EP4338275A1

EP4338275A1 - Power supply circuit, driver and controlling method

Info

Publication number: EP4338275A1
Application number: EP21947823.7A
Authority: EP
Inventors: Shuanghong Wang; Jianfeng Zhang; Hai ZHENG; Jiaqi Yang
Original assignee: Tridonic GmbH and Co KG
Current assignee: Tridonic GmbH and Co KG
Priority date: 2021-06-29
Filing date: 2021-08-10
Publication date: 2024-03-20
Also published as: CN117597857A; WO2023272885A1; EP4338275A4

Abstract

A power supply circuit, a driver and a controlling method. The power supply circuit includes: a buck circuit, configured to convert input DC (direct current) voltage into output DC voltage, the buck circuit comprising a diode, an inductor, a capacitor and a switch, the output DC voltage being outputted from output ports (VOUT+, VOUT -); a controller (MCU), configured to output a controlling signal (PWM signal) according to a feedback voltage (Vout), the feedback voltage being obtained by a voltage detection circuit being connected between one output port and ground port; and a MOS Drive circuit, configured to drive the switch of the buck circuit according to the controlling signal, an off state time (Toff) for each cycle of the controlling signal is set according to an output compare (OC) reference voltage.

Description

POWER SUPPLY CIRCUIT, DRIVER AND CONTROLLING METHOD

TECHNICAL FIELD

Embodiments of the present disclosure generally relate to the field of lighting, and more particularly, to a power supply circuit, a driver and a controlling method.

BACKGROUND

This section introduces aspects that may facilitate better understanding of the present disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
A buck circuit includes a diode, an inductor, a capacitor and a switch. A controlling signal may control the switch to be on and off, so that the buck circuit can convert an input voltage into an output voltage, which is lower than the input voltage. The controlling signal may be PWM (Pulse Width Modulation) signal.
SUMMARY
In the related art, an auxiliary winding is used to detect a current flowing through the inductor of the buck circuit, and zero cross detecting is performed on the detected current so as to control the switch according to the zero cross detecting result.
The inventor found that, the auxiliary wind will cause high cost in buck circuit.
In general, embodiments of the present disclosure provide a power supply circuit, a driver and a controlling method. In the embodiments, the power supply circuit uses an output compare (OC) reference voltage to set an off state time (Toff) for each cycle of the controlling signal, no auxiliary winding is needed to control the switch, thus to reduce the cost of buck circuit.
In a first aspect, there is provided a power supply circuit, includes:
a buck circuit, configured to convert input DC (direct current) voltage into output DC voltage, the buck circuit including a diode, an inductor, a capacitor and a switch, the output DC voltage being outputted from output ports (VOUT+, VOUT -) ;
a controller (MCU) , configured to output a controlling signal (PWM signal) according to a feedback voltage (Vout) , the feedback voltage being obtained by a voltage detection circuit being connected between one output port and a ground port; and
a MOS Drive circuit, configured to drive the switch of the buck circuit according to the controlling signal,
an off state time (Toff) for each cycle of the controlling signal is set according to an output compare (OC) reference voltage.
In one embodiment, an on state time (Ton) for each cycle of the controlling signal is determined by a detected peak current (Ipeak) that flows through the switch.
In one embodiment, the power supply circuit further includes:
a current peak limiter, configured to detect the peak current, and send a break signal to a break pin of the controller, the break signal makes the controlling signal to fall into a low level, so as to end the on state.
