US 8,526,202 B2Granted

System and method for synchronous rectifier

AssigneeBCD Semiconductor Manufacturing Limited

InventorsShaoHua Fang, BaoZhen Chen, Qiang Zong, YaJiang Zhu

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Abstract

A synchronous rectification circuit for a power converter includes a power switch coupled to a transformer and an output capacitor and a switching control circuit configured to provide a control signal to the power switch in response to a first state and a second state of the voltage across the power switch. In the switching control circuit, the second state is determined prior to the first state is determined. In an embodiment, the switching control circuit includes a voltage comparing unit configured to act in response to the first and second inputs. The voltage comparing unit is also configured to output a logic signal according to the voltage difference between the sensed voltage drop across the power switch and a reference threshold voltage. A logic processing circuit is coupled to the voltage comparing unit and configured to provide the first state and the second state of the voltage across the power switch.

Claims16

Dependent claims

A synchronous rectification circuit for a power converter, comprising:

a power switch including a body diode and configured as a synchronous rectifier for coupling to a transformer and an output capacitor;
and
a switching control circuit configured to provide a control signal in response to a first voltage signal related to a voltage across the power switch and a second voltage signal which is a delayed version of the first voltage signal;
wherein the switching control circuit is configured to provide the control signal to the synchronous rectifier without any other-input signal that provides information about an operating state of the power converter;
wherein the switching control circuit comprises:
first and second inputs, configured for coupling to a positive and a negative terminals of the power switch and sensing a voltage across that power switch;
an output terminal, configured for outputting a signal to drive the power switch;
a voltage comparing unit, configured to act in response to the first and second inputs, wherein the voltage comparing unit is configured to output a logic signal according to the voltage difference between the sensed voltage drop across the power switch and a reference threshold voltage;
and
a logic processing circuit, coupled to the voltage comparing unit and configured to provide the first voltage signal and the second voltage signal;
where the logic process unit comprises:
an input terminal for receiving an input signal;
a delay module coupled to the input terminal;
an SR flip-flop coupled to the input terminal and the delay module;
a NAND logic gate coupled to the input terminal and the SR flip-flop;
and
a drive circuit coupled to the output of the logic processing unit and the output terminal of the synchronous rectification circuit,
wherein the delay module is configured to provide a delay time between the determination of the first voltage signal and the second voltage.

15 more elements

A synchronous rectification circuit for a power converter, comprising:

a power switch including a body diode and configured as a synchronous rectifier for coupling to a transformer and an output capacitor;
and
a switching control circuit configured to provide a control signal in response to a first voltage signal related to a voltage across the power switch and a second voltage signal which is a delayed version of the first voltage signal;
wherein the switching control circuit is configured to provide the control signal to the synchronous rectifier without any other-input signal that provides information about an operating state of the power converter;
wherein the switching control circuit comprises:
first and second inputs, configured for coupling to a positive and a negative terminals of the power switch and sensing a voltage across that power switch;
an output terminal, configured for outputting a signal to drive the power switch;
a voltage comparing unit, configured to act in response to the first and second inputs, wherein the voltage comparing unit is configured to output a logic signal according to the voltage difference between the sensed voltage drop across the power switch and a reference threshold voltage;
and
a logic processing circuit, coupled to the voltage comparing unit and configured to provide the first voltage signal and the second voltage signal;
wherein the voltage comparing unit comprises:
a comparator having two inputs;
and
two voltage divider circuits, each of the voltage divider circuits including:
a resistor string configured as a voltage divider and coupled to the power switch, the resistor string is configured to set a desired offset voltage and to cause the voltage input from the voltage comparing unit to fall into a common mode input range of the comparator;
and
a diode coupled to the resistor string and configured to prevent current back flow,
wherein each of the two inputs of the comparator is coupled to an internal node of each of the voltage dividers, respectively.

15 more elements

A switching control circuit for a synchronous rectifier, comprising:

first and second inputs, configured for coupling to a positive and a negative terminals of a power switch including a body diode and configured as a synchronous rectifier and sensing a voltage across that power switch;
an output terminal, configured for outputting a signal to drive the power switch;
a voltage comparing unit, configured to act in response to the first and second inputs, wherein the voltage comparing unit is configured to output a logic signal according to the voltage difference between the sensed voltage drop across the power switch and a reference threshold voltage;
and
a logic processing circuit, coupled to the voltage comparing unit and configured to provide a control signal according to a first voltage signal related to a voltage across the power switch and a second voltage signal which is a delayed version of the first voltage signal, wherein the first voltage signal and the second voltage signal each includes a vibrating portion caused by switching of the body diode and the control signal is free from a vibrating portion.

2 more elements

A synchronous rectification circuit for a power converter, comprising:

a power switch, coupled to a transformer and an output capacitor;
and
a switching control circuit configured to provide a control signal to the power switch in response to a first voltage signal related to a voltage across the power switch and a second voltage signal which is a delayed version of the first voltage signal;
wherein the switching control circuit comprises:
first and second inputs, configured for coupling to a positive and a negative terminals of the power switch and sensing a voltage across that power switch;
an output terminal, configured for outputting a signal to drive the power switch;
a voltage comparing unit, configured to act in response to the first and second inputs, wherein the voltage comparing unit is configured to output a logic signal according to the voltage difference between the sensed voltage drop across the power switch and a reference threshold voltage;
and
a logic processing circuit, coupled to the voltage comparing unit and configured to provide the first voltage signal and the second voltage signal, where the logic process unit comprises:
an input terminal for receiving an input signal;
a delay module coupled to the input terminal;
an SR flip-flop coupled to the input terminal and the delay module;
a NAND logic gate coupled to the input terminal and the SR flip-flop;
and
a drive circuit coupled to the output of the logic processing unit and the output terminal of the synchronous rectification circuit,
wherein the delay module is configured to provide a delay time between the determination of the first voltage signal and the second voltage signal.

13 more elements

Dependencies are inferred from the claim text.

Facts

Application: 12/604,246Filed 22 Oct 2009
Published: US 2011/0096578 A128 Apr 2011
Granted: 3 Sep 2013
Term ends: Not estimatedThe source records don’t support an estimate.

Classification12 codes

H02M 3/33592
Having at least one active switching element at the secondary side of an isolation transformer: having a synchronous rectifier circuit or a…
H03K 5/08
Shaping pulses: by limiting; by thresholding; by slicing, i.e. combined limiting and thresholding
H03K 17/133
Modifications for switching at zero crossing: in field-effect transistor switches
H02M 3/33561
Using semiconductor devices only: having more than one ouput with independent control