Adaptive turn-off trigger blanking for synchronous rectification

A switching converter includes a synchronous rectifier and a synchronous rectifier driver that controls conduction of the synchronous rectifier. The synchronous rectifier driver turns OFF the synchronous rectifier in response to a turn-off trigger. The synchronous rectifier driver prevents the turn-off trigger from turning OFF the synchronous rectifier during a turn-off trigger blanking time that is adaptively set based on a conduction time of the synchronous rectifier.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electrical circuits, and more particularly but not exclusively to synchronous rectifiers.

2. Description of the Background Art

Rectifier diodes are employed in switching converters, such as flyback converters. Generally speaking, a flyback converter is a buck-boost converter where the output inductor is split to form a transformer. In a flyback converter, a primary-side switch is closed to connect the primary winding of the transformer to an input voltage source. Closing the primary-side switch increases the primary current and magnetic flux, stores energy in the transformer, and induces current on the secondary winding of the transformer. The induced current has a polarity that places a diode rectifier in reverse bias to block charging of an output capacitor. When the primary-side switch is opened, the primary current and magnetic flux drop, and the current on the secondary winding changes polarity to thereby forward bias the diode rectifier and allow charging of the output capacitor to generate a DC output voltage.

Many flyback converters employ diode rectifiers to generate the DC output voltage. The conduction loss of a diode rectifier contributes significantly to overall power loss, especially for low-voltage, high-current converter applications. The conduction loss of a diode rectifier is given by the product of its forward voltage drop and forward conduction current. By replacing the diode rectifier with a metal-oxide semiconductor field effect transistor (MOSFET) operated as a synchronous rectifier, the equivalent forward voltage drop can be lowered and, consequently, the conduction loss can be reduced. Unlike a diode rectifier, however, the conduction of the synchronous rectifier has to be actively controlled by additional circuit, such as a synchronous rectifier driver.

SUMMARY

In one embodiment, a switching converter includes a synchronous rectifier and a synchronous rectifier driver that controls conduction of the synchronous rectifier. The synchronous rectifier driver turns OFF the synchronous rectifier in response to a turn-off trigger. The synchronous rectifier driver prevents the turn-off trigger from turning OFF the synchronous rectifier during a turn-off trigger blanking time that is adaptively set based on a conduction time of the synchronous rectifier.

DETAILED DESCRIPTION

FIG. 1shows a schematic diagram of a flyback converter that may take advantage of embodiments of the present invention. In the example ofFIG. 1, the flyback converter includes a primary-side switch QPR, a synchronous rectifier QSR, a transformer T1, and an output capacitor COUT. In one embodiment, each of the primary-side switch QPR and the synchronous rectifier QSR comprises a MOSFET.

When the primary-side switch QPR is turned ON, the primary winding of the transformer T1is connected to the input voltage source VIN, resulting in a current IDS flowing through the primary-side switch QPR and the primary winding. When the primary-side switch QPR is turned OFF, the energy stored in the primary side winding is released to the secondary winding of the transformer T1. The induced current in the secondary winding turns ON the body diode of the synchronous rectifier QSR and a current ISR flows through the secondary winding to charge the output capacitor COUT. The synchronous rectifier QSR turns ON at onset of body diode conduction, thereby minimizing the forward voltage drop across the synchronous rectifier QSR by providing a low impedance current path in parallel with its body diode. To prevent current inversion, the synchronous rectifier QSR is turned OFF before the synchronous rectifier current ISR reaches zero.

FIGS. 2 and 3show waveforms of signals of a synchronous rectifier at heavy load condition and light load condition, respectively, when a fixed turn-off trigger blanking time is used.FIGS. 2 and 3show waveforms of the drain-to-source voltage VDS.SR (see101) of the synchronous rectifier, the synchronous rectifier current ISR (see102) through the synchronous rectifier, and the gate-to-source voltage VGS.SR (see103) of the synchronous rectifier.FIGS. 2 and 3also show the turn-off threshold VTH.OFF (see104) and the turn-on threshold VTH.ON (see105) of the synchronous rectifier. The synchronous rectifier turns ON when its drain-to-source voltage VDS.SR drops below the turn-on threshold VTH.ON, which is caused by conduction of body diode of the synchronous rectifier. Note that the conduction time of the synchronous rectifier begins with body diode conduction, not necessarily when the gate drive signal to the synchronous rectifier is asserted.

