Muti-level hybrid flying capacitor converter control

The present disclosure is directed to flying capacitor instability detection and correction in Multi-Level Hybrid Flying Capacitor (MLHFC) converters.

Reliable control of an MLHFC converter ensures that its flying capacitor is properly balanced under all conditions and prevents flying capacitor voltage drift. Flying capacitor instability may cause a converter component to sustain full output voltage or full input voltage, which may be higher than the component's maximum voltage rating. This results in reduced lifetime, performance, reliability, and in the end, permanent damage to the converter's power stage.

Flying capacitor balancing is typically achieved by adapting duty cycles of switch control signals in different switching phases. A fundamental issue with this approach is that the feedback sign from a change in duty cycle to a change in flying capacitor voltage depends on system parameters such as load current, input voltage, output voltage, switching frequency, and inductance value. Different feedback regions therefore exist. This is detrimental for control law because the feedback may accelerate, rather than compensate for, flying capacitor voltage drift.

Voltage Mode Control (VMC) methods overcome flying capacitor instability using techniques such as dual edge modulation. This technique cannot be applied to other control methods such as Current Mode Control (CMC) and Sliding Mode Control (SMC). CMC and SMC have better dynamic performance and overcurrent protection in comparison to VMC so are often preferred methods. It is thus important to have a flying capacitor instability detection and correction solution that works with various control schemes.

FIGS. 1A and 1B illustrate schematic diagrams of a control system and a Multi-Level Hybrid Flying Capacitor (MLHFC) converter in accordance with aspects of the disclosure.

FIGS. 2A and 2B illustrate waveforms comparing a known control system and the disclosed control system of FIGS. 1A and 1B in accordance with aspects of the disclosure.

FIG. 3 illustrates a flowchart of a method 300 of a control system controlling an MLHFC converter in accordance with aspects of the disclosure.

The present disclosure is directed to a control system that balances a flying capacitor voltage by detecting a feedback region without any additional measurements or assumptions, and then adapts a feedback sign accordingly.

FIGS. 1A and 1B illustrate schematic diagrams of a control system 100 and a Multi-Level Hybrid Flying Capacitor (MLHFC) converter 10 in accordance with aspects of the disclosure.

The MLHFC converter 10 may be any MLHFC converter. For example, the converter 10 may be a step-up (boost) converter, step-down (buck) converter, a step-down/step-up (buck/boost) converter, or the like. The topology of the MLHFC converter 10 may have any number of flying capacitors, levels, and phases. For example, the MLHFC converter 10 may comprise a single flying capacitor and have three-levels. Alternatively, the MLHFC converter 10 may comprise a plurality of flying capacitors, in which case, each of the flying capacitors corresponds with a respective pair of feedback region detector 120 and controller adjuster 130 as described below.

The control system 100 comprises a controller 110 , a feedback region detector 120 , and a controller adjuster 130 . At least one of the feedback region detector 120 and the controller adjuster 130 may be part of the controller 110 , or alternatively, separate from the controller 110 .

The controller 110 is configured to control an output of the MLHFC converter 10 and is controller-architecture independent. The MLHFC converter 10 may be a three-level flying capacitor boost converter 10 . The controller 110 may be configured to perform Sliding Mode Control (SMC), a Current Mode Control (CMC), a Voltage Mode Control (VMC), a Proportional Integral Derivative (PID) control, a hysteresis control, a non-linear control, or the like. The controller 110 may use a flying capacitor balancing loop, and the flying capacitor balancing loop may be part of the controller 110 , or alternatively, separate from the controller 110 . In the example shown, the controller 110 is configured to perform CMC and flying capacitor balance using a flying capacitor balancing loop 110 A. The CMC controller 110 shown is for a single capacitor three-level MLHFC converter 10 , and the flying capacitor balancing loop 110 A maintains the flying capacitor voltage V Cfly . The CMC controller 110 is described briefly below. Controllers 110 configured to perform CMC and flying capacitor balancing loops 110 A are known, and for the sake of brevity, further detailed descriptions are omitted.

The feedback region detector 120 is configured to detect a change in a feedback region of the MLHFC converter 10 by monitoring a property of a flying capacitor voltage V Cfly of the MLHFC converter 10 , and a property of a control signal S Cfly of the control system 100 . Using the CMC controller 110 shown, the property of the flying capacitor voltage V Cfly is a sign of a derivative of the flying capacitor voltage,

sgn ⁢ ( d ⁢ V Cfly d ⁢ t ) ,

of the MLHFC converter 10 . The property of the control signal is a sign of a derivative of the control signal,

sgn ⁢ ( d ⁢ S Cfly d ⁢ t ) ,

of the control system, 100 . Since the flying capacitor voltage V Cfly is sensed for its balancing, no additional hardware is required. Furthermore, in a digital controller, all control signals are readily available and can be further processed.

