[Technology Report]
New Techniques Enhance Efficiency Across All Loads
Governments around the world are moving from voluntary to mandatory power-supply efficiency efforts. Challenges may lie ahead, but designers now have methods to cope with them.
Explicitly, as output voltages shrink to match the needs of smaller-geometry ICs, the voltage drop across RDS(ON) becomes more and more significant. In addition, the power needed to drive the MOSFET gate cancels out some of the efficiency gained from a reduced forward-voltage drop.
Another efficiency limitation arises from dead-time delay, inserted by the synchronous controller to prevent “shootthrough,” or the switching overlap of the high-side and low-side MOSFETs that could have both conducting simultaneously. During this dead time, the low-side MOSFET’s parasitic body diode would generally act as a clamp on the negative inductor voltage swing.
However, the body diode is lossy and slow to turn off, which could result in a 1% to 2% efficiency penalty. Therefore, some designers parallel the lower MOSFET with a Schottky diode, which turns on at a lower voltage than the body diode.
Even with that kind of design, conduction losses during the dead time can become significant at high switching frequencies and especially at light loads. When load current is light, the current in the switching supply’s inductor discharges to zero. In that case, the power-supply designer has several options.
From a simplicity standpoint, it’s attractive to drive the lower MOSFET gate with the complement of the signal on the upper MOSFET’s gate. Alternatively, the switching controller could continue to hold the synchronous switch on until the beginning of the next cycle.
In that case, when the inductor current would start to flow in the reverse direction, the regulator’s controller could sense the inductor current’s zero crossing in each cycle. Then it either shuts off the synchronous rectifier or simply disables the synchronous rectifier at light loads.
Many advantages are possible when holding the synchronous switch off until the beginning of the next cycle. But there’s also at least one drawback in terms of efficiency. When the inductor current reverses, the synchronous rectifier pulls current from the output, storing the energy in the input bypass capacitor and replacing the lost output energy during the next half cycle. This dissipates power in all of the circuit’s parasitic resistances and switching inefficiencies.
One workaround has been pulse-skipping. The supply reverts to non-synchronous operation, with the Schottky doing the commutation. That re-introduces the diode drop as a drag on efficiency. The most efficient approach—and the most complex in terms of design—uses zero-crossing detection, or “valley control” (explained below).
Those are some of the essential limitations of older designs and some traditional ways of dealing with them. Next, let’s look at some of the most recent power-supply products that attempt to flatten the efficiency curve across all load regimes—and largely succeed in doing so.
POWER INTEGRATIONS One of the companies longest associated with power-supply efficiency, Power Integrations, introduced its EcoSmart technology in 2002 as part of its LinkSwitch LNK501, a 3-W constant-voltage/ constant-current (CV/CC) ac-dc switcher for portable devices. This development illustrated some new thinking (Fig. 3).
Traditional switching supplies placed the switching element on the low side of the transformer. But the LNK501 places it on the high side. Referencing the IC to the rectified dc input made it possible to derive all feedback information for an approximate constant-voltage and constant-current operation from the primary- side clamp circuit. Removing the sense resistor, along with the rest of the secondary-side current-sense circuit, cut secondary loss and increased efficiency by about 10%.
After a bootstrapping startup, the primary-side leakage inductance clamp (D5 and C4) provides power for the LNK501. The clamp is also the source for feedback information—that is, the voltage developed across C4 is an approximate representation of the output voltage transformed through the transformer turns ratio. Resistor R1 converts this reflected voltage to a current that’s applied to the IC’s control pin.
The operating modes are discontinuous flyback with voltage-mode control for the constant-voltage portion and current-limit operation for the constant-current portion of the output characteristic. Once the output voltage reaches regulation, control of the output transitions to constant voltage. If the output load increases beyond the peak power point and the output voltage falls, the reduced control-pin current lowers the internal current limit, providing an approximate constant-current output characteristic.
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