What Comes First: Devices or Topology

Topologies Comparison

Present Power Electronics Systems are built upon and resemble an INVERTED pyramid which is by definition unsustainable and ready to collapse at any moment. Why? At its bottom it has the buck converter as its foundation which than supports the next level single ended isolated converter like flyback and forward to culminate on the third level into bridge type converter with even more switching devices. Consequence: buck converter problems lead to collapse of the whole structure irrespective what switching devices are used.

The buck converter does have huge problems: device voltage stresses much higher than output regulated DC voltage and inherent hard switching operation. Moreover, the use of the inductor for filtering output ripple currents leads to inherent problems in scaling up the power due to the requirement to pass DC load current so that even modest DC load current and resulting air-gap just a fraction of millimeter leads to complete waste of the magnetic material and ineffective filtering. The new PWM/resonant switching method results in converter topologies which eliminate output inductor with DC bias and yet offering much more effective ripple current filtering.

The new switching methods and resulting converter topologies result in much reduce device voltage stresses. In the isolated converters this translated into reduction of transformer fluxes by an order of magnitude and consequent huge size reduction and efficiency improvements. The next example demonstrates that the optimum converter topology is of the primary importance for system performance and that actual switching devices utilized have only secondary importance in system improvements.

Converter Graphic

Comparison of the two switch buck converter with the three switch Cuk-buck2 demonstrates the huge differences in device stresses for 48V to 1V step down conversion. The synchronous rectifier diode in buck converter has 100V voltage stress versus 2V voltage stress in Cuk-buck converter. Hence a 10kW rated switch must be used instead of the 200W rated switch! Moreover, the diode is turned ON and turned- OFF at full 100A current in buck converter. In Cuk-buck2 the diode is turned both ON and OFF at zero current. Therefore, it does not even know that it is switching although it transfers huge current to the load. The duty ratio in buck converter is extremely small at 0.02 in order to make a 50V to 1V step-down buck converter. The Cuk-buck2 operates at 50% duty ratio and results in reduction of magnetics size by at least 50%.

Large Voltage Graphic

In addition to elimination of switching losses Cuk-buk2 also has an isolated extension which retains all the above advantages, while the isolated version of the buck converter, the forward converter, suffer from the same disadvantages as the buck converter.

Device performance depends 100% on topology and not the other way around!