The feedback region detector 120 is configured to detect the change in the feedback region from a Negative Feedback region (NFB) to a Positive Feedback region (PFB) when the sign of the derivative of the flying capacitor voltage,

sgn ⁢ ( dv Cfly d ⁢ t ) ,

and the sign of the derivative of the control signal,

sgn ⁢ ( d ⁢ S Cfly d ⁢ t ) ,

are not equal. Alternatively, the feedback region detector 120 is configured to detect the change in the feedback region from a PFB to a NFB when the sign of the derivative of the flying capacitor voltage,

sgn ⁢ ( dv Cfly d ⁢ t ) ,

and the sign of the derivative of the control signal,

sgn ⁢ ( d ⁢ S Cfly d ⁢ t ) ,

are equal.

The controller adjuster 130 is configured to, if the feedback region detector 120 detects a change in the feedback region, adjust the control system 100 by inversing a sign of the control signal S Cfly of the controller 110 to counteract instability due to the changed feedback region. The change in the feedback region of the MLHFC converter 10 may be from a NFB to a PFB, or from a PFB to a NFB. The controller adjuster 130 comprises an inverter 132 and a multiplexer 134 as an example configuration, and inverses the sign of the flying capacitor balancing by the multiplexer selecting, in response to a select signal from the feedback region detector 120 , an inverted version of the control signal output by the inverter 132 . The flying capacitor voltage V Cfly is thus balanced without switching between controllers or employing special modulation schemes.

The instability may result from an operating point change of the MLHFC converter 10 . Also, the instability may result from a parameter change of the MLHFC converter 10 . Since this instability can be counteracted, the feedback region detector 120 and controller adjuster 130 are therefore configured to operate independently of parameters of the MLHFC converter 10 , wherein the parameters may comprise a load current, an input voltage, an output voltage, a switching frequency, and/or an inductance.

The control system 100 is also independent of the control method. FIG. 1B shows a controller 110 configured to perform CMC to determine the state of the MLHFC converter switches.

The control signal, which dictates the switching decision, is provided by:

The control signal c CMC , which determines the states of the control switches of the MLHFC converter 10 , is then obtained by evaluating S_CMC.

Here, a constant frequency implementation is used. To that end, the control signal c CMC is set to 1 by a constant clock signal generated by a Finite State Machine (FSM), and reset to 0 if S_CMC>0. The FSM is used to select the appropriate group of MLHFC converter switches.

For a boost converter, the flying capacitor voltage is stabilized at

V o 2

for a three-level converter. In general, the flying capacitor voltages are stabilized at a fraction of the output voltage. The flying capacitor voltage balancing is achieved by the S Cfly control signal. As a first step, the sign is determined by the FSM assuming operation in the default NFB. This causes the time instants at which the reset of an SR latch in the controller 110 is triggered by S_CMC>0 to be different for the two sub-phases, for example, if in the first sub-phase, in which the first switching group is controlled, the flying capacitor is charged, in the second sub-phase, in which the second switching group is controlled, the flying capacitor is discharged. Consequently, the duty cycles of the two switching groups are modified in opposite directions, resulting in a balancing of the flying capacitor voltage V Cfly .

A core of the control system 100 is the feedback region detection and the subsequent adaptation of the sign of S Cfly , reversing the sign selected by the FSM if operation in the PFB is detected. In this implementation, the feedback region is detected as follows.

First, the sign of the slope of the flying capacitor voltage,

sgn ⁢ ( dv Cfly d ⁢ t ) ,

is detected.

Second, the sign of the slope of the flying capacitor balancing control signal,

sgn ⁢ ( d ⁢ S Cfly d ⁢ t ) ,

is detected.

The correct feedback region (FB) is then determined by the following:

FB = { PFB , sgn ⁡ ( d ⁢ v Cfly d ⁢ t ) ≠ sgn ⁡ ( d ⁢ S Cfly d ⁢ t ) ⁢ ⁢ and ⁢ ⁢ NFB NFB , sgn ⁡ ( d ⁢ v Cfly d ⁢ t ) = sgn ⁡ ( d ⁢ S C ⁢ f ⁢ l ⁢ y d ⁢ t ) ⁢ ⁢ and ⁢ ⁢ PFB Equation ⁢ ⁢ ( 3 )

If NFB is detected, the feedback sign as selected by the FSM is used, as it is assumed that operation is in the default negative feedback region NFB. Otherwise, if PFB is detected, the sign of S Cfly , as determined by the FSM, is reversed. The feedback region detector 130 may be implemented as digital or analog circuitry.