In one embodiment, the current peak limiter includes a first comparator (COMP_break) , which compares a voltage corresponding to the peak current with a first reference voltage (Ref) , when the voltage corresponding to the peak current is higher than the first reference voltage, the break signal with low level is generated by the first comparator.
In one embodiment, the controller includes:
a first timer (Tim 1) , configured to define each cycle of the controlling signal by counting, when the first timer is reset, a new cycle starts; and
a second timer (Tim 2) , configured to be reset when the off state of the controlling signal starts,
when a count value of the second timer reaches to a value corresponding to the output compare (OC) reference voltage, the first timer is reset, so as to end the off state of the controlling signal.
In one embodiment, when the controller controls the buck circuit to work under a CCM (continuous conduction mode) or a BCM (borderline conduction mode) , the off state time (Toff) is determined by a fixed ΔIpeak, inductance of the inductor and the feedback voltage (Vout) ,
wherein, the ΔIpeak represents the difference between an Ipeak upper limit and an Ipeak lower limit.
In one embodiment, when the controller controls the buck circuit to work under a DCM (discontinuous conduction mode) , the off state time (Toff) is determined by a fixed ΔIpeak, inductance of the inductor, an fixed output current (iLED) , the input DC (direct current) voltage (Vin) and the feedback voltage (Vout) , wherein, the ΔIpeak equals to an Ipeak upper limit.
In a second aspect, there is provided a driver, used for driving a lighting device, the driver includes the power supply circuit according to the first aspect of embodiments and a control circuit, the power supply circuit providing power to the lighting device, the control circuit communicates with the power supply circuit, the control circuit communicates with a peripheral device.
In a second aspect, there is provided a controlling method of a power supply circuit of the first aspect of embodiments. The controlling method including:
setting an off state time (Toff) for each cycle of the controlling signal according to output compare (OC) reference voltage.
According to various embodiments of the present disclosure, an off state time (Toff) for each cycle of the controlling signal is set according to an output compare (OC) reference voltage, no auxiliary winding is needed to control the switch, thus to reduce the cost of buck circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and benefits of various embodiments of the disclosure will become more fully apparent, by way of example, from the following detailed description with reference to the accompanying drawings, in which like reference numerals or letters are used to designate like or equivalent elements. The drawings are illustrated for facilitating better understanding of the embodiments of the disclosure and not necessarily drawn to scale, in which:
Fig. 1 is a diagram of a power supply circuit in accordance with the first aspect of embodiments of the disclosure;
Fig. 2 is a simple circuit topology of the power supply circuit in accordance with the first aspect of embodiments of the disclosure;
Fig. 3 is a full circuit topology of the power supply circuit in accordance with the first aspect of embodiments of the disclosure;
Fig. 4 is a diagram of the controller MCU in accordance with the first aspect of embodiments of the disclosure;
Fig. 5 is a diagram of working principle of the controller MCU in accordance with the first aspect of embodiments of the disclosure;
Fig. 6 is a Timing Sequence Diagram when the buck circuit works under a CCM or a BCM;
Fig. 7 is a Timing Sequence Diagram when the buck circuit works under a DCM;
Fig. 8 is a diagram of control method for Toff control with fixed ΔIpeak (or Ipeak) in DCM;
Fig. 9 is a diagram of state machine of Toff control for power supply circuit in accordance with the first aspect of embodiments of the disclosure;
Fig. 10 is a diagram of the diver.