In one embodiment, the turn-off trigger for turning OFF the synchronous rectifier is its drain-to-source voltage VDS.SR becoming greater than the turn-off threshold VTH.OFF. More particularly, as the synchronous rectifier current ISR reaches zero, the drain-to-source voltage VDS.SR of the synchronous rectifier rises above the turn-off threshold VTH.OFF, thereby turning OFF the synchronous rectifier. After the synchronous rectifier turns OFF, the synchronous rectifier current ISR flows through the body diode and the body diode becomes reverse biased when the synchronous rectifier current ISR reaches zero.

As illustrated inFIG. 2, the drain-to-source voltage VDS.SR (see101) of the synchronous rectifier severely oscillates after the synchronous rectifier turns ON. Because of this switching noise, there is a period after the synchronous rectifier turns ON when the drain-to-source voltage VDS.SR can exceed the turn-off threshold VTH.OFF, thereby causing the synchronous rectifier to prematurely turn OFF during its conduction time. To prevent this from occurring, a turn-off trigger blanking time (see106) is provided during which the synchronous rectifier is not turned-OFF regardless of the level of its drain-to-source voltage VDS.SR relative to the turn-off threshold VTH.OFF. That is, the turn-off trigger is blanked, i.e., disabled, during the turn-off trigger blanking time. The turn-off trigger blanking time represents the minimum conduction time, i.e., ON time, of the synchronous rectifier. In the example ofFIG. 2, the turn-off trigger blanking time (see106) is shorter than the conduction time of the synchronous rectifier (see107). In a typical SR driver integrated circuit (IC), a dedicated pin is required to program the turn-off trigger blanking time.

FIG. 3shows the waveforms ofFIG. 2at light load condition. Generally speaking, it is relatively difficult to select an optimal turn-off trigger blanking time. A turn-off trigger blanking time that is too long can cause synchronous rectifier current inversion at light load condition, deteriorating efficiency when the synchronous rectifier conduction time is short relative to the turn-off trigger blanking time. On the other hand, a turn-off trigger blanking time that is too short cannot effectively blank the switching noise on the drain-to-source voltage VDS.SR at heavy load condition.

Generally speaking, more switching noise is induced at heavy load condition, while less switching noise is induced at light load condition. In one embodiment, to obtain an efficient turn-off trigger blanking time and avoid problems associated with a fixed turn-off trigger blanking time, the turn-off trigger blanking time of the synchronous rectifier is adjusted to adapt to the load condition. The turn-off trigger blanking time may be introduced after the gate drive signal turns ON the synchronous rectifier. During the turn-off trigger blanking time, the turn-off trigger for turning OFF the synchronous rectifier is blanked to prevent the turn-off trigger from prematurely turning OFF the synchronous rectifier.

FIGS. 4 and 5show waveforms of signals of a synchronous rectifier at heavy load condition and light load condition, respectively, in accordance with an embodiment of the present invention. In the example ofFIGS. 4 and 5, the turn-off trigger blanking time (FIGS. 4 and 5, see106) is adaptively set to be equal to a fraction of the synchronous rectifier conduction time in the previous switching cycle. More particularly, in one embodiment, the waveforms ofFIGS. 4 and 5are the same as those ofFIGS. 2 and 3, respectively, except that the turn-off trigger blanking time is adaptively set to be 50% of the synchronous rectifier conduction time in the previous switching cycle.

FIG. 6shows a schematic diagram of a switching converter circuit in accordance with an embodiment of the present invention. In the example ofFIG. 6, the switching converter is a flyback converter300, which is the same as that ofFIG. 1with the addition of a synchronous rectifier (SR) driver200. The other components of the flyback converter300ofFIG. 6are as described with reference toFIG. 1.

