximumleakage, so they are almost entirely unsuitable as GDTs, as is. The core is good, but you have to rewind them.QuoteAnything with mu > 2000. Pulse transformers, CMCs and ferrite beads are typical. FBs may not be the best, and you'll want to test them for suitability.To the OP:You can measure a few circuit parameters and calculate exactly how much inductance, capacitance and resistance your circuit needs / is capable of, and what the rise/fall time and overshoot will be. Not only is this a solved problem, the solution is pretty simple. You only need to measure or look up a few things. Namely: GDT primary inductance (both secondaries open), series leakage inductance (primary inductance, both secondaries shorted), transistor gate capacitance and charge (note that Ciss != Qg(tot) / Vgs, there is a large difference and for a good reason).There are also preferred ways to drive the GDT. One example is using a push-pull controller, like the TL598: this has totem pole outputs, which are nearly capable of driving a GDT on their own. Some additions have to be made, such as a coupling capacitor, and supply clamping diodes. This will develop pretty reasonable drive voltages. Other methods (TL494 and related, with "uncommitted transistor" outputs) are not so suitable, and need more circuitry (e.g., a dual gate driver IC).Tim

author: klr5205
author_rank: Regular Contributor
timestamp: May 21, 2015, 01:02:14 pm
content: My first SSTC was based on this design. If you're willing to give hand-winding a try again, check to make sure that you don't have too many turns per winding on your GDT.Steve's drawing shows 16 turns but for my ferrite toroid (I forget the exact model - I salvaged it but eventually did some measurements and determined what it was), the core was saturating and my pretty square wave input was turning into practically a sine wave at the secondaries.14 turns per winding solved the problem entirely and I got a nice pretty square wave at the output.Just something else to try.Fun fact: My avatar is me holding a CFL near my sstc

author: BennVenn
author_rank: Regular Contributor
timestamp: May 21, 2015, 08:56:01 pm
content: You'll find a perfect toroid for a GDT in a CFL. It is almost what their purpose is in a ballast circuit. Might have trouble winding 16 turns x 3 (especially litz)A modern downlight transformer has a slightly larger toroid, more suited for your purpose. Operates at around 40khz depending on load and is good for 50watts.

author: johansen
author_rank: Super Contributor
timestamp: May 22, 2015, 05:42:29 am
content: i guess i disagree, that common mode chokes are wound for maximum leakage inductance.here's what i think is going on.most common mode chokes have a relatively non optimal core topology.i'm talking, a space available for winding copper in.. on the order of 7mm by 12mm.yet the core cross section is only 4mm square.*so naturally, when two coils are wound on a bobbin that offers them 6mm of creepage distance between the two coils.. yeah, there's a lot of leakage inductance, but there isn't really much that can be done about it. some common mode bobbins can be unwound without breaking the core.. and you'll find there is no air gap, the bobbin is in two pieces and is wound up on a machine.. these typically have 4 slots for copper to be wound in, and i suspect if you configure them psps you can get a relatively low leakage inductance.but you'll get a much lower leakage inductance if you get rid of the bobbin and wind say, cat 5 cable directly on the ferrite, feeding it through the core like you wind a toroid. this should result in half the copper physical length, and probably one fourth the leakage inductance or less.. simply because the turns are closer to the core and each other.*this is the same problem with CRT yoke cores. the core itself is light weight, low cross section, yet it has a relatively large diameter and its toplogy forces the copper path to be about 4 times as long as it would need to be if the cross section of the core were circular.so while the core appears to be no good.. low permiability, high leakage inductance.. most of that is the non optimal cross section.sometimes you can find datasheets on the core itself and.. they say they are good quality ferrite.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2015, 12:51:42 pm
content: Quote from: johansen on May 22, 2015, 05:42:29 amPerhaps it would be better to say: they certainly aren't wound in any manner which would achieve a reduction in leakage (interleaving, multi-filar, paralleled strands..). Maximum leakage, while retaining current compensation, for a given core geometry, would probably be something like, having a dense multilayer construction for each winding, so the length of core occupied by each winding is minimal (and the distance between windings is maximal), and on a toroid for example, they would be placed exactly opposite each other.The common "single layer on opposite sides of a toroid" construction gives a pretty close version of this, so those types probably achieve somewhere near "maximum leakage".The bank wound style (used for C, E and figure-eight type cores) is probably a little higher in coupling coefficient, since the windings are adjacent on a common leg.QuoteYeah, the figure-eight cores are solid, and that's how they can achieve mu up to 15k, even 20k, in ferrite. Pretty impressive, but a pain to wind. The bobbins are usually two-piece affairs, with a peculiar ratchet tooth shape going all the way around one side... gear drive, wound in place.The cores themselves do tend to have a small cross section given the size, which is mainly to accommodate the large amount of copper and insulating material; they hardly need any flux density handling at all, whereas flux is a key aspect of transformer design.Both considerations, together, mean you need that much more winding length for your GDT, which means more leakage.BTW, leakage for a "twisted pair" construction is essentially the stray inductance of the pair to begin with. (After all, leakage is the flux that's NOT in the core, so the presence or absence of the core makes almost zero effect.) Given that twisted pair is typically around 100 ohms (transmission line characteristic impedance), and mu_0 is 1.26 uH/m and Zo is 377 ohms, you can figure the inductivity is around sqrt(100/377) times mu_0, or in the 0.7 uH/m range. If you need 1m of twisted pair, you'll have around 0.7uH leakage.Which, in turn, if you have a nice low impedance driver (gate driver IC with < 5 ohm resistance?), means you effectively get a resonant tank between driven source, 0.7 uH series inductance, and whatever the gate capacitance equivalent is (10nF is pretty typical for power transistors of broadly this size; that would be 100nC at 10V). To minimize overshoot, you need to dampen that resonance, which has a characteristic impedance of Z = sqrt(L/C) = 8.3 ohms. A little extra series gate resistance might be handy (~4.7 to 10 ohms?).If you're driving a half bridge, you'll have the same thing from the other transistor, acting in parallel, and you'll end up with the driver chip working into a resonant impedance of 4.15 ohms. A pair of 2 ohm drivers (~3-6A rated capacity, note: use CMOS type, not BiCMOS "boosted" kinds), acting in series with the LC (leakage + gate capacitance), should be just about critically damped, and pretty reasonably fast.The risetime is around the LC time constant t = pi*sqrt(LC)/2, or 130ns. Pretty slow as switching supplies go, but if your switching frequency is under 100kHz, it'll be fine.QuoteYeah, rewinding with a good winding design (multifilar isn't bad -- good enough for modest transistor drive applications, but don't expect too much out of it) gives you the full performance of the core, without the limitations of whatever the original winding did. And the cores are intentionally optimized for just this sort of thing: a good pulse transformer has minimal current flow to ground (magnetizing current), just as a CMC wants minimum current flow through the AC line!Tim