Control of isolated power converters during transient load conditions

An isolated power converter includes primary side switch devices coupled to secondary side rectifying devices by a transformer and a controller. Responsive to a transient load condition, the controller switches the primary side switch devices at an initial switching period having a positive half cycle and a negative half cycle to transfer energy across the transformer during the positive half cycle and the negative half cycle. The positive half cycle and the negative half cycle of the initial switching period have the same initial duration. The controller is further operable to symmetrically reduce the duration of the positive half cycle and the negative half cycle for at least one subsequent switching period during the transient load condition.

TECHNICAL FIELD

The present application relates to isolated power converters and, and in particular relates to control of isolated power converters during transient load conditions.

BACKGROUND

Isolated bridge topologies such as the half-bridge (HB) or full-bridge (FB) can be paired with different rectifier configurations. The selection of rectifier configurations depends on the requirements of the design, including output voltage and current requirements. The current doubler ectifier uses a single secondary winding coupled with two inductors, while maintaining the equivalent voltage stress of a full-wave rectifier. Each inductor is energized once per positive or negative cycle of the switching period, and therefore require symmetrical pulses to balance current in both branches of the doubler. Accordingly, current doubler rectifiers are not typically used for powering systems with very dynamic load conditions, such as CPU (central processing unit) applications. Also, to obtain high efficiency, high value inductors often with lower saturation limits are conventionally used so the current imbalance during (dynamic) load transients may cause inductor saturation.

Current doubler rectifiers are beneficial in high power applications with limited load activity, such that fast dynamic response is not a primary concern. In such systems, the extra magnetic components are justifiable from an area and cost perspective. Further, with infrequent load transients, balancing the inductor currents is not a consideration. However, when used in lower power applications such as powering CPUs, handling the load transient while balancing the inductor currents is preferred to avoid saturation of one of the inductors and at the same time achieve faster transient response. One way to maintain current balance during a transient load condition is to lock the duty cycle for the primary side during each half cycle. However, the transient response becomes sluggish because the reaction time is up to one switching period. Accordingly, an improved control technique for isolated power converters during transient load conditions is needed.

SUMMARY

According to an embodiment of a method of controlling an isolated power converter, the method comprises: responsive to a transient load condition, switching primary side switch devices of the isolated power converter at an initial switching period having a positive half cycle and a negative half cycle to transfer energy across a transformer of the isolated power converter during the positive half cycle and the negative half cycle, the positive half cycle and the negative half cycle of the initial switching period having the same initial duration: and symmetrically reducing the duration of the positive half cycle and the negative half cycle for at least one subsequent switching period during the transient load condition.

According to an embodiment of an isolated power converter, the isolated power converter comprises primary side switch devices coupled to secondary side rectifying devices by a transformer and a controller. The controller is operable to: responsive to a transient load condition, switch the primary side switch devices at an initial switching period having a positive half cycle and a negative half cycle to transfer energy across the transformer during the positive half cycle and the negative half cycle, the positive half cycle and the negative half cycle of the initial switching period having the same initial duration; and symmetrically reduce the duration of the positive half cycle and the negative half cycle for at least one subsequent switching period during the transient load condition.

DETAILED DESCRIPTION

The embodiments described herein provide control techniques for isolated power converters such as current doubler rectifiers and full-wave rectifiers during transient load conditions. The techniques described herein balance the currents in current doubler rectifiers during transient load conditions, so that current doubler rectifiers can be used in applications with frequent load transients such as powering CPUs. The techniques described herein also avoid transformer core saturation in isolated DC-DC voltage converters such as full-bridge converters.

During a transient load condition in which an instantaneous or near instantaneous change in load current occurs, the primary side switch devices of the isolated power converter are switched at an initial switching period having a positive half cycle and a negative half cycle to transfer energy across the transformer of the isolated power converter during the positive half cycle and the negative half cycle. The positive half cycle and the negative half cycle of the initial switching period have the same initial duration. The initial duration can be selected as a function of the magnitude of the transient load condition. In some embodiments, different initial durations are assigned to different types of transient load conditions. This way, the initial duration of the positive half cycle and the negative half cycle can be optimized based on the type of transient load condition. The duration of the positive half cycle and the negative half cycle is then symmetrically reduced for at least one subsequent switching period during the transient load condition, to balance currents in a current doubler rectifier or avoid transformer core saturation in a full-bridge converter. Nonlinear control can use current or charge information for the secondary side to adjust the switching sequence of the primary side switch devices, to handle load transients as fast as possible while simultaneously limiting current imbalance. In the case of full-bridge converters, transformer core saturation is avoided.

Various embodiments of isolated power converters and control methods for isolated power converters are provided in the following detailed description and the associated figures. The described embodiments provide particular examples for purposes of explanation, and are not intended to be limiting. Features and aspects from the example embodiments may be combined or re-arranged, except where the context does not allow this.

FIG. 1illustrates an embodiment of an isolated power converter100within which the control techniques described herein may be implemented. The isolated power converter100has a primary side which includes primary side switch devices Q1-Q2in a half bridge configuration, a secondary side which includes secondary side rectifying devices SR1-SR2, a transformer102coupling the primary side switch devices Q1-Q2to the secondary side rectifying devices SR1-SR2, and a controller104for controlling operation of the converter100. According to this embodiment, the secondary side rectifying devices SR1-SR2are configured as a current doubler rectifier having two output inductor windings LO1, LO2coupled to the transformer102.

