Controlling an active clamp switching power converter circuit based on a sensed voltage drop on an auxiliary winding

An active clamp switching power converter circuit includes a sensing circuit that generates a sensed auxiliary winding voltage of an auxiliary winding around the core. A primary side controller controls switching of a power switch coupled to a primary winding to control current through the primary winding and controls switching of the active clamp switch based on the sensed voltage to control leakage energy when the power switch is turned off. The primary side controller regulates timing of the switching to achieve a zero voltage switching condition prior to turning on the power switch for power efficient operation.

BACKGROUND

The present disclosure relates generally to a switching power converter circuit, and more specifically, to an active clamp switching power converter that uses sensing of an auxiliary winding.

An active clamp switching power converter circuit regulates power delivered to a load by controlling switching of a power switch and reduces power loss by controlling switching of an active clamp switch. However, a challenge exists in controlling timing of the switching for power efficient operation in a manner that does not depend on complex external circuits or high voltage components.

SUMMARY

A switching power converter circuit comprises a transformer coupled between an input terminal receiving an input voltage and an output terminal supplying an output voltage. The transformer includes a core, a primary winding around the core coupled to the input terminal, and a secondary winding around the core coupled to the output terminal. The switching power converter circuit further comprises a sensing circuit comprising an auxiliary winding around the core of the transformer. The sensing circuit generates a sensed voltage based on an auxiliary winding voltage across the auxiliary winding. The switching power converter circuit also comprises a power switch coupled to the primary winding of the transformer that controls current through the primary winding. The switching power converter circuit further comprises an active clamp switch to control leakage energy of the switching power converter when the power switch is turned off. The switching power converter circuit further comprises a secondary side switch to control secondary side current through the secondary winding. A secondary side controller controls switching of the secondary side switch. A primary side controller controls switching of the power switch to regulate the output voltage and controls switching of the active clamp switch to turn on the active clamp switch following the power switch turning off and to turn off the active clamp switch based on detected timing of a voltage drop occurring in the sensed voltage satisfying a predefined detection condition.

In another embodiment, a method is provided for controlling the switching power converter. In a first switching cycle, a primary side controller detects a sensed voltage value that is based on an auxiliary winding voltage across an auxiliary winding of a transformer. In the first switching cycle, the controller detects a time interval between the active clamp switch being turned on and the voltage drop in the sensed voltage satisfying a predefined detection condition. In a subsequent switching cycle, the controller controls a turn-off time of the active clamp switch relative to the turn-on time of the active clamp switch such that timing of the voltage drop in the sensed voltage satisfying the predefined detection condition in the subsequent switching cycle is closer to a target timing. The power switch is turned on a substantially fixed time period after the active clamp switch is turned off.

DETAILED DESCRIPTION OF EMBODIMENTS

The Figures (FIG.) and the following description relate to the preferred embodiments of the present invention by way of illustration only. It should be noted that from the following discussion, alternative embodiments of the structures and methods disclosed herein will be readily recognized as viable alternatives that may be employed without departing from the principles of the present disclosure.

Figure (FIG. 1illustrates a switching power converter circuit100in accordance with one embodiment. The switching power converter circuit100generates a regulated output voltage Vout to power a load (e.g., an electronic device (not shown)). In an embodiment, the switching power converter circuit100includes an active clamp flyback circuit in which leakage energy is recycled to reduce power loss relative to a conventional flyback circuit. The switching power converter circuit100may furthermore generate feedback signals for controlling timing of switching for efficient power operation of the switching power converter circuit100and to ensure switching occurs shortly after a zero voltage switching (ZVS) condition is achieved.

In an embodiment, the switching power converter circuit100includes a transformer T1, a power switch S1, an active clamp switch S2, a clamping capacitor Cc, a sensing circuit120, a primary side controller110, an output filter capacitor C1, a current sensor140, a secondary side switch S3, and a secondary side controller150.

