# Patent

## Title
Zero-crossing detection circuit

## Abstract

In one embodiment, a zero-crossing detection circuit can include: (i) a first detection circuit configured to detect a current through a main transistor of a main circuit of a switching power supply, and to generate a voltage sense signal that represents the current through the main transistor; (ii) a second detection circuit configured to detect if quasi-resonance occurs in the main circuit, the second detection circuit being configured to generate at least one pulse signal when the quasi-resonance is detected; and (iii) a control circuit configured to receive the at least one pulse signal and the voltage sense signal, to turn the main transistor off when the current through the main transistor reaches a predetermined value, and to turn the main transistor on when the at least one pulse signal is active.

## Background
RELATED APPLICATIONS
This application claims the benefit of Chinese Patent Application No. 201410039224.3, filed on Jan. 26, 2014, which is incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
The present invention generally relates to the field of power electronics, and more particularly to zero-crossing detection circuitry.
BACKGROUND
Switch mode power supplies can efficiently convert electrical power from a source to a load, or to several different loads, with each corresponding to a different output. The main transistor of a switching-mode supply can switch between on and off states at a given operating frequency, and voltage regulation can be achieved by varying the ratio of the on-to-off time of the main transistor. Switch mode power supplies may have relatively high power conversion efficiency, as compared to other types of power converters. Switch mode power supplies may also be substantially smaller and lighter than a linear supply due to the smaller transformer size and weight. Switch mode power supplies can be used in many applications, such as LED driver circuits.
SUMMARY
In one embodiment, a zero-crossing detection circuit can include: (i) a first detection circuit configured to detect a current through a main transistor of a main circuit of a switching power supply, and to generate a voltage sense signal that represents the current through the main transistor; (ii) a second detection circuit configured to detect if quasi-resonance occurs in the main circuit, the second detection circuit being configured to generate at least one pulse signal when the quasi-resonance is detected; and (iii) a control circuit configured to receive the at least one pulse signal and the voltage sense signal, to turn the main transistor off when the current through the main transistor reaches a predetermined value, and to turn the main transistor on when the at least one pulse signal is active.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of an example zero-crossing detection circuit, in accordance with embodiments of the present invention.
FIG. 2 is schematic block diagram of an example control circuit in a zero-crossing detection circuit, in accordance with embodiments of the present invention.
FIG. 3 is schematic block diagram of an example detection circuit in a zero-crossing detection circuit, in accordance with embodiments of the present invention.