The transient response techniques described herein control switching of the primary side switch devices Q1-Q2, and indirectly control the secondary side rectifying devices SR1-SR2as the secondary side rectifying device control signals are generated as a function of the primary side switch device control signals. The secondary side rectifying devices SR1-SR2are shown as transistor switch devices inFIG. 1, but instead may be implemented as diodes which have no synchronous rectification (SR) control signals. If the secondary side rectifying devices SR1-SR2are implemented as transistor switch devices, control of the secondary side rectifying devices SR1-SR2follows the switches on the primary side.

In either configuration, an input power source Vinprovides power to the isolated power converter100and the converter100supplies output power to a load which is generically represented as a resistor RL, The input power source Vinis provided to the primary side of the converter100, which couples it to the transformer102using the primary side switch devices Q1-Q2. Each of the primary side switch devices Q1-Q2has an associated driver within a driver stage. The driver stage and related driver circuitry are not illustrated for ease of illustration, and any standard driver stage/circuitry may be used. The primary side switch devices Q1-Q2are oriented in a half-bridge configuration inFIG. 1.

The primary side switch devices Q1-Q2are illustrated inFIG. 1as enhancement-mode metal-oxide semiconductor field-effect transistors (MOSFETs), but other switch types may be used. For example, junction field-effect transistors (JFETs), bipolar junction transistors (BJTs), insulated gate bipolar transistors (IGBTs), high electron mobility transistors (HEMTs), or other types of power transistors may be preferred in some applications. The primary side switch devices Q1-Q2may be integrated on the same semiconductor die, may each be provided on separate dies, or may otherwise be spread across a plurality of semiconductor dies. The corresponding driving circuitry (not shown) may be integrated on the same semiconductor die(s) as their corresponding primary side switch devices Q1-Q2, or may be provided on separate dies.

The transformer102has a primary winding P with N1 turns, a secondary winding S with N2 turns, and a core106. The transformer102ofFIG. 1also includes a leakage inductance, which is not a separate component but which models stray inductance that is not included in the windings P, S. Presuming the effect of the leakage inductance to be insignificant, the ratio N1/N2 determines the ratio of the rectified voltage Vrectto the input voltage VABof the transformer102.

Operation of the isolated power converter100is described next in more detail during both non-transient and transient load conditions. A non-transient load condition means that the load current iLremains relatively unchanged, whereas a transient load condition means that an instantaneous or near instantaneous change in load current has occurred. The controller104is equipped to operate in both a non-transient mode during which the load current remains relatively unchanged, and in a transient mode during which instantaneous or near instantaneous changes in load current occur.

In general, the controller104is responsible for controlling the primary side switch devices Q1-Q2and the secondary side rectifying devices SR1-SR2(if implemented as transistors) to supply the necessary power (voltage VOand current IL) to the load. This includes generating PWM waveforms that control the primary side switch devices Q1-Q2and also the secondary side rectifying devices SR1-SR2(if implemented as transistors). The PWM waveforms that control the primary side switch devices Q1-Q2and the secondary side rectifying devices SR1-SR2(if implemented as transistors) are generated to ensure that the load is supplied adequate power, and this generation is typically based upon the output voltage VOand/or the load current IL. Conventional techniques are used to generate baseline PWM waveforms, based upon load requirements.

For example, a proportional, integral and derivative (PID) controller108included in or associated with the main controller104may use the output voltage VO, a reference voltage Vrefand the output of a standard AVP (adaptive voltage positioning) unit109to adaptively determine duty cycle. A digital pulse width modulator (DPWM)110may use the duty cycle information provided by the PID controller108to generate the PWM waveforms that control switching of the primary side switch devices Q1-Q2and the secondary side rectifying devices SR1-SR2(if implemented as transistors). Because such techniques are well-known, they will not be described further herein. Instead, the following description focuses on techniques for modifying the PWM waveforms to provide current balance in the output inductor windings LO1, LO2of the secondary side current doubler rectifier during transient load conditions. To this end, a transient control unit112and a supervisor unit114included in or associated with the main controller104implement the transient control techniques described herein.

The controller104and its constituent parts may be implemented using a combination of analog hardware components (such as transistors, amplifiers, diodes, and resistors), and processor circuitry that includes primarily digital components. The processor circuitry may include one or more of a digital signal processor (DSP), a general-purpose processor, and an application-specific integrated circuit (ASIC). The controller104may also include memory, e.g., non-volatile memory such as flash that includes instructions or data for use by the processor circuitry, and one or more timers. The controller104inputs sensor signals such as signals corresponding to VOand IL.

Detailed operation of the isolated power converter100is described next with reference toFIG. 2.FIG. 2illustrates various waveforms associated with operation of the isolated power converter100in both the non-transient and transient modes. These waveforms include voltage VABacross the primary winding P of the transformer102, currents ILo1, ILo2in the respective output inductor windings LO1, LO2of the current doubler rectifier, total current Io,totdelivered by the isolated power converter100to the load (Io,tot=ILo1+ILo2), and voltage VOacross the output capacitor COof the isolated power converter100.FIG. 2also shows a transient load condition in which the load current iLchanges from a first (lower) target value to a second (higher) target value iL2. During this transition in the target current, the controller104operates in the transient mode in which the transient control unit112and the supervisor unit114control switching of the primary side switch devices Q1-Q2and the secondary side rectifying devices SR1-SR2. Before and after the transition, the controller104operates in the non-transient mode in which the PID controller108and the DPWM110control switching of the primary side switch devices Q1-Q2and the secondary side rectifying devices SR1-SR2.

