INTEGRATED SWITCH MODE POWER SUPPLY CONTROLLER AND SWITCH MODE POWER SUPPLY USING THE SAME

[0001] This application claims the benefit of Chinese Patent Application No. 201310017681.8, filed on Jan. 17, 2013, which is incorporated herein by reference in its entirety.

[0002] The present invention relates to power supplies, and more particularly to an integrated switch mode power supply controller, and an associated switch mode power supply.

[0003] 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.

[0004] In one embodiment, an integrated switch mode power supply controller can include: (i) a multiplexing pin configured to receive a detection voltage signal; (ii) a switch mode power supply configured to receive a DC input voltage, and to operate in a switching cycle having first, second, and third time intervals; (iii) where during the first time interval, the detection voltage signal is proportional to the DC input voltage, and a current compensation signal is configured to be generated according to the detection voltage signal to obtain a peak current flowing through an inductor in the switch mode power supply; (iv) where during the second time interval, the detection voltage signal is proportional to an output voltage of the switch mode power supply, and a discharging duration of current through the inductor is determined based on the detection voltage signal; and (v) where during the third time interval, the detection voltage signal is proportional to a voltage across a power transistor of the switch mode power supply, the power transistor being turned on at a local minimum according to the detection voltage signal.

[0009] 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.

[0010] Integrated circuits or “chips” are widely used in various electronic devices or modules, and such chips are becoming increasingly miniaturized and portable. As a result, increased performance and size requirements (e.g., relatively high density and levels of integration, along with relatively small size) come into play in many applications. In one application, a number of controller chips and an integrated driving circuit can be realized by utilising a relatively small number of peripheral circuits.

[0011] Referring now to FIG. 1, shown is a schematic block diagram of an example switch mode power supply. In this particular case, the switch mode power supply is realized by an isolated flyback converter with an output current regulating circuit. The main structure or power stage circuit of the switch mode power supply can thus utilize a flyback topology, and may include transformer T, power transistor Q, output diode D_OUT, and output capacitor C_OUT. Controller chip 101 can utilize a peak current control method, and may control the periodic turning-on and turning-off of power switch (e.g., transistor) Q to maintain output current I_OUT as substantially constant.

[0012] Detection resistor R_CS can connect between power switch Q and ground to sample current flowing through primary side winding N_P of transformer T. Sample signal I_CS can be provided at pin CS of controller chip 101 via voltage V_CS. Auxiliary winding N_A can be coupled to transformer T, and may be used to sample current information of secondary side winding N_S. Voltage signal V_AUX can be used to obtain detection voltage V_S via a divider resistor network formed by resistors R₁ and R₂. Detection voltage signal V_S can be provided to controller chip 101 by way of pin FB. In addition, voltage signal V_AUX can be filtered by diode D₀ and capacitor C₀, and a voltage can be generated to supply power to controller chip 101 via pin VCC.

[0013] Due to signal transfer delay of controller chip 101, as well as circuit delays due to control logic and driving, the peak current of primary side winding N_P and the peak current obtained by detection voltage V_CS on detection resistor R_CS can be different. Thus, the peak value as indicated by detection voltage V_CS may not match the actual peak current of primary side winding N. For example, the timing of the actual peak event of primary side winding N_P may be different than that represented by detection voltage V_CS. Inaccurate peak current can result in various adverse control effects, and may influence the adjustment accuracy of output current I_OUT. Therefore, this example switch mode power supply can also include compensation circuitry to compensate for the peak current. Specifically, resistors R_FF1 and R_FF2 can connect to DC input voltage V_PWR, power switch device Q, and detection resistor R_CS, in order to compensate for detection voltage V_c. The voltage on the common node of resistors R_FF1 and R_FF2 can be utilized as a compensated correct peak current signal, and may be provided to controller chip 101 via pin COMP.

[0014] Controller chip 101 can utilise current information and reference voltage V_REF that indicates a desired output current/voltage to provide driving signal DRV through pin SW. For example, the switch state of power switch Q can be controlled such that output current I_OUT can be at or above a predetermined (e.g., reference) level, even when influenced by various factors (e.g., load conditions, etc.). However, drawbacks of this approach include the number of pins, including compensation pin COMP, and a relatively complex control solution, which can work against chip minimization goals, and may increase production costs.