FIGS. 2A and 2B illustrate waveforms comparing a known control system and the control system 100 of FIGS. 1A and 1B . More specifically, FIG. 2A illustrates a comparison of converter waveforms during load change, the left-hand side with the known control system, and the right-hand side with the control system 100 . FIG. 2B illustrates a comparison of control system waveforms during this load change, again, the left-hand side of the known control system, and the right-hand side of the control system 100 .

Consider first the known control system during the load change—the converter waveforms on the left-hand side of FIG. 2A , and the control signals on the left-hand side of FIG. 2B . The load changes result in different feedback regions.

More specifically, a transient response to load drop from 0.5 A to 1 mA is followed by a load step back to 0.5 A for a MLHFC converter 10 . A negative feedback between the difference in duty cycle and the flying capacitor voltage V Cfly can be observed at a load current of 0.5 A. This is the typical operating region considered when designing a flying capacitor balancing loop and analyzing the stability of the MLHFC converter 10 . As can be seen in the figures, flying capacitor balancing in known systems achieves regulation of the flying capacitor voltage V Cfly to the required value of

V o 2

under this load condition. However, after the load current I L drops to 1 mA, the feedback region changes. The flying capacitor voltage V Cfly increases, although S Cfly (i.e., the control signal responsible for balancing) is negative. Consequently, the MLHFC converter 10 loses output voltage regulation, the flyting capacitor voltage V Cfly drifts, and the stress on the switches increases. Moreover, it can be seen that regulation and flying capacitor balancing are restored after the load increases back to 0.5 A, indicating that the balancing works as intended for the NFB.

Compare now the disclosed control system 100 during the load change—the converter waveforms on the right-hand side of FIG. 2A , and the control signals on the right-hand side of FIG. 2B . Again, the load changes result in different feedback regions.

Output voltage regulation and a balanced flying capacitor voltage V Cfly are achieved by the disclosed control system 100 regardless of the load current i load . To ease in an understanding of the right-hand side waveforms, the following encircled numerals represent those in the illustrated waveforms. At first, the sign of S Cfly , is still configured by default for the NFB. {circle around (1)} After the load current i load drops from 0.5 A to 1 mA, {circle around (2)} the flying capacitor voltage V Cfly initially increases, {circle around (3)} while the flying capacitor balancing control signal S Cfly is negative. The feedback region detector 120 detects this change of the feedback region by the different signs of the slopes {circle around (4)} of the integral of S Cfly

( i . e . , d ⁢ S Cfly d ⁢ t )

and {circle around (5)} of the flying capacitor voltage

( i . e . , dV Cfly d ⁢ t ) .

The feedback sign should be reversed because the feedback region detector 120 has detected that the MLHFC converter 10 is in the wrong feedback region and going away from balance. Consequently, {circle around (6)} the feedback sign of S Cfly is reversed and {circle around (7)} the flying capacitor voltage v Cfly (t) is again balanced at

V

o

2

.

A similar behavior can be observed as {circle around (8)} the load current i load increases back to 0.5 A. At first, {circle around (9)} the sign of S Cfly , is still configured for the PFB. Due to the same signs of the slopes {circle around (10)} of the integral of S Cfly and {circle around (11)} of the flying capacitor voltage V Cfly , the flying capacitor voltage V Cfly is going away from balance, so {circle around (12)} the feedback sign of S Cfly is reversed again, {circle around (13)} which results in flying capacitor voltage balancing.

FIG. 3 illustrates a flowchart of a method 300 of a control system controlling a MLHFC converter 10 in accordance with aspects of the disclosure.

At Step 310 , a controller 110 controls an output of the MLHFC converter 10 .

At Step 320 , a feedback region detector 130 detects a change in a feedback region of the MLHFC converter 10 .

At Step 330 , a controller adjuster 120 adjusts, in response to the detected change, the control system 100 to counteract instability.

The control system 100 enables industrial use of MLHFC converters 10 in existing or future systems by eliminating flying capacitor imbalance resulting in converter instabilities. The control system 100 disclosed herein has many advantages. Flying capacitor balancing is maintained under all conditions. It is not necessary to measure inductor current to detect the feedback region. It is not necessary to know converter parameters, operating conditions, or feedback region boundaries. The control system 100 is independent of the control scheme (VMC, CMC, SMC, etc.). And the same control system 100 can be used in all feedback regions, that is, for all load conditions. This control system 100 enables the use of lower rated components in the power stage, allowing for a smaller silicon footprint for the same conductance of the switches, therefore ultimately reducing cost.

The control system 100 may be comprised of any suitable circuit elements such as bipolar transistors, Metal-Oxide Semiconductor Field-Effect Transistors (MOSFETs), or the like.

The techniques of this disclosure may also be described in the following examples.