During an energy transfer interval within a positive half-cycle of the input power source Vin, primary side switch device Q1is conducting via a corresponding PWM signal, thereby producing a positive voltage +VABacross the primary winding P of the transformer102. During an energy transfer interval within a negative half-cycle of the input power source Vin, primary side switch device Q2is conducting via a corresponding PWM signal, thereby providing a negative voltage −VABacross the primary winding P of the transformer102. Energy circulation intervals occur between successive energy transfer intervals. For PWM control, a so-called dead time occurs during the energy circulation intervals in which none of the primary side switch devices Q1-Q2are conducting and no voltage is provided across the primary winding P of the transformer102, Current does not flow in the primary side during energy circulation intervals under PWM control, only in the secondary side. The operational details of the isolated power converter100are described herein in the context of PWM control for ease and simplicity of explanation. However, those skilled in the art will readily understand that the techniques described herein equally apply to PSM control.

With a standard PWM-based approach, the controller104switches the primary side switch devices Q1-Q2at a fixed (constant) first switching period TS1and variable duty cycle D during non-transient load conditions to transfer energy across the transformer102during first (non-transient mode) energy transfer intervals which are separated by energy circulation intervals. The PID controller108determines the variable duty such that the ratio of each energy transfer interval TenergyTxto the fixed switching period TS1is less than unity i.e. TenergyTx/TS1<1. Accordingly, as shown inFIG. 2, ample dead time is provided between energy transfer intervals to allow the controller104to react to a transient load condition.

Transient Mode

During a transient load condition, the controller104switches the primary side switch devices Q1-Q2and the secondary side rectifying devices SR1-SR2of the current doubler rectifier at a second (transient mode) initial switching period TS2_intdifferent than the first (non-transient mode) switching period TS1so as to transfer energy across the transformer102during second (transient mode) energy transfer intervals each of an initial duration THC,max, and such that any energy circulation interval separating the transient mode energy transfer intervals is shorter than the energy circulation intervals separating the non-transient mode energy transfer intervals. Each switching period in the transient mode has two energy transfer intervals, one of which is a positive half cycle of the switching period (when Q1is on and Q2is off) and a negative half cycle of the switching period (when Q2is on and Q1is off). Energy is transferred across the transformer102of the isolated power converter100to the current doubler rectifier during the positive half cycle and the negative half cycle of each switching period.

The initial transient mode switching period TS2_intmay be greater than or less than the non-transient mode switching period TS1. If the initial transient mode switching period TS2_intis less than the non-transient mode switching period TS1, the primary side switch devices Q1-Q2are switched at a higher switching frequency in the transient mode than in the non-transient mode.

The controller104may detect a transient load condition e.g. based on VOand/or IL. In response to a transient load condition, the transient control unit112determines the initial transient mode switching period TS2_intbased on the initial duration THC,maxof the energy transfer intervals i.e. the positive and negative half cycles in the transient mode which correspond to the width of the ON time pulses applied to the primary side half bridge switch devices Q1-Q2. In some case, the initial duration THC,max, of the positive and negative half cycles for the initial switching period TS2_intin the transient mode can be the same for all transient load conditions. In other cases, the initial duration THC,maxof the positive and negative half cycles for the initial switching period TS2_intin the transient mode can be determined as a function of the magnitude of the transient load condition. This way, different THC,maxvalues can be assigned to different types of transient load conditions. The controller104can determine the type of transient load condition e.g. based on VOand/or IL, and the transient control unit112can select the corresponding THC,maxvalue assigned to the positive and negative half cycles of the initial switching period TS2_intin the transient mode.

In the transient mode, the transient control unit112adjusts the primary side PWM sequence to obtain fast transient response while balancing the currents ILo1, ILo2in the respective output inductor windings LO1, LO2of the current doubler rectifier. In response to a transient load condition, the transient control unit112switches the primary side switch devices Q1-Q2of the half bridge, as well as secondary side rectifying devices SR1-SR2of the current doubler rectifier at an initial switching period TS2_inthaving a positive half cycle and a negative half cycle to transfer energy across the transformer102during the positive half cycle and the negative half cycle. The transient control unit112ensures the positive half cycle and the negative half cycle of the initial switching period TS2_inthave the same initial duration THC,max.

