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Gate Characteristics
Half Bridge
Bi-Polar Drive
Primary AC Coupling
Saturation
Inductance
Leakage Inductance
Leakage + X-Conduction

Practical Design
Part Selection

Part Selection

This part of the guide gives some selection criteria for the magnetic cores and wire required to construct a GDT. Some suitable parts, available from common sources, are also listed.

Before selecting parts, you need to decide what winding technique and design you want to use.

Magnetic Cores

The best performance for GDTs can be achieved by using toroidal (doughnut) shaped cores. Toroids have the advantage of a continuous flux path, resulting in low flux leakage and therefore a lower leakage inductance. Toroids also are available in a wide range of sizes.

Other core shapes, like EE, EI, CI and pot cores, can all be used but with reduced performance due to increased leakage inductance. If these cores are all that is available, then they must be ungapped i.e. no space between the core halves. The cores should be fixed in place to ensure that they are held together firmly. Introducing an air gap dramatically reduces the AL, reducing the inductance and increasing the magnetsing current.

Core Material

Ideally, the material needs to be ferrite with a relaively high permeability / AL value, preferably higher than 4000 nH/N^2, but the higher the better. Also, the material should be rated for operation into the MHz region.

Suitable manufacturers / materials include:

• Epcos - materials list, "broadband transformers, up to 3MHz"
• Ferroxcube - signal processing ferrite materials list

However, generic power grade ferrite can be made to work, albeit with lower performance.

Avoid powdered iron cores, from manufactuers such as Micrometals as these are designed for DC choke applications with a low AL and losses that increase significantly with frequency. These toroids are most often found in the outputs of PC power supplies where they are used for output filtering and coupling the outputs. Don't be tempted to use them!

Also, unless you can't find any other cores, avoid using EMI supression toroids as the material is designed to have losses in the MHz region to reduce the amount high frequency noise.

Also avoid materials like Ferroxcube 3R1 which are designed to be used in magnetic amplifier (magamp) regulators.

Sources of Cores

Farnell (UK)

• Cat # 3056960 (datasheet), Ferroxcube 3E5. It's a bit small (5mm inner diameter) but an AL of 3470nH/N^2 is good.
• Cat # 3056971, Ferroxcube 3E25, OD = 24mm, ID = 13mm, AL of 3828nH/N^2.

RS (UK)

Wire Type

Because one winding of the GDT is connected to the source of the high side MOSFET, it will assume the DC rail voltage when that device is turned on. Therefore, the insulation needs to be able to withstand 350V in a typical SSTC application.

ECW (Enamelled Copper Wire) is a good choice. The insulation varnish is specified in Grades, the number of which determine the number of coats of varnish that the wire has received. So Grade 2 wire has received two coats of insulation. Grade 1 insulation is good for a couple of kV IIRC and is man enough for a GDT.

The advantage in using ECW, otherwise called transformer wire or magnet wire, is that the insulation is phisically thin. This enables high coupling between the windings, reducing the leakage inductance. You may be able to use some of the leftovers from winding your Tesla Coil secondary if it is thick enough.

CAT5 Ethernet cable has been used by several people for their GDTs with some success. It is best to remove the outer sheath, which will allow the wires to get closer to the core, reducing leakage.

Normal hookup wire isn't ideal, but can be used.

Wire Current

As for the current that the wire must withstand, there are two components to consider: Total wire current = magnetising current + gate charge/discharge current.

The gate current peaks on the switching transitions where the MOSFET gate capacitance has to be rapidly charged and discharged. To calculate the RMS current it is better to speak in terms of gate capacitance charge. We know that:

Q = CV
and
I = Q / t

therefore

I = ( C * V ) / t

where:
I = current in Amps
C = MOSFET gate capacitance in Farads
V = gate voltage change in Volts
t = time that the voltage is applied for in seconds

So for a -12V to +12V, 100kHz system driving a 15nF gate, the RMS current would be

I = ( C * V ) / t
= ( 15e-9 * 24 ) / 5e-6
= 72mA

Assuming a magnetising current of a similar magnitude, you could reasonably assume an RMS current through the wire of 150mA.

There will be some current due to the interwinding capacitance, but we shall ignore this for our purposes.

Wire Diameter & Current Density

The formulae for current density is

CD = I / XSA

where:
CD = current density in A/mm^2
I = current in Amps
XSA = cross sectional area of the wire in mm^2

A typically quoted maximum current density for wires is 5A/mm^2. So, for a current of 150mA, the minimum wire area would be:

XSA = I / CD
= 0.15 / 5
= 0.03 mm^2

Given that:

XSA = pi * r^2

the diameter is given by:

d = 2 * SQRT [ XSA / pi ]
= 2 * SQRT [ 0.03 / 3.142 ]
= 0.195 mm

where:
d = diameter of wire in mm
SQRT [ ] = square root of contents of brackets

So thin wire should be acceptable for GDT applications give the currents involved.