DETAILED DESCRIPTION

The present disclosure will now be discussed with reference to several example embodiments. It should be understood that these embodiments are discussed only for the purpose of enabling those skilled persons in the art to better understand and thus implement the present disclosure, rather than suggesting any limitations on the scope of the present disclosure.
As used herein, the terms “first” and “second” refer to different elements. The singular forms “a” and “an” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises, ” “comprising, ” “has, ” “having, ” “includes” and/or “including” as used herein, specify the presence of stated features, elements, and/or components and the like, but do not preclude the presence or addition of one or more other features, elements, components and/or combinations thereof. The term “based on” is to be read as “based at least in part on. ” The term “one embodiment” and “an embodiment” are to be read as “at least one embodiment. ” The term “another embodiment” is to be read as “at least one other embodiment. ” Other definitions, explicit and implicit, may be included below.
First aspect of embodiments
A power supply circuit is provided in a first embodiment.
Fig. 1 is a diagram of a power supply circuit in accordance with the first aspect of embodiments of the disclosure. Fig. 2 is a simple circuit topology of the power supply circuit in accordance with the first aspect of embodiments of the disclosure. Fig. 3 is a full circuit topology of the power supply circuit in accordance with the first aspect of embodiments of the disclosure.
As shown in Fig. 1, Fig. 2 and Fig. 3, a power supply circuit 1 includes:
a buck circuit 10, configured to convert input DC (direct current) voltage (received from input ports INPUT+ and INPUT-) into output DC voltage, the buck circuit 10 includes a diode, an inductor, a capacitor and a switch Q1, the output DC voltage being outputted from output ports (VOUT+, VOUT -) ;
a controller (MCU, also denoted as 20) , configured to output a controlling signal (PWM signal) according to a feedback voltage (Vout) , the feedback voltage is obtained by a voltage detection circuit 21 being connected between one output port (VOUT-) and a ground port; and
a MOS Drive circuit 30, configured to drive the switch Q1 of the buck circuit 10 according to the controlling signal.
As shown in Fig. 3, the MOS Drive circuit 30 may drive the switch Q1 according to the controlling signal. For example, the MOS Drive circuit 30 includes a Totem pole circuit.
In Fig. 3, usage of each pin of the controller MCU is explained as follows:
Vin: DC input voltage detection.
Vout: LED output voltage detection, for example, the Vout pin may receive the feedback voltage (Vout) .
Isns: Rshunt mean voltage detection
PWM: MOS Gate Driver (to the MOS Drive circuit) , for example, the controlling signal may be outputted from the PWM pin.
BREAK: PWM breaker trigger, for example, a break signal maybe received by the BREAK pin.
Ref: Current limit reference, for example, the Ref pin receives a first reference voltage.
In at least one embodiment, an off state time (Toff) for each cycle of the controlling signal is set according to an output compare (OC) reference voltage.
In at least one embodiment, an on state time (Ton) for each cycle of the controlling signal is determined by a detected peak current (Ipeak) that flows through the switch Q1.
As shown in Fig. 3, the power supply circuit 1 further includes:
a current peak limiter 40, configured to detect the peak current, and send the break signal to a break pin (i.e., the BREAK pin in FIG. 3) of the controller 20, the break signal makes the control signal to fall into a low level, so as to end an on state of the switch Q1.
As shown in Fig. 3, the current peak limiter 40 includes a first comparator (COMP_break) 41, which compares a voltage corresponding to the peak current and the first reference voltage (denoted as Ref, which is also received by the Ref pin of the controller 20) . When the voltage corresponding to the peak current is higher than the first reference voltage, the break signal with low level is generated by the first comparator. Therefore, the first reference voltage defines the Ipeak upper limit. The voltage corresponding to the peak current can be obtained by a resistor denoted as R-shunt 1.
Fig. 4 is a diagram of the controller MCU in accordance with the first aspect of embodiments of the disclosure. Fig. 5 is diagram of working principle of the controller MCU in accordance with the first aspect of embodiments of the disclosure.
As shown in Fig. 4, number ① represents “Falling edge trigger” . Number ②represents “Capable for external reference or internal DAC module” , which means the comparator COMP in the controller MCU shown in Fig. 4 may replace the first comparator (COMP_break) 41 in Fig. 3, so as to use MCU internal comparator, thus to use less component and save space. Number ③ in Fig. 5 represents “Use OC (output compare) reference to set the Toff holding time” .