The transient control unit112then symmetrically reduces the duration of the positive half cycle and the negative half cycle for at least one subsequent switching period during the transient load condition. InFIG. 2, the new duration of the positive and negative half cycles is labelled THC,adjfor each subsequent switching period after the initial switching period TS2_int. Also inFIG. 2, only one complete switching period occurs after the initial switching period TS2_intbefore the output current Io,totof the isolated power converter100reaches a peak current limit Ipk. When the peak current limit Ipkis reached or expected to be reached, the transient control unit112either terminates the present PWM pulse (e.g. last transient mode pulse Q1inFIG. 2) or allows the present PWM pulse to complete before halting the PWM sequence. In yet another embodiment, if the remaining time (before Ipk is reached) is less than twice the initial duration THC,maxor less than twice the initial duration THC,maxplus twice the minimum duration THC,min, the remaining time is divided evenly for each half cycle. The resulting half cycle duration is less than the initial calculated value, but may be greater than the minimum value THC,minin the second scenario. In each case, the PWM sequence remains halted once Ipkis reached, with the primary side switch devices Q1-Q2off, and secondary side rectifying devices SR1-SR2on or off, until the output current Io,totof the isolated power converter100drops to the new target value iL2after which point the supervisor unit controller114permits the PID controller108and the DPWM110to resume primary side switching in the non-transient mode as previously explained herein.

FIG. 3illustrates one embodiment of the transient mode control technique. The controller104enters the transient mode responsive to detecting a transient load condition e.g. based on VOand/or IL(Block200). The transient control unit112switches the half bridge primary side switch devices Q1-Q2and the secondary side rectifying devices SR1-SR2of the current doubler rectifier at an initial switching period TS2_inthaving a positive half cycle and a negative half cycle to transfer energy across the transformer102during the positive half cycle and the negative half cycle (Block202). The positive half cycle and the negative half cycle of the initial switching period TS2_inthave the same initial duration THC,max. The transient control unit112determines whether the total current Io,totdelivered by the isolated power converter100is expected to reach a peak current limit Ipkin less than twice the initial duration during the next switching period (Block204). The isolated power converter100includes ADCs (analog-to-digital converters)116,118,120for measuring the inductor currents iLo1, iLo2and the total current Io,totdelivered by the isolated power converter100to enable the transient control unit112to make this determination. The transient control unit112can calculate the time remaining to the peak current limit Ipkbased on the current measurements, based on the new target load current iL2, based on the calculated stored charge in capacitor CO, etc.

If the transient control unit112determines that the total current Io,totdelivered by the isolated power converter100is expected to reach the peak current limit Ipkin less than twice the initial duration THC,maxassigned to the positive and negative half cycles during the next switching period, then the transient control unit112reduces the duration of the positive half cycle and the negative half cycle for the subsequent switching period to the same minimum duration THC,mindetermined for the positive half cycle and the negative half cycle (Block206). Thus, according to this embodiment, the transient control unit112reduces the duration of the positive half cycle and the negative half cycle from the maximum initial value THC,maxto the minimum assigned value THC,minin a single step.

The minimum assigned value THC,minis selected so that the last pulse applied in the transient mode is narrow enough (smaller than HC,max) so that the last pulse does not create too much imbalance. The transient mode does not begin at THC,minbecause the primary side switch devices Q1-Q2would be switched at a much higher frequency at the beginning of the transient load condition in this case, increasing switching losses of the system. The selection of THC,minand the transition to THC,minis a trade-off between switching losses and current imbalance, and depends on the requirements placed on the system in which the transient mode control technique is used.

In some cases, the change to THC,minmay occur before the condition in Block204is satisfied and more than one complete switching period may complete before the total current Io,totdelivered by the current doubler rectifier reaches the peak current limit Ipk. In these cases, the transient control unit112maintains the minimum duration THC,minof the positive half cycle and the negative half cycle for each subsequent switching period while the total current Io,totdelivered by the isolated power converter100continues to ramp up toward Ipkduring the transient load condition.

FIG. 4illustrates another embodiment of the transient mode control technique. The controller104enters the transient mode responsive to detecting a transient load condition e.g. based on VOand/or IL(Block300). The transient control unit112switches the half bridge primary side switch devices Q1-Q2and the secondary side rectifying devices SR1-SR2of the current doubler rectifier at an initial switching period TS2_inthaving a positive half cycle and a negative half cycle to transfer energy across the transformer102during the positive half cycle and the negative half cycle (Block302). The positive half cycle and the negative half cycle of the initial switching period TS2—inthave the same initial duration THC,max. The transient control unit112determines whether the total current Io,totdelivered by the isolated power converter100is expected to reach the peak current limit Ipkin less than twice the initial duration during the next switching period e.g. as described above in connection withFIG. 3(Block304).

If the transient control unit112determines that the total current Io,totdelivered by the current doubler rectifier is expected to reach the peak current limit Ipkin less than the initial duration THC,maxassigned to the positive and negative half cycles during the next switching period, the transient control unit112reduces the duration of the positive half cycle and the negative half cycle for the next switching period by half to THC,max/2 (Block306). Several scenarios are possible in this case. The positive half cycle (Q1) may terminate before THC,max/2, the positive half cycle (Q1) may complete but there is no negative half cycle (Q2), or the positive half cycle (Q1) may complete but the negative half cycle (Q2) terminates before THC,max/2.

However, if the transient control unit112determines that the total current Io,totdelivered by the isolated power converter100is not expected to reach the peak current limit Ipkin less than the initial duration THC,maxassigned to the positive and negative half cycles during the next switching period, the transient control unit112reduces the duration of the positive half cycle and the negative half cycle for the next switching period by half i.e. THC,max/2 and completes one full switching period (Block308). After this switching period, the transient control unit112determines whether the total current Io,totdelivered by the isolated power converter100is expected to reach the peak current limit Ipkin less than THC,max/2 (Block310).