[0015] In one embodiment, an integrated switch mode power supply controller can include: (i) a multiplexing pin configured to receive a detection voltage signal; (ii) a switch mode power supply configured to receive a DC input voltage, and to operate in a switching cycle having first, second, and third time intervals; (iii) where during the first time interval, the detection voltage signal is proportional to the DC input voltage, and a current compensation signal is configured to be generated according to the detection voltage signal to obtain a peak current flowing through an inductor in the switch mode power supply; (iv) where during the second time interval, the detection voltage signal is proportional to an output voltage of the switch mode power supply, and a discharging duration of current through the inductor is determined based on the detection voltage signal; and (v) where during the third time interval, the detection voltage signal is proportional to a voltage across a power transistor of the switch mode power supply, the power transistor being turned on at a local minimum according to the detection voltage signal.

[0016] Referring now to FIG. 2A, shown is a schematic block diagram of an example switch mode power supply with a controller, in accordance with embodiments of the present invention. In this particular example, switch mode power supply 200 can include a power stage circuit and integrated switch power controller 202. For example, integrated switch power controller 202 may be a chip or a circuit module, and may be integrated with one or more other elements of switch mode power supply 200, such as on a same chip or on a printed-circuit board (PCB). In this case, the power stage circuit can employ a flyback topology. A flyback converter can be used in AC/DC or DC/DC conversion with galvanic isolation between the input and any outputs. The flyback converter is typically a boost converter with the inductor split to form a transformer (e.g., T) such that the voltage ratios are multiplied with an additional advantage of isolation. While a flyback topology is shown in this particular example, any other suitable power stage circuit topologies (e.g., buck, buck-boost, Sepic, etc.) can be employed in certain embodiments.

[0017] Integrated switch power controller 202 can include multiplexing pin FB. A “pin” can provide an external connection to the integrated circuit, module, circuit portion, or chip that forms integrated switch power controller 202. Multiplexing pin FB can receive detection voltage signal V_S generated by auxiliary winding N_A and the resistor divider network (e.g., resistors R₁ and R₂). For example, integrated switch power controller 202 can control the power stage circuit (e.g., including transistor Q) to operate in a quasi-resonant mode according to detection voltage signal V_S, and the inductance current can be in a discontinuous current mode (DCM). In DCM operation, the current through the inductor (e.g., primary winding N_P or secondary winding N_S) can fall to zero during a portion of the switching cycle.

[0018] In a quasi-resonant zero-current/zero-voltage switch (ZCS/ZVS), each switch cycle can deliver a quantized “packet” of energy to the converter output, and switch (e.g., transistor Q) turn-on and turn-off can occur at zero current and/or voltage, potentially resulting in a lossless switch. Quasi-resonant switching or “valley switching” can reduce electromagnetic interference (EMI) in the power supply. For example, by switching the transistor when the voltage across the transistor is at a minimum (e.g., in the valley or “bottom”) can minimize hard switching effects that may cause EMI. Also, by switching when a valley is detected, rather than at a fixed frequency, a natural frequency jitter can spread out the radio frequency (RF) emissions spectrum, and may reduce overall EMI.

[0019] Referring now to FIG. 2B, shown is a waveform diagram of example operation of the switch mode power supply shown in FIG. 2A, in accordance with embodiments of the present invention. Each working period or switching cycle of switch mode power supply 200 can be divided into three time intervals based on the operating states of power switch device Q and transformer T. During the first time interval (e.g., from time t₀ to time t₁), when power transistor Q is turned on, the inductance current flowing through primary side winding N_P can continuously rise, and primary side winding N_P can continuously store energy. During this first time interval, a value (e.g., absolute value) of detection voltage signal V_S can be determined as indicated below by formula (1).

$\begin{array}{cc}V_S=V_{\mathrm{PWR}}\times \frac{N_A}{N_P}\times \frac{R_2}{R_1+R_2}& \left(1\right)\end{array}$

[0020] Since the number of turns of primary side winding N_P and auxiliary winding N_A, as well as the values of resistors R₁ and resistor R₂ are constant values, detection voltage signal V_S may be considered as proportional to input voltage V_PWR of the switch mode power supply during this time interval.

[0021] Also during the first time interval, the inductance current flowing through primary side winding N_P of transformer T can generate a current sample signal V_CS on detection resistor R_CS connected between power transistor Q and ground. Current sample signal V_CS can be provided to integrated switch power controller 202 via pin CS. Due to inherent delays between the control and driving circuitry, current sample signal V_CS may not accurately indicate the inductance current peak of primary side winding N. Thus in particular embodiments, integrated switch mode power supply controller 202 can compensate for current sample signal V_CS based on detection voltage signal V_S. In this way, an accurate voltage signal (e.g., accurate in time and/or amplitude) that denotes the peak current information of the primary side winding N_P can be obtained.