The transformer T1is coupled between an input terminal receiving an input voltage Vin (e.g., a rectified AC voltage) and an output terminal supplying the output voltage Vout to the load. The transformer T1includes a magnetic core. On a primary side115of the switching power converter circuit100, a primary winding Np has a first polarity and wraps around the core of the transformer T1in a first direction. The primary winding Np is coupled to the input terminal at a first end of the primary winding Np. On a secondary side145of the switching power converter circuit100, a secondary winding Ns has a second polarity (different than the first polarity) and wraps around the core of the transformer T1in a second direction (different than the first direction). The secondary winding Ns is coupled to the output terminal.

The power switch S1is coupled to the primary winding Np of the transformer T1at a second end of the winding Np such that a voltage Vx is across the power switch S1. The power switch S1may be a transistor such as a MOSFET, a bipolar junction transistor, or some other type of switch. The power switch S1is configured to control the primary current Ip through the primary winding Np by enabling the primary current Ip to flow from the input voltage Vin through the primary winding Np to ground when the power switch S1is on, and by preventing the primary current Ip from flowing to ground when the power switch S1is off.

When the power switch S1is off, some energy stored in the leakage inductance of the transformer causes energy loss. The active clamp switch S2controls the flow of this leakage energy when the power switch S1is turned off to recycle leakage energy back to the primary winding Np and reduce power loss. The active clamp switch S2couples between the second end of the primary winding Np (at voltage node Vx) and the input voltage Vin through the clamping capacitor Cc. The active clamp switch S2may be a transistor such as a MOSFET, a bipolar junction transistor, or some other type of switch.

The secondary side controller150controls switching of the secondary side switch S3. Particularly, the secondary side controller150controls switching of the secondary side switch S3so that the secondary side switch S3turns on after a voltage V_S3across the secondary side switch begins to decrease below zero. The secondary side controller150turns off the secondary side switch S3when a secondary side current Is decreases to zero (or is within a predefined range of zero). The secondary side controller150detects the voltage V_S3and detects the secondary side current Is via the current sensor140. In one embodiment, the current sensor140sends a signal to the secondary side controller150when the secondary side current Is is within a predefined range of zero.

The sensing circuit120includes an auxiliary winding Na that wraps around the core of the transformer T1, a control supply capacitor C2, a clamping diode D2, and a detection circuit130. The auxiliary winding Na is wrapped around the core of the transformer T1in a same direction and has the same polarity as the secondary winding Ns. The sensing circuit120is connected to a collector supply voltage Vcc at a collector supply terminal with a positive terminal Vcc+ and a negative terminal Vcc−. The clamping diode D2is coupled between the positive terminal Vcc+ of the supply terminal between and one end of the auxiliary winding Na to clamp the sensed voltage V_sense to the negative terminal Vcc− voltage when the change in current through the auxiliary winding is sufficiently high for the clamping diode D2to become forward biased. The sensed voltage V_sense represents the voltage across the auxiliary winding Na. The sensed voltage V_sense is monitored by the detection circuit130. In one embodiment, the detection circuit130may include a high pass filter. The detection circuit130monitors the sensed voltage V_sense and provides a detection signal to the primary side controller110when the sensed voltage V_sense satisfies a predefined detection condition indicative of the secondary side switch S3turning off. For example, the predefined detection condition may be satisfied when the sensed voltage V_sense experiences a voltage drop of at least a predefined amount over a predefined time interval during a period when the power switch S1is turned off and the active clamp switch S2is turned on. The predefined amount of voltage drop may be dependent on the circuit design characteristics. For instance, an amount of leakage inductance and magnetizing inductance found on the primary side115may affect the amount of the voltage drop. In some embodiments, the predefined amount of voltage drop may be between 3 and 4 volts. In some embodiments, the predefined time interval over which the voltage drop takes place may be between 100 nanoseconds (ns) to 400 ns. The predefined detection condition occurs and the secondary side switch S3turns off a substantially fixed time period before the ZVS condition occurs. Thus, by detecting this voltage drop at the primary side controller, a timing of the ZVS condition can be predicted and switching of power switch S1and active clamp switch S2may be controlled accordingly.