If the transient control unit112then determines that the total current Io,totdelivered by the current doubler rectifier is expected to reach the peak current limit Ipkin less than THC,max/2, the transient control unit112reduces the duration of the positive half cycle and the negative half cycle for the next switching period by half again to THC,max/4 (Block312). The same scenarios described above are again possible in this case. The positive half cycle (Q1) may terminate before THC,max/4, the positive half cycle (Q1) may complete but there is no negative half cycle (Q2), or the positive half cycle (Q1) may complete but the negative half cycle (Q2) terminates before THC,max/4.

However, if the transient control unit112determines that the total current Io,totdelivered by the current doubler rectifier is not expected to reach the peak current limit Ipkin less than THC,max/2 during the next switching period, the transient control unit112again reduces the duration of the positive half cycle and the negative half cycle for the next switching period by half to THC,max/4 and completes one full switching period (Block316). The process of reducing the duration of the positive half cycle and the negative half cycle for a subsequent switching period continues until the duration reaches a minimum duration THC,mindetermined for the positive half cycle and the negative half cycle, or until the total current Io,totdelivered by the isolated power converter100reaches the peak current limit Ipk.

If the duration of the positive half cycle and the negative half cycle were reduced to THC,minbut the total current Io,totdelivered by the current doubler is not expected to reach the peak current limit Ipkfor at least one more complete switching period, the transient control unit112would maintain the minimum duration THC,minof the positive half cycle and the negative half cycle for each subsequent switching period while the total current Io,totcontinues to ramp up toward Ipkduring the transient load condition. In general, the duration of the positive half cycle and negative half cycle can be symmetrically reduced by the same amount each subsequent switching cycle in the transient mode. That amount can be something value other than a 50% reduction. For example, the converter may be operating at a pulse duration greater than the minimum duration and then split the remaining time. In a specific non-limiting example, the converter may be operating at ½ THC,maxand ¼ THC,maxis THC,min. However, the equivalent of ⅓ duration remains before Io,totreaches Ipk, so the converter uses a symmetric positive and negative half cycle duration of ⅓ THC,maxfor the next switching period.

In many of the embodiments described herein, reduction of the positive and negative half cycles based on timing criteria are meant to illustrate possible implementation examples. However, reduction can be autonomous and independent of time without loss of benefit. For example, the transient control unit112may enter the transient mode and switch the primary side and secondary side switching devices Q1-Q2, SR1-SR2with a switching period of TS2_int. After one complete switching period, the positive and negative half cycles are reduced symmetrically to result in a new switching period TS2bsuch that TS2bis less than TS2—int. After one complete switching period of duration TS2b, the positive and negative half cycles are again reduced symmetrically to result in a new switching period TS2csuch that TS2cis less than TS2b. After one complete switching period of duration TS2c, the positive and negative half cycles are again reduced symmetrically. This process of symmetric half cycle reduction continues until the current Io,totof the isolated power converter100reaches a peak current limit Ipk, or until the duration of each half cycle reaches its minimum value THC,min. There are then three possible operation scenarios with this embodiment. The current of the isolated power converter100may reach the peak current limit Ipkbefore the positive and negative half cycles have been reduced to the minimum value of THC,min. The current Io,totof the isolated power converter100may reach the peak current limit Ipkafter one complete switching period where the duration of the positive and negative half cycles equal THC,min. The duration of the positive and negative half cycles may reduce symmetrically to the minimum value of THC,minprior to the current Io,totof the isolated power converter100reaching the peak current limit Ipk. In this case, the isolated power converter100operates with positive and negative half cycles of duration THC,minuntil the current Io,totreaches the peak current limit Ipk.

According to another embodiment of the transient mode control technique, the transient control unit112symmetrically reduces the duration of the positive half cycle and the negative half cycle for at least one subsequent switching period during a transient load condition by comparing a measured voltage parameter of the isolated power converter100to stored values associated with different switching period durations and reducing the duration of the positive half cycle and the negative half cycle to the switching period duration associated with the stored value that most closely matches the measured voltage parameter. For example, a lookup table of delta VOvalues, voltage excursion values, minimum voltage values, etc. can be accessible by the transient control unit112. Or a user may input a voltage threshold that justifies entering the transient mode of operation. In each case, the transient control unit112can decide when to symmetrically reduce the duration of the positive half cycle and the negative half cycle based on a voltage parameter so that enough time is provided for one complete switching period, so that the transient control unit112has sufficient time to perform the calculations used to indicate how many more maximum half cycles THC,maxremain before the pulse width should be scaled back to ensure the desired amount of current balancing.

Transitioning from transient control to PWM can be problematic if the PWM pulse is applied to the phase that has the higher current. This may cause even more imbalance and may lead to saturation. In one embodiment, a current balance pulse can be inserted of a duration Tlbal. The current balance pulse is applied to the half cycle with the lowest inductor current. After the current balance pulse, the PID108resumes steady-state operation. According to another embodiment, the current balance pulse is not applied and instead the first PWM pulse is directly to the half-cycle with lower current.