DETAILED DESCRIPTION
Reference may now be made in detail to particular embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention may be described in conjunction with the preferred embodiments, it may be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents that may be included within the spirit and scope of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it may be readily apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, processes, components, structures, and circuits have not been described in detail so as not to unnecessarily obscure aspects of the present invention.
A switching power supply can generate a substantially stable output voltage by controlling a ratio of an on time and off time of a power switch (e.g., a power transistor) with a pulse-width modulation (PWM) control signal. Transistors (e.g., a bipolar-junction transistor [BJT], a MOS transistor, etc.) may be used as power switches in switching power supplies because of relatively low saturation voltage drop characteristics.
One type of switching power supply is a synchronous step-down converter. After a main switch (e.g., power transistor) is turned on at an initial time of each switching period, an inductor current may rise. Energy at an input terminal of the converter may be converted to magnetic energy to be stored in the inductor, and the main switch may be turned off after remaining on for a certain duty cycle. The inductor current may gradually decrease via a synchronous transistor, and the magnetic energy may be converted to electric energy to be released to an output terminal of the converter, thus completing a converter switching period. When a load (e.g., a light-emitting diode [LED], etc.) on the converter is relatively large, the inductor current may not return to zero during an entire switching cycle or period. In this case, the converter can generally operate in a continuous conduction mode (CCM). When the load is light, the inductor current may gradually decrease to zero during a “freewheeling” process of the synchronous transistor, and the synchronous transistor can be turned off. This can lower power consumption and improve conversion efficiency, and in this case, the converter can operate in a discontinuous conduction mode (DCM).
In DCM operation, a zero-crossing detection circuit may be utilized for determining whether/when the inductor current crosses zero. When the inductor current is detected to be zero by a zero-crossing comparator, the synchronous transistor may be turned off to lower power consumption, and to improve conversion efficiency. In a DC-DC converter, a zero-crossing detection circuit can be implemented by an RC differential circuit, an auxiliary winding, a float R-type circuit, and so on. When an RC differential circuit including a resistor and a capacitor coupled in series is used to detect a current, extra losses may be generated due to a current flowing through the resistor. Also, and product costs may be relatively high due to use of a high voltage capacitor to sustain a high voltage. When a zero-crossing detection circuit with an auxiliary winding is used to detect a current, circuit interference can occur, and circuit volume may be relatively high, also resulting in high product costs. When a float R-type circuit with a floated resistor is used to detect current, a control circuit applied may also be floated. As a result, the circuit can also suffer from interference, and product cost can be relatively high.
In particular embodiments, a zero-crossing detection circuit can detect if quasi-resonance occurs in a main circuit through a detection circuit. This can be based on quasi-resonance occurring when an inductor current in the main circuit crosses zero, and in response may generate a pulse at a quasi-resonance moment, in order to realize zero-crossing detection. Further, the zero-crossing detection circuit may have a relatively simple circuit structure, as well as relatively high detection precision. This approach may also have relatively low product costs as compared to circuits that directly detect the inductor current.
In one embodiment, a zero-crossing detection circuit can include: (i) a first detection circuit configured to detect a current through a main transistor of a main circuit of a switching power supply, and to generate a voltage sense signal that represents the current through the main transistor; (ii) a second detection circuit configured to detect if quasi-resonance occurs in the main circuit, the second detection circuit being configured to generate at least one pulse signal when the quasi-resonance is detected; and (iii) a control circuit configured to receive the at least one pulse signal and the voltage sense signal, to turn the main transistor off when the current through the main transistor reaches a predetermined value, and to turn the main transistor on when the at least one pulse signal is active.
Referring now to FIG. 1, shown is a schematic block diagram of an example zero-crossing detection circuit, in accordance with embodiments of the present invention. In this particular example, zero-crossing detection circuit 104 can detect if an inductor current in main circuit 102 crosses zero. In this example, main circuit 102 is a flyback circuit, and the inductor current to be detected may be a current through secondary winding Ns of transformer T. When main circuit 102 is implemented in other topologies (e.g., buck, boost, etc.), zero-crossing detection circuit 104 can detect if the inductor current crosses zero.
Zero-crossing detection circuit 104 may include detection circuit 106, control circuit 108, and detection circuit 110. Detection circuit 106 can be used to detect a current through main transistor Q of main circuit 102, in order to obtain a voltage sense signal (e.g., Vs) that represents the current through main transistor Q. Detection circuit 110 can be used to detect if quasi-resonance occurs in main circuit 102. For example, when quasi-resonance occurs, detection circuit 110 can generate a pulse signal (e.g., pulse signals fr), or a group of pulse signals.
