Power Supply Design Tips

On this page are some of the lessons I've learned in my 25 years designing switchmode converters in many power supply design projects. I learned most of them the hard way. I hope you find these tips useful in your efforts to solve, or understand, some of the issues involved in switchmode converter power supply design.

Transformer Design Issues
Leakage inductance between the primary and secondary windings of the transformer is a real problem in switchmode power supplies. It is manifested at it's worst, in single ended supplies. These are the one switch versions of both flyback and forward converters.

Here's the problem: when the switch turns off, the switch side of the transformer flies back to twice the dc input voltage. However, the leakage inductance will superimpose a spike on top of the flyback voltage. The spike voltage is dependent on the value of the parasitic circuit capacitance's, and the value of the leakage inductance of the transformer. If the leakage inductance values are large, the amplitude of the spike will be high, and could exceed the voltage withstand capabilities of the switch. Adios mosfet!.... So now you need a snubber, but the energy in the spike is high also, so the snubber cap must be large enough to store all that energy. The energy stored in the snubber cap must be dissipated in a resistor as heat during the next switching period. Saved the mosfet, but now efficiency has taken a beating, and the temperature in the enclosure has gone way up due to all that heat we're losing in the snubber resistor. Now we have to buy a fan to cool the whole thing off. Costs are up, reliability is down, and the guys in manufacturing don't want a fan.

The solution to this problem is to design the transformer for the lowest possible leakage inductance and to select a mosfet with the lowest output capacitance (Coss). The root cause of excessive leakage inductance is poor coupling between primary and secondary windings. To increase the coupling you should select transformer cores with wide winding windows. This allows you to build a transformer with fewer layers, improve coupling, and reduce leakage inductance. Cores with narrow windows are not suitable for switchmode transformers. Cores such as TDK EER series with their wide windows are excellent choices. Cores such as the PQ series are not; they force you to build transformers with narrow windings and many layers.

Once you've got the right core, and have designed for minimum turns, interleave the windings. That is wind 1/2 of the primary, then all of the secondaries, then the other 1/2 of the primary. Interleaving alone will cut leakage inductance by 1/2.

DO NOT LET THE CORE SATURATE!!

More is being written, this page is in constant flux. Topics to be added soon are:

Snubber circuit design
Power supply grounding
EMI/RFI: Who's the culprit?
More transformer design