The transient mode control techniques described previously herein can be extended to full-bridge converters with current doubler rectifiers. An exemplary full-bridge converter with current doubler rectifier400is illustrated inFIG. 5. InFIG. 5, the primary side includes four switch devices Q1-Q4. The transient mode control techniques described previously herein are directly applicable to the full-bridge topology shown inFIG. 5, by substituting full-bridge switching operation for half-bridge control. This means that during an energy transfer interval within a positive half-cycle of the input power source Vin, primary side switching devices Q1and Q3are conducting via respective PWM signals, thereby producing a positive voltage +VABacross the primary winding P of the transformer102. During an energy transfer interval within a negative half-cycle of the input power source Vinprimary side switching devices Q2and Q4are conducting via respective PWM signals, thereby providing a negative voltage −VABacross the primary winding P of the transformer102. Otherwise, operation of the isolated power converters100,400shown inFIGS. 1 and 5is the same. In these systems, the initial duration THC,maxof the positive half cycle and the negative half cycle for the first switching period in the transient mode is selected to avoid saturation of the transformer core. The transient mode control techniques described previously herein also can be extended to other rectifier topologies.

FIG. 6illustrates an embodiment of a full-bridge converter with full-wave rectifier500within which the control techniques described herein may be implemented. The illustrated full-wave rectifier500has a center-tap rectifier configuration, but the techniques described herein also apply to other rectifier topologies that use the same signals as the center-tap configuration; including a full-bridge configuration. The full-bridge converter with full-wave rectifier500has a primary side which includes primary side switch devices Q1-Q4, a secondary side which includes secondary side rectifying devices SR1-SR2, a transformer502coupling the primary side switch devices Q1-Q4to the secondary side rectifying devices SR1-SR2, and a controller504for controlling operation of the full-wave rectifier500.

The transient mode control techniques described herein control the switching of the primary side switch devices Q1-Q4, and indirectly control the secondary side rectifying devices SR1-SR2as the secondary side rectifying device control signals are generated as a function of the primary side switch device control signals. The secondary side rectifying devices SR1-SR2are shown as transistor switch devices inFIG. 1, but instead may be implemented as diodes which have no synchronous rectification (SR) control signals. If the secondary side rectifying devices SR1-SR2are implemented as transistor switch devices, the secondary side rectifying devices SR1-SR2follow the corresponding switches on primary side.

In either configuration, an input power source Vinprovides power to the full-wave rectifier500and the full-wave rectifier500supplies output power to a load which is generically represented as a resistor RL. The input power source Vinis provided to the primary side of the full-wave rectifier500, which couples it to the transformer502using the primary side switch devices Q1-Q4. Each of the primary side switch devices Q1-Q4has an associated driver within a driver stage. The driver stage and related driver circuitry are not illustrated for ease of illustration, and any standard driver stage/circuitry may be used. The primary side switch devices Q1-Q4are oriented in a full-bridge configuration inFIG. 6.

The primary side switch devices Q1-Q4at are illustrated inFIG. 6as enhancement-mode metal-oxide semiconductor field-effect transistors (MOSFETs), but other switch types may be used. For example, junction field-effect transistors (JFETs), bipolar junction transistors (BJTs), insulated gate bipolar transistors (IGBTs), high electron mobility transistors (HEMTs), or other types of power transistors may be preferred in some applications. The primary side switch devices Q1-Q4may be integrated on the same semiconductor die, may each be provided on separate dies, or may otherwise be spread across a plurality of semiconductor dies. The corresponding driving circuitry (not shown) may be integrated on the same semiconductor die(s) as their corresponding primary side switch devices Q1-Q4, or may be provided on separate dies.

The transformer502has a primary winding P with N1 turns, secondary windings S1, S2 with N2 turns each, and a core506. The transformer502ofFIG. 6also includes a leakage inductance, which is not a separate component but which models stray inductance that is not included in the windings P, S1, S2. The secondary windings S1, S2 are connected at a center tap inFIG. 6. A rectified voltage node is coupled to this center tap. Presuming the effect of the leakage inductance to be insignificant, the ratio N1/N2 determines the ratio of the rectified voltage Vrectto the input voltage VABof the transformer502.

Operation of the full-wave rectifier500is described next in more detail during both non-transient and transient load conditions. The controller504is equipped to operate in both a non-transient mode during which the load current remains relatively unchanged, and in a transient mode during which instantaneous or near instantaneous changes in load current occur.

In general, the controller504is responsible for controlling the primary side switch devices Q1-Q4and the secondary side rectifying devices SR1and SR2(if implemented as transistors) to supply the necessary power (voltage VOand current IL) to the load. This includes generating PWM waveforms that control the primary side switch devices Q1-Q4and also the secondary side rectifying devices SR1and SR2(if implemented as transistors). The PWM waveforms that control the primary side switch devices Q1-Q4and the secondary side rectifying devices SR1and SR2(if implemented as transistors) are generated to ensure that the load is supplied adequate power, and this generation is typically based upon the output voltage VOand/or the load current IL. Conventional techniques are used to generate baseline PWM waveforms, based upon load requirements.

For example, a proportional, integral and derivative (PID) controller508included in or associated with the main controller504may use the output voltage VOto adaptively determine duty cycle. A digital pulse width modulator (DPWM)510included in or associated with the main controller504may use the duty cycle information provided by the PID controller508to generate the PWM waveforms that control switching of the primary side switch devices Q1-Q4and the secondary side rectifying devices SR1and SR2(if implemented as transistors). Because such techniques are well-known, they will not be described further herein. Instead, the following description focuses on the unique aspects of this invention, which are directed to techniques for modifying the PWM waveforms to prevent saturation of the transformer core106during transient load conditions. To this end, the controller504includes a transient auxiliary control and protection unit512for implementing the transformer core saturation avoidance techniques described herein.