Control circuit 108 can receive pulse signal fr and voltage sense signal Vs representing a current through main transistor Q. When the current through the main transistor increases to a level of a predetermined value, control circuit 108 can turn main transistor Q off. When pulse signal fr is active, control circuit 108 can turn main transistor Q on. For example, detection circuit 106 can include transistor Q1 and sampling resistor Rs with a terminal coupled to ground. Transistor Q1 can connect between main transistor Q and sampling resistor Rs.
Referring now to FIG. 2, shown is schematic block diagram of an example control circuit in a zero-crossing detection circuit, in accordance with embodiments of the present invention. For example, control circuit 108 can include comparator A1 and RS flip-flop 206. Comparator A1 may have a non-inverting input terminal connected to a first terminal of sampling resistor Rs for receiving voltage sense signal Vs, and an inverting input terminal that receives compensation voltage signal Vc. Also, an output terminal of comparator A1 can generate an off control signal at a reset terminal of RS flip-flop 206.
A set terminal of RS flip-flop 206 may receive an on control signal, and an output terminal can generate driving signal VQ1 to control on/off of transistor Q1. For example, when a last pulse signal (e.g., in every pulse signal group) is active, control circuit 108 can turn transistor Q1 on. Control circuit 108 can also include timer 204-1 and AND-gate 202. Timer 204-1 may have an input terminal that receives driving signal VQ1 for effectively delaying driving signal VQ1. For example, the delay of driving signal VQ1 can be such that a last pulse signal in a pulse signal group (e.g., a plurality of pulses) is utilized in generating a high level portion of driving signal VQ1.
AND-gate 202 may have input terminals that connect to an output terminal of timer 204-1 and pulse signals fr. An output terminal AND-gate 202 can generate the on control signal at the set terminal of RS flip-flop 206. Control circuit 108 can also include transconductance operational amplifier Gm and compensation capacitor Cc. Transconductance operational amplifier Gm may have an inverting input terminal for receiving feedback signal Vfb, which can represent an output voltage of main circuit 102. A non-inverting input terminal of transconductance operational amplifier Gm can receive reference voltage signal Vref, and an output terminal can connect to the inverting input terminal of comparator A1. Compensation capacitor Cc may be connected to the output terminal of transconductance operational amplifier Gm, and to ground.
Referring now to FIG. 3, shown is schematic block diagram of an example detection circuit in a zero-crossing detection circuit, in accordance with embodiments of the present invention. Detection circuit 110 can include driving resistor Rg2, switch S1, switch S2, transistor Q2, detection resistor Rd, comparator A2, delay circuit 302, and pulse generator 304. Driving resistor Rg2 can connect to DC voltage Vcc of main transistor Q, and to a gate of transistor Q2.
Switch S1 can connect to DC voltage Vcc, and to main transistor Q and transistor Q1, and may receive detection signal VQs, which can represent a source voltage of main transistor Q. Switch S1 can be controlled by switch control signal Ts1, where waveform of switch control signal Ts1 may be obtained by delaying driving signal VQ1. Switch S2 can connect to driving resistor Rg2, and to main transistor Q and transistor Q1. Switch S2 can be turned off by switch control signal Ts2, where the operating waveforms of switch control signal Ts2 and driving signal VQ1 may be complementary.
A source of transistor Q2 can receive DC voltage Vcc, and a drain can connect to detection resistor Rd. Detection resistor Rd may have a first terminal for generating detection signal Vz, and a second terminal connected to ground. Comparator A2 may have an inverting input terminal receiving detection signal Vz, a non-inverting input terminal receiving threshold signal Vzx, and an output terminal generating trigger signal Vzx. Pulse generator 304 can receive trigger signal Vzx (either directly or via delay circuit 302), and may generate pulse signal(s) fr. In this particular example, transistor Q2 is a P-type MOS transistor; however, other types of transistors can also be employed in certain embodiments. Detection circuit 110 can also include delay circuit 302. Delay circuit 302 can receive trigger signal Vzx, and may output delayed trigger signal Vzxd to pulse generator 304.
Referring back to FIG. 2, control circuit 108 can also include inverter 208. Inverter 208 can connect to an output terminal of RS flip-flop 206, and can generate switch control signal Ts2, which can control on/off of switch S2. Control circuit 108 may also include timer 204-2. Timer 204-2 may have an input terminal connected to an output terminal of RS flip-flop 206, and may generate switch control signal Ts1 to control on/off of switch S1.
Referring back to FIG. 1, parasitic capacitors CQ and CQ1 can connect in parallel between source and drain of main transistor Q, and the source and drain of transistor Q1, respectively. Main circuit 102 can include transformer T, an RCD snubber, freewheeling diode Df, output capacitor Co, main transistor Q, driving resistor Rg1, and DC voltage Vcc. For example, the RCD “snubber” may be utilised to suppress voltage and/or current transients.
Transformer T can include primary winding Np, secondary winding Ns, magnetizing inductance Lm, and leakage inductance Ls. The RCD snubber can connect to primary winding Np of transformer T, and magnetizing inductance Lm can connect to primary winding Np in parallel. Leakage inductance Ls may have a first terminal for receiving a DC input voltage of main circuit 102, and a second terminal connected to a first terminal of magnetizing inductance Lm.
