Valley-to-valley switching in quasi-resonant mode for drivers

Abstract

A method for controlling power supplied to a load (13), the method comprising: Determining whether a turn-on time of a switch (T0) is adjusted from a first time corresponding to a first voltage valley in an oscillating voltage at a node of the switch (T0) to a second time corresponding to a second voltage valley in the oscillating voltage; when adjusting the on-time of the switch (T0) from the first time to the second time, gradually adjusting the on-time of the switch (T0) from the first time to the second time, wherein the gradual adjustment of the on-time comprises gradually increasing or decreasing a switching period from a switching period leading to the first time to a switching period leading to the second time, so that the on-time assumes a plurality of intermediate times corresponding to voltages outside a voltage valley in the oscillating voltage at the node of the switch (T0); gradually adjusting one or more switch operating parameters for the switch (T0) to keep a power supplied to the load (13) substantially constant during the gradual adjustment of the on-time from the first time to the second time and to keep an average amount of current flowing to the load approximately at a target current level or an average amount of voltage across the load approximately at a target voltage level.

Description

[0001] The disclosure relates to controlling a current or voltage at a load with a driver.

[0002] A driver can be used to control the amount of current flowing through a load to maintain a target average current flowing through the load or to maintain a target average voltage across the load. According to one example, the driver can control how long current flows through the load and how often current flows through the load to maintain the desired average current flowing through the load or to maintain the desired average voltage across the load.

[0003] US 2002 / 0 085 394 A1 describes a method for operating a flyback converter in quasi-resonant mode, wherein a switch of the flyback converter is switched on at the beginning of each control period when a voltage across the switch reaches one of several local minima (valleys). In the known method, various parameters are determined during a control period of the switch in which the switch was switched on in a specific one of the several valleys. Based on these parameters, the duty cycles and switching frequencies are determined that would occur if the power consumption remained unchanged but if the switch were switched on in a different one of the several valleys. Based on the duty cycles and switching frequencies determined in this way, a decision is then made as to which of the valleys the switch should be switched on in the next control period.

[0004] US 6 341 073 B1 describes a method for operating a flyback converter in quasi-resonant operation. A new control period of a switch should begin whenever a local minimum (valley) of a voltage across the switch is present. In an optimization process, which can take place, for example, when starting up the flyback converter, power losses that can occur in the individual valleys when switched on are determined in order to then switch the switch on in the valley for which the minimum power loss was determined. In the optimization process, a first of the valleys is first determined by varying the switch's turn-off time. Based on measurements obtained when determining the first of the valleys, the positions of further valleys are then determined. The valley with the lowest power losses is then determined in order to operate the flyback converter based on this valley.

[0005] The object underlying the invention is to provide an optimized method for controlling power supplied to a load using a switch, a corresponding driver system, and a driver. This object is achieved by a method according to claim 1, a driver system according to claim 6, and a driver according to claim 12.

[0006] This disclosure describes methods for maintaining high switching efficiency for a switch that a driver turns on and off to control the average current flowing through a load (referred to as the average load current) or to control the average voltage level at the load (referred to as the average load voltage). For example, the driver turns the switch on and off to control the amount of current flowing through the load such that the average load current or the average load voltage is approximately equal to the target current level or the target voltage level.

[0007] According to one example, when the load current or load voltage needs to change due to a change in the target current level or target voltage level, the driver adjusts the time at which the switch turns on (e.g., adjusts the switch's turn-on time or on-time) to a time when the least amount of energy is required to turn the switch on while allowing the amount of current required to achieve the average load current level or average voltage level to flow through the load. Additionally, in this example, the driver promptly adjusts one or more switch operating parameters to compensate for the sudden change in the amount of current that would otherwise flow through the load due to the adjustment in the switch's turn-on time (e.g., the time the switch is turned on).

[0008] According to another example, when the load current or voltage requires a change due to a change in the target current level or voltage level, the driver gradually adjusts the switch's on-time from one voltage valley to another voltage valley and through an intermediate on-time, which may be a less optimal on-time (e.g., a time when high switching efficiency of the switch is not achieved). According to this example, the driver gradually adjusts the on-time while allowing the appropriate amount of current to flow through the load to achieve the average load current level or the average load voltage level.Furthermore, in this example, a driver feedback control loop gradually adjusts one or more switch operating parameters as the driver adjusts the on-time to compensate for changes in the amount of current that would otherwise flow through the load due to the switch on-time adjustment.

[0009] According to one example, the disclosure describes a method for controlling power supplied to a load, the method comprising determining whether or not to adjust an on-time of a switch from a first time corresponding to a first voltage valley of an oscillating voltage at a node of the switch to a second time corresponding to a second voltage valley of the oscillating voltage at the node of the switch, setting one or more switch operating parameters for the switch such that when the on-time is adjusted from the first time to the second time, an amount of power (: "power") supplied to the load is substantially the same immediately before the adjustment and immediately after the adjustment, adjusting the on-time of the switch to the second time based on the determination to adjust the on-time, and adjusting the one or more switch operating parameters for the switch based on the determined one or more switch operating parameters.

[0010] According to one example, the disclosure describes a driver system for controlling power supplied to a load, the driver system comprising a transformer having a primary side including a first winding and a secondary side including a second winding, the load being connected to the secondary winding, a switch connected to the first winding, and a driver coupled to the switch and configured to determine whether or not to adjust an on-time of the switch from a first time corresponding to a first voltage valley in an oscillating voltage at a node of the switch to a second time corresponding to a second voltage valley of the oscillating voltage at the node of the switch, setting one or more operating parameters for the switch such that when the on-time is adjusted from the first time to the second time, an amount of power,supplied to the load is substantially the same immediately before the adjustment and immediately after the adjustment, and adjusting the on-time of the switch to the second time based on the determination to adjust the on-time, and setting the one or more switching operating parameters for the switch based on the determined one or more switch operating parameters.

[0011] According to one example, the disclosure describes a driver for controlling power supplied to a load, the driver comprising a controller configured to determine whether or not to adjust an on-time of a switch from a first time corresponding to a first voltage valley in an oscillating voltage at a node of the switch to a second time corresponding to a second voltage valley in the oscillating voltage at the node of the switch, and to set one or more switch operating parameters for the switch such that when the on-time is adjusted from the first time to the second time, an amount of power supplied to the load immediately before the adjustment and immediately after the adjustment is substantially the same, and a pulse width modulation (PWM) unit configured to adjust the on-time of the switch to the second time based on the determination,adjust the switch-on time, adjust, and set the one or more switch operating parameters for the switch based on the specified one or more switch operating parameters.,

[0012] According to one example, the disclosure describes a method for controlling power supplied to a load, the method comprising determining whether or not to adjust an on-time of a switch from a first time corresponding to a first voltage valley of an oscillating voltage at a node of the switch such that an average amount of current flowing through the load or an average amount of voltage across the load is approximately equal to a target current load level or a target load voltage level.: "target current load level"), determining a second time corresponding to a second voltage valley of the oscillating voltage, gradually adjusting the switch on time from the first time to the second time over an intermediate time corresponding to a voltage outside a valley of the oscillating voltage at the node of the switch based on the determination to adjust the on time, and gradually adjusting the one or more switch operating parameters for the switch to keep the amount of power supplied to the load approximately constant during the gradual adjustment of the switch on time from the first time to the second time.

[0013] According to one example, the disclosure describes a driver system for controlling power supplied to a load, the driver system comprising a transformer having a primary side including a first winding and a secondary side including a second winding, the load being connected to the second winding, a switch coupled to a first winding, and a driver coupled to the switch and configured to determine whether to adjust an on-time of the switch from a first time corresponding to a first voltage valley in an oscillating voltage at a node of the switch such that an average amount of current flowing through the load or an average amount of voltage across the load is approximately equal to a target load current level or a target load voltage level, determining a second time,which corresponds to a second voltage valley of the oscillating voltage, gradually adjusting the switch-on time from the first time to the second time over an intermediate time corresponding to a voltage outside a valley of the oscillating voltage at the node of the switch, based on the determination to adjust the switch-on time, and gradually adjusting one or more switch operating parameters for the switch to keep the amount of power supplied to the load approximately constant during the gradual adjustment of the switch-on time from the first time to the second time.

[0014] According to one example, the disclosure describes a power control driver configured to determine whether or not to adjust a switch-on time from a first time corresponding to a first voltage valley of an oscillating voltage at a node of the switch such that an average amount of current flowing through the load or an average amount of voltage at the load is approximately equal to a target load current level or a target load voltage level, determining a second time corresponding to a second voltage valley of the oscillating voltage, gradually adjusting the switch-on time from the first time to the second time over an intermediate time corresponding to a voltage outside a valley of the oscillating voltage at the node of the switch based on the determination to adjust the switch-on time,and gradually adjusting one or more switch operating parameters for the switch to keep the amount of power supplied to the load approximately constant during the gradual adjustment of the switch on time from the first time to the second time.