As shown in Fig. 4, the controller MCU (also denoted as 41 in Fig. 3) includes:
a first timer (Tim 1) , configured to define each cycle of the controlling signal by counting, when the first timer is reset, a new cycle starts; and
a second timer (Tim 2) , configured to be reset when the off state of the controlling signal starts (for example, the break signal with a low level is sent to the break pin of the controller MCU) .
As shown in Fig. 5, when a count value of the second timer reaches to a value corresponding to the output compare (OC) reference voltage, the first timer (Tim 1) is reset, so as to end the off state of the controlling signal.
In at least one embodiment, the first reference voltage may be sampled voltage. The first reference voltage determines the maximum value of the current flowing through the switch Q1. The output compare (OC) reference voltage (denoted as OC ref) is a parameter in controller 20 for controlling the off state time (Toff) , for example, the output compare (OC) reference voltage can be stored in controller 20 and updated.
In at least one embodiment, the first reference voltage and the output compare (OC) reference voltage is the same or different.
Fig. 6 is Timing Sequence Diagram when the buck circuit works under a CCM or a BCM.
In at least one embodiment, as shown in Fig. 6, when the controller controls the buck circuit to work under a CCM (continuous conduction mode) or a BCM (borderline conduction mode) , the off state time (Toff) is determined by a fixed ΔIpeak, inductance of the inductor and the feedback voltage (Vout) . The ΔIpeak represents the difference between an Ipeak upper limit and an Ipeak lower limit. Ipeak upper limit and Ipeak lower limit respectively represent the maximum value and the minimum value of the current flowing through the switch Q1.
As shown in Fig. 6, the first comparator’s output is connected to BREAK pin, therefore output current can be controlled cycle by cycle.
As the Ipk upper limit is got, the following formula (1) can be used to calculate the target of Toff (i.e. Toff _target) , so as to set a fixed ΔIpeak:
For example: in step 1, when Ipeak upper limit is set, the first reference voltage can be determined by Ipeak upper limit; in step 2, Toff _target can be calculated by using the ΔIpeak, the inductance L and the detected Vout, according to the above mentioned formula; in step3, the output compare (OC) reference voltage can be determined according to the calculated Toff _target, the output compare (OC) reference voltage stored in the controller MCU 20 can be undated by using the determined output compare (OC) reference voltage.
As shown in Fig. 6, V_Rshunt represents a voltage on the resistor R-shunt 1 in Fig. 3.
Fig. 7 is Timing Sequence Diagram when the buck circuit works under a DCM.
As shown in Fig. 7, when the controller 20 controls the buck circuit 10 to work under a DCM (discontinuous conduction mode) , the off state time (Toff) is determined by a fixed ΔIpeak, inductance L of the inductor, an fixed output current (iLED) , the input DC (direct current) voltage (denoted as Vin) and the feedback voltage (Vout) . The ΔIpeak equals to an Ipeak upper limit.
Where, the fixed ΔIpeak, the inductance L of the inductor and the fixed output current (iLED) are fixed value. The input DC (direct current) voltage (Vin) and the feedback voltage (Vout) are variables. Vout may also be low site voltage.
As shown in Fig. 7, 1/2ΔIpeak value is larger than I_LED. Buck circuit would go into DCM (discontinuous conduction mode) . Off-time controlled in dependency of Vin and Vout, as depending on stored parameters (e.g. in Look up table) . The stored parameters may be the fixed ΔIpeak, the inductance L of the inductor and the fixed output current (iLED) . iLED refers to a mean current flowing through a load (e.g., the load is an LED) connected between the output ports VOUT+ and VOUT-. iLED has the same meaning as I_LED in Fig. 7.
Fig. 8 is diagram of control method for Toff control with fixed ΔIpeak (or Ipeak) in DCM.
As shown in Fig. 8, the control method for Toff control with fixed ΔIpeak (or Ipeak) in DCM includes:
step 1. the controlling signal with maximum on state time Ton (denoted as PWM_ton (max) ) of the switch Q1 is sent to buck circuit 10;
step 2. a reference for peak of current (denoted as Ipeak_ref) is generated according to a current selection signal, where Ipeak_ref corresponds to the above mentionedΔIpeak or Ipeak upper limit;
step 3. the buck circuit 10 generates an output voltage Vout according to Ipeak_ref, PWM_ton (max) and Toff (max) , where Toff (max) is a pre-set constant value, which is around 50 micro seconds and could be defined by designer;
step 4. Vout and bus voltage are sent to an ADC converter to generate VBus_adc and Vout_adc which are digital signals;
step 5. a CCM or DCM calculation is performed based on VBus_adc and Vout_adc, to obtain an actual off state time (Toff) of the switch Q1.
Step 5 may include two sub-steps:
