Converters / Planar transformers
System EL specializes in printed circuit layout design for power conversion (power management): converters, power supplies DC/DC, AC/DC, inverters, etc. PCB layout in power conversion applications must meet many additional requirements besides being a simple physical realization of the schematic netlist. It is necessary to create optimal operating conditions for sensitive electronic circuitry in a challenging electromagnetic environment. Some aspects tend to call for contradictory approach, so reaching a satisfactory solution is even more difficult. The most important issues are:
1. Trace width and thickness adequate to conducted current.
2. Isolation distance between nodes with large voltage differential
3. Minimalization of fast changing current loops to reduce parasistic layout inductance and emitting of magnetic field affecting other parts of the circuit
4. Minimalization of fast changing voltage surfaces to reduce parasitic layout capacitance and emitting of electric field affecting other parts of the circuit.
5. Reduction of layout inductance around high power switching to minimize oscillations, voltage spikes and commutation losses.
6. Using layout for shielding sources of electromagnetic emission and parts of the circuit which may be affected.
7. Reducing EMI though proper layout geometry.
8. Separation of ground planes for control and power.
9. Using multilayer layout arrangements for optimal power conversion and control (most complex projects require up to 8 layers).
10. Cooling of SMT components directly via pcb metallization
11. Meeting safety requirements through proper creepage and clearance distance
12. Mechanical solutions associated with heatsinks and large power components
13. Magnetic components designed in PCB technology (planar magnetics), realized as separate components or with windings embedded in the main board.
Due to above requirements PCB layout in power conversion circuits frequently becomes a critical part of the project with big impact on performance, reliability and durability. System EL and cooperating consultants designed many advanced switched mode power conversion systems, we invite you to take advantage of our experience.
Planar transformers are commonly used in high frequency switched mode power supplies. The windings are manufactured using PCB or flex circuit technology, sometimes also using stamped copper foil or wire. Their main advantages are:
1. Low cost – manufacturing a winding in PCB technology may be much less expensive than an actual process of winding (very difficult to automate, especially with high frequency switching transformers).
2. Very low probability of quality problems – the PCB manufacturing process is highly repeatable and reliable while quality control is simple. On the other hand, winding transformers in traditional technology, applying insulation covering all require spaces with proper margin, wire termination, etc., may easily be performed with errors, sometimes in a way difficult to detect in testing of finished component.
3. Stable parameters – planar transformer geometry is governed by a PCB design and the board itself is manufactured with an accuracy counted in microns. As a result leakage inductance and intrawinding capacitance are very stable and repeatible component to component. It is not possible to control a winding process in traditional transformers with this level of accuracy, so higher variability of some parameters is unavoidable. Leakage inductance and intrawinding capacitance have a significant impact on power transistor commutation, oscillations, voltage spikes and EMI
4. High operational frequency – planar transformers are naturally composed of thin layers of copper, which reduces power losses associated with eddy currents present in high frequency operation. Achieving similar results in traditional transformers requires using thin copper foil, which complicates assembly and increases cost.
5. Transformer embedded in main board – in some situations it is possible to realize (completely or partially) windings directly in in the main PCB. All that is needed to have a fully functional transformer is to place a core in the PCB cutout.
6. Low profile – planar technology enables very low profile design, which may be a very desirable feature in some system geometries.