The controller504and its constituent parts may be implemented using a combination of analog hardware components (such as transistors, amplifiers, diodes, and resistors), and processor circuitry that includes primarily digital components. The processor circuitry may include one or more of a digital signal processor (DSP), a general-purpose processor, and an application-specific integrated circuit (ASIC). The controller504may also include memory, e.g., non-volatile memory such as flash that includes instructions or data for use by the processor circuitry, and one or more timers. The controller504inputs sensor signals such as signals corresponding to VOand IL, e.g. as provided by an ADC514.

Detailed operation of the full-wave rectifier500is described next with reference toFIG. 7.FIG. 7illustrates various waveforms associated with operation of the full-bridge converter and full-wave rectifier500in both the non-transient and transient modes. These waveforms include voltage VABacross the primary winding P of the transformer502, current iLin the output inductor LOof the full-wave rectifier500, voltage VOacross the output capacitor COof the full-wave rectifier500, and magnetic flux density B of the transformer core506.FIG. 7also shows a transient load condition in which the load current changes from a first (lower) target value iL1to a second (higher) target value iL2and the corresponding difference ΔIO. During this transition in the target current, the controller504operates in the transient mode. Before and after the transition, the controller504operates in the non-transient mode.

During an energy transfer interval within a positive half-cycle of the input power source Vin, primary side switch devices Q1and Q3are conducting via respective PWM signals, thereby producing a positive voltage +VABacross the primary winding P of the transformer502. During an energy transfer interval within a negative half-cycle of the input power source Vin, primary side switch devices Q2and Q4are conducting via respective PWM signals, thereby providing a negative voltage −VABacross the primary winding P of the transformer502. Energy circulation intervals occur between successive energy transfer intervals. For PWM control, a so-called dead time occurs during the energy circulation intervals in which none of the primary side switch devices Q1-Q4are conducting and no voltage is provided across the primary winding P of the transformer502. Current does not flow in the primary side during energy circulation intervals under PWM control, only in the secondary side. For phase shift modulation (PSM) control, primary side switch devices Q1and Q2conduct circulating current; or primary side switch devices Q3and Q4conduct circulating current during energy circulation intervals. As such, current circulates in both the primary and secondary sides during energy circulation intervals under PSM control. The operational details of the full-wave rectifier500are described herein in the context of PWM control for ease and simplicity of explanation. However, those skilled in the art will readily understand that the techniques described herein equally apply to PSM control.

With a standard PWM-based approach, the controller504switches the primary side switch devices Q1-Q4at a fixed (constant) first switching period TS1and variable duty cycle D during non-transient load conditions to transfer energy across the transformer502during first (non-transient mode) energy transfer intervals which are separated by energy circulation intervals. The PID controller508determines the variable duty such that the ratio of each energy transfer interval TenergyTxto the fixed switching period TS1is less than unity i.e. TenergTx/TS1<1. Accordingly, as shown inFIG. 7, ample dead time is provided between energy transfer intervals to allow the controller504to react to a transient load condition.

Transient Mode

During a transient load condition, the transient auxiliary control and protection unit512included in or associated with the controller504switches the primary side switch devices Q1-Q4of the full-wave rectifier500at a second (transient mode) initial switching period TS2adifferent than the first (non-transient mode) switching period TS1to transfer energy across the transformer502during second (transient mode) energy transfer intervals of a duration Ton,max, and such that any energy circulation interval separating the transient mode energy transfer intervals is shorter than the energy circulation intervals separating the non-transient mode energy transfer intervals.

Each switching period in the transient mode has two energy transfer intervals, one of which is a positive half cycle of the switching period (when Q1and Q3are on and Q2and Q4are off) and a negative half cycle of the switching period (when Q2and Q4are on and Q1and Q3are off). Energy is transferred across the transformer502of the full-wave rectifier500during each the positive half cycle and the negative half cycle of each switching period.

The initial transient mode switching period TS2amay be greater than or less than the non-transient mode switching period TS1. If the initial transient mode switching period TS2ais less than the non-transient mode switching period TS1, the primary side switch devices Q1-Q4are switched at a higher switching frequency in the transient mode than in the non-transient mode.

The controller504may detect a transient load condition e.g. based on VOand/or IL. In response to a transient load condition, the transient auxiliary control and protection unit512determines the initial transient mode switching period TS2abased based on the duration Ton,maxof the energy transfer intervals in the transient mode which correspond to the width of the ON time pulses applied to the primary side switch devices Q1-Q4of the full-wave rectifier500. The duration Ton,maxof the transient mode energy transfer intervals is determined to avoid saturation of the transformer core506. If the transient mode energy transfer intervals were to exceed Ton,max, the magnetic flux density B in the transformer core506would increase/decrease to its positive/negative saturation limit.

