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AN-6003 Dataheets PDF



Part Number AN-6003
Manufacturers Fairchild Semiconductor
Logo Fairchild Semiconductor
Description Shoot-through
Datasheet AN-6003 DatasheetAN-6003 Datasheet (PDF)

www.DataSheet4U.com www.fairchildsemi.com AN-6003 “Shoot-through” in Synchronous Buck Converters Jon Klein Power Management Applications Abstract The synchronous buck circuit is in widespread use to provide “point of use” high current, low voltage power for CPU’s, chipsets, peripherals etc. In the synchronous buck converter, the power stage has a “high-side” (Q1 below) MOSFET to charge the inductor, and a “Low-side” MOSFET which replaces a conventional buck regulator’s “catch diode” to provid.

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www.DataSheet4U.com www.fairchildsemi.com AN-6003 “Shoot-through” in Synchronous Buck Converters Jon Klein Power Management Applications Abstract The synchronous buck circuit is in widespread use to provide “point of use” high current, low voltage power for CPU’s, chipsets, peripherals etc. In the synchronous buck converter, the power stage has a “high-side” (Q1 below) MOSFET to charge the inductor, and a “Low-side” MOSFET which replaces a conventional buck regulator’s “catch diode” to provide a low-loss recirculation path for the inductor current. V IN PWM CONTROLLER 2. Adaptive gate drive: This circuit looks at the VGS of the MOSFET that’s being driven off to determine when to turn on the complementary MOSFET. Theoretically, adaptive gate drives produce the shortest possible dead-time for a given MOSFET without producing shootthrough. H igh-S ide Q1 VO U T L1 In practice, a combination of adaptive and fixed produces the best results, and is typically what is in today’s PWM controllers and gate drivers as shown in Figure 2 D1 BOOT RG V IN +5 C BO O T + – D C GD PW M 1V + D elay SW H D RV R G A TE G C GS S D C GD LDR V PW M R G A TE G C GS S D elay + 1V PG ND Low -S ide Q2 Figure 1. Synchronous Buck output stage Q1 Shoot-through is defined as the condition when both MOSFETs are either fully or partially turned on, providing a path for current to “shoot through” from VIN to GND. To minimize shoot-through, synchronous buck regulator IC’s employ one of two techniques to ensure “break before make” operation of Q1 and Q2 to minimize shoot-through: 1. Fixed “dead-time”: A MOSFET is turned off, then a fixed delay is provided before the lowside is turned on. This circuit is simple and usually effective, but suffers from its lack of flexibility if a wide range of MOSFET gate capacitances are to be used with a given controller. Too long a dead-time means high conduction losses. Too short a dead time can cause shoot-through. A fixed dead-time typically must err on the “too long” side to allow high CGS MOSFETs to fully discharge before turning on the complementary MOSFET. Q2 Figure 2. Typical Adaptive Gate drive Even though there apparently is a “break before make” action by the controller, shoot-through can still occur when the High-side MOSFET turns on, due to Gate Step. Shoot-through is very difficult to measure directly. Shoot-through currents persist for only a few nS, hence the added inductance in a current probe drastically affects the shoot-through waveform. Shoot-through manifests itself typically as increased ringing, reduced efficiency, higher MOSFET temperatures (especially in Q1) and higher EMI. This paper will provide analytical techniques to predict shoot-through, and methods to reduce it. 04/25/2003 Shoot-through in Synchronous Buck Regulators AN-6003 “Gate Step” – The shootthrough culprit VGS 6 5 SW NODE VOLTAGE 25 20 15 LS MOSFET GATE Not exactly. Most shoot-through occurs when the high-side MOSFET is turned on. The h.


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