Crossover losses

Another way of reducing the reverse recovery current shoot-through is simply to ensure that the boost diode is carrying no forward current at the moment when the switch starts to turn ON. The diode then blocks reverse voltage instantly. In other words, running the Boost in DCM or BCM (boundary conduction mode, i.e., at the critical boundary) will produce higher peak currents, but smaller inductors (yes, if r is large, the size of any inductor typically reduces ), and perhaps much better efficiency too, because now, the turn-on crossover loss becomes zero. 

FIGURE 27.11 (See color insert following page S88.) H2 permeability as a function of temperature and RH. Upper limit (solid line) defined by crossover losses (assuming no contribution from O2 crossover), lower Umit (dotted Une) defined by electrode ionomer film-transport requirements, and data are for wet and dry Nafion 1100 EW-based membranes. (Reproduced from Gasteiger, H.A. and Mathias, M. F., in Proceedings of the Symposium on Proton Conducting Membrane Fuel Cells III, 2003. The Electrochemical Society of America. With permission from The Electrochemical Society, Inc.)... 

Therefore, we realize that the only thing holding us back at any moment of time from going to even higher frequencies are the switching losses. This term is in fact rather broad — encompassing all the losses that occur at the moment when we actually switch the transistor (i.e. from ON to OFF and/or OFF to ON). Clearly, the crossover loss mentioned earlier is... 

In principle, the low-side mosfet does not have any significant crossover loss because there is virtually no overlap between its V and I waveforms — it switches (changes state) only when the voltage across it is almost zero. Therefore, typically, the high-side mosfet is selected primarily on the basis of its high switching speed (low crossover loss), whereas the low-side mosfet is chosen primarily on the basis of its low drain-to-source on-resistance, Rds (low conduction loss). 

Note Note that to be precise, this particular term more correctly should be called the crossover loss, as was first pointed out in Chapter 1. The crossover loss (i.e. specifically attributable to the V-I overlap) is not necessarily the entire switching loss taking place in the switch, as we will see. 

We realize that our ability to avoid integration (and use simpler arguments to calculate the crossover loss) is just a piece of good luck here — specific to the case of an inductive load. 

However, unlike the crossover loss, the conduction loss is not frequency-dependent. It does depend on duty-cycle, but not on frequency. For example, suppose the duty cycle is 0.6 then in a measurement interval of say, one second, the net time spent by the switch in the ON-state is equal to 0.6 seconds. But we know that conduction loss is incurred only when the switch is ON. So in this case, it is equal to a x 0.6, where a is an arbitrary proportionality constant. Now suppose the frequency is doubled. Then the net time spent in... 

---