The primary side controller110controls switching of the power switch S1and controls switching of the active clamp switch S2. Particularly, the primary side controller110controls switching of the power switch S1so that the power switch S1has on-times and off-times appropriate to maintain a regulated output voltage Vout. Furthermore, the primary side controller110controls switching of the active clamp switch S2so that it has an on-time during the off-time of the power switch S1(i.e., so that the on-times of the power switch S1and the active clamp switch S2are non-overlapping), to enable recycling of the leakage energy when the power switch S1is turned off. The primary side controller110furthermore controls the relative timings of the on-times and off-times of the active clamp switch S2and the power switch S1to enable the power switch S1to turn on in each switching cycle a short time after a ZVS condition is achieved (i.e., the voltage Vx=0), which enables power efficient switching. The primary side controller110achieves these switching conditions by receiving the detection signals from the detection circuit130indicative of the ZVS condition and controlling switching accordingly as described in further detail below with reference toFIG. 3.

FIGS. 2A-2Dillustrate a model of a primary side115of a switching power converter circuit (e.g., the switching power converter circuit100) during different stages of a switching cycle with leakage inductance, magnetizing inductance, and parasitic capacitance modeled as circuit elements, according to one embodiment. In the models200,202,204,206the leakage inductance of the primary winding Np is modeled as an inductor Lr, the magnetizing inductance is modeled as an inductor Lm, a drain-source parasitic capacitance of the power switch S1is modeled as a capacitor Coss1, a body diode of the power switch S1is modeled as BD1, a drain-source parasitic capacitance of the active clamp switch S2is modeled as a capacitor Coss2, and a body diode of the active clamp switch S2is modeled as BD2. These models200,202,204,206will be referenced in describing the operation of the switching power converter circuit100below.

FIGS. 2A-Dwill be described together withFIG. 3, which provides waveforms illustrating the operation of the switching power converter circuit100ofFIG. 1in accordance with the primary side models ofFIGS. 2A-2D. Five time frames (T1, T2, T3, T4, and T5) are illustrated that collectively make up one switching cycle. The time frames are not necessarily fixed and some of the time frames may vary between cycles.

During time frame T1, Vg1has a high value (i.e., the power switch S1is turned on), Vg2has a low value (i.e., the active clamp switch S2is turned off), and Vg3has a low value (i.e., the secondary side switch S3is turned off). Operation of the switching power converter circuit100during time frame T1is illustrated by the primary side model200inFIG. 2A. In this time frame T1, the BD1of the power switch S1stops conduction as a result of the voltage Vx across the power switch S1being substantially zero (coupled to ground). The voltage across the primary winding Np (i.e., the voltage Vp) is the input voltage Vin, which causes energy to be stored in the primary winding Np and the primary winding primary current Ip to increase. The primary current Ip is equal to a leakage inductance current ILr and equal to a magnetizing inductance current ILm (i.e., Ip=ILr=ILm). The leakage inductance current ILr and the magnetizing inductance current ILm increase linearly with a slope dependent on the input voltage Vin as described in the following equation:

i⁢p⁡(t)=i⁢p⁡(t0)+V⁢i⁢nLm⁡(1+β)⁢(t1)(1)β=LrLm(2)
where t0is a starting time of time frame T1and t1is an end time of time frame T1. The voltage across the secondary winding Ns (i.e., the voltage Vs) is zero, blocking the transfer of energy to the load. In this state, energy is supplied to the load via the output filter capacitor C1. The auxiliary winding voltage Va is proportional to the secondary winding voltage Vs, and is therefore zero, which means the sensed voltage V_sense is also zero.

