Gate Characteristics
Half Bridge
Bi-Polar Drive
Primary AC Coupling
Saturation
Inductance
Leakage Inductance
Leakage + X-Conduction
Practical Design
Part Selection
SSTC Gate Drive Transformer (GDT) Design Guidelines
Thanks go to Richie Burnett for sharing his notes on SSTC design - credit is shared with him for the content here, although any errors are most likely mine. If you have any comments or corrections, please email me: james at pawson dot plus dot com
This guide is primarily aimed at Solid State Tesla Coil builders, but the notes apply equally to half/full bridge designs e.g. induction heating applications.
Quick Start
- Quick Tips - A brief summary for those that want to lash something up quickly. The rest of this guide is worth a read, especially if you are new to SSTCs
The Theory
- Gate characteristics of MOSFETs and IGBTs
- The half bridge and the requirement for isolated gate drive
- Why using bi-polar drive signals is useful in a half bridge
- AC coupling of the drive signal to the GDT primary is also necessary
- Saturation flux density is definitely something to be aware of
- How to calculate inductance of a winding on ferrite core
- Leakage inductance: know your enemy!
- Leakage inductance and cross conduction in the half bridge
Practical GDT Design
- Practical design to reduce leakage in a GDT
- Part selection
- Some existing designs have been added to this thread on 4hv.org
- I've put some specific GDT construction notes on my Stubby SSTC page
- I've made a basic GDT design spreadsheet in Excel that allows calculation of the critical parameters, available for download soon (.zip)
- The GDT for Titch was designed in this spreadsheet, here is a screenshot
Driver Circuits
It should be noted that the GDT merely couples the drive power to the gate of the transistor. As a result, whatever devices used to drive the GDT primary need to be able to supply the high peak currents required to rapidly charge and discharge the transistor gate.
MOSFET gate drivers are ideal for this task, such as the following devices
- Texas Instruments UCC37322 non-inverting and UCC37321 inverting drivers are the most popular choices, with a 9A peak drive current capability.
- For the more difficult, higher capacitance IGBT gates, Ixys make a selection of high current gate drivers, including the popular IXDD414
- Maxim also make a selection of gate driver ICs
For higher power drive of MOSFETs, although not entirely necessary, is a half bridge of low voltage, high current small MOSFETs. Cross conduction in these smaller transistors can occur if the gate drive circuitry is badly designed. Steve Ward has an example of this circuit.
Jan Wagner has some good design notes on SSTC gate drive design.
Further Reading
- This guide only deals with bipolar drive, AC coupled GDTs - the main choice for half and full bridge SSTCs. For further information on different gate drive configurations see http://focus.ti.com/lit/ml/slup169/slup169.pdf - an excellent paper from Texas Instruments on high speed MOSFET gate drive techniques. Page 30 onwards covers transformer coupled gate drive, but the rest of the paper is well worth a read! It also covers driving transistors with a signal that varies in duty cycle but that still requires isolation
- This page contains some info on basic magnetic principles and is worth a read.
- This article by Keith Billings discusses some aspects of high side gate driving