[0022] During the second time interval (e.g., from time t₁ to time t₂), when power switch device Q is turned off, the stored energy of primary side winding N_P of transformer T can be transformed to secondary side winding N_S. The inductance current of secondary side winding N_S can continuously decrease until reaching zero, and the stored energy of secondary side winding N_S can be transferred to the load (e.g., at V_OUT). Primary side winding N_P can thus be within a demagnetization time interval, and the value of the detection voltage signal V_S can be approximated as below in formula (2).

$\begin{array}{cc}V_S=\frac{N_A}{N_S}\times V_{\mathrm{OUT}}\times \frac{R_2}{R_1+R_2}& \left(2\right)\end{array}$

[0023] Since the number of turns of secondary side winding N_S and auxiliary winding N_A, as well as the values of resistors R₁ and R₂ are constant values, detection voltage signal V_S may be proportional to output voltage V_OUT at the output of the power stage circuit.

[0024] Therefore, whether output voltage V_OUT is in an overvoltage state (e.g., when output voltage V_OUT is raised above its upper design or a predetermined limit) can be determined according to detection voltage signal V_S and a reference voltage (to provide the predetermined limit). Thus, when output voltage V_OUT is in an overvoltage state, power transistor Q can be turned off. In addition, during the second time interval, the length or duration of the discharge time of the inductance current of secondary side winding N_S can also be obtained based on detection voltage signal V_S.

[0025] During the third time interval (e.g., from time t₂ to time t₄), after the inductance current of secondary side winding N_S is reduced to zero, power transistor Q is off, and resonance can occur at the inductance of primary side winding N_P, power transistor Q, and parasitic capacitance. For example, if power transistor Q is a MOSFET transistor, detection voltage signal V_S can denote drain-source voltage V_DS of power transistor Q. During the resonance time, the value of detection voltage signal V_S can be determined as indicated in formula (3) below.

$\begin{array}{cc}V_S=\frac{N_A}{N_S}\times \left(V_{\mathrm{OUT}}+V_{\mathrm{DS}}\right)\frac{R_2}{R_1+R_2}& \left(3\right)\end{array}$

[0026] Since the number of turns of auxiliary winding N_A and secondary side winding N_S, and the values of resistors R₁ and R₂ are constant values, output voltage V_OUT can be maintained as substantially constant. Also, the amplitude of detection voltage signal V_S can be proportional to, or approximated as, drain-source voltage V_DS of power transistor Q. By using a “bottom moment” or minimum (e.g., a local minimum or valley) of drain-source voltage V_DS as determined by detection voltage signal V_S, power transistor Q can be turned on at this local minimum time. In this way, quasi-resonant switching can be realized, and may reduce turn-on power consumption of power transistor Q and enhance overall system efficiency (e.g., by reducing EMI).

[0027] In particular embodiments, the integrated switch power controller can receive a detection voltage signal (e.g., V_S) of the auxiliary winding via a multiplexing pin (e.g., FB). Quasi-resonant switching or control, peak current compensation, and overvoltage protection, can be realized, as described above. In addition, the power switch device/transistor can be turned on based on a local minimum voltage or valley/bottom voltage across its drain and source (V_DS). This approach can reduce power consumption, protect the system, and enhance system reliability. The use of the multiplexing pin can also reduce the number of chip pins, and the size of the chip, and production costs.

[0028] Referring now to FIG. 3, shown is a schematic block diagram of an example integrated switch power controller, in accordance with embodiments of the present invention. Example integrated switch power controller 300 can include multiplexing pin FB, output pin SW, and current sample pin CS. Current sample pin CS can receive current sample signal V_CS that may be obtained via sample resistor R_CS, as shown in FIG. 2A.

[0029] Multiplexing pin FB can receive detection voltage signal V_S that may be obtained by auxiliary winding N_A. Integrated switch power controller 300 can perform a corresponding process on detection voltage signal V_S, and power transistor Q in the power stage circuit can be driven by driving signal V_G via output pin SW. In this way, the output electrical signal of the power-stage circuit can be maintained as substantially constant. For example, the control mode of integrated switch mode power supply controller 300 may be a peak current control mode, while the driving mode of the power stage circuit can be a quasi-resonant driving mode.

[0030] Detection voltage signal V_S received by multiplexing pin FB can pass through at least four “paths” as part of the process performed by integrated switch power controller 300 on detection voltage signal V_S. In a first path (discharge time sampling), referring back to FIG. 2B, within the second time interval (e.g., time t₁ to time t₂), inductance current I_NS of secondary winding N_S of transformer T may gradually reduce to zero. Since output current I_GUT of switch mode power supply 200 is the average value of inductance current I_NS, the peak value of inductance current I_NS can be inferred according to the current sample signal V_CS. Thus if the discharge time length T_DIS (time t₁ to time t₂), or the discharge duration, can be obtained, then the value of output current I_GUT can be accordingly calculated.

