Active snubber for synchronous rectifier

Details Active snubber for synchronous rectifier Active snubber for synchronous rectifier

2003-02-20

2004-08-03

Han, Jessica (Department: 2838)

Electric power conversion systems

Current conversion

Including automatic or integral protection means

C363S021060, C363S021140

Reexamination Certificate

active

06771521

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to power supplies and, more specifically, to an active snubber circuit for synchronous rectifier to reduce the turn-off voltage spike and high frequency ringing associated with synchronous rectifier.
BACKGROUND OF THE INVENTION
Synchronous rectification is widely applied in a low voltage and high current DC/DC converter. As it is known in the art, in the customarily used half-wave synchronous rectifier, synchronous MOSFET is selectively turned on and off in synchronicity with the secondary voltage of an isolate power transformer. In this way the secondary voltage of the transformer is rectified and transferred to the output. However, there exists a high voltage spike and high frequency ringing between the drain and the source of the freewheeling synchronous MOSFET. Such voltage spike is caused by the poor reverse recovery characteristic of the body diode of the freewheeling synchronous MOSFET, and imposes severe stress on the freewheeling synchronous MOSFET. As a result, higher rated MOSFET are needed to prevent the breakdown of the freewheeling synchronous MOSFET. Since higher voltage rated MOSFET has larger conduction resistor leading to higher conduction loss of the freewheeling synchronous MOSFET, several attempts have been made to suppress the high voltage spikes and high frequency ringing associated with additional circuit.
FIG. 1
shows a RC snubber including a capacitor C and a resistor R, which has been utilized widely in the synchronous rectifier to suppress the voltage spike and voltage ringing. As shown in
FIG. 1
, the RC snubber is coupled across the freewheeling synchronous MOSFET S
1
. The capacitor C decreases the rising speed of the voltage across the freewheeling synchronous MOSFET S
1
and absorbs the voltage spike energy. The resistor R provides necessary damping to reduce the voltage ringing. Since the energy stored in the capacitor C is discharged fully via the resistor R during each switching cycle, the RC snubber brings large energy dissipation. The symbol T represents the isolate power transformer. The symbol N
1
and N
2
represent the primary winding and the secondary winding of the isolate power transformer T.
FIG. 2
shows a RCD clamping circuit implemented in a half-wave synchronous rectifier. A clamping capacitor C is coupled across a freewheeling synchronous MOSFET S
1
through a diode D. Therefore, the voltage spike of the freewheeling synchronous MOSFET S
1
can be clamped by the capacitor C. And a resistor R is placed across the clamping capacitor C and the output capacitor Co. By this way, the excessive voltage spike energy stored in the capacitor C is transferred to the output capacitor Co through the resistor R. That is to say, a portion of spike energy is recovered. Accordingly, the RCD snubber circuit brings lower energy dissipation in comparison with the RC snubber circuit in FIG.
1
.
Another commonly used method applied in diode rectifier to reduce the voltage spike is active snubber circuit.
FIG. 3
illustrates a schematic diagram of a full-bridge diode rectifier employing an active snubber. The active snubber branch consisting of an active switch Sa and a snubber capacitor Ca connected in series is coupled across the output terminal of the rectifier. The gate drive signal for the active switch Sa is derived from the primary side circuit via a logic circuit, an isolate transformer T
1
and a driving circuit. The driving circuit keeps the active switch Sa being turned on during a time interval that the secondary winding of the power transformer outputs a high voltage level. The reverse recovery energy of the rectifier diode is transferred to the snubber capacitor through the body diode Da of the active switch Sa. And during the conduction interval of the active switch Sa, the excessive reverse recovery energy stored in the snubber capacitor Ca is transferred to the output filter without any dissipation. Therefore, the active snubber is an effective lossless snubber circuit. However, since the drive signal of the active switch Sa is derived from the primary side via an additional isolated driving circuit, the complexity and the component cost are increased greatly.
A synchronous rectifier has been utilized widely in a low voltage and high current DC/DC converter. However there exists a high voltage spike across the synchronous rectifier due to the poor reverse recovery characteristic of the body diode of the synchronous rectifier. It is seen that the active snubber brings perfect performance when it is applied to the diode rectifier, except that the complicated driving for the active switch. Accordingly, in order to eliminate the voltage spike, the present invention implements an active snubber circuit in the synchronous rectifier circuit, and proposes a driving circuit for the active switch, which has a simple structure and uses fewer components.
It is therefore attempted by the applicant to deal with the above situation encountered with the prior art.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to propose an active snubber circuit for synchronous rectifier to reduce the turn-off voltage spike and high frequency ringing associated with synchronous rectifier.
It is therefore another object of the present invention to propose an active snubber circuit for synchronous rectifier, in which a driving circuit for the active switch has a simple structure and uses fewer components.
In accordance with the first aspect of the present invention, the active snubber is coupled across a synchronous rectifier having a first synchronous MOSFET and a second synchronous MOSFET coupled to a transformer in a power converter. The active snubber includes a series-coupled active switch and first snubber capacitor which is coupled between a drain terminal and a source terminal of the first synchronous MOSFET, a gate driver operative to keep the active switch conducting a specified period of time during a non-conduction interval of the first synchronous MOSFET. The gate driver is composed of an auxiliary winding, a capacitor, and a resistor, wherein the auxiliary winding and the capacitor are connected in series and then coupled across the resistor in parallel, which is coupled between the gate and the source of the active switch.
Preferably, the gate driver further includes a diode coupled across the capacitor.
Preferably, the active switch is a N-channel MOSFET.
Preferably, the active switch is a P-channel MOSFET.
Preferably, the auxiliary winding is derived from the transformer.
Preferably, the power converter further includes an output filter circuit having at least an output inductor and an output capacitor, and the auxiliary winding is derived from the output inductor.
Preferably, the active snubber further includes a first auxiliary diode coupled across the active switch.
Preferably, the active switch includes a parasitical body diode.
Preferably, a series-coupled branch including a second auxiliary diode and a second snubber capacitor is further coupled between a drain terminal and a source terminal of the second synchronous MOSFET, and the second snubber capacitor is coupled in parallel with the first snubber capacitor.
Preferably, the synchronous rectifier is selected from the group consisting of: a half-wave synchronous rectifier; a center-tapped synchronous rectifier; and a current doubler synchronous rectifier.
Preferably, the synchronous rectifier further includes a synchronous switch post regulator circuit.
In accordance with the second aspect of the present invention, the active snubber is coupled across a synchronous rectifier having a first synchronous MOSFET and a second synchronous MOSFET coupled to a transformer in a power converter. The active snubber includes: a series-coupled active switch and first snubber capacitor which is coupled between a drain terminal and a source terminal of the first synchronous MOSFET, an auxiliary winding for driving the active switch to keep the active switch conducting a specified period of time during a non-conduction i

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