[0015] The details of one or more methods of this disclosure are set forth in the accompanying drawings and the following description. Other features, objects, and advantages of this disclosure will be apparent from the description and drawings, as well as from the claims. Fig. 1 is a circuit diagram illustrating an example of a driver system according to one or more examples described in this disclosure. Fig. Figure 2 is a graphical diagram used to illustrate example ways in which a driver can increase or decrease the amount of current flowing through windings of a transformer. Fig. Figure 3 is a graphical diagram illustrating the behavior of a voltage at a switch node when the switch is turned off and when a current flowing through a winding coupled to the load becomes zero. Fig. 4 is a flowchart illustrating a first example method according to this disclosure. Fig. 5 is a flowchart illustrating a second example method according to this disclosure.

[0016] A driver, such as a light-emitting diode (LED) driver or a power supply, to name two examples, may be employed to control the amount of current flowing through a load connected to the driver (e.g., one or more LEDs in the example where the driver is an LED driver) such that the average amount of current flowing through the load (referred to as the average load current) or the average voltage level across the load (referred to as the average load voltage) is at a target current level or a target voltage level. In this disclosure, the voltage level across the load refers to the potential across the load.

[0017] According to some examples, the load is connected to a flyback converter and a switch. However, topologies other than a flyback converter are possible, and the methods described in this disclosure are not intended to be limited to a flyback converter topology. The driver turns the switch on and off to control the amount of current flowing through the switch to control the average load current or average load voltage.

[0018] When current flows through the load and the switch is turned on, a voltage at a node of the switch is at a certain voltage level. When the current flowing through a coil (e.g., an inductor) connected to the load reaches zero and the switch is turned off, a voltage at the switch node tends to oscillate, creating an oscillating voltage with voltage peaks and valleys at the switch node. The node of the switch where the voltage can oscillate is called the switch node, and a voltage at the switch node is called the switch node voltage.

[0019] According to one example, the switch may be a transistor; when the switch is on, the transistor is in resistive mode or active mode; and when the switch is off, the transistor is in off mode. According to examples where the switch is a transistor, the node of the switch at which the voltage oscillates may be the drain node of the transistor. In other words, the switch node may be the drain node of the transistor, and the switch node voltage may be the voltage at the drain node of the transistor (i.e., the drain node voltage). However, the examples described in this disclosure are not limited to a transistor being the switch.

[0020] According to some examples, the difference between the switch node voltage at an oscillating voltage valley and the switch node voltage when the switch is on is the smallest compared to the difference between the switch node voltage at other voltage levels outside an oscillating voltage valley and the switch node voltage when the switch is on. For example, suppose the voltage at the switch node when the switch is on is X, and the voltage at an oscillating voltage valley at the switch node when the switch is off is Y. In this example, Y minus X is smaller than a voltage outside a valley at the switch node during oscillation minus X.

[0021] Accordingly, turning the switch on in an oscillating voltage valley can maximize the switching efficiency of the switch because the voltage at the switch node in an oscillating voltage valley when the switch is off is closest to the voltage at the switch node when the switch is on. In other words, the least amount of energy is required to turn the switch back on when the switch is turned on in an oscillating voltage valley compared to turning the switch back on outside of an oscillating voltage valley, and the difference between the switch node voltage when the switch is on and the switch node voltage during oscillation in a voltage oscillation valley is smallest. Turning the switch on (e.g., turning on the transistor as an example) in a voltage valley is called quasi-resonant switching.

[0022] According to some examples, the amount of current that must flow through the load may change to achieve a target current or voltage level. For example, in the example where the load is one or more LEDs, a user may dim or brighten the LEDs, requiring a decrease or increase, respectively, in the amount of current that must flow through the load. Changes in the amount of current that must flow through the load may require adjusting the time at which the switch is turned on, possibly to a time when the oscillating voltage is at a voltage level outside of a valley. However, adjusting the time at which the switch is turned on to a voltage level outside of a valley may result in inefficient switching.For example, turning the switch on at a voltage level outside a valley may require more energy for the switch node voltage to return to the voltage level at which the switch is turned on compared to the energy required for the switch node voltage to return to the voltage level when the switch is turned on at a time when the oscillating voltage has a voltage level in a valley.

[0023] There are multiple peaks and valleys in the oscillating voltage, and in some cases, if the time at which the switch is to be turned on changes (i.e., the switch's turn-on time changes), it may be possible to turn the switch on in a subsequent or preceding valley. In this case, switching efficiency is maintained or only slightly affected because the switch still turns on in a voltage valley, albeit a different voltage valley. However, such sudden changes in the time at which the switch turns on cause a step change (i.e., a relatively large instantaneous change) in the amount of current flowing through the load if no compensation is provided.

[0024] Gradual jumps in the amount of current flowing through the load may be undesirable, and gradual changes in the amount of current flowing through the load may be desirable. For example, in the context of lighting, when LEDs are dimmed or brightened, a gradual jump in the amount of current flowing through the LEDs will result in flickering of the LEDs, which observers may perceive as undesirable. Viewers may generally prefer gradual changes in LED output when LEDs are dimmed or brightened.

[0025] In the methods described in this disclosure, a driver that controls the amount of current flowing through a load may determine whether a change in the time at which a switch is turned on (e.g., whether an adjustment of the switch's on time) is required to adjust the amount of current flowing through the load such that the average load current or voltage is approximately equal to a target current or voltage level. In the methods described in this disclosure, the current flowing through the load flows through the switch when the switch is on. If the driver determines that a change in the time at which the switch is turned on (e.g., the on time) is required, the driver may implement one or more example methods described in this disclosure.

[0026] Furthermore, according to some examples, the driver turns on the switch prior to adjustment at a time corresponding to a first voltage valley of the oscillating voltage. In other words, according to some examples, the driver may be configured to turn on the switch prior to changing the amount of current flowing through the load at a time of a first voltage valley of the oscillating voltage. It should be understood that the "first voltage valley" does not necessarily mean the very first instance in which a valley is present, but is instead used to distinguish the voltage valleys. For example, there may be multiple voltage valleys, and a first voltage valley, as used in this context, means that it refers to one of these voltage valleys.A second stress valley means that it refers to another of these stress valleys and it can be a preceding stress valley or a subsequent stress valley with respect to the first stress valley.

[0027] According to one example, the driver may determine a time corresponding to a second voltage valley of the oscillating voltage and adjust the time at which the switch is turned on (e.g., adjusting the turn-on time) to the time of the second voltage valley. The second voltage valley may be a voltage valley that precedes or follows the first voltage valley, based on whether an increase or decrease in current through the load is required to achieve the target current or voltage level. As described above, changing the time at which the switch is turned on from one voltage valley to another may cause a step-wise jump in the amount of current through the load.

[0028] Accordingly, in this example, the driver can compensate for a sudden change in the amount of current flowing through the load. For example, the driver can instantaneously adjust one or more switch operating parameters so that the amount of power supplied to the load immediately before the switch on time changes is the same as the amount of power supplied to the load immediately after the switch on time changes. In other words, because the current flowing through the load can instantaneously jump when the switch turn-on time is adjusted from one voltage valley to another, the driver can instantaneously adjust one or more operating parameters to counteract the jump in current from the change in the switch turn-on time. This example method can be referred to as valley-to-valley switching with power compensation.

[0029] An example of a switch operating parameter is a duty cycle, which specifies the duration the driver keeps the switch on, by turning the switch off after the runtime. Another example of a switch operating parameter is a peak current amplitude threshold of the current flowing through the switch, where the driver turns the switch off when the current flowing through the switch reaches the amplitude threshold.

[0030] As another example, instead of adjusting the switch's turn-on time from a time corresponding to one voltage valley to a time corresponding to another voltage valley, the driver can gradually adjust the switch's turn-on time to a different voltage valley. For example, the driver can adjust the switch's turn-on time from a voltage valley turn-on time to a different voltage valley turn-on time, and through an intermediate turn-on time that is not in a voltage valley, and at a voltage level of the oscillating voltage where suboptimal switching efficiency occurs. For example, when the LEDs are dimmed, the driver can gradually increase the time at which the switch is turned back on to reduce the amount of current flowing through the load.In this example, the gradual increase in the time at which the switch is turned on again leads to cases where the switch turns on and the oscillating voltage is not in a voltage valley, resulting in suboptimal switching efficiency.

[0031] According to the methods described in this disclosure, according to some examples, the driver may gradually adjust the turn-on time (i.e., gradually adjust the switch turn-on time) until the turn-on time is in a voltage valley, allowing high switching efficiency (i.e., until the switch turn-on time is associated with a voltage valley time). However, gradually adjusting the switch turn-on time to a time associated with a voltage valley requires changes in the amount of current flowing through the load.

[0032] According to some examples, a feedback control loop of the driver may already be configured to adjust switch operating parameters to compensate for changes in the input voltage and other factors that affect the amount of current flowing through the load. This control loop of the driver may not be fast enough to compensate for incremental changes in the current flowing through the load, as is the case in the example above. Accordingly, in the example above, the driver may adjust the one or more switch operating parameters instantaneously.However, this driver control loop can be fast enough to compensate for gradual changes in current, such as those that would occur due to a gradual adjustment of the switch's turn-on time from the first turn-on time when the oscillating voltage is in a valley, to a second time when the oscillating voltage is at a valley, and through an intermediate turn-on time when the oscillating voltage is outside a valley. Due to the gradual adjustment of the switch turn-on time, this example method can be referred to as valley-to-valley soft handover switching.