FIG. 7shows a schematic diagram of the SR driver200in accordance with an embodiment of the present invention. In the example ofFIG. 7, the SR driver200is implemented as an integrated circuit with a DRAIN pin for connecting to a drain of the synchronous rectifier QSR, a SOURCE pin for connecting to a source of the synchronous rectifier QSR, a GND pin for connecting to a ground reference, a GATE pin for connecting to the gate of the synchronous rectifier QSR, a VDD pin for receiving a supply voltage, and a VIN pin for receiving an input voltage source. In the example ofFIG. 7, the SR driver200includes a green circuit205for minimizing standby power consumption by shutting down the gate drive signal to the synchronous rectifier QSR when the synchronous rectifier conduction time, which may be detected from an SR_COND signal, is shorter than a predetermined threshold.

FIG. 8shows waveforms of signals of the flyback converter300in accordance with an embodiment of the present invention. With reference toFIGS. 6 and 7,FIG. 8shows waveforms of the SR_COND signal (see161) that is output by a set/reset latch203, an ARM signal (see162) that is output by an amplifier206, a TURN_ON_ALLOW signal (see163) that is output by a turn-on trigger blanking circuit201(e.g., fixed blanking time of 1 μs), a TURN_ON_TRG signal (see164) that is output by an AND gate210, a TURN_OFF_ALLOW signal (see165) that is output by a turn-off trigger blanking circuit204, a TURN_OFF_TRG signal (see166) that is output by an AND gate212, and a gate drive signal (see167) at the GATE pin of the synchronous rectifier QSR. In the example ofFIG. 8, the turn-off trigger blanking time (indicated by the TURN_OFF_ALLOW signal) is adaptively selected to be 50% of the detected conduction time of the synchronous rectifier QSR in the previous switching cycle. Advantageously, because the turn-off trigger blanking time is adaptively selected, the integrated circuit package of the SR driver200does not need a dedicated pin for programming the turn-off trigger blanking time.

FIG. 8further shows waveforms of the current IDS through the primary-side switch QPR (see151), the voltage from the DRAIN pin to the SOURCE pin (see152), the synchronous rectifier current ISR through the synchronous rectifier QSR (see153), the gate drive signal to the primary-side switch QPR (see154) relative to the gate drive signal to the synchronous rectifier QSR (see155), and the drain-to-source voltage VSR.DS of the synchronous rectifier (see156). It is to be noted that the voltage from the DRAIN pin to the SOURCE pin of the SR driver200is not exactly the same as the drain-to-source voltage VSR.DS of the synchronous rectifier because of stray inductance on the drain of the synchronous rectifier QSR.

In one embodiment, the turn-off trigger blanking time (FIG. 8, see165) is introduced after the gate drive signal (FIG. 8, see167) is asserted to turn ON the synchronous rectifier QSR. This results in the turn-off trigger blanking time being extended for an adaptively-set period of time to prevent the synchronous rectifier QSR from being turned OFF by switching noise that occurs after the synchronous rectifier QSR is turned ON.

Referring toFIG. 7, the SR driver200detects the drain-to-source voltage of the synchronous rectifier QSR from the DRAIN and SOURCE pins. The amplifier211compares the drain-to-source voltage VDS.SR of the synchronous rectifier to the turn-on threshold VTH.ON to detect the start of body diode conduction. When the amplifier211detects that the body diode of the synchronous rectifier QSR starts to conduct, the amplifier211, through the AND gate210, clocks the flip-flop202to assert the gate drive signal at the GATE pin and thereby turn ON the synchronous rectifier QSR. This advantageously minimizes power loss by conducting through the channel, instead of the body diode, of the synchronous rectifier QSR. The amplifier206compares the drain-to-source voltage VDS.SR of the synchronous rectifier QSR to a high threshold VTH.HGH to detect end of body diode conduction. When the amplifier206detects end of body diode conduction, the amplifier206resets the set/reset latch203to de-assert the SR_COND signal.

