IGBT Gate Drive Transformer (GDT) Design, Construction and Test

A guide on how to calculate gate drive transformer, how to wind gate drive transformers and test gate drive transformers. Gate drive transformers are also called GDT for short. The most important aspects is insuring low leakage inductance, no core saturation and having enough peak drive current available for your MOSFETs or IGBTs. This video guide primarily focuses on Tesla coil inverters in the range of 30 to 500 kHz, but it can be used in all kinds of inverters, power supplies, drivers etc.

GDT calculation spreadsheet: https://docs.google.com/spreadsheets/d/1wT125P1bht__SJjPawmC46sLOFMn8neapgBCj-5y1dw/edit?usp=sharing

All pictures, calculations and additional information, will eventually be made accessible as part of the DRSSTC Design Guide.

Gate Drive Transformers vs. High Side Drivers

A Gate Drive Transformer is a simple, cheap and robust alternative to High Side Drivers. Simplicity does however come with some trade-offs like less precise control and higher losses due to that.

GDT advantages

  • A transformer is more robust than a silicon die
  • Less sensitive to spurious noise and high dV/dt pulses
  • Cheap
  • With drivers of opposite-polarity and symmetrical output, no ac coupling capacitor is needed.
  • There is no need for any isolated DC to DC Converter
  • There is practically no propagation delay time in a transformer to carry signals from primary side to the secondary side.
  • Several thousand volts of isolation can be built-in between windings by proper design and layouts.
  • Provides excellent protection of the driver circuit in case of a violent fault condition on the bridge where a MOSFET or IGBT explodes.

GDT disadvantages

  • Pay attention on the leakage inductance and isolation.
  • No strong turn off capability, but can be implemented with a single turn-off diode and optional resistor.
  • Minimize the leakage inductance.
  • Follow Faraday’s law – keep V*T constant, otherwise, saturation.
  • Keep enough margin from saturation – the worst case happens with transient load at high line.
  • High permeability ferrite – minimize the magnetizing current.
  • They can be used only for AC signals.
  • Large duty ratios cannot be handled by the transformer without being saturated by net DC, unless AC coupling capacitors are employed in series.
  • Large foot print.

HSD advantages:

  • HSD IC turn-off is 70 ns faster [2], lowering the switching losses
  • The HSD IC keeps safe and enough dead time between high and low side MOSFETs.
  • Protective features like: undervoltage lockout, desaturation, detection and fault processing.
  • Small foot print.

HSD disadvantages

  • An isolated DC to DC Converter is needed.
  • Complicated circuits .
  • Understanding these HSD ICs, their strengths and limitations, is of paramount importance. Different configurations for particular topologies call for specific application knowledge.
  • There is no efficiency difference between the HSD IC and GDT solutions.
  • Desaturation does not work reliably fast enough for soft switching applications. Trying to correct thousands of Ampere for a IGBT rated for much less in hard switching, like in a DRSSTC.

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