The RCD snubber can include capacitor C, resistor R, and clamping diode D. Capacitor C and resistor R can connect in parallel with a first common node coupled to a first terminal of leakage inductance Ls, and a second common node coupled to a cathode of clamping diode D. An anode of clamping diode D can connect to a second terminal of magnetizing inductance Lm. Secondary winding Ns of transformer T, freewheeling diode Df, and output capacitor Co can be coupled in a loop, and a load (e.g., an LED load) can connect across terminals of output capacitor Co.
Main transistor Q may have a drain connected to a second terminal of magnetizing inductance Lm, a source connected to zero-crossing detection circuit 104, and a gate connected to a first terminal of driving resistor Rg1. A second terminal of driving resistor Rg1 can connect to DC voltage Vcc. For example, a DC voltage input circuit of main circuit 102 can include an AC voltage source and a rectifier bridge, as shown. An AC voltage output from the AC voltage source can be rectified by the rectifier bridge.
An output terminal of the rectifier bridge can generate a DC input voltage provided to the main circuit 102. The positive end of the DC input voltage can connect to the first terminal of leakage inductance Ls, and a negative end can connect to ground. Referring to FIGS. 1-3, when main transistor Q and transistor Q1 are turned on, primary winding Np of transformer T in main circuit 102 can begin storing energy. The inverting input terminal of transconductance operational amplifier Gm can receive feedback signal Vfb, which may represent the output voltage of the main circuit 102 (e.g., Vfb is a dividing signal of the output voltage). Also, the non-inverting input terminal of transconductance operational amplifier Gm can reference voltage signal Vref.
When the load at the output terminal of the main circuit 102 is relatively heavy, Vfb may be less than Vref. In this case, the current generated by transconductance operational amplifier Gm can charge compensation capacitor Cc, and a voltage across compensation capacitor (e.g., compensation voltage signal Vc) may be positive. Sampling resistor Rs can sense the current through main transistor Q, in order to generate voltage sense signal Vs. Thus, voltage sense signal Vs can represent the current through main transistor Q. When Vs increases (e.g., linearly) to reach a level of Vc, comparator A1 can output a high level, RS flip-flop 206 can be reset, and driving signal VQ1 output from RS flip-flop 206 can go low to turn off Q1.
When driving signal VQ1 is low, switch control signal Ts1 generated by timer 204-2 can be high, and switch control signal Ts2 generated via inverter 208 may also be at a high level. When switch control signals Ts1 and Ts2 received by detection circuit 110 are high, switches S1 and S2 can be on. In this case, the voltage of detection signal VQs representing the source voltage of main transistor Q can be equal to DC voltage Vcc, and the voltage of driving signal VQ2 of transistor Q2 can be equal to that of detection signal VQs. Thus, driving signal VQ2 may be equal to DC voltage Vcc, and transistor Q2 can be turned on.
As the voltage of detection signal VQs equals DC voltage Vcc, the gate-source voltage (VQg-VQs) of main transistor Q may be almost zero, too low to supply a driving voltage, and main transistor Q may be off. As a result, the current through main transistor Q can be zero, and Vs may be zero. In this case parasitic capacitors CQ and CQ1 respectively connected in parallel with main transistor Q and transistor Q1 can be in resonance with leakage inductance Ls, in order to generate a resonance current.
The inductor current through secondary winding Ns of transformer T can cross zero, the primary side circuit can begin to resonate, and current can flow through parasitic capacitor CQ, transistor Q2, and detection resistor Rd. Detection signal Vz can detect the current through transistor Q2 by detection resistor Rd, which can be the current through parasitic capacitor CQ of main transistor Q under a resonant state. For example, transistor Q2 can be implemented by a P-type MOS transistor; however, other types of transistors can also be employed in certain embodiments.
Comparator A2 may have an inverting input terminal that receives detection signal Vz, and a non-inverting input terminal that receives threshold signal Vth. When Vz is less than Vth, the primary-side circuit of transformer T can be in a resonant state. Parasitic capacitor CQ connected in parallel with main transistor Q can be in resonance with leakage inductance Ls, trigger signal Vzx output from comparator A2 can be high, and delayed trigger signal Vzxd can also be high. When Vzxd is at a high level, pulse signal generator 304 can generate pulse signal fr. When Vzxd again goes high, pulse generator 304 can generate another pulse signal fr. Thus, a pulse group or plurality of pulses can be generated by pulse generated 304.
Control circuit 108 can receive pulse signal(s) fr triggered during the resonance state. Driving signal VQ1 can be processed by timer 204-1, and then by a logical AND operation with pulse signal(s) fr. When both signals are high, RS flip-flop 206 can be set, and VQ1 may transition to a high level to turn on transistor Q1. Thus, the voltage of detection signal VQs can be equal to zero, transistor Q2 can be turned off, and main transistor control signal VQg at the gate of main transistor Q can be equal to Vcc. Therefore, main transistor Q can also be turned on, and voltage sense signal Vs can begin increasing to reach a level of Vc, which may result in VQ1 again going low. In this way, by repeating the above-described operation process, zero-crossing detection of main circuit 102 can be realized.
The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilise the invention and various embodiments with modifications as are suited to particular use(s) contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