The input voltage Vineffects the slew rate of the magnetic flux density in the transformer core506. An increase in Vincorrespondingly increases the slew rate of the magnetic flux density. The transient auxiliary control and protection unit512may adjust the initial duration Ton,maxof the energy transfer intervals in the transient mode accordingly. For example, higher Vintranslates to narrower initial Ton,maxpulses in the transient mode. By adjusting the initial duration Ton,maxof the energy transfer intervals in the transient mode based on a new input voltage magnitude for the full-bridge converter with full-wave rectifier500, saturation of the transformer core506may be avoided for the new input voltage magnitude during the transient load condition. Because the initial switching period TS2afor the transient mode is derived from an initial duration Ton,maxselected to avoid transformer core saturation in this full-wave rectifier embodiment, the transient auxiliary control and protection unit512also adjusts the initial switching period TS2abased on the newly determined duration of the transient mode energy transfer intervals.

Various embodiments for determining the initial duration Ton,maxare described in more detail later herein. Switching period TS1is determined in a wholly different manner in the non-transient mode. In the non-transient mode, switching period TS1is fixed (constant) and determined based on various system parameters. The variable duty cycle of the PWM signals applied to the primary side switch devices Q1-Q4during the non-transient mode is determined based on e.g. the output voltage VOand the switching frequency. As such, frequency is not used to provide regulation on the output in the non-transient mode, but switching frequency will change in the transient mode so that the necessary energy transfer is provided for the output inductor LO.

The variable duty cycle (D) and ON time of the primary side switch devices Q1-Q4are related by switching period in the non-transient mode as given by Ton=D*TS1. The maximum duty cycle Dmax may be set by the user, e.g. based on transformer saturation (Volt-seconds) limits.

In the transient mode, the maximum duty cycle Dmax translates to an initial duration Ton,maxwhich avoids saturation of the transformer core106with excessive Volt-seconds. The transient auxiliary control and protection unit512included in or associated with the controller504uses the initial duration Ton,maxof the ON time pulses applied to the primary side switch devices Q1-Q4to determine the initial switching period TS2aused in the transient mode. Ideally, the transient auxiliary control and protection unit512sets the initial transient mode switching period TS2aequal to twice the duration of the transient mode energy transfer intervals i.e. TS2a=2*Ton,maxas shown inFIG. 7. In this configuration, there is no dead time between the positive and negative half cycles of the voltage VABapplied to the primary coil P of the transformer502during the initial switching period TS2a. In a non-ideal setting, the initial transient mode switching period TS2a, may be set equal to twice the initial duration Ton,maxof the transient mode energy transfer intervals plus dead time i.e. TS2a=2*Ton,max+2 energy circulation intervals to ensure proper operation of the primary side switch devices Q1-Q4. In general, the transient auxiliary control and protection unit512ensures the positive half cycle and the negative half cycle of the initial switching period TS2ahave the same initial duration Ton,max.

In each case, there is little to no dead time between the positive and negative half cycles of the transformer primary coil voltage VABin transient mode as compared to non-transient mode. As a result, a constant or nearly constant voltage is applied across the output inductor LOof the full-wave rectifier500and the inductor ramp current ILramps up in a linear or mostly linear manner. A square-wave for the voltage VABacross the primary coil P of the transformer502yields a constant ramp of the inductor iLas represented by the following equations:
diL/dt=(Vrect−V0)/L(1)
Vrect=Vin/N(full-bridge),  (2)
Vrect=Vin/2/N(half-bridge)  (3)
where Vrectis the rectified voltage on the secondary side of the full-wave rectifier500.

The transient auxiliary control and protection unit512then symmetrically reduces the duration of the positive half cycle and the negative half cycle for at least one subsequent switching period TS2b, TS2c, etc. during the transient load condition. The new (reduced) duration of the positive and negative half cycles for at least one subsequent switching period TS2b, TS2c, etc. can be determined as previously described herein e.g. in connection withFIGS. 3 and 4. For example, the transient auxiliary control and protection unit512can reduce the duration of the positive half cycle and the negative half cycle from the maximum initial value Ton,maxto a minimum assigned value Ton,minin a single step as previously described in connection withFIG. 3. In another example, the transient auxiliary control and protection unit512can reduce the duration of the positive half cycle and the negative half cycle by a predetermined amount (e.g. 50%) for each subsequent switching period until the duration reaches a minimum duration determined for the positive half cycle and the negative half cycle or until the inductor ramp current ILreaches a peak current limit Ipk_tfmrdetermined for the transformer502.

When the peak current limit Ipkis reached or expected to be reached, the transient auxiliary control and protection unit512either terminates the present PWM pulse (PlastinFIG. 7) or allows the present PWM pulse to complete before halting the PWM sequence as previously described herein. In either case, the PWM sequence remains halted, with the primary side switch devices Q1-Q4off, and the secondary side rectifying devices SR1-SR2on or off, until the output current ILof the full-wave rectifier500drops to the new target value iL2after which point the controller504resumes primary side switching in the non-transient mode as previously explained herein.

In the transient mode, the inductor current iLincreases linearly or nearly linearly until the peak current value ipk_Ttfmrof the transformer502is reached. The transient auxiliary control and protection unit512may monitor the inductor current iLand compare the monitored inductor current to a predetermined threshold to determine when the peak current value ipk_tfmrof the transformer502is reached. The peak current value ipk_tfmrof the transformer502may be determined based on the input voltage Vin, load step ΔIOand output inductor, and is set so that area A and area B inFIG. 7are ideally equal or nearly equal. Various techniques are well known in the voltage converter arts for measuring output voltage and inductor current, and therefore no further explanation is provided.