During time frame T2, Vg1has a low value (i.e., the power switch S1is turned off), Vg2has a low value (i.e., the active clamp switch S2is turned off), and Vg3transitions from a low value to a high value (i.e., the secondary side switch S3turns on). Operation of the switching power converter circuit100during this time frame T2is illustrated by the primary side model202inFIG. 2B. The primary current Ip charges up the capacitor Coss1of the power switch S1(approximately linearly). Following the secondary side switch S3turning on when V_S3drops below ground, the energy stored in transformer T1is transferred to the load and the secondary side current Is re-charges the output filter capacitor C1.

During time frame T3, Vg1has a low value (i.e., the power switch S1is turned off), Vg2has a high value (i.e., the active clamp switch S2is turned on), and Vg3has a high value (i.e., the secondary side switch S3is turned on). Operation of the switching power converter circuit100during this time frame T3is represented by the primary side model204inFIG. 2C. At the start of time frame T3, the voltage Vx across the power switch S1is the sum of the input voltage Vin and a voltage across the clamping capacitor Cc and the body diode BD2of the active clamp switch S2starts to conduct. The inductor Lm and the inductor Lr resonate with both the clamping capacitor Cc and the capacitor Coss2of the active clamp switch S2until the secondary winding voltage Vs is greater than the output voltage Vout. During the time frame T3, the magnetizing inductance current ILmat a time t decreases linearly transferring energy to the output as described by the following equation:

i⁢L⁢m⁡(t)=i⁢L⁢m⁡(t2)-N⁢V⁢o⁢u⁢tL⁢m⁢(t-t2)(3)
where t2is a starting time of time frame T3, t is a time during time frame T3, and NVout is a reflected voltage with N representing a turn ratio (i.e., Np/Ns) between the primary winding Np and the secondary winding Ns. With the inductor Lr and the clamping capacitor Cc resonating during the transformer T1reset, the leakage inductance current ILris described by the following equations:
iLr(t)=A2·sin(ω2t+φ2)  (4)

During time frame T4, Vg1has a low value (i.e., the power switch S1is turned off), Vg2has a high value (i.e., the active clamp switch S2is turned on), and Vg3has a low value (i.e., the secondary side switch is turned off). Still referring to the primary side model ofFIG. 2C, at t3, the leakage inductance current ILrand the magnetizing inductance current ILmhave equal and opposite current slope directions. After t3, the voltage across the inductor Lr is affected by only the magnetizing inductance current ILm. This results in a voltage drop of the sensed voltage V_sense and a voltage drop of the secondary winding voltage Vs. The voltage Vx across the power switch S1remains at a level exceeding the input voltage Vin. Here, the voltage Vx is the sum of the input voltage Vin and a voltage across the clamping capacitor Cc where the voltage across the clamping capacitor Cc substantially equals the voltage NVout across the primary winding. The voltage across the inductor Lm is described by the following equation:

VL⁢m=VC⁢c·VLr=Lr·dd⁢t⁢i⁢L⁢m⁡(t)(9)
where VCcis the voltage across the clamping capacitor and VLris the voltage across the inductor Lr.

During time frame T5, Vg1has a low value (i.e., the power switch S1is turned off), Vg2has a low value (i.e., the active clamp switch S2is turned off), and Vg3has a low value (i.e., the secondary side switch is turned off). Operation of the switching power converter circuit100during this time frame T5is represented by the primary side model206inFIG. 2D. The leakage inductance current ILrand the magnetizing inductance current ILmcharge up the capacitor Coss2of the active clamp switch S2until the voltage across the capacitor Coss2is equal to the voltage across the clamping capacitor Cc. As a result, the voltage Vx decreases and equals the input voltage Vin when the voltage across the capacitor Coss2equals the voltage across the clamping capacitor Cc. The leakage inductance current ILr and the magnetizing inductance current ILm discharge the capacitor Coss1of the power switch S1. As a result, the voltage Vx continues to decrease to zero. The ZVS condition is achieved when Vx reaches substantially zero.