## Claims

1. A zero-crossing detection circuit, comprising:
a) a first detection circuit configured to detect a current through a main transistor of a main circuit of a switching power supply, and to generate a voltage sense signal that represents said current through said main transistor, wherein said first detection circuit comprises a first transistor that is coupled to a source of said main transistor;
b) a second detection circuit configured to detect if quasi-resonance occurs in said main circuit, said second detection circuit being configured to generate at least one pulse signal when said quasi-resonance is detected; and
c) a control circuit configured to receive said at least one pulse signal and said voltage sense signal, to turn said main transistor off when said current through said main transistor reaches a predetermined value, and to turn said main transistor on by turning said first transistor on when said at least one pulse signal is active.

2. The zero-crossing detection circuit of claim 1, wherein said first detection circuit comprises a sampling resistor coupled between said first transistor and ground.

3. The zero-crossing detection circuit of claim 2, wherein said control circuit comprises:
a) a first comparator having a non-inverting input terminal coupled to receive said voltage sense signal across said sampling resistor, an inverting input terminal coupled to receive a compensation voltage signal, and an output terminal configured to generate an off control signal; and
b) an RS flip-flop having a reset terminal coupled to receive said off control signal, a set terminal coupled to receive an on control signal, and an output terminal configured to generate a first driving signal to control said first transistor.

4. The zero-crossing detection circuit of claim 3, wherein said control circuit is configured to turn said first transistor on in response to a last of said at least one pulse signal.

5. The zero-crossing detection circuit of claim 4, wherein said control circuit further comprises:
a) a first timer configured to delay said first driving signal such that only a last pulse signal of said at least one pulse signal is in a high level portion of said first driving signal; and
b) an AND-gate having input terminals coupled to an output terminal of said first timer and said at least one pulse signal, and an output terminal configured to generate said on control signal.

6. The zero-crossing detection circuit of claim 3, wherein said control circuit further comprises:
a) a transconductance operational amplifier having an inverting input terminal configured to receive a feedback signal, a non-inverting input terminal configured to receive a reference voltage signal, and an output terminal coupled to said non-inverting input terminal of said first comparator; and
b) a compensation capacitor having a first terminal coupled to said output terminal of said transconductance operational amplifier, and a second terminal coupled to ground.