[0031] In one example, the falling time point (e.g., the time at which the signal begins to decrease) of voltage signal V_S can be determined during each switch period (e.g., time t₂), and the discharge duration (e.g., R_DIS) of each switch cycle can be inferred therefrom. Various implementations of discharge time sample circuit 301 (see, e.g., Chinese patent application CN201210233135.3) can be supported in particular embodiments.

[0032] In a second path (peak current compensation) of processing detection voltage signal V_S, peak current information of primary winding N_P can be obtained via current sample pin CS, and can be compensated by detection voltage signal V_S. In this way, errors generated due to inherent delays of the system can be compensated for to obtain correct or accurate peak current information. Current compensation circuit 302 can receive detection voltage signal V_S to generate corresponding current compensation signal V_COMP to output to peak current generating circuit 303. Peak current generating circuit 303 can receive a pulse-width modulation (PWM) control signal, and may superimpose current compensation signal V_COMP and current sample signal V_CS received via current sample pin CS. Peak sampling can be performed on the superimposed calculation result in order to obtain peak current I_PK.

[0033] Average current generating circuit 304 can calculate a current average value I_AVG according to discharge time length or duration T_DIS and peak current I_PK. Here, current average value I_AVG may denote output current information of the switch mode power supply. Error calculation circuit 305 can receive current average value I_AVG and current reference I_REF denoting a desired output current level, and may calculate an error therebetween to obtain error signal V_ERROR. Turning-off (OFF) signal generating circuit 306 can generate turn-off control signal S_OFF according to error signal V_ERROR. Any suitable implementation of current compensation circuit 302 (see, e.g., Chinese patent application CN201210216958.5), average current generating circuit 304, error calculation circuit 305, and OFF signal generating circuit 306 (see, e.g., Chinese patent application CN201210233135.3) can be supported in particular embodiments.

[0034] In a third path (bottom turning-on power device) of processing detection voltage signal V_S, bottom/valley or minimum voltage level detection circuit 307 can receive detection voltage signal V_S, and detect a zero time of detection voltage signal V_S (e.g., as time t₃ in FIG. 2B). After a certain or predetermined time delay, power switch device Q can again be turned on at time t₄ (e.g., by V_G going high). Bottom detection circuit 307 can also include a comparator that receives detection voltage signal V_S and the zero indication, and may generate zero signal S_ZERO. ON signal generating circuit 308 can generate ON signal S_ON after a certain or predetermined time delay according to zero signal S_ZERO. For example, ON signal generating circuit 308 can include a single-pulse or one-shot generating circuit.

[0035] In a fourth path (output voltage overvoltage protection) of processing detection voltage signal V_S, within each switch period (e.g., from time t₁ to time t₂ of FIG. 2B) detection voltage signal V_S can be approximately proportional to output voltage V_OUT. Thus, overvoltage detection circuit 309 in overvoltage protection circuit 313 can compare detection voltage signal V_S against reference voltage V_REF to determine the overvoltage condition. For example, overvoltage detection circuit 309 can include a comparator, and an overvoltage signal S_OVP can be generated after the comparison result passes through overvoltage signal generating circuit 310. For example, overvoltage signal generating circuit 310 can include a single-pulse generating circuit.

[0036] Logic circuit 311 can receive OFF signal S_OFF, ON signal S_ON and overvoltage signal S_OVP, and may generate control signal PWM. When output voltage V_OUT is not in an overvoltage condition, ON signal S_ON can control power switch device Q to turn on at time t₀. OFF signal S_OFF can control power switch device Q to be turned off at time t₁. When output voltage V_OUT is in an overvoltage condition, overvoltage signal S_OVP can control logic circuit 311 to disable the switch pulse of control signal PWM. Otherwise, integrated switch power controller 300 might enter into a locked state until the switch mode power supply is turned on again.

[0037] When the load is a light-emitting diode (LED), the power supply system can be protected by using overvoltage protection circuit 313 during abnormal conditions, such as when the LED is short-circuited or open-circuited. Driving circuit 312 can receive control signal PWM, and may generate a corresponding driving signal V_G to control the switching actions of power switch device Q via output pin SW.

[0038] Various example implementations of an integrated switch power controller in particular embodiments, and a switch mode power supply using the same, have been described herein. Those skilled in the art will recognize that the present invention is not limited to the above disclosed embodiments, and other suitable circuit structures based on the principles of the present invention are also suitable for embodiments of the present invention. For example, the topology of the switch mode power supply is not limited to flyback topology, and any suitable topology, such as other isolated or non-isolated topologies, or also applicable to the present invention.

[0039] 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 utilize the invention and various embodiments with modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.