POWER ELECTRONIC p
ONICS
FEA
FEAT REATURE
Im roving buck converter light-load efficiency with diode emulation mode ome Johnston fromIntersil Corporation explores how
Jerom ohnston f om Intersil Corporation explores how to iowto improve systempowprove system pow ov
he buck converter switching regulator topology has evolved over the years as designers added new improvements to enhance efficiency and increase overall performance. Figure 1(a) shows an early buck converter using a diode rectifier during the off state of themain power switch. To achieve higher efficiency, designersmodified the buck topology by replacing the diode with a sync FET as shown in Figure 1(b). However, it
T
introduced undesirable side effects under light load conditions. To overcome this, diode emulationmode was added to enhance the sync FET design. Figure 2(a) illustrates the single
transistor buck controller u sing a diode rectifier.When the switch is conducting, current builds up in the inductor. The amount of current is a function of the voltage across the inductor and time the switch is closed (ON time). The ratio of the time the switch is ON to the time it’s OFF is used to regulate the output voltage. When the switch is open (OFF), the current continues to flow in the inductor as shown in Figure 2(b). When the power switch is off, the diode provides the path for the inductor current T his is a practical solution for regulating higher output voltages. But, with the need for lower output voltages and higher output currents, this has become less practical due to the diode losses. Los proportional to the voltage diode times themagnitude
.
of the current drop of the ses were
during the duty cycle when the current flowed through the diode. To improve efficiency, the standard diode was
replaced with a Schottky diode featuring lower forward voltage drop (0.4 volts versus 0.7 volts), but this also has its limits. To improve efficiency even further, the diode function was replaced with a
synchronous FET or sync FET because it is only ON during the OFF time of themain power switch. When the buck converter is switching with nominal output load, the inductor current is always zero or greater as shown in Figure 3.
Under normal load conditions, the inductor current is always positive, flowing fromthe inductor’s input to output. The current is composed of a DC portion, but it also has an AC component known as ripple current. When the sumof the DC and AC components’ inductor current remains positive for the entire switching period, the conv erter is said to
be operating in continuous-conduction- mode (CCM). However, if the inductor current under light load conditions
becomes negative or zero, the converter is operating in discontinuous-conduction- mode (DCM).
Figure 4 Figure 4 Figure 1 (a) & (b) negative. Unlike the standard DC/DC buck
Figure 1: (a) buck converter with diode rectifier, and (b) buck converter with sync FET
Figure 2 (a) & (b)
Figure 1: (a) buck converter with diode rectifier, and (b) buck converter with sync FET
regulator with a diode rectifier, the sync FET causes the current in the inductor to flow backwards during DCM, stealing energy fromthe output filter capacitor. This behaviour reduces the light-load efficiency because of the conduction loss as the low-sideMOSFET sinks the inductor current when it would bemore efficient to prevent this current from flowing at all.
Manymodern controllers include
Figure 2: (a) Single transistor buck controller with diode rectifier, and (b) controller with switch open flows current to the inductor
Figure 2: (a) Single transistor buck controller with diode rectifier, and (b) controller with switch open flows current to the inductor
Figure 3
circuitry that avoids the DCMconduction loss bymaking the low-side sync FE T emulate the current-blocking behaviour of a diode. This smart-diode operation is called diode emulationmode (DEM) and functions to turn the sync FET off when the circuitry senses that the inductor current is starting to flow in the wrong direction. This circuitrymonitors the voltage across the RDS(ON) of the low- side sync FET and turns off the FET when adverse conditions occur.
For example, the ISL8117 60V buck
Figure 3: Buck converter switching with inductor current that is always greater than zero
k onverte h inductor
greater than zero
Figure 4: Buck converter n light current load
DC Figure 5: Buck Figure 5: Buc
converter’s sync FET allows negative current flow
converter’s sync FET allows negative current flow
Figure 4: Buck converter iin light current load condition operatcondition operating ining in DCM
cr the voltage across the sync FET and always
controller fromIntersil Corporation offers a mode option in which DEMcircuitry can be enabled to enhance light load efficiency. When enabled, the DEMcircuitry examines
activates DEMif it signals that the inductor current is going negative for eight
consecutive PWMcycles while the LGATE pin is high (sync FET is Using detection over
prevents noise from ON).
activating DEM. If the ISL8117 enters DEM mode, the switching frequency of th e controller also decreases. Both of these actions increase efficiency by not allowing negative current flow and by reducing unnecessary gate-driver switching losses. The frequency reduction is proportional to the reduction of load current.
In the single-switch buck converter, which uses a diode rectifier, the inductor current could never go negative because the diode allowed current flow in only one direction. Therefore, when the converter was under light load conditions, the current during DCMwill appear as shown in Figure 4.
Figure 5 illustrates what happens when the buck converter’s diode is replaced
/ ELECTRONICS ELECTRONICS Figure 5 Figure 5
If your buck regulator application requires excellent light-load efficiency, you’ll want to consider the selection of a controller or regulator that offers DEM. Avoiding DCMconduction loss and reducing unnecessary gate-drive r
switching losses will help your next power supply designmeet its performance specification targets.
www.intersil.com
Intersil Corporation
www.intersil.com
T: +1 408-432-8888 ELEC RO ELECTRONICS CS | SEPTEMBER 2016 SEP EMBER 201 23 23 ower efficiency wer efficiency
light load conditions - with a sync FET and is
the current goes operating under
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