Sunday, January 24, 2016
Phasing a Pentafiliar Gate Drive Transformer (GDT)
So you've made yourself a sweet Half Bridge SSTC or DRSSTC, and now you want to increase your coil's performance by moving to a Full Bridge. Adding the extra transistors, diodes, resistors, and doing all the additional layout is really the hard part, but I get asked all the time how to phase a GDT for a full bridge as if it is this mysterious perilous endeavor. It really isn't any more difficult than doing a GDT for a Half Bridge.... IF you have a Dual Channel Oscilloscope. Unlike with a simple Trifiliar GDT, you cannot rely on process of elimination alone to determine the phasing. This time you really do need a signal source and a scope. Now I should say, you can simply use two Trifiliar GDTs in parallel, but you must really try hard to balance the impedances or your bridge can fail catastrophically from cross-conduction ("shoot through"). I find it simply isn't worth the risk. Not to mention, you need to compare phases between the two trifiliar GDTs, and that really isn't much easier, but it could be done provided you have a signal source and a meter that can measure differential ac voltages. That's a topic for another day, and one I'm not inclined to broach honestly.
What I've done here is show the results of properly phasing a Pentafiliar GDT through images. I also include an image set at the end that shows what happens when you probe a winding with the "wrong" phasing. This is of course what you'd see as you are determining the phases. I wanted it to very clear what is going on and what you're looking for, so you have to see what the "wrong" result looks like to know what the "right" result is.
I didn't show the process of winding the GDT, or pairing off individual windings with an ohmmeter/continuity meter, but that's the same as with a Trifiliar GDT, you just do it with four more wires.
I've already labeled my windings "A" and "B", with the indicator on the "Gate" side wire. B is antiphase of A, and of course to reverse the phase of a winding we just swap the wires. Pay attention to which wire the scope probe is on, even though I do state it clearly. We keep the first channel on a single arbitrarily chosen winding the entire time, making sure to never reverse the connections to that winding. From this we have a stable comparison point to reference the second channel against.
I also used my 1MHz sine discrete signal generator because I wanted to show that these cores I use and my techniques really are good at 1MHz like I claim them to be! You might note that compared to the first image, when the second probe is added the amplitude drops by half. This is because my 1MHz source is a very high impedance source, around 4Kohm impedance, so the low impedance GDT windings are loading it significantly.