It’s impossible to understand the essentials of maximizing switching-supply efficiency across all potential loads without understanding the sources of circuit losses. Fortunately, it isn’t necessary to consider all possible circuit configurations. The analysis that follows looks at a basic step-down (buck) voltage regulator (Fig. 1). Allowing for circuit differences, there are parallels in all switching-regulator topologies.
In a non-synchronous buck-regulator circuit, the forward-voltage drop across the low-side rectifier diode is in series with the output voltage. Naturally, the diode’s losses seriously impact efficiency. For example, in a regulator stepping down a 12-V intermediate bus voltage to 3.3 V, the 0.4-V forward voltage of a Schottky diode represents roughly a 12% loss. The greater the step-down, the worse the situation.
So the first step in pursuing efficiency is synchronous rectification—which is essentially replacing the diode with a switch, usually another MOSFET. This substitutes the diode junction’s constant voltage drop with the MOSFET’s conduction and switching losses and adds complexity, though it gains something on the order of 4% in efficiency. However, a new efficiency limitation arises from dead-time delay, inserted by the synchronous controller to prevent “shoot-through,” with both MOSFETs conducting simultaneously (Fig. 2).
The good thing about dead time is that during the time that top-side MOSFET is conducting, the low-side MOSFET’s parasitic body diode generally acts as a clamp on the negative inductor voltage swing. Unfortunately, like the Schottky, the body diode is lossy (and slower to turn off), and this could result in a 1% to 2% efficiency penalty.
So what do you do? Some designers actually “put back” the Schottky, which goes into conduction at a lower voltage than the body diode, in parallel with the lower MOSFET. The MOSFET has lower conduction losses than the lone Schottky, and the Schottky helps during dead time.
Even with that approach, though, there are still efficiency problems at light loads when switching frequencies are high. That’s because, under light loads, the current in the switching supply’s inductor discharges to zero. This leads to a deeper investigation of dead time.
Recall the situation. For the least complexity, you simply would want to drive the lower MOSFET gate with the complement of the signal on the upper MOSFET’s gate. Or even more simply, you could continue to hold the lower MOSFET on until the beginning of the next cycle. In that kind of design, when the inductor current began to flow in the reverse direction, the regulator’s controller might be designed to sense the inductor current’s zero crossing in each cycle, and it would either shut off the synchronous rectifier or disable it at light loads.
That keeps the controller simple, but consider this: when the inductor current reverses, the synchronous rectifier must pull current from the output, storing the energy in the input bypass capacitor and replacing the lost output energy during the next half cycle. That’s not good because it dissipates power in all of the circuit’s parasitic resistances and switching inefficiencies.
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