GDT wiring oscillation for proper mosfets (AC switch) sequence - Page 1
Forum section: electronics / projects

author: eneuro
author_rank: Super Contributor
timestamp: February 02, 2015, 08:49:40 pm
content: I've made simple prototype of GDT which I'd like to use to switch ON/OFF AC mosfets switch.To do this on control 5V circuit side I've something like H bridge, so I can send swithing sequence by applying +5V on one of the yellow toroid wires and 0V on another, than do reverse 0V on first and 5V on second-like changing direction of DC motor, so I can make this GDT input to oscillate at choosen frequency, etc.However, I want of course at the begining of the cycle (0*-180*) get positive output on red (+) marked GDT output wire, so chave to choose which of those GDT (yellow) wires (lets say top or bottom) connect to +5V first?I've shown how wiring is made by marking them on toroid (there is around 1:3 or 1:2 turns ratio, but it is not important while even 10V on secondary will be fine).Of course I will try myself derive from physics laws direction of output current based on rising to 5V lets say bottom yellow input wire, while top is pulldown to 0V, but if someone already did such GDTs could help solve this puzzle it could be nice to verify this?It must be very simple, but too less sleep doesn't help today

author: eneuro
author_rank: Super Contributor
timestamp: February 02, 2015, 09:46:36 pm
content: Hopefully, found detailed explanation undertransformer phasingTransformer Phasing: The Dot Notation and Dot ConventionI've got lucky and those voltages are in phase, so no worry during programming and no wires changing needed

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2015, 10:48:33 pm
content: NNNnnnnnooooooo! There's so many wrongs about your transformer1. Core appears to be #26 powdered iron. mu = 75, and it's about as lossy as a steel brick. While it's still possible to get good transformer action out of it, your gate driver will have a hard time dealing with the large magnetizing current. Expect to need hundreds of turns!2. The windings are opposed, giving maximal leakage inductance. Expect a coupling factor of... maybe 0.95? Which is pretty atrocious for GDT duty.As for phasing, this is easy to remember:Observe that there is one opening through the core. You can enter it from one side or the other.Each time a turn passes through the opening, it makes one "turn". (So a straight wire going on for infinity, but threaded through the core, consists of one turn. The trick is, it makes a "wrap" around the core because, whether or not you've kept the wire close to the core as a single wrap, or it goes on straight for all of infinite space -- it's still making a complete loop around the core, and that's all that matters.)If you denote one side of that opening as "positive", the other is "negative". Any wires entering through the "positive" side will all have a positive sense, together.As for making a good GDT, first, hunt for a good hi-mu ferrite core, ungapped. You might have something salvageable from a common mode filter, or a car amp's DC-DC converter, or EMI beads/rings, or something like that. These will all be moderate to high permeability examples, and will make good transformers.Second, minimize leakage. Leakage inductance arises directly because there is space separating the wires of the windings. The inductance due to this space has almost zero to do with the core -- it's almost independent of the core being there or not. The best way to minimize this is to wind the coils on top of each other, closely (alternating layers), or by using twisted pair.Tim

author: moffy
author_rank: Super Contributor
timestamp: February 03, 2015, 03:34:33 am
content: "The voltage-time product, V x dt is quite useful. The size and cost of a pulse transformer is roughly proportional to this product."from:http://www.butlerwinding.com/store.asp?pid=36625

author: eneuro
author_rank: Super Contributor
timestamp: February 03, 2015, 07:03:58 am
content: For the moment wanted to ensure I will get right voltage phase on this simple GDT output (random core from old PC power supply).Quote from: moffy on February 03, 2015, 03:34:33 amThere is 7:15 turns ratio and do not care too much about it since maybe even old car battery 12V as power supply can be used, so limiting GDT output voltage maximum to safe mosfets gate +/-20Vgs levels was more important than type of toroid core used.Yep, I found also this app note earlier, but do not want triger triac directly using GDT:TRIAC CONTROL BY PULSE TRANSFORMERI know that designing GDTs is tricky:A Guide to Designing Gate-Drive TransformersAnyway to switch ON/OFF AC mosfets switch to triger two thyristors SCR's in anti parallel or make "light show" with a few wats light bulb (up to 100W) probably doesn't need rocket science

author: eneuro
author_rank: Super Contributor
timestamp: February 04, 2015, 04:50:38 pm
content: Quote from: T3sl4co1l on February 02, 2015, 10:48:33 pmThis white-yellow core (from old PC powr supply I guess) doesn't perform great, however was able with those a few turns on its primary (about 8 turns) output at 200kHz about 6V and at 400kHz 5V in open circuit with 1N4148 diode and capacitor.Anyway why only 6Vmax without any load @ 200kHz and about 4.5Vmax @ 400kHzI expected at least 10VMaybe .... made a mistake and 15turns connected to as primary and 8 turns as secondary?I will try another core tomorow.Anyway AC mosfets switch with two BUZ78 (http://pdf.datasheetcatalog.com/datasheet/siemens/BUZ78.pdf) in series showed not so bad RDSON close to its typical nominal 6.5Von per mosfet , while 13.8ohm/2= 6.9ohm at ONLY 4Vgs at 400kHz switching frequency AND with 1N4148 in series as rectifier to mosfets Vgs supply...However, when tried to drive this AC switch without this additional rectifier 1N4148 diode,my crappy multimeter showed @ 200kHz switching frequency 12k ohm resistance on AC mosfets switch terminals and @ 400kHz looked like no resistance drop, but maybe its to fast and this bloody multimeter was not able to notice any AC switch fast swithving ON/OFF or .. this GDT with prety random core dosen't switch it ON/OFF @ 400kHz.... of course scope waveforms needed, but it is always interesting try to predict what happends first and see if it fits into theoryFor the moment RDSOn looks fine while threshold voltage was crossed, but I do not like this quite low <<10V GDT output without any load...BTW: 5V H bridge (3 phase but used only its two phases in this experiment) is made of BC327/BC337 0.8Amax transistors and its gate trigered from 5V using 4.7k resistor probably limits this maksimum switching current below 0.5A or less, but ATTiny running at 8MHz was for sure programmed for 400kHz:Quote

author: moffy
author_rank: Super Contributor
timestamp: February 06, 2015, 12:32:56 am
content: Transformers have resonances, which are effected by the load, you have to use a scope on both the driving and output waveforms.

author: eneuro
author_rank: Super Contributor
timestamp: February 06, 2015, 03:01:58 pm
content: Yep, removed part of one side of this small transformer so I have close to 11V on its output from 5V LM7805 logic levelNo rpoblem to switch ON/OFF 230VAC light bulb and everything is at room temperature while for testing 20W light bulb was used so less than 100mA wirh AC mosfets switch 5.5 Ohm each of course power loses in this case very small-less than 0.100 W per each mosfet:Now fun part and time to connect scope to 230VAC mains Hall current sensor (SS495A at safe 5V levels galvanic insulated) and will see if I will be able to drive AC current with this GDT with a few additional lines of AVR ATTiny85 code.I was looking for something like PWM mode of GDT but didn't found anything interestingGDT input is powered by H bridge 5V and I can easy set up to 1MHz switching frequency.For the moment at 400kHz and 1N4148 diode with 100k resistor on mosfets gate fully opens those mosfets and gate voltage is 10.5V ....