In an example control scheme, the primary side controller110may control when it turns off power switch S1based on sensed values or a predefined timing scheme. In an embodiment, the primary side controller110determines to turn off the power switch S1based on detecting when the primary current Ip reaches a threshold level. For example, the threshold level may be detected by measuring the voltage across a resistor (not shown) placed in between the source connection of the power switch S1and ground. When the voltage across this resistor equals the threshold level, the power switch S1is turned off. In another embodiment, the power switch S1is turned off after a predetermined amount of time, which may be estimated to correspond to the amount of time for the primary current Ip to reach the threshold level. In another example control scheme, the primary side controller110controls when to turn on the power switch S1. For instance, the power switch S1is turned on after a fixed time period after the active clamp switch S2turns off. This fixed time period is denoted Tb inFIG. 3. In some embodiments, the time period Tb may be a fixed time period based on output capacitance circuit characteristics of the switching power converter circuit100.

In another example control scheme, the primary side controller110controls the active clamp switch S2to turn on a short time after the power switch S1turns off in each switching cycle. For example, the primary side controller110may turn the active clamp switch S2on after the voltage Vx across the power switch S1ramps up to a level above the input voltage Vin, which may be detected by V_sense exceeding a predefined value based on circuit design characteristics. The primary side controller110adjusts the on-time of the active clamp switch S2to turn off the active clamp switch S2based on the timing of the voltage drop occurring in the sensed voltage V_sense satisfying the predefined detection condition in a previous switching cycle. To detect the predefined detection condition, the primary side controller110senses the sensed voltage V_sense after the active clamp switch S2turns on (and before the power switch S1turns back on). The primary side controller110may then compare the timing of the detected predefined detection condition relative to a target timing and adjust the timing of turning off the active clamp switch S2in subsequent switching cycles so that the timing of the detected predefined detection condition occurring and the active clamp switch S2turning off in the subsequent cycle is closer to the target timing.

InFIG. 3, time period Ta represents a time between detecting the secondary side switch S3turning off and the active clamp switch S2turning off. The substantially fixed time period Ta+Tb represents the time between the sensed voltage V_sense satisfying the predefined detection condition and the ZVS condition taking place. The substantially fixed time period Ta+Tb may be a fixed time period based on circuit characteristics of the switching power converter circuit100. Under the described control scheme, the primary side controller110detects Ta or Tb (or both) in each switching cycle and adjusts the on-time of the active clamp switch S2in subsequent switching cycles so as to maintain Tb (and correspondingly Ta since Ta+Tb is constant) within a predefined range of a target time period. As a result, the power switch S1can be controlled to turn on after the ZVS condition is reached, thereby enabling efficient switching.

For example, in one control scheme, if the time Ta between the sensed voltage V_sense satisfying the predefined detection condition and the active clamp switch S2turning off is above a target time period in a current switching cycle, then the turn-off time of the active clamp switch S2is adjusted to turn off the active clamp switch S2earlier in the next switching cycle (thus decreasing the on-time of the active clamp switch S2). If the time Ta between the sensed voltage V_sense satisfying the predefined detection condition and the active clamp switch S2turning off is below the target time period in the current switching cycle, then the turn-off time of the active clamp switch S2is adjusted to turn off the active clamp switch S2later in the next switching cycle (thus increasing the on-time of the active clamp switch S2).

In an alternative control scheme that operates equivalently given the fixed value of Ta+Tb, the primary side controller110may adjust switching based on detecting the time Tb between the off-time of the active clamp switch S2and the on-time of the power switch S1. For example, the primary side controller110adjusts the turn-off time of the active clamp switch S2to occur later in the next switching cycle if the detected time Tb is greater than a target time period, and adjust the turn-off time of the active clamp switch S2earlier in the next switching cycle if the detected time Tb is less than the target time period. In an efficient switching power converter circuit, the target time period for Tb is a fixed time period between the turn-off time of the active clamp switch S2and the turn-on time of the power switch S1.