Understand the tradeoffs of switched-mode power design

November 30, 2016 // By EDN Europe
Martin Brabham, Managing Director, Stadium Stontronics
The switched-mode power supply has made an incredible contribution to energy efficiency in electrical and electronic systems. Old, linear power supplies struggled to waste less than half of the energy supplied to them. The introduction of switching based on pulsewidth modulation (PWM) quickly showed the power of a move away from linear supplies for all but the smallest voltage converters, such as the simple low-dropout regulators used close to the point of delivery for sensors and other sensitive mixed-signal circuitry.

But there are many subtleties in power-converter design, which have led to numerous variations on the idea of PWM switching, each designed to eke more efficiency out of the circuitry for the chosen application. Although PWM-based switching proved to be more efficient than linear conversion from its introduction, the basic technique has issues.

The most fundamental is the use of hard switching: an approach that leads to the power transistors that deliver energy to the load being turned off while both current and voltage are above zero. This behaviour can lead to large losses during the transition from the on state to the off state. This is not the only problem. The high dV/dt and dI/dt levels that result from hard switching cause significant electromagnetic interference (EMI).

The key to reducing the losses of hard switching is to try to reduce current or voltage to zero or as close as possible before activating or deactivating the power path. The breakthrough in efficiency came with the adoption of resonant switching, an approach that makes switching at zero current or voltage a practical option.

The resonant architecture takes advantage of reactive passive components in the switching path. Combinations of capacitors and inductors result in frequency resonances that cause the voltage and current to oscillate around a zero point. Some designs adopted the approach of quasi-resonance. In this strategy the voltage across the primary winding is monitored actively. The switching of the power transistor is synchronized to a point where current and voltage is at its minimum. This technique reduces the turn-on switching loss but has little effect on energy wastage during the turn-off phase.

Soft-switching converters addressed the issue of turn-off losses using resonant tank circuits. The switching strategy remains under the control of the core PWM algorithm but the voltage and current waveforms are handled in such a way that either is pushed towards zero just before the power transistor switches state.

One form is zero-voltage switching (ZVS); the other is zero-current switching (ZCS). A typical ZCS strategy pushes the current to zero at turn-on and reduces the current as much as possible before turning the transistor back off. The design does incur losses at turn-on, particularly in MOSFETs because of the capacitive effects in the transistor’s body diode.

Another issue with ZCS is that a high rate of change in voltage can couple to the gate-drive circuitry through the power transistor’s Miller capacitance. The effect slows down the switching rate and can cause the device to bounce between states in the worst case. But this does not mean ZCS is a poor choice. Larger power supplies that use insulated gate bipolar transistors (IGBTs) for their isolation and energy-handling ability can make efficient use of ZCS because they are prone to large tail currents when switched off and do not suffer the same issues as MOSFETs with a ZCS strategy.

Resonant ZVS on the other hand sees advantages with MOSFET-based converters thanks to the ability of the architecture to eliminate capacitance-related losses during the turn-on phase. The ZVS architecture is similar to that