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Dealing with magnetics despite its mystery

6 Nov 2017
Smail Haddadi, Field Applications Engineer, TT Electronics: Magnetic components have many real-world subtleties beyond their basic equations; start your search with application-specific standard parts, then leverage vendor support and insight.
Dealing with magnetics

For most students in electrical engineering programs, their first close encounter with “magnetics” begins with the basic AC transformer in step-up and step-down arrangement, extends to the use of inductor in L-C filter and resonant circuits, and may even include use of the transformers for signal isolation of a power-supply feedback loop. Beyond those basic applications, many electronics engineers regard anything “magnetic” as an almost magical aspect of design, one best left to specialists in order to avoid complications and headaches.

In some ways, that’s unfortunate, but it also makes sense. Magnetic components in a wide range of configurations are a critical part of almost every electronic system’s design. They are embedded and pervasive, though often invisible to the user and even the designer.

Consider just a few of their many applications:

  • Obviously, as the primary-side transformer between AC mains and rest of a power subsystem;
  • Within AC/DC or DC/DC switching power supplies, in the feedback path from secondary side to primary side;
  • To provide isolation and level translation in for MOSFET gate-driver circuits;
  • Inductors are used to manage the flow of current and charge/discharge cycles to regulate output in power supplies.

But the roles of magnetics go beyond power supplies, such as:

  • The coil of electromechanical relays (yes, they are still very widely used, for many reasons) and solenoids;
  • Coupling power into an Ethernet cable, in a power over Ethernet (PoE) installation;
  • To steer DC current into one path while blocking AC from that path;
  • In RF circuits, to properly bias a PIN diode when it is being used as a switch;
  • In single-stage and multistage LC filters, to establish low-pass, band-pass, and high-pass topologies with varying bandwidths, cutoff frequencies, passband/stopband ripple, and attenuation slopes.

Here’s the magnetics challenge: once you get rough-out the design using the basic equations which govern electromagnetism – and there are just a few – reality hits hard as you need to translate these equations into actual products. That’s easy at first, but soon gets very difficult. Some specifications, such as DC resistance, are straightforward as they are based on wire gauge and length. Others, such as inductance get more complicated, as they are dependent on wire gauge, but also winding topology, leakage current, physical winding form, and many other variables which are hard to model and involve tricky compromises.

The result is that most users find it easier, quicker, and smarter to rely on the expertise of a magnetics vendor to guide them to the best component, or at least show a few and clarify the inevitable tradeoffs. Add to this the issue of any relevant regulatory requirements such as insulation and spacing which meet high-voltage mandates, and very quickly having an experience guide via selection charts, applications engineers, and others who ate better versed in the nuances of magnetics is a good idea.

Two examples make this very clear. First, consider a basic flyback transformer used in a switching DC/DC supply in conjunction with its optocouplers. The top-tier specifications are fairly easy to meet, but the added-on ones become a real challenge: AEC-Q200 Grade 0 qualification (the most stringent), including -40°C to +155°C rating so it can be used in any locale and anywhere in the vehicle, high saturation-current capability and low-leakage inductance performance; small footprint; compatibility with designated optocouplers; and high isolation between primary and secondary windings.

Clearly, not just any transformer with the appropriate turns ratio will suffice. That’s where the HA00-10043ALFTR from TT Electronics is an excellent choice. Not only does it meet all the “hard specifications” for electrical, thermal, and mechanical performance, it is housed in a 10‑pin small-outline style (SOIC) IC (SOIC) package with a small 12.0 mm × 12.5 mm footprint and low 6.3 mm profile, Figure 1. Yet it’s a standard product and fully optimised for a well-defined set of difficult requirements, Reference 1.

HA00-10043ALFTR flyback transformer for switching power supplies

Figure 1: The HA00-10043ALFTR flyback transformer for switching power supplies is AEC-Q200 certified and targets severe high-temperature automotive and industrial applications; it is compatible with Broadcom (Avago) ACPL-32JT and ACPL-302J optocoupler ICs.

 

Similarly, a PoE design requires many passive components: diodes, transformers, inductors, capacitors, and resistors, plus discrete transistors and controller/regulator ICs. TT Electronics supports the reference designs from the vendors of these ICs, such as the DAK86 reference design for a PoE powered device (PD) from Power Integrations, Reference 2. This design uses their DPA423G power-conversion switch and controller, with inductors which have the appropriate values, of course, as well as DC resistance, current-handling capacity, and switching frequency rating, such as those in the HM79 series of medium-power surface-mount inductors from TT Electronics, Figure 2.

HM79 series of medium-power surface-mount inductors  

Figure 2: Members of the HM79 series of medium-power surface-mount inductors from TT Electronics are well-suited for use in the DAK86 reference design from Power Integrations for a Power over Ethernet (PoE) Powered device (PD).

 

In short, there’s no need to try to go it alone when it comes to selecting transformers and inductors which are optimised for defined application niches. Further, if your requirements are truly unique, as is sometimes the case, a top-tier vendor can often suggest a modification to an existing part, or even a fully custom part, that meets the complex objectives of your design.

For more information visit: www.ttelectronics.com/magnetics.

 

References

  1. TT Electronics, Edgar C. Taculog, High Isolation Voltage Flyback Transformer
  2. Power Integrations, Engineering Prototype Report for EP-86 – 6.6 W Multi-Class Powered Device (PD) for Power over Ethernet (PoE) Using DPA-Switch® (DPA423G)
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