Optimal phase-shifted control for a series resonant converter

[0002] The present invention relates generally to resonant power converters and, more particularly, to a series resonant converter controlled to operate in a super-resonant switching mode in order to achieve zero-voltage switching under all operating conditions.

[0003] Classical optimal control involves operating a full-bridge switching converter in a full square wave mode of operation. That is, both diagonal pairs of switching devices are switched on and off simultaneously. Unfortunately, classical optimal control results in higher than desirable switching losses and less than desirable dynamic performance for some applications. For example, for x-ray generation, classical optimal control does not meet strict performance criteria. Such criteria include the following: a fast generator voltage rise time in order to avoid an excess radiation dose to patients; minimal generator voltage ripple in order to avoid imaging jitter problems; avoidance of voltage overshoots which would create an additional radiation dose to patients; high audiosusceptibility, or line voltage ripple rejection, to avoid jitter and to ensure a tightly regulated output; independence of generator with respect to line voltage variations for consistent imaging performance; maintenance of a constant output voltage for high-quality successive images; reduction of high frequency currents flowing from the power converter back into the dc bus capacitors; and compliance with electromagnetic interference regulations.

[0004] Accordingly, it is desirable to provide a control for a series resonant converter which results in a very fast response time while maintaining zero-voltage switching operation. It is furthermore desirable that such a control meet strict performance criteria such as those set forth hereinabove.

[0005] An optimal phase-shifted control for a series resonant converter comprises an instantaneous monitoring of state variables (resonant capacitor voltage, resonant inductor current and dc output voltage) and implementation of a control law for providing a quasi-squarewave-with-maximum-coasting (QSWMC) mode of operation. The control law uses the instantaneous resonant inductor current, the instantaneous resonant capacitor voltage and the dc output voltage to determine the optimal switching events in order to operate on a desired control trajectory. QSWMC operation for optimal phase-shifted control comprises a repetitive set of four forcing functions across the resonant tank circuit and thus four repetitive switching modes. The QSWMC converter operates at a minimized frequency in a super-resonant mode (i.e., above the resonant tank frequency) with zero-voltage switching, minimized electrical stresses, and reduced electromagnetic interference due to nearly sinusoidal resonant tank currents.

[0006] FIG. 1 schematically illustrates an exemplary series resonant power converter to which optimal phase-shifted control according to the invention is applicable;

[0007] FIGS. 2A, 3A, 4A and 5A, respectively, schematically illustrate the four operating modes of QSWMC for the exemplary circuit of FIG. 1; and

[0008] FIGS. 2B, 3B, 4B and 5B, respectively, illustrate the equivalent circuits therefor;

[0009] FIGS. 6A-6D, respectively, illustrate state diagrams corresponding to the operating modes of FIGS. 2A-5A, respectively;

[0010] FIG. 7 graphically illustrates two possible state plane trajectories corresponding to the four modes of FIGS. 6A-6D, respectively, and a transition between the two trajectories;

[0011] FIG. 8 is a block diagram illustrating an exemplary embodiment of phase-shifted optimal control according to the present invention;

[0012] FIG. 9 is a block diagram illustrating a closed-loop proportional-integral (PI) controller with an optimal phase-shifted control inner loop according to a preferred embodiment of the present invention; and

[0013] FIG. 10 graphically illustrates fast voltage rise time, without overshoot, using the control approach illustrated in FIG. 9.

[0014] FIG. 1 illustrates an exemplary series resonant converter 10 to which optimal phase-shifted control 11 according to the present invention is applicable. Converter 10 comprises a full-bridge configuration having switching devices 12 (Q1) and 14 (Q2) connected in series in one converter phase leg and switching devices 16 (Q3) and 18 (Q4) connected in series in the other converter phase leg. A resonant tank circuit 20, comprising a series combination of a resonant inductor 22 (Lr) and resonant capacitor 24 (Cr), is connected in series with a transformer primary winding 38 between nodes A and B joining the switching devices in each converter phase leg. FIG. 1 shows anti-parallel diodes 26-29, respectively, coupled across switching devices 12, 14, 16 and 18, respectively. A snubber capacitor 30-33, respectively, is also shown as being coupled across each switching device 12, 14, 16 and 18, respectively. A dc input voltage 36 is connected across the full-bridge. The primary winding 38 of an isolation transformer 40 is connected in series with the resonant tank circuit 22 and 24. A secondary winding 42 of transformer 40 is coupled to a full-bridge rectifier 44 which is coupled across the illustrated parallel combination of an output capacitance 48 and an electrical load 46.