1 ^st sub-step: to determine the operation mode is CCM or DCM, for example, as shown in Fig. 7, when 1/2ΔIpeak value is larger than I_LED, it is determined that the mode is DCM;
2 ^nd sub-step: to calculate Toff based on VBus_adc and Vout_adc.
For example, a lookup table can be used to calculate Toff based on VBus_adc and Vout_adc. For another example, a formula (like formula (1) ) related with VBus and Vout can be used to calculate Toff. Other methods in existing technologies can be used to calculate Toff based on VBus_adc and Vout_adc
step 6. a reference for Toff (denoted as Toff_ref) is generated according to the current selection signal, for example, the current selection signal determines iLED, Ipeak_ref can be obtained as iLED fixed, then Toff_ref corresponding to iLED and Ipeak_ref can be determined by using a look-up table;
step 7. a PI (Proportional-Integral) control is performed based on a difference between Toff_ref and the actual Toff, so as to update the Toff in controlling signal, and send the updated Toff to the buck circuit 10.
After several cycles of step 3, step 4, step 5 and step 7, output voltage Vout will become stable.
Fig. 9 is diagram of state machine of Toff control for power supply circuit in accordance with the first aspect of embodiments of the disclosure.
As shown in Fig. 9, when the output voltage is stable, a slow Toff control can be used; when the output voltage is not stable, a fast Toff control can be used. The update rate for PI control loop in fast Toff control is higher than the update rate for PI control loop in slow Toff control.
For example, 2 sets of PI parameters can be defined, one is used for fast Toff control, and another is used for slow Toff control. Fast T_off control and slow T_off control may correspond to the PI control of step 7 in Fig. 8.
Fast Toff control can be enabled at start up or when the output voltage is in unstable state. Slow Toff control can be enabled when the output voltage is in unstable state.
According to the first aspect of embodiments, the power supply circuit 1 has the following advantages:
1. Digital solution –flexible usage in different products (Hardware parameters could be configured in Software list) ;
2. Cost saving –no additional auxilary winding for Zero cross detection;
3. Space saving –less component (capable for using MCU internal comparator) ;
4. Support CCM/DCM performance –wider operation window for output power.
The power supply circuit 1 has the following Characteristics:
1. Use formula in DCM operation –calculation performed for control;
2. Totem circuit is used to Drive MOSFET;
3. Vin &Vout detection is needed, no measurement of average LED current.
Second aspect of embodiments
A controlling method of a power supply circuit. The power supply circuit is provided in the first aspect of embodiments. The same contents as those in the first aspect of embodiments are omitted.
Third aspect of embodiments
A driver is provided in the third aspect of embodiments.
Fig. 10is a diagram of the diver. As shown in Fig, 10, the driver 50 includes:
an EMI filter 51, which filter the Electromagnetic Interference;
a boost PFC circuit 52, which convert the input AC power into DC power;
a DC-DC convertor 53, which uses a buck circuit to convert the DC voltage of the boost PFC circuit 52 into an output voltage, the output voltage is used to drive a lighting device, for example, the lighting device is LED;
a controller 54 controls the DC-DC convertor 53; and
a control circuit 55 communicates with the controller 54. The control circuit 54 communicate with a peripheral device via an interface. For example, the peripheral devices may be dimmers, sensors, controllers, security device, etc. The interface maybe DALI (Digital Addressable Lighting Interface) .
In the driver 50, the controller 54 corresponds to the MCU in Figs. 1, 2 and 3. The DC-DC convertor 53 corresponds to the buck circuit, the MOS Drive circuit and the Current peak limiter in Fig. 3.
The driver 50 may supply direct current (DC) power to the lighting device. The driver 50 may be an LED driver, the lighting device may be an LED device.
An output power, output voltage or output current of the lighting device may be changed from a minimum to maximum value according to dimming signal, e.g. 1-10V, which is received via DALI (Digital Addressable Lighting Interface) , NFC (Near Field Communication) , Bluetooth etc.. Preferably the DC-DC-converter supplying the lighting device will change its output parameters (current and/or voltage) depending on the dimming signal.
Further, while operations are depicted in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Likewise, while several specific implementation details are contained in the above discussions, these should not be construed as limitations on the scope of the present disclosure, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable sub-combination.
Although the present disclosure has been described in language specific to structural features and/or methodological acts, it is to be understood that the present disclosure defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