[0033] In both valley-to-valley switching with power compensation and valley-to-valley switching with soft handover, once the switch's on-time is set to a voltage valley, the driver can further adjust other operating parameters of the switch so that the average current through the load or the average voltage across the load equals the target load current level or the target load voltage level. In this way, high switching efficiency can be achieved while allowing the average current through the load or the average voltage across the load to equal the target current level or the target voltage level.

[0034] Fig. 1 is a circuit diagram illustrating an example of a driver system according to one or more examples described in this disclosure. For example, Fig. 1 shows a driver system 10 including a driver 12 and a load 13. Examples of the driver system 10 include a printed circuit board with the illustrated components and the driver 12, as well as a connector for plugging into a power source, such as an AC input source.

[0035] Driver 12 may be embodied in an integrated circuit (IC) chip. Alternatively, one or more units of driver 12 may be embodied in other IC chips. The units of driver 12 are illustrated as functional blocks to simplify understanding, and they may be combined or separated within driver 12. The units of driver 12 may be implemented as hardware or software, or as firmware running on hardware.

[0036] In the Fig. 1, the load 13 contains a plurality of light-emitting diodes (LEDs) connected in series. Accordingly, in Fig. 1, the driver 12 represents an example of an LED driver. Another example of a driver includes a power supply, where the load 13 is a laptop computer. In general, the driver 12 may be a circuit configured to output power, and the load 13 may be a device that consumes power. For ease of description, the methods explained in this disclosure are described from the perspective of an LED driver system. However, the methods described in this disclosure are not to be understood as so limiting and may be extended to other driver systems.

[0037] The LEDs of load 13 light up when a current flows through them. As shown, flyback converter 14 outputs the current to load 13, which causes the LEDs of load 13 to light up. For example, an AC input source (e.g., 85 to 305 V AC) is connected to driver system 10. The AC voltage of the input source is rectified by a full-wave rectifier (e.g., the four diodes coupled to the AC input source) and filtered with a capacitor. In examples where the input source is a DC input source, the rectifier may not be required, and the capacitor used in Fig. 1 connected to the rectifier can optionally be connected to the DC input source.

[0038] Driver 12 is connected to the full-wave rectifier and thus receives power from the input source. For example, the local voltage for driver 12 (referred to as VCC) is generated from the voltage output by the full-wave rectifier. The components and units of driver 12 are powered by the VCC generated from the output voltage of the full-wave rectifier.

[0039] Similarly, flyback converter 14 receives power from an input source via the full-wave rectifier. It should be understood that the methods described in this disclosure are described in connection with a flyback converter for ease of illustration. The methods described in this disclosure may be extended to other converter topologies.

[0040] As shown, the flyback converter includes a transformer having a primary side comprising a primary winding (referred to as winding 16 for simplicity), a secondary winding (referred to as winding 18 for simplicity), and a sense winding (referred to as winding 20 for simplicity). Winding 16 is connected to the full-wave rectifier and a switch TO. In the Fig. 1, the switch TO is an "n" type transistor, and therefore, the winding 16 is shown connected to the drain node of the switch TO. For ease of description, the examples are described with respect to the primary side winding 16 connected to a drain node of the switch TO. However, the methods described in this disclosure are not so limited, and the switch TO may be a "p" type transistor. According to some examples, the transistor TO may be a power transistor such as a power metal-oxide-semiconductor field-effect switch (MOSFET), a gallium nitride (GaN) FET, an insulated-gate bipolar transistor (IGBT), or other types of transistors. According to some examples, the switch TO may not be a transistor and may be other types of switches.

[0041] In general, switch T0 may have a first node coupled to winding 16, a second node that controls whether switch T0 is on or off, and a third node through which switch T0 outputs current. For example, in examples where switch T0 is a transistor, the first node of switch T0 coupled to winding 16 may be a drain node of the transistor, the second node of switch T0 that controls whether switch T0 is on or off may be a gate node of the transistor, and the third node of switch T0, through which switch T0 outputs current, may be a source node of the transistor.

[0042] In the methods described in this disclosure, the driver 12 may control when the switch TO is turned off and on to control the amount of current flowing through the load 13 such that the average amount of current flowing through the load 13 (referred to as average load current) or the average voltage level across the load 13 (referred to as average load voltage) is set to a load current or load voltage level (e.g., at a target load current level or target load voltage level). In the method described in Fig. In the example shown in Figure 1, the output voltage indicates the voltage level at the load 13 (e.g., the potential across the LEDs), and the current flowing through the load 13 is the current flowing through the LEDs, which causes the LEDs to light up.

[0043] When driver 12 turns on switch TO, current flows through winding 16 (also called winding 16) and into switch TO and through the resistor connected to the current sense (CS) pin of driver 12 to ground (GND). The current through winding 16 cannot change instantaneously, and therefore the current through winding 16 increases slowly (and in some examples, it increases linearly). The current flowing through winding 16 creates a magnetic field, and when the current flowing through winding 16 is turned off, the magnetic field induces a current in winding 18 (also called inductor 18) in the second side, discharging the magnetic field. The current from winding 18 flows through load 13 and illuminates the LEDs.

[0044] When driver 12 turns off switch TO, current stops flowing into switch TO. However, due to the inductance of winding 16, the current flowing through winding 16 does not stop immediately. Instead, the current through winding 16 slowly decays (and, according to some examples, decays linearly) by flowing through the capacitor shown to the left of winding 16 and within flyback converter 14. As the current flows through winding 16, the current creates a magnetic field. As the current is turned off, the magnetic field creates a secondary current through winding 18, which also slowly decays (e.g., linearly due to the inductance) through load 13, discharging the magnetic field.

[0045] If the driver 12 turns the switch TO back on before the current through the winding 16, and in some examples, the load 13, has reached zero (i.e., an amplitude of zero amperes (0A)), the driver 12 may be considered to be operating in a continuous conduction mode (CCM). If the driver 12 turns the switch TO back on after the current through the winding 16 and, according to some examples, the load 13, has reached zero, the driver 12 may be considered to be operating in a discontinuous conduction mode (DCM).As will be described in more detail, the methods described in this disclosure may operate in the DCM, with the driver 12 determining when to turn the switch TO back on after the current through the winding 16 and, according to some examples, the load 13, reaches zero, such that the average load current or the average load voltage is equal to the specified load current level or the specified load voltage level.

[0046] For example, driver 12 may control the amount of current flowing through load 13 based on how often driver 12 turns on switch T0 and how long driver 12 turns on switch T0. As an example, if a higher average load current is required, driver 12 may decrease the amount of time driver 12 waits after the current through winding 16 and / or load 13 reaches zero before turning on switch T0, or driver 12 may leave switch T0 turned on for a longer period of time, or a combination of these two measures.As another example, if a lower average load current is required, the driver 12 may increase the length of time the driver 12 waits after the current through the winding 16 and / or the load 13 reaches zero before turning on the switch T0, or the driver 12 may leave the switch T0 turned on for a shorter duration, or a combination of these two measures.

[0047] Fig. Figure 2 is a graphical representation used to illustrate exemplary ways in which a driver can increase or decrease the amount of current flowing through windings of a transformer. For example, Fig. 2 the current flowing through winding 16, which is coupled to switch TO, as well as the current flowing through winding 18, which is coupled to load 13. For example, the current Ilade flows through winding 16 and switch TO. Lenlade is the current flowing through winding 18. The current flowing through winding 18 is as shown in Fig. 1, with a capacitor low-pass filtered, which is connected to the load 13, and the low-pass filtered current flows through the load 13. Accordingly, a representation of the current flowing through the load may differ from that in Fig. 2 illustrates the current flowing through the winding 18.

[0048] In the Fig. 2, driver 12 initially turns on switch T0, allowing a current to flow through winding 16 and through switch T0, creating a magnetic field. At the end of the on-period, driver 12 turns off switch T0, and the magnetic field induces a current to flow through load 13, which, due to the inductance of winding 18, decreases linearly until the current through winding 18 reaches zero to dissipate the magnetic field. However, in the example shown, driver 12 does not turn switch T0 back on immediately. Instead, there is a delay before driver 12 turns switch T0 back on. Fig. For example, Figure 2 illustrates the switching period. The switching period is the duration between one operation in which driver 12 turns on switch TO and another operation in which driver 12 turns on switch TO (i.e., the on-time of switch TO).

[0049] Based on the Fig. 2, there may be at least two ways in which the driver 12 can control the amount of current flowing through the load 13. In a first way of controlling the amount of current flowing through the load 13, the driver 12 can increase or decrease the switching period, which in effect increases or decreases the time at which the driver 12 turns on the switch TO (i.e., increases or decreases the on-time of the switch T0). The switching period indicates the frequency at which the driver 12 turns on the switch TO.