[0015] For purposes of analysis of the optimal phase-shifted control algorithm described herein, the switching devices are assumed to be ideal, i.e., having no forward voltage drop and an instantaneous turn-on and turn-off. Exemplary suitable switching devices include insulated gate bipolar transistors, metal oxide semiconductor field effect transistors, and bipolar transistors. Advantageously, with this optimal phase-shifted control algorithm, each of the switching devices can be turned on and off independently of the other switching devices, the only limitation being that two switching devices in each respective phase leg cannot be turned on simultaneously in order to avoid destructive shoot-through.

[0016] One phase leg operates as the controlled leg which modulates to regulate the output voltage. This phase leg is also referred to herein as the leading phase leg because this is the phase leg which initially changes its switching state during a switching cycle. The other phase leg is referred to herein as the lagging phase leg. The switching devices in the lagging phase leg are controlled to switch at (or slightly before) the natural current zero crossing of the resonant circuit. The phase-shifted optimal control forces the series resonant converter to operate at a frequency higher than the natural frequency of the resonant tank components (L_R, C_R). This super-resonant mode of operation has no turn-on switching losses. The simple capacitor snubbers 30-33 comprise a lossless turn-off snubber. (The capacitor snubbers 32 and 34 for the lagging phase leg can alternatively be omitted, if desired, since the switching loss for this leg is very low.) The main switching devices Q1-Q4 have significantly reduced switching losses as compared to their hard-switched counterparts. In addition, there are reduced electrical stresses and reduced electromagnetic interference (EMI) due to the nearly sinusoidal resonant tank currents.

[0017] This optimal phase-shifted control comprises an instantaneous monitoring of state variables (resonant capacitor voltage, resonant inductor current and output voltage) of the converter and implementation of a control law for providing a quasi-squarewave-with-maximum-coasting (QSWMC) mode of operation. As represented in FIG. 1, monitoring of resonant inductor current I_Lris accomplished using any suitable current sensing technique, as indicated by a reference numeral 35; and monitoring of resonant capacitor voltage V_Crand monitoring of the output voltage V_outare accomplished using any suitable voltage sensing technique, as indicated by the reference numerals 37 and 39, respectively.

[0018] There are four repeated operating modes of operation for the series resonant converter using the QSWMC approach for optimal phase-shifted control. FIGS. 2A, 3A, 4A and 5A illustrate the four operating modes, respectively; and FIGS. 2B, 3B, 4B and 5B, respectively, illustrate the corresponding equivalent circuits therefor. The bold lines indicate the paths of current flow in each mode. The voltage across nodes A and B is either +V_In, −V_In, or zero, depending upon which combination of transistors and diodes in each phase leg is conducting.

[0019] The four-mode squarewave control can be viewed as an L-C resonant tank with a repetitive set of forcing functions. The V_Cr-Z₀I_Lrstate plane trajectory is a circle with its center at the forcing function voltage. Different initial conditions (V_CR[0], I_LR[0]) change the angular location on the radial arc, but not the center of the state plane trajectory.

[0020] The solutions for the resonant capacitor voltage and resonant inductor currents are: $\begin{array}{cc}{V}_{\mathrm{CR}}\left(t\right)=V_F\left(1-\cos \left(\frac{t}{\sqrt{L_R{C}_R}}\right)\right)& \mathrm{Eqn}.1\\ I_{\mathrm{LR}}\left(t\right)=\frac{V_F}{\sqrt{\frac{L_R}{C_R}}}\sin \left(\frac{t}{L_R{C}_R}\right)& \mathrm{Eqn}.2\end{array}$

[0021] V_Fis the forcing voltage for each of the four different operating modes. The solutions to equations 1 and 2 are circles when drawn in a V_CR-Z₀I_LRstate plane, where Z₀is the characteristic impedance of the resonant tank, $\sqrt{\frac{L_R}{C_R}}.$

[0022] As illustrated in FIGS. 2A and 2B, in the first mode, switching devices Q1 and Q4 are on; the voltage V_ABis equal to the dc supply voltage V_in; the resonant tank voltage V_tank(i.e., across the resonant capacitance and resonant inductance) is given by: V_tank=V_in−V_out/N, where N represents the transformer turns ratio; and the resonant current flows in the direction shown.

