New Industry Products

TDK Introduces a Series of Compact Transformers Aimed at DC/DC Converters

February 11, 2021 by Gary Elinoff

The new surface mounted devices will also serve in gate driving circuitry and in industrial electronic and e-mobility applications

The four-member EPCOS E10 EM series includes four compact models, all featuring dimensions of 11.7 x 13.15 x 11.35 square millimetres.


TDK Introduces a Series of Compact Transformers Aimed at DC/DC Converters Figure
Image courtesy of TDK


The Members of the Series

The transformers are designed for use in Full Bridge, Half Bridge, Flyback or Two Switch Flyback topologies. The flyback diagram is labeled as two switch flyback, and vice versa. For the interested reader, Texas Instruments provides a detailed explanation of two switch flybacks. TDK provides a detailed datasheet.


The B78307A2276A003, B78307A9741A003 and B78307A2385A003 

These three transformers are appropriate for full bridge and half bridge topologies 

  Primary to Secondary turns ratio Primary Inductance Primary leakage Primary DC Resistance Secondary DC Resistance Primary to Secondary Capacitance
B78307A2276A003 1 to 1.08 ≥ 100µH 0.35µH 350mΩ 410mΩ 9pF
B78307A9741A003 1 to 0.76 ≥ 100µH 0.45µH 360mΩ 300mΩ 9pF
B78307A2385A003  1 to 3.67 ≥ 10µH 0.1µH 200mΩ 700mΩ 6pF


The B78307A2338A003 

This transformer is aimed at flyback and two switch flyback applications. It has a 4:1 turns ratio, and the data sheet suggests that either winding serves as the primary, rendering a 1:4 turns ratio.

  • In the 1:4 use case, primary inductance is 2µH ±10% and ISAT is 4 amps
  • In the 4:1 use case, primary inductance is 25µH ±10% and ISAT is 1 amp

In both cases, primary to secondary capacitance is 4pF.



Members of the EPCOS EM series provide:

  • Basic Winding Insulation (protection for as long as the barrier is intact) for a working voltage of 500 VRMS
  • Reinforced insulation (equivalent to two levels of isolation),  of 300 VRMS.
  • Triple insulated wire
  • UL1446 class 155(F) electrical insulation system 
  • N1 / N2 creepage ≥ 6 mm, clearance ≥ 5.5 mm (cumulative, as core is conductive, free floated between N1 and N2) 
  • [N1,N2] / [core] creepage ≥ 3 mm, clearance ≥ 2.75 mm
  • Top surface to core creepage and clearance is greater than 2 mm 


Other Technical Considerations

These transformers will typically operate at frequencies ranging from 100 to 500 kHz

High voltage AC test and AC type type for primary to secondary are at 3000 VAC (50 Hz, 1 seconds) and 3750 VAC (50 Hz, 60 seconds), respectively

Partial discharge voltages for N1/N2, inception and extinction, are greater than 900 and 700 volts, respectively (type tests)



TDK’s  EPCOS E10 EM series will find application largely in:

  • DC/DC conversion within industrial electronic devices
  • Gate driver circuits 


Other Physical Considerations

  • These transformers feature MnZn ferrite cores
  • They operate at temperatures over a -40 to +150℃ temperature range
  • The units weigh approximately 2 grams


Regulatory Concerns

  • These compact transformers feature a minimum primary to secondary clearance of 5.5 mm clearance and  6 mm creepage and thus comply with the IEC 60664-1 standard with regard to creepage and clearance
  • Qualified to AEC-Q200 Rev. D 
  • RoHS compatible
  • Complies with elements of IEC 61558;  61800, IEC 60664-1. Datasheet provide full details
  • IEC 60664 to pollution degree P2