I want to design a pulse transformer to drive a MOSFET.

Parameters are as follows:

  • Voltage = 15V
  • Current = 2A
  • Switching Frequency : 400kHz.
  • Ratio 1:1.
  • Duty Cycle = 50%

I know the following formulas, but I don't know their chronology for implementation.

  • V=Ldi/dt
  • L = N^2Acmu/lg
  • AcAw = L I I/KBmJ [Area Product]
  • N = LI/BmAc
  • lg = N^2Acmu/L

This MOSFET driver will be general as I don't have any application in my mind for this yet.

1) Please help to step by step finding each parameters and suitable core selection.

2) Is it necessary to have any application, while designing a pulse transformer?

3) Pulse transformer is not a transformer, but a coupled inductor. So design should be from inductor's perspective. Am I right?

4) Is there a way to select a ferrite core from wattage? Like this one would be 15 x 2 =30 Watts.


1 Answer 1


Step 1. Saturation. Calculate your worst case scenario voltage-time product, in your case peak voltage times max duty cycle divided by minimum frequency. Apply that to Vt=NAB. Adjust number of turns accordingly. This assumes you have already chosen a suitable core here. If not, you will at least know by now what area-turn product to look for. This sets your core size.

Step 2. Does that number of turns even fit on the core with the wire thinkness needed to give low enough series resistance? If not, select a larger core and recompute for your new A.

Step 3. Inductance and magnitization current. What are the limitations on your driver? Calculate Lprim by AlN^2. Calculate peak magnitization current by Ipk=UpkDmax/(Lprim,min*fmin). Can your driver support that? Great, you are probably done. If not, increase number of turns and or Al (different/larger core)

This should cover the basics for you.

You will however need to consider many more issues with transformer driven gate drivers.

How do you avoid staircase saturation? Do you run a capacitor in series? Do you have a reset winding? Do you limit the duty cycle?

How much droop can you accept due to leakage inductance?

  • \$\begingroup\$ How to choose core.Is there any method to choose core from power . \$\endgroup\$
    – Israr
    Commented Oct 1, 2017 at 0:09
  • \$\begingroup\$ @IsrarSayed Not for a gate driver no. \$\endgroup\$
    – winny
    Commented Oct 1, 2017 at 10:47
  • \$\begingroup\$ So Area Product would do? \$\endgroup\$
    – Israr
    Commented Oct 1, 2017 at 20:42
  • \$\begingroup\$ Most cases yes. Say above >1 MHz you have more things to consider too but 400 kHz is well within reason. \$\endgroup\$
    – winny
    Commented Oct 2, 2017 at 8:30

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