[0023] In the second mode, as illustrated in FIGS. 3A and 3B, devices D2 and Q4 are conducting; the voltage V_ABis equal to zero; and the resonant tank voltage V_tankis given by: V_tank=−V_out/N.

[0024] In the third mode, as illustrated in FIGS. 4A and 4B, devices Q2 and Q3 are on; the voltage V_ABis equal to −V_in; and the resonant tank voltage V_tankis given by: V_tank=−V_in+V_out/N. The resonant current reverses direction as shown.

[0025] In the fourth mode, as illustrated in FIGS. 5A and 5B, devices D1 and Q3 are on; the voltage V_ABis equal to zero; and the resonant tank voltage V_tankis given by: V_tank=+V_out/N.

[0026] As illustrated in FIGS. 6A-6D, each of the four modes are respectively represented by four sets of circles in the state plane, the centers of which are located at the appropriate forcing voltage. The radii correspond to the specific initial conditions on the resonant components. Increasing time corresponds to subtending the curves in a clockwise direction. Operation at a steady state operating condition corresponds to clockwise subtending each of the four operating modes and appropriately transitioning between one operating mode and another. The operating modes operate on a symmetrical trajectory in the state plane. This optimal phase-shifted control approach removes the high frequency dynamics associated with the resonant tank components.

[0027] FIG. 7 shows two steady-state trajectories 50 and 52, and the time optimal switching condition 54 to transition between the trajectories. In particular, the optimal phase-shifted control trajectories transition between operating conditions in the time optimal manner for the QSWMC operating modes. In this case, transitioning between steady-state trajectory 50 and steady-state trajectory 52 is accomplished by extending the operating time in Mode 1 until it reaches the new trajectory. Larger-size trajectories correspond to increasing load currents and operation closer to the resonant frequency.

[0028] The control law for phase-shifted optimal control uses the instantaneous inductor current I_Lrand instantaneous resonant capacitor voltage V_Crto determine the optimal time to perform switching events, i.e., turning on and off switching devices Q1-Q4. The desired control trajectory is determined by the control law as described hereinbelow.

[0029] The control law employs calculation of an instantaneous distance squared function. This distance calculation is from the point (−V_out/N,0) when I_Lris positive, and from (V_out/N,0) when I_Lris negative. This radius-squared value is continuously calculated using the control law as follows:

[0030] RADIUS²_CONTROL−V_CONTROL≦0  Eqn.3

[0031] where

[0032] RADIUS²_CONTROL=(Z₀*I_Lr)²+(V_Cr+V_out/N)²for I_Lr>0  Eqn.4

[0033] RADIUS²_CONTROL=(Z₀*I_Lr)²+(V_Cr−V_out/N)²for I_Lr<0  Eqn.5

[0034] FIG. 8 is a block diagram representing an exemplary implementation 60 of phase-shifted optimal control 11 in accordance with a preferred embodiment of the present invention. The monitored resonant inductor current I_Lris provided as an input to the non-inverting input of a comparator 62. The inverting input is shown as being referenced to ground. The output of comparator 62 is provided to a switching device 64, the drain of which is shown as being provided to the non-inverting input of an operational amplifier 66. The value V_out/N is provided to each input of the operational amplifier through a resistor 68 and 70, respectively, each having the value R. The operational amplifier also has a feedback resistor 72, also shown as having the value R. The monitored resonant capacitor voltage V_Cris provided through a resistor 73 to the inverting input of another operational amplifier 74. The output of operational amplifier 66 is also provided through a resistor 76 to the inverting input of operational amplifier 74 which has a feedback resistor 78. The output of operational amplifier 74 is provided to a squarer 80. The resonant inductor current I_Lris provided to a squarer 82. The outputs of squarers 80 and 82 are provided through resistors 84 and 86, respectively, to the inverting input of an operational amplifier 88 having a feedback resistor 90. The output of operational amplifier 88 is provided to the inverting input of a comparator 92. A control voltage V_controlis provided to the non-inverting input of comparator 92. The output is provided to a clocked D flip-flop 94, the outputs of which are provided to the gate drivers (not shown) for switching devices Q1 and Q2. The resonant inductor current I_Lris provided to a zero-crossing detector 96, the output of which is provided to a T flip-flop 98. The outputs of flip-flop 98 are provided to the gate drivers (not shown) for switching devices Q3 and Q4. The D output of flip-flop 94 is tied to the Q output of flip-flop 98 to properly synchronize the control circuit.