[0050] If the amount of current flowing through the load 13 needs to increase so that the average load current or the average load voltage is approximately equal to the set load current level or the load voltage level (e.g., the target load current level or the target load voltage level), the driver 12 can turn on the switch T0 more frequently (i.e., decrease the switching period). If the amount of current flowing through the load 13 needs to decrease so that the average load current or the average load voltage is approximately equal to the set load current level or the load voltage level, the driver 12 can turn on the switch T0 less often (i.e., increase the switching period).

[0051] In a second way to control the amount of current flowing through load 13, driver 12 can increase or decrease the switch on-time (i.e., increase or decrease t n ). For example, if the amount of current flowing through load 13 needs to increase so that the average load current or voltage is approximately equal to the set load current level or voltage level, driver 12 can increase the switch on-time. If the amount of current flowing through load 13 needs to decrease so that the average load current or voltage is approximately equal to the set load current level or voltage level, driver 12 can decrease the switch on-time.

[0052] Increasing or decreasing the switch on-time also affects the switching period because the switch off-time (toff) may be constant or vary only slightly. For example, driver 12 may not be able to adjust the switch off-time because the switch off-time is a function of the inductance of the transformer, which includes the primary side containing winding 16 and the secondary side containing winding 18, as well as capacitors connected to the transformer.

[0053] Accordingly, when the switch on-duty changes, the switching period (i.e., the on-time of the switch TO) changes accordingly, meaning that the switching period is a function of the switch on-duty. As described in more detail below, according to some examples, the calculations required to determine when the switch TO should be turned on may be complicated. Therefore, according to some examples, the methods described in this disclosure may approximate the time at which the switch TO should turn on based on an assumption that the switching period is a function of the switch on-duty or that the switch on-duty has little influence on the switching period. However, the methods described in this disclosure are not so limited.The methods described in this disclosure are also applicable to situations where the effect of changes in switch on time on the switching period is taken into account.

[0054] In this way, if the driver 12 is not capable of processing computationally intensive tasks, the driver 12 may implement the example methods that assume the switching period is not a function of the switch on-time. If the computational capability of the driver 12 is capable of processing computationally intensive tasks, the driver 12 may implement example methods that account for the effect of the switch on-time on the switching period.

[0055] As explained above, one way in which driver 12 controls the amount of current flowing through load 13 is by adjusting the switch on-time. There may be at least two ways in which driver 12 adjusts the switch on-time. According to one example, driver 12 may set the switch on-time, and after the switch on-time has elapsed, driver 12 may turn off switch TO. As another example, driver 12 may set a peak current threshold amplitude of Ilade, where Ilade may be a current flowing through the transformer and switch TO. According to this example, driver 12 may turn off switch TO when Ilade reaches the peak current threshold amplitude, as determined by the voltage across the resistor connected to the CS pin of driver 12.

[0056] Whether the driver 12 sets the duration during which the switch TO is turned on (i.e., the duty cycle) can be a matter of design choice. For example, for power supplies, the driver 12 may set the duty cycle based on the peak current threshold amplitude to enable more constant power transfer. If the driver 12 sets the duration during which the switch TO is turned on based on the specified duration, the driver 12 can minimize reactive power. Real power refers to components that consume power, and reactive power refers to the transfer of power back and forth between the components and the input source. Setting the duration for which the switch TO is turned on based on the specified duration causes the components to appear as a constant resistive load, providing a better real-world factor (i.e.,ensures better consumption of active power and minimizes reactive power). Therefore, whether better constant power transfer is needed or whether better active factors are needed can determine whether driver 12 sets the current duration during which switch TO is on to set the duty cycle, or whether driver 12 sets the peak current threshold amplitude to set the duty cycle.

[0057] In discontinuous conduction mode (DCM), driver 12 can turn on switch T0 at any time after the current through winding 16 (inductor 16) has reached zero (i.e., has reached an amplitude of zero amperes). However, there may be certain times when re-turning switch T0 is more beneficial than at other times.

[0058] Fig. 3 is a graphical diagram illustrating the behavior of a voltage at a circuit node when the switch is turned off and when no current flows through the winding 16. For example, Fig. 3 the behavior of a voltage VAUX, which indicates the voltage at the circuit node of the switch TO (shown as a dashed line), which corresponds to a graphical representation of the Icharge and lentlade (which is similar to the one in Fig. 2 illustrated Ilade and lentlade are superimposed.

[0059] For example, the voltage VAUX may be a representation of the voltage at the switch node of switch T0 (e.g., the drain node in examples where switch T0 is a transistor) connected to winding 16. In general, the voltage VAUX illustrates the behavior of the voltage at the switch node of T0 connected to winding 16. As in Fig. As shown in Figure 3, the switch node voltage (e.g., the drain voltage of switch TO) is at a constant low voltage level during the switch on-period. This constant low voltage level is referred to as the duty cycle switch node voltage level. Although switch TO is off, current still flows through winding 16 during the switch off-period, and the switch node voltage is at a relatively constant high voltage level (there may be a slight droop).

[0060] However, when the current through winding 16 reaches zero and switch TO is turned off, the switch node voltage begins to oscillate with a period Tosc. The frequency of this oscillation (i.e., 1 / Tosc) is a function of the components connected to switch TO, and driver 12 may not be able to control the frequency of the oscillation. During turn-on (e.g., when driver system 10 is connected to the input source), the turn-on controller 26 of driver 12 can determine the oscillation period Tosc.

[0061] The voltage oscillation at the switch node creates voltage peaks and valleys. For example, Fig. 3, the oscillating voltage (i.e., the voltage at the switch node of the switch when (1) the current through the winding 16 is zero and (2) the switch TO is off) includes a voltage valley 36 and a voltage trough 40 and a voltage spike 38. The voltage valley 36 occurs at time 0.5*Tosc after toff, the voltage valley 40 occurs at time 1.5*Tosc after toff, and the voltage spike 38 occurs at time Tosc after toff.

[0062] Fig. 3 also illustrates three hypothetical switching periods: switching period A, switching period B, and switching period C, where each switching period illustrates a time at which the switch TO is turned on again (i.e., each illustrates a turn-on time of the switch TO). In Fig. 3, switching period A is at the time of voltage valley 36, switching period B is at the time of voltage peak 38, and switching period C is at the time of voltage valley 40. According to some examples, the switching efficiency of switch TO can be maximized if driver 12 turns switch TO back on at a voltage valley 36 or a voltage valley 40.

[0063] In the methods described in this disclosure, the amount of energy required to re-enable the switch TO may be related to the difference between the duty-cycle switch node voltage level and the voltage level of the oscillating voltage. The smaller the difference between the duty-cycle switch node voltage level and the voltage level of the oscillating voltage, the smaller the amount of energy required to allow the switch node voltage to return to the duty-cycle switch node voltage level when the switch TO is re-enabled. The amount of energy required to allow the switch node voltage to return to the duty-cycle switch node voltage level when the switch TO is re-enabled is indicated by a switching efficiency of the switch TO.

[0064] In Fig. 3, the voltage difference 42 represents the difference between the voltage at the voltage valley 36 and the duty cycle switch node voltage level (e.g., the voltage at the drain node of switch T0 when switch T0 is turned on). The voltage difference 44 represents the difference between the voltage at the voltage peak 38 and the duty cycle switch node voltage level, and the voltage difference 46 represents the difference between the voltage at the voltage valley 40 and the duty cycle switch node voltage level.

[0065] As shown, voltage difference 42 is the smallest voltage difference between the duty cycle switch node voltage level and any other voltage level of the oscillating voltage. Therefore, if driver 12 turns switch TO on again when the oscillating voltage is at voltage valley 36, the least amount of energy would be required to return the switch node voltage to the duty cycle switch node voltage level, resulting in the highest switching efficiency. As shown, voltage difference 46 is the next smallest voltage difference between the duty cycle switch node voltage level and any other voltage level (except voltage level 36) of the oscillating voltage.Therefore, if driver 12 were to turn switch TO back on when the oscillating voltage is at voltage valley 40, the next smallest amount of energy would be required to return the switch node voltage to the duty cycle switch node voltage level, resulting in nearly the highest switching efficiency. As shown, voltage difference 44 is the largest voltage difference between the duty cycle switch node voltage and any other oscillating voltage level. Therefore, if driver 12 were to turn switch TO back on when the oscillating voltage is at voltage peak 38, the greatest amount of energy would be required to return the switch node voltage to the duty cycle switch node voltage level, resulting in the lowest switching efficiency.

[0066] Accordingly, to achieve high switching efficiency, driver 12 may be configured to turn on switch T0 at a time at a voltage valley of the oscillating voltage. For example, a user may set the brightness level of the LEDs to the highest level by setting the load current level to the highest level. In this example, driver 12 may be configured to turn on switch T0 at a time of voltage valley 36. In other words, driver 12 may set the turn-on time of switch T0 equal to switching period A. Conversely, a user may set the brightness level of the LEDs to the lowest level by setting the load current level to the lowest level. In this example, driver 12 may be configured to turn on switch T0 at a time of voltage valley 40.In other words, the driver 21 can set the turn-on time of the switch T0 equal to the switching period C.