Still referring toFIG. 7, the SR_COND signal is output by the set/reset latch203. The SR_COND signal is received by the turn-off trigger blanking circuit204to generate a turn-off trigger blanking signal, which disables the AND gate212to prevent resetting of the gate drive signal to the synchronous rectifier QSR. More particularly, in the example ofFIG. 7, the turn-off trigger is when the drain-to-source voltage VDS.SR of the synchronous rectifier QSR, as detected from the DRAIN pin and SOURCE pin, exceeds the turn-off threshold VTH.OFF. When the drain-to-source voltage VDS.SR of the synchronous rectifier QSR rises above the turn-off threshold VTH.OFF, the amplifier209generates an output signal that clears the flip-flop202to de-assert the gate drive signal at the GATE pin and thereby turn OFF the synchronous rectifier QSR. The turn-off trigger blanking circuit204outputs a turn-off trigger blanking signal that gates, i.e., enables/disables, the AND gate212to prevent the turn-off trigger from turning OFF the synchronous rectifier QSR during the turn-off trigger blanking time. In one embodiment, the turn-off trigger blanking circuit204generates the turn-off trigger blanking signal by charging a timing capacitor during the synchronous rectifier conduction time. The peak of the timing capacitor is sampled and held. Half of the sampled signal may be compared with the timing capacitor voltage to generate the turn-off trigger blanking signal.

FIG. 9shows a schematic diagram of a turn-off trigger blanking circuit204in accordance with an embodiment of the present invention. In the example ofFIG. 9, the SR_COND signal is delayed by a delay circuit U2(e.g., 30 ns delay) and a delay circuit U3(e.g., 20 ns delay) to ensure proper sample and hold. A timing capacitor C4is charged by a current source12according to the SR_COND signal. The peak voltage of the timing capacitor C4is proportional to the synchronous rectifier conduction time for each switching cycle. The peak value of the timing capacitor C4voltage is sampled and stored in a capacitor C2. Then, half of the voltage on the capacitor C2is compared to the voltage on the timing capacitor C4in the next switching cycle to generate the TURN_OFF_ALLOW signal. The TURN_OFF_ALLOW signal rising edge is delayed from the SR_COND signal rising edge by half of the synchronous rectifier conduction time of the previous switching cycle. The TURN_OFF_ALLOW signal, i.e., the turn-off trigger blanking signal, is output by the gate U5.

FIG. 10shows a schematic diagram of a flyback converter300A in accordance with an embodiment of the present invention. The flyback converter300A is a particular embodiment of the flyback converter300ofFIG. 6. The flyback converter300A comprises previously discussed components, namely the transformer T1, the primary-side switch QPR, the synchronous rectifier QSR, and the SR driver200. The flyback converter300A receives, filters, and rectifies an AC input to generate an input voltage across the primary winding of the transformer T1. In the example ofFIG. 10, the flyback converter300A includes a controller IC301for controlling the switching operation of the primary-side switch QPR by, e.g., pulse-width modulation (PWM), on the primary side of the transformer T1. The SR driver200controls the conduction of the synchronous rectifier QSR on the secondary side.

In one embodiment, the integrated circuit package of the SR driver200comprises a plurality of pins. The integrated circuit package of the SR driver200includes a DRAIN pin that is connected to the drain of the synchronous rectifier QSR, a SOURCE pin that is connected to the source of the synchronous rectifier QSR, and a GATE pin that is connected to the GATE of the synchronous rectifier QSR. In the example ofFIG. 10, the DRAIN pin is connected to the drain of the synchronous rectifier QSR by way of an external resistor REXT. With adaptive turn-off trigger blanking, the integrated circuit package of the SR driver200does not have a dedicated pin for programming the turn-off trigger blanking time.

Circuits and methods of switching converters with adaptive turn-off trigger blanking have been disclosed. While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.