[0035] FIG. 9 shows an implementation of a phase-shifted optimal controller (such as that represented by block diagram 60 of FIG. 8) as part of an inner control loop 100 in a closed-loop proportional-integral (PI) controller 102. In particular, the PI controller comprises a summer 104 and a PI controller block 106. The summer provides the difference between a reference voltage V_refand the feedback V_outfrom the inner loop 100. The output of PI controller block 106 is the input control voltage V_controlto the phase-shifted optimal controller inner loop 100.

[0036] FIG. 10 shows the series-resonant power converter fast voltage rise time, without overshoot, using the control approach shown in FIG. 9.

[0037] Advantageously, the optimal phase-shifted control algorithm described herein provides a very fast responding power converter, while maintaining zero-voltage switching operation. This provides fast voltage rise times and rapid transitions from one DC operating condition to other DC operating conditions.

[0038] As another advantage, with QSWMC, no current is re-circulated back to the input voltage source (V_In). In addition, the lagging switching devices turn off at a simultaneous zero-voltage and zero-current condition, such that the switching losses for these devices are very low.

[0039] The phase-shifted optimal control circuit has excellent intrinsic electromagnetic interference characteristics. The input voltage, shown at 36 of FIG. 1, is typically created from a rectified AC voltage source. This causes the input voltage Vin to have a 120 Hz component of ripple for a 60 Hz power source. When the phase-shifted optimal control circuitry operates with the AC input voltage ripple component, it has the effect of moving around, or dithering, the switching frequency of the series resonant power converter. This is because the phase-shifted optimal controller is regulating the energy in the resonant tank to operate on the correct switching trajectory. For Modes 1 and 3 (see FIG. 7), the input voltage is one component of the driving voltage across the resonant tank, or alternatively the “centers” for this region of subtending arcs. These centers are thus also modulating at a 120 Hz rate. This has the effect of modulating between trajectories at a 120 Hz rate, thereby modulating the switching frequency of the series resonant converter. The result is to create a spread spectrum of switching frequencies, and hence a spread-spectrum of high frequency currents drawn from the input voltage bus, and a spread spectrum of currents flowing through the resonant tank. As a result, the magnitude of current from a single large component at a fixed frequency is reduced to a much lower level of current modulated around the same frequency. The conducted and radiated EMI are reduced, and compliance with EMI conducted and radiated regulations is easier.

[0040] As another advantage, the converter in QSWMC operates at the minimum possible frequency while maintaining zero-voltage switching. The optimal phase-shifted control algorithm ensures operation in a super-resonant manner, i.e., at a frequency above the tank resonant frequency. Thus, the switching frequency is lower (for same kV, mA) than for either full squarewave or quasi-squarewave operation. The lower frequency of operation further reduces the switching losses.

[0041] The lagging phase leg (Q3-Q4) turns on and off with both a zero-voltage and zero-current condition, thus having no turn-on or turn-off losses. However, the semiconductor devices in this phase leg conduct a larger rms current, and thus have higher conduction losses.

[0042] The leading phase leg (Q1-Q2) operates in a zero-voltage switching manner. Hence, there are no turn-on losses, but still some turn-off losses. The switching devices in this phase leg conduct current for less time than those of the lagging phase leg, and thus have lower conduction losses.

[0043] Still another advantage of optimal phase-shifted control is an inherent audiosusceptibility improvement (Δv_out/Δv_in) due to the inherent feedback mechanism resulting from monitoring the state variables.

[0044] While the preferred embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those of skill in the art without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.