[0067] It goes without saying that Fig. 3 illustrates two voltage valleys for ease of illustration and description. Generally, multiple voltage valleys and multiple voltage peaks may be present. Therefore, according to some examples, driver 12 may be configured to switch the switch TO at any of these voltage valleys.

[0068] In the above examples, driver 12 may determine the time at which switch TO is to be turned on based on when a voltage valley occurs in the oscillating voltage. Accordingly, driver 12 may be required to determine when the voltage valleys occur.

[0069] Referring again to Fig. 1, the transformer has a sensing side that includes a winding 20. Winding 20 can sense the voltage across winding 16. Because winding 16 is connected to the switch node of switch TO (e.g., in the example where switch TO is a transistor, the drain node) where the oscillating voltage appears, winding 20 can sense the voltage at the switch node of switch TO and supply the voltage to driver 12. For example, driver system 12 includes a voltage divider 22 connected to the zero crossing (ZCD) pin of driver 12. Therefore, driver 12 receives, via the ZCD pin, a fraction of the voltage that follows the switch node voltage, which is the voltage at the node of switch TO connected to flyback converter 14 (e.g., the drain node of switch TO). Voltage divider 22 can be optional or included with driver 12.

[0070] For example, if a voltage oscillation is present at the switch node, the voltage oscillation may be sampled by winding 20, divided by voltage divider 22, and coupled to the ZCD pin. A ZCD unit 30 of driver 12 may determine when the sampled voltage drops below zero (e.g., moves from above zero to below zero) and outputs a signal to controller 24 of driver 12 indicating that the sampled voltage has dropped below zero. Based on the time the sampled voltage drops below zero, controller 24 may determine the time at which a voltage valley will occur in the oscillating voltage. For example, the voltage valley of the oscillating voltage may occur at one-quarter of the oscillation period. According to some examples, turn-on controller 26 may have already determined the oscillation period (Tosc) during turn-on.Therefore, when the ZCD unit 30 outputs a signal to the controller 24 indicating that the sampled voltage has crossed zero volts, the controller 24 can determine that a voltage valley will occur in the oscillating voltage approximately Tosc / 4 later. Because the sampled voltage follows the voltage at the switch node, the controller 24 can determine that a voltage valley will occur in the oscillating voltage at the switch node approximately Tosc / 4 later from the time the sampled voltage crossed zero from a voltage greater than zero.

[0071] In the methods described in this disclosure, the voltage at the switch node begins to oscillate when the current through winding 16 reaches zero. Although driver 12 may not be able to determine the exact time at which the oscillation began, driver 12 may still be able to determine when a voltage valley will occur based on the zero-crossing detection of the sampled voltage from winding 20.

[0072] Once driver 12 determines the time at which switch TO is to be turned on to achieve high switching efficiency, driver 12 can also determine the on-duty time (ton). Again, driver 12 may not be able to control the off-duty time (toff). Furthermore, the time at which switch TO is to be turned on determines the switching period (e.g., switch TO turns on at every switching period, which is determined based on when high switching efficiency is realized).

[0073] To set the duty cycle, the current sensing (CS) unit 34 receives a voltage indicative of the amount of current flowing through the switch TO. For example, as shown, the current flows through the switch TO and through a resistor connected to the CS pin of the driver 12, producing a voltage at the CS pin that is proportional to the amount of current flowing through the switch TO. The CS unit 34 may output a signal to the controller 24 indicative of the amount of current flowing through the switch. In response, the controller 24 may set the switch's duty cycle such that an average load current or voltage is approximately equal to the set load current or voltage.

[0074] In this way, the controller 24 can determine the switching period of the switch T0 (i.e., the time when the switch T0 is turned on again after being turned off, also referred to as the on-time), as well as how long the switch T0 should be turned on (i.e., the on-time). The controller 24 can then cause the pulse width modulation (PWM) unit 32 to turn on the switch T0 at appropriate times based on the specified switching period and to keep T0 turned on for an appropriate duration based on the specified on-time.

[0075] Accordingly, the CS unit 34, the controller 24, and the PWM unit 32 form a control loop to adjust the duty cycle for the given switching period, as determined using output signals from the ZCD unit 30 and the turn-on controller 26, to control the amount of current flowing through the load 13 so that the average load current or the average load voltage is approximately equal to the set load current level or the set voltage level. For example, the current flowing through the winding 16 is controlled by controlling when the switch T0 turns on and off, which controls the amount of current flowing through the load 13. According to some examples, the internal temperature of the driver may require adjustments to the duty cycle.Accordingly, the temperature sensor 28 may output a signal indicative of the internal temperature to the controller 24, and the controller 24 may use the internal temperature as one of the variables for determining the duty cycle.

[0076] However, a user can modify the set load current level or the set voltage level. For example, the set current level may be at a level where the LEDs are brightest, and the user can dim the LEDs, or the set load current level may be at a level where the LEDs are less bright, and the user can make the LEDs brighter. In this example, the control loop may adjust the duty cycle such that the average load current or the average load voltage is approximately equal to the set load current level or the set voltage level (i.e., the modified level set by the user, also referred to as the target load current level or target load voltage level). In some cases, there may be a limit as part of the control loop on how far the controller 24 can adjust the duty cycle.

[0077] If the controller 24, as part of the control loop, can no longer adjust the duty cycle to achieve the required average load current or average load voltage, the controller 24 can adjust the switching period (e.g., adjust the on-time). According to some examples, the controller 24 does not have to wait until the controller 24 can no longer adjust the duty cycle to achieve the required average load current or average load voltage, and can adjust the switching period (e.g., the on-time) to an earlier point.

[0078] Adjusting the switching period can lead to switching inefficiencies. For example, referring to Fig. 3, assume that the switching period is initially equal to switching period A, and the user dimmed the LEDs to such a level that the controller 24 had to adjust the switching period (e.g., increase the switching period). In this example, in order to achieve the set load current or the set load voltage level, the controller 24 may increase the switching period from switching period A to switching period B. As previously described, switching period B provides the lowest switching efficiency because the difference 44 is the largest difference between the oscillating voltage and the duty cycle switch node voltage level.

[0079] The example methods described in this disclosure provide ways to maintain high switching efficiency while allowing the required current to flow through the load 13 such that the average load current or average load voltage is equal to the target load current level or target load voltage level. According to a first example method, the controller 24 adjusts the switching period such that the switching period changes instantaneously from a first voltage valley to a second voltage valley. In this first example method, the controller 24 also adjusts a switch operating parameter (e.g., the duty cycle by adjusting time or peak current) such that the power output to the load remains the same immediately before adjusting the switching period from the first voltage valley to the second voltage valley and immediately after adjusting the switching period from the first voltage valley to the second voltage valley.After the driver 12 adjusts the switching period to the second voltage valley, if the user further modifies the target load current level or the target load voltage level, the controller can adjust the duty cycle as part of the control loop and immediately switch the voltage valleys if the controller 24 cannot achieve the target load current level or the target load voltage level by adjusting the duty cycle.

[0080] According to a second example method, the controller 24 adjusts the switching period to a switching period corresponding to a voltage valley via an intermediate switching period (i.e., an intermediate on-time). This intermediate switching period may be at a voltage level outside a valley of the oscillating voltage, resulting in switching inefficiencies. However, in the second example method, the controller 24 may gradually adjust the switching period until the switching period is at a voltage valley. In this second example method, the control loop may gradually adjust the switch operating parameters to correct for changes in the amount of current flowing through the load 13 that cause a deviation from the target load current level or the target load voltage level.Similar to the above, after the driver 12 adjusts the switching period to the second voltage valley, if the user further modifies the target load current level or the target load voltage level, the controller 24 can adjust the duty cycle as part of the control loop, and gradually adjust the switch turn-on time to another voltage valley if the controller 24 cannot achieve the target load current level or the target load voltage level by adjusting the duty cycle.

[0081] In the second example method, during the gradual adjustment of the switch on-time, the controller 24 may adjust the switch on-time to the intermediate time corresponding to the out-of-valley voltage of the oscillating voltage. Further, during the gradual adjustment of the one or more switch operating parameters, the controller may adjust the one or more switch operating parameters such that the amount of power supplied to the load 13 is maintained approximately constant when adjusting the on-time to the intermediate time corresponding to the out-of-valley voltage during the adjustment of the on-time from the first time to the second time.

[0082] The first and second example methods are described in more detail below, but in general, both example methods adjust the switching period so that the switch turns on at a voltage valley of the oscillating voltage at the switch node (i.e., they adjust the turn-on time so that the switch T0 turns on at a voltage valley). Both the first and second example methods may be advantageous over other methods because the first and second example methods adjust the turn-on time to equal a time at which a voltage valley is present, whereas other methods adjust the turn-on time to equal a time at which no voltage valley can be present. In other words, both the first and second example methods use valley-to-valley switching, whereas other methods operate with a single-valley setting.Because both the first and second example methods use valley-to-valley switching, and switching at a valley is called quasi-resonant switching, both the first and second example methods can use quasi-resonant switching.

[0083] The first example method uses an open-loop system in which the controller 24 instantaneously adjusts the on-time from the time of one voltage valley to the time of another voltage valley. Instead of sampling the resulting current flowing through the switch T0 and gradually adjusting the amount of current flowing through the load 13, the controller 24 instantaneously determines the switch operating parameters necessary to maintain the power supplied to the load 13 equal immediately before and immediately after the on-time adjustment. In this sense, the first example method can be considered valley-to-valley switching with power compensation.

[0084] In the second example method, the controller 24 does not immediately adjust the on-time from the time of one voltage valley to the time of another voltage valley. Rather, the controller 24 gradually adjusts the on-time from the time of one voltage valley through an intermediate on-time at which the oscillating voltage is not in a voltage valley, until the on-time is at a time of another voltage valley. This gradual adjustment of the on-time to another voltage valley causes the load current level or the load voltage level to deviate from the target load current level or the target voltage level.In this second example method, instead of immediately determining the required switch operating parameters in an open-loop scheme, the second example method relies on the feedback control loop to gradually adjust the switch operating parameters to correct deviations of the load current level or the load voltage level from the target load current level or the target load voltage level. In this sense, the second example method can be considered valley-to-valley switching with soft handover.

[0085] According to some examples, for the first example method, there may be no intermediate time when there is a reduction in switching efficiency, and the driver 12 may not necessarily determine whether the turn-on time has reached the voltage valley because the driver 12 jumps to the voltage valley immediately compared to the example method. According to some examples, for the second example method, the intensity of the flicker occurring during valley-to-valley switching may be minimal due to the gradual adjustment of the turn-on time compared to the first example method.

[0086] The following describes the first example method in more detail. For the purpose of illustration and simplifying the description, the following explains the example of adjusting the duty cycle from switching period A to switching period C and vice versa. Hereinafter, the duty cycle when the switching period is switching period A is denoted as ton,1, and switching period A is denoted as T1. When the duty cycle is switching period C, the switching period is denoted as ton,2, and switching period C is denoted as T2.

[0087] As described above, an instantaneous change in the switching period results in a gradual jump in power. In the lighting process, this gradual jump in power appears as flicker and negatively impacts the light quality. Accordingly, to avoid flicker, the power immediately before the switching period change and the power immediately after the switching period change should be the same (i.e., to achieve a constant power transition in the switching period).

[0088] In the first example method, a switch operating parameter (e.g., the duty cycle) required to maintain power constant when adapting from one voltage valley to another is based on the current duty cycle, the current switching period, and the parasitic oscillation period (Tosc). For example, the power supplied to load 13 is equal to: P=12*Vin2*ton2 / L / T

[0089] In equation 1, Vin (input voltage) and L (inductance) are constant, which means that ton 2 / T should be kept constant. In other words, ton 2 / T immediately before adjusting the switching period from T1 to T2 or from T2 to T1 and immediately after adjusting the switching period from T1 to T2 or from T2 to T1 should be equal. Accordingly, an increase or decrease in the switching period T by Tosc results in: ton2=ton1*sqrt((T1+Tosc) / T1)=ton1*sqrt(1+Tosc / T1) ton1=ton2*sqrt((T2+Tosc) / T2)=ton2*sqrt(1−Tosc / T2)

[0090] Equation 2 is for the increase in on-time from switching period A to switching period C (i.e., increase from T1 to T2). Equation 3 is for the decrease in on-time from switching period C to switching period A (i.e., decrease from T2 to T1). Furthermore, as described above, the on-time (e.g., ton1 and ton2) can affect the on-time (e.g., T1 and T2). In other words, the on-time is a function of the on-time; however, Equation 2 and Equation 3 are constructed based on the approximation that the switching period is not a function of ton (i.e., T = f(ton). However, Equations 2 and 3 are based on an approximation that assumes that T != f(ton)).

[0091] Controller 24 determines a turn-on time that compensates for the change in switching period during a valley transition in quasi-resonant switching. According to some examples, controller 24 may be a digital controller with limited processing capabilities, and performing the square root operation may not be practical for controller 24. Instead of performing the square root operation, controller 24 may, according to some examples, determine the square root using lookup tables. According to some examples, instead of using lookup tables, the controller may approximate the square root using its Taylor series: (1+x)0.5=1+(1 / 2)x−(1 / 8)x2+(1 / 16)x3−(5 / 128)x4+(7 / 256)x5

[0092] To reduce the computational work that controller 24 must perform, controller 24 can calculate the first two terms of the Taylor series and stop, with the result being an approximation of the actual square root value. In this case, equations 2 and 3 simplify to: ton2=ton1*(1+Tosc / T1 / 2)=ton1*(T1+Tosc / 2) / T1; ton1=ton2*(1−Tosc / T2 / 2)=ton2*(T2−Tosc / 2) / T2

[0093] In each of Equations 5 and 6, the latter form of the equations (i.e., ton1*(T1+Tosc / 2) / T1 for Equation 5 and ton2*(T2-Tosc / 2) / T2 for Equation 6) is better for fixed-point numbers because it reduces quantization noise by first multiplying and then dividing the result. Furthermore, the first example methods are based on various approximations. Therefore, the final determined duty cycle may not be exactly correct, but the feedback control loop of driver 12 corrects any deviations from the exact values.

[0094] To test how well this first example method works, it is possible to measure the output voltage across the switch. If the measurement shows that the duty cycle (ton) and the on-time (switching period) change instantaneously in a step-like jump at the same time, this first example method can be very useful. However, such an instantaneous change in the step-like jump at the same time is not required for the first example method to be effective.

[0095] Furthermore, as described above, there can be two ways to set the duty cycle: (1) setting the duty cycle and (2) setting the peak current. Equations 1-3, 5, and 6 are applicable to examples where the controller 24 sets the duty cycle time to set the duty cycle. The following describes the equations for the example where the controller 24 sets the peak current threshold amplitude to set the duty cycle.

[0096] If the duty cycle is set by the peak current, the equation for power is: P=12*L*Ipk2 / T

[0097] Similar to above, L is constant, and therefore Ipk 2 / T is kept constant, so that the power immediately before the on-time change and immediately after the on-time change are the same. Ipk is the peak current threshold amplitude. Furthermore, the on-time (i.e., the switching period T) is a function of Ipk; however, the equations are constructed based on the approximation that the switching period is not a function of Ipk (i.e., T = f(Ipk), however, the equations are based on the approximation that T != f(Ipk)).

[0098] Similar to equations 2 and 3: Ipk2=Ipk1*sqrt((T1+Tosc) / T1)=Ipk1*sqrt(1+Tosc / T1) Ipk1=Ipk2*sqrt((T2+Tosc) / T2)=Ipk2*sqrt(1+Tosc / T2)

[0099] Similar to equations 5 and 6 with the Taylor series approximation: Ipk2=Ipk1*(1+Tosc / T1 / 2)=Ipk1*(T1+Tosc / 2) / T1; Ipk1=Ipk2*(1−Tosc / T2 / 2)=Ipk2*(T2−Tosc / 2) / T2

[0100] The following describes the second example procedure in more detail. In this example, switching from one valley to another occurs not in a single step, but rather using a slow sweep. In this example, the control loop adjusts the switch operating parameters (e.g., duty cycle based on time or peak current) so that the average load current or average load voltage approximately equals the set load current level or set load voltage level.

[0101] In the second example method, there may be a target valley N and a further switching period as the switching period decreases, and a target valley N and a further switching period as the switching period increases. For ease of understanding, voltage valley 36 is referred to as N1 and voltage valley 40 is referred to as N2.

[0102] If the switching period needs to be increased, the controller 24 can set a switching period to the time of N1 (voltage valley 36) and can set the target valley to N2 (voltage valley 40). As explained above, the turn-on controller 26 can determine the Tosc, from which the controller 24 can determine the timing of multiple voltage valleys after the ZCD unit 32 detects the zero crossing time.

[0103] In this example, because the set switching period (N1) is less than the time of N2, driver 12 can maintain switching at the time of N1. Controller 24 can gradually increase the switching period from N1 to N2, allowing the feedback control loop to compensate for the amount of current flowing through load 13 to increase the switching period. When controller 24 determines that the voltage valley at N2 has been reached by the gradual increase in the switching period, controller 24 sets the switching period to N2.

[0104] If the switching period needs to be reduced, the controller 24 can set the switching period to time N2 and set the target valley to N1. In this example, because the set switching period (N2) is greater than the time of N1, the driver 12 can maintain switching at the time of N2. The controller 24 can gradually reduce the switching period from N2 to N1, allowing the feedback control loop to compensate for the amount of current flowing through the load 13 to reduce the switching period. When the controller 24 determines that the voltage valley at N1 has been reached by gradually reducing the switching period, the controller 24 sets the switching period to N1.

[0105] For example, once the PWM unit 32 has determined the time between valleys N1 and N2 (e.g., Tosc if N1 and N2 are adjacent valleys with N1 before N2), it may employ a mechanism to generate a timing for the switching period based on the time during which N1 is detected, plus or minus an adjustable delay. If a change from N1 to N2 is desired, the delay may be gradually increased from 0 to Tosc. If a change from N2 to N1 is desired, the delay may be gradually decreased from Tosc to 0. For example, to change from N1 to N2, the controller 24 may cause the PWM unit 32 to gradually increase the switching time from N1 plus a delay between 0 and Tosc. The original switching period may be N1 + 0, then N1 + 0.01, then N1 + 0.02, and so on until the switching period is equal to N1 + Tosc, which is equal to N2.In the reverse case from N2 to N1, the original switching period may be equal to N2 - 0, then N2 - 0.01, then N2 - 0.02, and so on until the switching period is equal to N2 - Tosc, which is equal to N1.

[0106] In the above example, an increase or decrease in the switching period by 0.01 is provided merely for ease of understanding and is not to be understood as limiting. A finer or coarser increase or decrease in the switching period is possible. For example, in this second example method, the increase or decrease in the switching period can be viewed as an analog increase or decrease in the switching period, in which case the switching period can be located at many intermediate switching periods between N1 and N2 in the rising switching period or between N2 and N1 in the falling switching period. The first example method can be viewed as a digital increase or decrease in the switching period, in which case the switching period jumps from N1 to N2 or vice versa, in which case the switching period does not assume a different intermediate switching period.

[0107] Furthermore, the switching period can be monitored by measuring the voltage across the switch, similar to the first example method. This second example method may work well if measurements show that the switching period does not jump, but rather sweeps from one valley to another. However, the second example method may work well even if there are only minor or otherwise imperceptible jumps in the switching period.

[0108] For example, the second example method may be configured as follows: The switch on-time may be set to a first time associated with voltage valley 36, and controller 24 may determine to adjust the switch on-time from the first time associated with voltage valley 36 such that an average amount of current flowing through load 13 or an average amount of voltage across load 13 is approximately equal to a target load current level or a target load voltage level. In this example, controller 24 may determine a second time associated with voltage valley 40. However, instead of gradually changing the on-time from the first time to the second time, controller 24 may gradually adjust the on-time in smaller increments.Conceptually, this can be viewed as the controller adjusting the turn-on time such that the switch's turn-on time is briefly at a time corresponding to a non-voltage valley. For example, the turn-on time can be viewed as being set such that the turn-on time corresponds to each of the points on the oscillation between voltage valley 36 and voltage valley 40, where in the first example method, the turn-on time jumps from the time corresponding to voltage valley 36 to the time corresponding to voltage valley 40. In other words, the first example method can be viewed as a digital turn-on time adjustment, while the second example method can be viewed as an analog turn-on time adjustment.

[0109] For example, the controller 24 may gradually adjust the switch's on-time such that, during the gradual adjustment of the switch's on-time, the controller 24 adjusts the switch's on-time to an intermediate time corresponding to a voltage outside a valley of the oscillating voltage at the switch node (e.g., the voltage spike 38). During the gradual adjustment, the feedback loop of the controller 24 adjusts the operating parameters to keep the power level approximately constant.For example, during the gradual adjustment of the one or more switch operating parameters, the controller may adjust the one or more switch operating parameters such that the amount of power supplied to the load is approximately constant when the on-time is adjusted from the first time to the second time to the intermediate time corresponding to the voltage outside a valley during the adjustment of the on-time.

[0110] For example, assume that in the first and second example methods, the switch's on-time is set to a first time corresponding to the voltage valley 36. Further, assume that the user changes the amount of current that should flow through the load 13 (sets the target current level). In this case, the feedback loop of the controller 24 adjusts the switch's on-time while maintaining the same on-time. However, at some point, the feedback loop of the controller 24 may no longer adjust the on-time to achieve the target current level.

[0111] In the first example method, controller 24 causes a stepwise jump from the first time associated with voltage valley 36 to a second time associated with voltage valley 40 and adjusts one or more switch operating parameters so that the performance is substantially the same. In this way, there is very little flicker. Furthermore, the feedback loop of controller 24 continues adjusting the switch's on-time until the current through load 13 is equal to the target current level.

[0112] In the second example method, the controller 24 gradually adjusts the on-time from the first time corresponding to the voltage valley to a second time corresponding to the voltage valley 40, and gradually adjusts the switch operating parameters to keep the amount of output power approximately constant during the gradual adjustment of the on-time. Furthermore, the feedback loop of the controller 24 maintains the adjustment of the switch on-time until the current through the load 13 is equal to the target current level.

[0113] In this way, adjustment of the amount of current flowing through load 13 is possible while maintaining high switching efficiency. For example, if the feedback loop of controller 24 is capable of adjusting the turn-on time so that the current flowing through load 13 is equal to the target current level, then controller 24 may not adjust the switch's turn-on time. However, if the feedback loop of controller 24 is not capable of adjusting the turn-on time so that the current through load 13 is equal to the target current level, then controller 24 may adjust the turn-on time. To maintain switching efficiency, it may be desirable to set the turn-on time to a voltage valley and then allow the feedback loop of controller 24 to further adjust the turn-on time so that the current through load 13 is equal to the target current level.

[0114] The first example procedure performs a jump in turn-on time from one voltage trough to another. If no additional steps are taken, there would be flicker due to the instantaneous change in current. To account for this, the controller determines what some of the switch operating parameters should be so that performance is essentially the same. In this way, the turn-on time can jump from one voltage trough to another, but the performance remains the same and therefore there is no flicker. The feedback loop can then take over and adjust the turn-on time to achieve the target current level.

[0115] The second example method performs a gradual adjustment of the on-time from one voltage valley to another. During this gradual adjustment, the on-time is temporarily outside of a voltage valley, but the feedback loop is able to control the on-time so that the power level remains constant. Then, once the controller 24 sets the on-time to a voltage valley, the feedback loop can take over and adjust the on-time to achieve the target current level.

[0116] Although the above examples were described with a current level, the methods are not limited thereto and can be extended to a target voltage level. Likewise, according to some examples, as described above, the target current level or a target voltage level can be based on the average amount of current flowing through the load 13 or the average amount of voltage across the load 13.

[0117] Fig. 4 is a flowchart illustrating a first example method according to this disclosure. In Fig. 4, the controller 24 may determine whether or not to adjust a turn-on time of the switch TO from a first time corresponding to a first voltage valley of an oscillating voltage at a node of the switch TO (e.g., a drain node) (e.g., a time corresponding to the voltage valley 36 or 40) to a second time corresponding to a second voltage valley in the oscillating voltage at the node of the switch TO (e.g., a time corresponding to the voltage valley 40 or 36) (50).

[0118] For example, the controller 24 may determine whether to adjust the on-time (i.e., the on-time of the switch TO) based on whether an amount of current flowing through the load is adjusted such that an average amount of current flowing through the load or an average amount of voltage across the load is approximately equal to a target load current level or a target voltage level. Furthermore, the oscillating voltage occurs at a drain node (as an example) of the switch TO when the switch TO is off and no current flows through a winding connected to the switch TO (e.g., winding 16).

[0119] The controller 24 may set one or more switch operating parameters for the switch such that when the on-time is adjusted from the first time to the second time, the amount of power supplied to the load immediately before the adjustment and immediately after the adjustment is substantially the same (52). According to some examples, the one or more switch operating parameters include a time duration that determines how long the switch is turned on. According to some examples, the one or more switch operating parameters include a peak current threshold amplitude that determines how long the switch is turned on. Further, according to some examples, the controller 24 may implement an open-loop (LOC) system.: "open loop scheme") to set the operating parameters, wherein the controller 24 can set the one or more switch operating parameters without having to determine an amount of current flowing through the switch.

[0120] The controller 24 may adjust the turn-on time of the switch to the second time based on the determination to adjust the turn-on time (54). The controller 24 may adjust the one or more switch operating parameters for the switch based on the one or more determined switch operating parameters (56). According to some examples, the controller 24 may cause the PWM unit 32 to adjust the time at which the switch is to be turned on (e.g., adjust the turn-on time) concurrently with adjusting the one or more switch operating parameters.

[0121] Fig.5 is a flowchart illustrating a second example method according to this disclosure. The controller 24 may determine whether or not to adjust a switch-on time from one having a first voltage valley to an oscillating voltage at a node of the switch at a first time such that an average amount of current flowing through the load or an average amount of voltage across the load is approximately equal to a target load current level or a target load voltage level (60). Further, the oscillating voltage occurs at the drain node of the switch when the switch is off and no current flows through the coil connected to the switch.

[0122] The controller 24 may determine a second time corresponding to a second trough of the oscillating voltage (62). For example, the controller 24 may determine the second time corresponding to the second trough of the oscillating voltage based on a time at which a current through the winding 16 reaches zero (e.g., based on an indication from the ZCD unit 30). The controller 24 may gradually adjust the switch on time from the first time to the second time over an intermediate time corresponding to a voltage outside of a trough of the oscillating voltage at the node of the switch based on the determination to adjust the on time (64).The controller 24 may gradually adjust the one or more switch operating parameters for the switch to keep the amount of power supplied to the load approximately constant during the gradual adjustment of the switch on time from the first time to the second time (66).

[0123] For example, during the gradual adjustment of the switch on-time, the controller 24 may adjust the switch on-time to the intermediate time corresponding to the voltage outside a voltage valley of the oscillating voltage at the switch node. Further, during the gradual adjustment of the one or more switch operating parameters, the controller may adjust the one or more switch operating parameters to keep the amount of power supplied to the load approximately constant when the on-time changes from the intermediate time corresponding to the voltage outside a voltage valley during the adjustment of the on-time from the first time to the second time.

[0124] According to one example, the controller may gradually adjust the on-time of the switch from the first time to the second time with a feedback control loop that measures the amount of current flowing through the load during the gradual adjustment of the on-time of the switch from the first time to the second time. According to some examples, the one or more switch operating parameters include a time duration that determines how long the switch is turned on. According to some examples, the one or more switch operating parameters include a peak current threshold amplitude that determines how long the switch is turned on.

[0125] According to one or more examples, the described functions may be implemented in hardware, software, firmware, or any combination thereof. When implemented in software, the functions may be stored as one or more instructions or code on a computer-readable medium and executed by a hardware-based computing device (e.g., the controller 24 of the driver 12). Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media. In this way, computer-readable media may generally correspond to tangible, non-transitory computer-readable storage media.The data storage media may be any available media accessible by one or more computers or one or more processors to retrieve instructions and / or data structures for implementing the methods described in this disclosure. A computer program product may include a computer-readable medium.

[0126] By way of example and without limitation, such computer-readable storage media may include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, flash memory, or any other medium that can be used to store the desired program code in the form of instructions or data structures and that can be accessed by a computer. It is understood that computer-readable storage media and data storage media do not contain carrier waves, signals, or other transient media, but instead refer to non-transitory, tangible storage media. As used herein, floppy disks and diskettes include compact discs (CDs), laser discs, optical discs, DVDs (Digital Versatile Discs), floppy disks, and Blu-ray discs, with floppy disks typically being magnetically reproducing, while floppy disks are non-volatile.: "discs") optically reproduce data using lasers. Combinations of the above are also included in the area of ​​computer-readable media.

[0127] Instructions may be executed by one or more processors (e.g., the controller 24 of the driver 12), such as one or more central processing units (CPUs), digital signal processors (DSPs), general-purpose microprocessors, application-specific integrated circuits (ASICs), field-programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuits. Accordingly, the term "processor" or "controller" as used herein may refer to any of the structures described above or any other structure suitable for implementing the methods described herein. Furthermore, the methods could be implemented entirely in one or more circuits or logic elements.

[0128] The methods of this disclosure may be implemented in a wide variety of devices or apparatus, including an integrated circuit (IC) or a set of ICs (e.g., a chipset). Various components, modules, or units are described in this disclosure to highlight functional aspects of devices configured to perform the disclosed methods, but they do not necessarily require implementation by different hardware units. Rather, as described above, different units may be combined into one hardware unit or provided by a collection of interoperable hardware units, including one or more processors as described above, in conjunction with suitable software and / or firmware.

Claims

[1] A method for controlling power supplied to a load (13), the method comprising: Determining whether a turn-on time of a switch (T0) is adjusted from a first time corresponding to a first voltage valley in an oscillating voltage at a node of the switch (T0) to a second time corresponding to a second voltage valley in the oscillating voltage; when adjusting the on-time of the switch (T0) from the first time to the second time, gradually adjusting the on-time of the switch (T0) from the first time to the second time, wherein the gradual adjustment of the on-time comprises gradually increasing or decreasing a switching period from a switching period leading to the first time to a switching period leading to the second time, so that the on-time assumes a plurality of intermediate times corresponding to voltages outside a voltage valley in the oscillating voltage at the node of the switch (T0); gradually adjusting one or more switch operating parameters for the switch (T0) to keep a power supplied to the load (13) substantially constant during the gradual adjustment of the on-time from the first time to the second time and to keep an average amount of current flowing to the load approximately at a target current level or an average amount of voltage across the load approximately at a target voltage level. [2] The method of claim 1, wherein gradually adjusting the one or more switch operating parameters comprises gradually adjusting with a feedback control loop that measures a current flowing through the switch (T0) during the gradual adjustment of the on-time from the first time to the second time. [3] Method according to one of claims 1 to 2, wherein the one or more switch operating parameters comprise a time duration which determines how long the switch (T0) is switched on. [4] A method according to any one of claims 1 to 3, wherein the one or more switch operating parameters comprise a peak current threshold amplitude that determines how long the switch (T0) is turned on. [5] A method according to any one of claims 1 to 4, wherein the oscillating voltage occurs at a drain node of the switch (T0) when the switch (T0) is turned off and when no current flows through a winding (16) connected to the switch (T0). [6] A driver system for controlling power supplied to a load (13), the system comprising: a transformer having a primary side having a first winding (16) and a secondary side having a second winding (18), the load (13) being connected to the second winding (18); a switch (T0) coupled to the first winding (16); and a driver (12) coupled to the switch (T0) and configured to: determining whether to adjust a turn-on time of the switch (T0) from a first time corresponding to a first voltage valley in an oscillating voltage at a node of the switch (T0) to a second time corresponding to a second voltage valley in the oscillating voltage; when adjusting the on-time of the switch (T0) from the first time to the second time, gradually adjusting the on-time of the switch from the first time to the second time, wherein the gradual adjustment of the on-time comprises gradually increasing or decreasing a switching period from a switching period leading to the first time to a switching period leading to the second time, so that the on-time assumes a plurality of intermediate times corresponding to voltages outside a voltage valley in the oscillating voltage at the node of the switch; and gradually adjust one or more switch operating parameters for the switch (T0) to keep a power supplied to the load (13) approximately constant during the gradual adjustment of the switch-on time of the switch (T0) from the first time to the second time and to keep an average amount of current flowing to the load approximately at a target current level or an average amount of voltage across the load approximately at a target voltage level. [7] The driver system of claim 6, wherein the transformer further comprises a sensing side including a third winding (20), and wherein the driver (12) is configured to set the second time based on the third winding (20) measuring the voltage at the node of the switch (T0) and a zero crossing unit (30) determining when the sensed voltage crosses zero. [8] A driver system according to any one of claims 6 and 7, wherein the driver (12) is adapted to gradually adjust the one or more switch operating parameters with a feedback control loop that measures a current flowing through the switch during the gradual adjustment of the switch-on time of the switch (T0) from the first time to the second time. [9] A driver system according to any one of claims 6 to 8, wherein the one or more switch operating parameters comprise a time duration that determines how long the switch (T0) is switched on. [10] A driver system according to any one of claims 6 to 9, wherein the one or more switch operating parameters comprise a peak current threshold amplitude that determines how long the switch (T0) is turned on. [11] A driver system according to any one of claims 6 to 10, wherein the oscillating voltage occurs at a drain node of the switch (T0) when the switch (T0) is turned off and when no current flows through the first winding (16). [12] A driver for controlling power supplied to a load (13), the driver (12) comprising: a controller (24) designed to: determining whether a turn-on time of a switch (T0) is adjusted from a first time corresponding to a first voltage valley in an oscillating voltage at a node of the switch (T0) to a second time corresponding to a second voltage valley in the oscillating voltage; to gradually adjust the on-time of the switch (T0) from the first time to the second time, wherein the gradual adjustment of the on-time comprises gradually increasing or decreasing a switching period from a switching period leading to the first time to a switching period leading to the second time, so that the on-time assumes a plurality of intermediate times corresponding to voltages outside a voltage valley in the oscillating voltage at the node of the switch (T0); and gradually adjust one or more switch operating parameters for the switch (T0) to keep a power supplied to the load (13) approximately constant during the gradual adjustment of the switch-on time of the switch (T0) from the first time to the second time and to keep an average amount of current flowing to the load approximately at a target current level or an average amount of voltage across the load approximately at a target voltage level. [13] The driver of claim 12, wherein the controller (24) for gradually adjusting the one or more switch operating parameters is configured for gradually adjusting with a feedback control loop that measures a current flowing through the switch (T0) during the gradual adjustment of the on-time of the switch (T0) from the first time to the second time. [14] A driver according to any one of claims 12 and 13, wherein the one or more switch operating parameters comprise a time duration that determines how long the switch is turned on. [15] A driver according to any one of claims 12 to 14, wherein the one or more switch operating parameters comprise a peak current threshold amplitude that determines how long the switch (T0) is turned on. [16] A driver according to any one of claims 12 to 15, wherein the oscillating voltage occurs at a drain node of the switch (T0) when the switch (T0) is off and when no current flows through the first winding (16). [17] A driver according to any one of claims 12 to 16, wherein the controller (24) is configured to determine the second time associated with a second voltage valley in the oscillating voltage based on a time at which a current through a winding (16) connected to the switch (T0) reaches zero.

Images