7. The zero-crossing detection circuit of claim 3, wherein said second detection circuit comprises:
a) a driving resistor having a first terminal coupled to a DC voltage supply of said main transistor, and a second terminal coupled to a gate of a second transistor;
b) a first switch having a first terminal coupled to said DC voltage supply, and a second terminal coupled to said main transistor and said first transistor and configured to receive said first detection signal, wherein said first switch is controllable by a first switch control signal;
c) a second switch having a first terminal coupled to said second terminal of said driving resistor, and a second terminal coupled to said main transistor and said first transistor, wherein said second switch is controllable by a second switch control signal, wherein said second switch control signal and said first driving signal are complementary;
d) said second transistor having a source configured to receive said DC voltage supply, and a drain coupled to said first terminal of a detection resistor, said detection resistor having a first terminal configured to generate said second detection signal, and a second terminal coupled to ground;
e) a second comparator having an inverting input terminal configured to receive said second detection signal, a non-inverting input terminal configured to receive a threshold signal, and an output terminal configured to generate a trigger signal; and
f) a pulse generator having an input terminal configured to receive said trigger signal, and an output terminal configured to generate said at least one pulse signal.

8. The zero-crossing detection circuit of claim 7, wherein said second detection circuit further comprises a delay circuit having an input terminal configured to receive said trigger signal, and an output terminal configured to generate a delayed trigger signal.

9. The zero-crossing detection circuit of claim 7, wherein said control circuit further comprises an inverter having an input terminal coupled to said output terminal of said RS flip-flop, and an output terminal configured to generate said second switch control signal.

10. The zero-crossing detection circuit of claim 7, wherein said control circuit further comprises a second timer having an input terminal coupled to said output terminal of said RS flip-flop, and an output terminal configured to generate said first switch control signal.

11. The zero-crossing detection circuit of claim 1, wherein said main transistor is configured in a flyback switching power supply topology.

12. The zero-crossing detection circuit of claim 11, wherein said main circuit comprises:
a) a transformer having a primary winding, a secondary winding, a magnetizing inductance, and a leakage inductance, said magnetizing inductance being coupled to said primary winding, said leakage inductance having a first terminal configured to receive a DC input voltage of said main circuit, and a second terminal coupled to a first terminal of said magnetizing inductance;
b) an RCD snubber coupled to said primary winding, said RCD snubber comprising a capacitor, a resistor, and a clamping diode, wherein said capacitor and said resistor are coupled in parallel with a first common node coupled to a first terminal of said leakage inductance, and a second common node coupled to a cathode of said clamping diode, and wherein an anode of said clamping diode is coupled to a second terminal of said magnetizing inductance;
c) said secondary winding of said transformer, a freewheeling diode, and an output capacitor being coupled in a loop, wherein a load is coupled across said output capacitor; and
d) said main transistor having a drain coupled to said second terminal of said magnetizing inductance, a source coupled to said zero-crossing detection circuit, and a gate coupled to a first terminal of a driving resistor, wherein a second terminal of said driving resistor is coupled to a DC voltage.

13. The zero-crossing detection circuit of claim 1, wherein a drain of said first transistor is connected to said source of said main transistor.

14. The zero-crossing detection circuit of claim 13, wherein said drain of said first transistor is directly connected to said source of said main transistor without any other devices therebetween.

15. The zero-crossing detection circuit of claim 1, wherein a gate of said main transistor is coupled to a DC voltage supply through a driving resistor.

16. The zero-crossing detection circuit of claim 15, wherein a first terminal of said driving resistor is directly connected to said DC voltage supply, and a second terminal of said driving resistor is directly connected to said gate of said main transistor.

17. The zero-crossing detection circuit of claim 1, wherein a drain of said main transistor is coupled to a transformer.

18. The zero-crossing detection circuit of claim 17, wherein said drain of said main transistor is directly connected to a primary winding of said transformer.