Claims

A power supply circuit, comprising:

a buck circuit, configured to convert input DC (direct current) voltage into output DC voltage, the buck circuit comprising a diode, an inductor, a capacitor and a switch, the output DC voltage being outputted from output ports (VOUT+, VOUT -) ;

a controller (MCU) , configured to output a controlling signal (PWM signal) according to a feedback voltage (Vout) , the feedback voltage being obtained by a voltage detection circuit being connected between one output port and a ground port; and

a MOS Drive circuit, configured to drive the switch of the buck circuit according to the controlling signal,

an off state time (Toff) for each cycle of the controlling signal is set according to an output compare (OC) reference voltage.
The power supply circuit according to claim 1, wherein,

an on state time (Ton) for each cycle of the controlling signal is determined by a detected peak current (Ipeak) that flows through the switch.
The power supply circuit according to claim 2, wherein,

the power supply circuit further comprises:

a current peak limiter, configured to detect the peak current, and send a break signal to a break pin of the controller, the break signal makes the controlling signal to fall into a low level, so as to end the on state.
The power supply circuit according to claim 3, wherein,

the current peak limiter comprises a first comparator (COMP_break) , which compares a voltage corresponding to the peak current with a first reference voltage (Ref) , when the voltage corresponding to the peak current is higher than the first reference voltage, the break signal with low level is generated by the first comparator.
The power supply circuit according to claim 4, wherein,

the controller comprises:

a first timer (Tim 1) , configured to define each cycle of the controlling signal by counting, when the first timer is reset, a new cycle starts; and

a second timer (Tim 2) , configured to be reset when the off state of the controlling signal starts,

when a count value of the second timer reaches to a value corresponding to the output compare (OC) reference voltage, the first timer is reset, so as to end the off state of the controlling signal.
The power supply circuit according to claim 2, wherein,

when the controller controls the buck circuit to work under a CCM (continuous conduction mode) or a BCM (borderline conduction mode) , the off state time (Toff) is determined by a fixed ΔIpeak, inductance of the inductor and the feedback voltage (Vout) ,

wherein, the ΔIpeak represents the difference between an Ipeak upper limit and an Ipeak lower limit.
The power supply circuit according to claim 2, wherein,

when the controller controls the buck circuit to work under a DCM (discontinuous conduction mode) , the off state time (Toff) is determined by a fixed ΔIpeak, inductance of the inductor, an fixed output current (iLED) , the input DC (direct current) voltage (Vin) and the feedback voltage (Vout) ,

wherein, the ΔIpeak equals to an Ipeak upper limit.
A driver, used for driving a lighting device, the driver comprises the power supply circuit according to any one of claims 1-7 and a control circuit, wherein,

the power supply circuit providing power to the lighting device,

the control circuit communicates with the power supply circuit,

the control circuit communicates with a peripheral device.
A controlling method of a power supply circuit, the power supply circuit comprising:

a buck circuit, configured to convert input DC (direct current) voltage into output DC voltage, the buck circuit comprising a diode, an inductor, a capacitor and a switch, the output DC voltage being outputted from output ports (VOUT+, VOUT -) ;

a controller (MCU) , configured to output a controlling signal (PWM signal) according to a feedback voltage (Vout) , the feedback voltage being obtained by a voltage detection circuit being connected between one output port and a ground port; and

a MOS Drive circuit, configured to drive the switch of the buck circuit according to the controlling signal,

the controlling method comprising:

setting an off state time (Toff) for each cycle of the controlling signal according to output compare (OC) reference voltage.
The controlling method of the power supply circuit according to claim 9, wherein,

an on state time (Ton) for each cycle of the controlling signal is determined by a detected peak current (Ipeak) that flows through the switch.
The controlling method of the power supply circuit according to claim 10, wherein,

the power supply circuit further comprises:

a current peak limiter,

the method further comprises:

the current peak limiter detects the peak current, and sends a break signal to a break pin of the controller,

wherein, the break signal makes the control signal to fall into a low level, so as to end the on state.
The controlling method of the power supply circuit according to claim 11, wherein,

the current peak limiter comprises a first comparator (COMP_break) , which compares a voltage corresponding to the peak current with a first reference voltage, when the voltage corresponding to the peak current is higher than the first reference voltage, the break signal with low level is generated by the first comparator.
The controlling method of the power supply circuit according to claim 12, wherein,

the controller comprises:

a first timer (Tim 1) , configured to define each cycle of the controlling signal by counting, when the first timer is reset, a new cycle starts; and

a second timer (Tim 2) , configured to be reset when the off state of the controlling signal starts,

when a count value of the second timer reaches to a value corresponding to the output compare (OC) reference voltage, the first timer is reset, so as to end the off state of the controlling signal.
The controlling method of the power supply circuit according to claim 10, wherein,

when the controller controls the buck circuit to work under a CCM (continuous conduction mode) or a BCM (borderline conduction mode) , the off state time (Toff) is determined by a fixed ΔIpeak, inductance of the inductor and the feedback voltage (Vout) ,

wherein, the ΔIpeak represents the difference between an Ipeak upper limit and an Ipeak lower limit.
The controlling method of the power supply circuit according to claim 10, wherein,

when the controller controls the buck circuit to work under a DCM (discontinuous conduction mode) , the off state time (Toff) is determined by a fixed ΔIpeak, inductance of the inductor, an fixed output current (iLED) , the input DC (direct current) voltage (Vin) and the feedback voltage (Vout) ,

wherein, the ΔIpeak equals to an Ipeak upper limit.