Gate drive transformer; 1.2% leakage inductance excessive? / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Gate drive transformer; 1.2% leakage inductance excessive?

Move Thread

LAN_403

Dinges

Wed Dec 30 2009, 06:37PM

Registered Member #2511 Joined: Mon Dec 07 2009, 02:46AM
Location:
Posts: 36

As one of the most critical parts of inverters is the gate drive transformer (GDT) I decided to give that a go first and measure the results.

The core was a pot-core of material 3H1 (Philips material, permeability u=2300). Not sure what the 'u150' printed on the core means. It was wound with 14 turns primary on the coil form, then 'glued' together/stabilized with a coating of nailpolish. Then a layer of cellotape. Over that was wound the first secondary coil of 13 turns. Then again nailpolish and cellotape, followed by the final secondary coil of also 13 turns.

The former was placed inside both ferrite halves and tightly clamped together.

Then I proceeded to measure inductance of the primary (with both secondaries open), by adding a 1uF capacitor in parallel and measuring resonance frequency; frequency was 15.5 kHz, yielding an inductance of 105uH. With both secondaries shorted (soldered the ends together) the inductance is still 1.3uH... or about 1.2% of the inductance of the primary.

From this site (http://thedatastream.4hv.org/gdt_practical.htm) I understand that .02% leakage inductance should be possible... 100nH or so... or about 1/10th of my leakage.So I think my leakage inductance is excessive for use as a GDT?

I intend to remove the windings and try again with trifilar wound coils, even though I'd rather have a little bit more insulation between the primary and secondary side of the transformer than just a few layers of lacquer, but if the price is too much leakage inductance, then it will have to be done with just that tiny layer of insulation.... (it's professional motor rewinding wire, IH2L, class H temperature rated, 2 layers of lacquer. It's top-grade material used for professionally repairing induction motors)

Questions:

1) is the leakage inductance excessive for use in a 100kHz induction heater (~1kW)
2) is the core material (3H1) suitable for the purpose of a GDT?
3) is the core shape suitable?
4) what are your comments on this coil?
5) would you feel safe with trifilar wound coils, with only 4 layers of lacquer between you and the grid? Personally I'd rather have some extra insulation, but maybe I worry too much.

Any advice and comments would be very much appreciated.

Peter.

Dr. Dark Current

Wed Dec 30 2009, 06:46PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

To test it, connect the primary to your gate drive circuit (drive with the expected running frequency) and connect the secondaries to your switching devices' gates (clamped with zener diodes!) and monitor the ringing on them. Increase gate resistors value until the ringing is damped enough and then look at your rise and fall times if they are acceptable. You can usually decrease the fall times with discharging schottky rectifiers in parallel to your gate resistors, without the ringing getting much worse.

Otherwise about the values, I don't know.

I make my GDTs with trifilar twisted PVC insulated solid-core wires, or if I need to get the leakage inductance as low as possible, I use screened stereo audio cable with the screenings in parallel as the primary, and the inner conductors as the secondaries. I don't trust the insulation of enameled wires for mains-voltage applications.

Dinges

Wed Dec 30 2009, 07:16PM

Registered Member #2511 Joined: Mon Dec 07 2009, 02:46AM
Location:
Posts: 36

Hm. Just made another attempt, using trifilar wound 0.25mm magnet wire on a ferrite core of unknown material (most likely RFI choke in its previous life....

).

9 turns trifilar wound yielded an inductance (open) of 17.5uH (Fres=38kHz with 1.0uF capacity). With the secondaries shorted, the inductance dropped to 0.28uH (Fres=300kHz with 1.0uF), so that would mean even relatively more leakage inductance (1.6%) than my first attempt at a GDT....

Peter.

IamSmooth

Wed Dec 30 2009, 07:35PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Why don't you just buy this premade gate drive 1:1:1: transformer from Mouser.com

part number 673-P0584NL

Dinges

Wed Dec 30 2009, 08:36PM

Registered Member #2511 Joined: Mon Dec 07 2009, 02:46AM
Location:
Posts: 36

Ok, now we seem to be going somewhere.... Using another toroid with its original winding (1:1, not 1:1:1) the inductance was 1.95mH and leakage inductance (only one secondary present) 0.37uH, or about 0.02%. After removing turns till there were 10 left, inductance measured 550uH and leakage inductance 0.23uH, or about 0.04%; that isn't half bad, I think?

Tomorrow I will try experimenting with screened wire and trifilar wound coils a bit more. From what I understand about the topic, a good GDT will prevent a lot of problems later, and subsequent debugging.

Imsmooth: the reason I don't buy the GDT is the same reason I don't buy an induction heater but build one

1. cheapness
2. education
3. challenge

Thanks for that link though, is interesting to have a look at the datasheet for that pulse transformer (http://ww2.pulseeng.com/products/datasheets/P515.pdf). Checked out Farnell a while ago to see what they had to offer, but wasn't much. The one you linked to had about .1% leakage inductance; some others I could get from Farnell were 0.2-0.3%. Seems that, with some effort, it's possible to homebrew a better product oneself?

Though it's hard for me to judge how much real-world improvement those smaller leakage inductances yield - when does 'good' become 'good enough'?....

Peter.

IamSmooth

Wed Dec 30 2009, 09:20PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I have to look up the part number, but Amidoncorp.com has a nice ferrite core for making a good gate drive transformer. I will get it to you later this evening when I am home.

Part number is FT-114A-77, ferrite core for drive

Steve Conner

Thu Dec 31 2009, 09:23AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

When you're testing leakage inductance, remember that you're also measuring the stray inductance of whatever test leads and capacitors you add to the setup. With a good GDT that can be significant.

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob