Design of a PWM/PFM hybrid modulation switching circuit for a four-switch buck-boost converter

Based on the application background of fuel cell, the conversion efficiency of the four-switch Buck-Boost converter using PWM (Pulse width modulation) is low at light load, and the conversion efficiency of the four-switch Buck-Boost converter using PFM (Pulse frequency modulation) is low at heavy load. A PWM/PFM hybrid modulation circuit using digital module to detect induced current and feedback voltage is presented. The circuit structure is simple and easy to integrate. It has the effect of fast selection according to inductance current and feedback voltage. The circuit works in PWM mode by default. After soft startup, PWM/PFM mode can be switched according to the load. Finally, simulation is carried out in FSBB converter. The experimental results show that the PWM/PFM hybrid modulation circuit has the function of multi-load lead switching mode and protection mechanism. Simulation in FSBB converter shows that the converter efficiency reaches 91.99% and the maximum load current is 2A. This helps improve the efficiency of Buck-Boost converters.

For FSBB converter, many scholars have proposed a variety of control strategies to improve converter efficiency and reduce converter loss.For example, by changing the mode control strategy, using PWM/PFM hybrid modulation, improve the performance of the circuit module and so on [1][2][3].
PWM and PFM each have their own advantages.PWM can provide precise voltage or current control, but in some cases can be a waste of energy.PFM can reduce the frequency of the signal as it approaches the target value, thus reducing power consumption.Hybrid modulation combines the advantages of both to achieve efficient energy utilization.Therefore, the research of PWM/PFM dual-mode converter has always been hotspot.Seong Joong Kim et al. proposed a technique to perform automatic seamless switching between PWM/PFM modes by using time-based PWM/PFM control [4].Yan Chen et al. proposed an automatic PWM/PFM hybrid mode conversion technique for buck DC-DC converter [5].Shin et al. achieved direct and seamless switching between PWM and PFM by forming a negative feedback loop with compensator in PFM operation to keep the control voltage level constant [6].The PWM/PFM control technique proposed by Kai Luo et al. uses a multiplexer to combine a PWM modulator and a PFM modulator to simplify the circuit [7].In the above article, the control conversion is more complicated, requiring more modules and increasing the occupied area ratio.
To solve these problems, a novel PWM/PFM hybrid modulation selection circuit is proposed for FSBB converter.The circuit is simple in structure and consists of PWM detection circuit, PFM detection circuit and mode switching circuit.This paper first analyzes the principle and structure of FSBB, then analyzes the modulation characteristics of PWM and PFM and designs the circuit.The circuit will perform state transfer according to the preset state of the control signal, and then coordinate the relevant signal actions to complete the specific operation.The circuit can automatically select the best modulation mode according to the actual load.The complexity of the circuit is reduced effectively and the integration is convenient.Finally, the circuit is placed in the FSBB converter, and the output of the converter is basically stable under light load and heavy load mode.

Structure description
For the hybrid modulation applied to FSBB, the FSBB principle should be started first. is the output voltage.Under ideal conditions, through the principle of inductance volt-second balance, the lossless voltage gain can be expressed as [8]:  , a new mode called Buck Boost mode is proposed.Since the converter contains three working modes, namely, Buck, Boost and Buck Boost, the three-mode control can achieve more accurate control, thus improving the system performance.
The closed-loop control methods of FSBB converter can be divided into two categories: constant frequency control and ripple control.Among them, fixed frequency control is a type of Pulse Width Modulation (PWM), in which the switching frequency is kept constant on a clock basis [10].Ripple control is Pulse Frequency Modulation (PFM), mainly including Constant On/Off Time (COT/CFT) control, hysteresis control, etc. PFM and PWM has a fundamental difference, no clock and switching frequency is not fixed [11].FSBB converters using PWM/PFM hybrid modulation technology take full advantage of both PWM and PFM.It achieves multiple advantages such as high efficiency, dynamic range extension and energy saving.This makes the converter suitable for a variety of applications, especially power management systems that need to balance high efficiency and energy utilization.

Analysis of PWM and PFM
PWM technology has many advantages.First, because the switching period is fixed, the output ripple is relatively small, resulting in better voltage stability at higher output voltages.Secondly, the control of PWM technology is simple, has good linearity, can be widely used in current and voltage control mode, and can also ensure high efficiency in heavy-duty mode, thus greatly improving the energy utilization of the system [12].However, PWM technology also has some shortcomings.For example, in the light load mode, the work efficiency will have a relatively large decline, resulting in a significant reduction in the utilization of system energy, which will affect the stability of the system.The single PWM modulation mode only considers the conversion efficiency of the switching tube loss: Because the output current OUT I is relatively large under heavy load, the conversion efficiency under PWM mode is relatively ideal, but under light load, the proportion of switching loss in the total loss increases, and the conversion efficiency is low.
Compared with pulse width modulation, pulse frequency modulation is a more advanced modulation method.The on-frequency of the switching tube is used to adjust the output voltage.Usually, adaptive modulation is adopted.The feedback control system monitors the output voltage in real time and adjusts the on-frequency of the switching tube to maintain the stability of the output voltage.
PFM modulation and PWM modulation have the same characteristics, both use constant pulse width generator instead of sawtooth wave, and use voltage-controlled oscillator (VCO) to change the frequency.However, the practical application of PFM is much less than that of PWM.When the output voltage changes, the system's loop feedback compares the sampled signal with the error signal of the internal reference voltage.And according to its offset control pulse frequency generator generates pulse of fixed frequency, so that PFM generates driving switch signal with fixed pulse width and period change.In continuous conduction mode, the modulation system M of PFM modulation mode is: The larger the M , the lower the frequency, the smaller the loss of the switching tube.In PFM mode, M increases with the increase of OUT R .The smaller the M , the lower the loss of the power switching tube, so the higher the conversion efficiency under light load.The advantages of PFM control mode are obvious.In the case of light load, you can increase the switching frequency, so that the conversion efficiency is much higher than the PWM mode.In addition, it is able to set a higher maximum frequency for better frequency characteristics and voltage regulation.However, its defects can't be ignored, because its load adjustment range is narrow, and the cost of the filter circuit will increase accordingly.
In view of the advantages and disadvantages of PWM and PFM, PWM/PFM dual-mode modulation is proposed, as shown in Figure 3.The design of the switching logic requires determining when to switch from PWM mode to PFM mode and vice versa.This is usually based on parameters such as output voltage, load current or input voltage.It is common to use a comparator to compare these parameters against a preset threshold.For example, when the output voltage is above a certain threshold, switch to PWM mode.When the PFM mode is lower than a certain threshold, the PFM mode is switched.
The specific implementation process is as follows: First, the converter works in PWM mode by default until the soft start is complete.Using PWM modulation to control the pulse width of the carrier, the amplitude of the output signal can be controlled.PWM modulation is to control the output voltage by constantly changing the pulse width, the wider the pulse width, the larger the output voltage.Then, after the soft start is complete, choose whether to switch to PFM by comparing the size of the inductive current IL and the reference current IREF.The mode switching circuit outputs the PFM signal to the oscillator, changing the frequency of the oscillating signal.PFM modulation is a technique based on frequency control that allows finer adjustment by changing the frequency of the pulse.In PFM modulation, the higher the pulse frequency, the larger the output voltage, and the lower the pulse frequency, the smaller the output voltage.Finally, by mixing PWM and PFM, the amplitude and frequency of the signal can be controlled simultaneously, so as to obtain a more flexible and accurate output signal.Specifically, PWM modulation can be used to control the coarse adjustment of the output voltage, and PFM modulation can be used to control the fine adjustment of the output voltage, thus achieving more accurate voltage regulation.

PWM/PFM circuit design 2.3.1 PFM detection circuit
As shown in Figure 4, the PFM detection circuit uses a comparator to compare the feedback voltage VFB of the voltage loop with the reference voltage VREF.In general, it is considered that when the constant current source is working, that is, when the feedback voltage is small, it is heavy load when the feedback voltage is large.When the system is operating in PFM mode, the VPFM signal is high.At this point, the system's COMP comparator continuously compares the load feedback voltage to a fixed reference voltage of the VREF.When the feedback voltage VFB<VREF, the system's COMP comparator will automatically adjust to PWM mode to ensure the stability of the output voltage.At this point, the system will issue a VPWMS signal.In PFM mode, the system adjusts the switching frequency of the switching tube according to the feedback voltage of the load to maintain the stability of the output voltage and avoid problems such as overload or overcurrent in the system.In this way, the system can achieve efficient power transmission and energy management under different operating states.

PWM detection circuit
In PWM mode, the system can effectively control the trigger of the master-slave D-flip-flop, so as to realize the two-stage frequency division.And the maximum load of the inductor can be monitored in real time to ensure that it does not exceed the predefined minimum load.If so, the VPFMS signal is activated.
The PWM detection circuit is shown in Figure 5.The PWM detection circuit is a detection and holding circuit composed of a trigger that the inductor peak current exceeds the limit.The circuit is used to detect whether the inductor current exceeds the preset value.According to the inductance current condition IL , the system adopts a PWM mode to control the output voltage.When IL IREF  , it is determined that the system is not suitable for continuing to work in PWM mode, and the system needs to be converted to another mode at this time, so the VOLVP signal is converted from low level to high level.
VOLVP is the output of IL and IREF through the comparator.In addition, it can also be used as a zero signal for the D flip-flop.The D trigger of the latter stage consists of Q1, Q2 and Q3.When the peak inductor current is less than the threshold current for four consecutive cycles, the VPFMS signal is triggered.This VPFMS signal will instruct the system to switch to PFM mode to continue operating in order to maintain the stability of the output voltage.

PWM/PFM switching circuit
When the system receives VPFMS and VPWMS signals, it needs to determine whether to use PWM mode or PFM mode according to the actual situation.If higher energy efficiency and lower load requirements are required, the system can choose to operate in PFM mode.Conversely, if greater stability and higher load requirements are required, the system can choose to operate in PWM mode.
Figure 6 is the PWM/PFM switching circuit flow chart, and VSS is the soft start voltage.Figure 7 shows the PWM/PFM switching circuit.When the system is just started and the soft start is not complete, the system works in PWM mode by default.After the soft start is complete, the system switches modes based on site requirements.When the system works in PWM mode, PWM=1, PFM=0.When VPFMS=1, the system enters PFM mode.In this case, PWM=0 and PFM=1.Comparison of the soft start voltage with the reference voltage produces a set signal RESET.When the protection mechanism is triggered, RESET=0.VPFMS and VPWMS are input signals of the circuit, while PWM and PFM are output signals of the circuit.When the input signal changes (VPFMS, VPWMS changes), the output of the circuit is judged; When PWM=0, PFM=1, if VPWMS changes from 0 to 1, PWM=1, PFM=0, otherwise unchanged; When PWM=1, PFM=0; when VPFMS changes from 0 to 1, PWM=0, PFM=1.Then switch PWM and PFM modes according to the actual load.When the system triggers the protection mechanism, the system reduces the working frequency or stops working.When the system returns to normal operation, the modulation mode is restored to the initial PWM mode, and the mode judgment is performed again.

3.Results and Discussion
The PWM/PFM hybrid modulation circuit proposed in this paper is based on the process of 0.18μm BCD.This work achieves the circuit architecture of the proposed PWM/PFM hybrid modulation by Cadence spectre.Figure 8 shows the simulation diagram of PFM detection circuit.VFB is Buck-Boost output voltage feedback signal and VREF is reference voltage.It is assumed that the circuit is operating in PFM mode and VPFM is a high signal.When the feedback voltage VFB VREF  , the system's comparator will automatically adjust to PWM mode to ensure the stability of the output voltage.At this point, the system will issue a VPWMS signal. .CLK is a periodic 1us signal, and VOLVOP is set to a periodic 10us signal.As can be seen from the simulation diagram, when the inductor peak current is less than the threshold current for 4 consecutive cycles, the VPFMS high level signal will be triggered.The simulation results show that when the system is just started, the RESET signal is 1, and the system sets the initial state PWM mode for initial configuration and startup.When the stability loop is established, the RESET signal becomes 0 and the system starts to work normally.At this time, the system dynamically switches the PWM/PFM modulation mode according to the actual load to ensure that the system can work at the best efficiency.In PFM mode, if VPWMS changes from 0 to 1, the mode switches to PWM.In PWM mode, if VPFMS changes from 0 to 1, the mode switches to PFM.When the system triggers the protection mechanism, the reset signal is reset to 1.At this time, the system will remain in PWM mode to ensure stable operation and avoid further damage.Before the system can return to normal operation, the fault must be detected and repaired, and the RESET signal must be reset to 0. Enables the system to switch modulation again and operate at optimal efficiency.

4.Conclusions
Based on the application background of fuel cell system, a new PWM/PFM hybrid modulation circuit is proposed to solve the problem of low PWM efficiency under light load and low PFM efficiency under heavy load.The circuit consists only of PFM detection circuit, PWM detection circuit and PWM/PFM mode switching circuit.The new PWM/PFM hybrid modulation circuit can automatically select the best modulation mode by detecting inductive current and feedback voltage.It is simulated in a FSBB converter.Through analysis and simulation, a conclusion can be drawn: 1. Simple circuit structure, easy to integrate.2. The modulation mode can be quickly selected by sensing inductive current and feedback voltage.
3. Simulation in FSBB converter, the converter efficiency can reach 91.99%, and the maximum load current is 2A.
Figure 1.shows FSBB converter.In the figure, Buck_ H and Buck_ L constitute Buck unit, and the duty cycle 1 d of the unit is defined.Boost_ H and Boost_ L constitute the Boost unit and define the duty cycle 2 d of the unit.L and C are inductance and capacitance respectively; IN V is the input voltage and OUT V

Figure 2 .
Figure 2. Basic topology of FSBB converter In order to solve the problem of frequent switching between Buck and Boost modes in FSBB converters when IN OUT V V 

Figure 3 .
Figure 3. Topology of PWM/PFM dual mode modulation Finally, by mixing PWM and PFM, the amplitude and frequency of the signal can be controlled simultaneously, so as to obtain a more flexible and accurate output signal.Specifically, PWM modulation can be used to control the coarse adjustment of the output voltage, and PFM modulation can be used to control the fine adjustment of the output voltage, thus achieving more accurate voltage regulation.

Figure 4 .
Figure 4. PFM detection circuit structure diagramIn PFM mode, the system adjusts the switching frequency of the switching tube according to the feedback voltage of the load to maintain the stability of the output voltage and avoid problems such as overload or overcurrent in the system.In this way, the system can achieve efficient power transmission and energy management under different operating states.

Figure 8 .
Figure 8. PFM detection circuit simulation diagram Figure9shows the PWM detection circuit simulation.The system works in PWM mode by default, and determines whether the system is suitable for continuing to work in PWM mode according to the inductor current IL condition.If it is not suitable, the system needs to be converted to another mode.CLK is the output signal of the oscillator module, and the square wave signal is used to represent the output VOLVP of the comparator.VOLVP from high to low indicates L REF I I 

Figure 9 .
Figure 9. PWM detection circuit simulation diagram Figure 10 is the simulation diagram of PWM/PFM switching circuit, where VSS represents the soft start signal, and the reference signal is set to 4.5V.When VSS is less than 4.5V, the circuit starts, and PWM is pulled up while PFM is pulled down.After starting, the circuit enters the normal transition state.

Figure 10 .
Figure 10.PWM/PFM switching circuit simulation diagramThe simulation results show that when the system is just started, the RESET signal is 1, and the system sets the initial state PWM mode for initial configuration and startup.When the stability loop is established, the RESET signal becomes 0 and the system starts to work normally.At this time, the system dynamically switches the PWM/PFM modulation mode according to the actual load to ensure that the system can work at the best efficiency.In PFM mode, if VPWMS changes from 0 to 1, the mode switches to PWM.In PWM mode, if VPFMS changes from 0 to 1, the mode switches to PFM.When the system triggers the protection mechanism, the reset signal is reset to 1.At this time, the system will remain in PWM mode to ensure stable operation and avoid further damage.Before the system can return to normal operation, the fault must be detected and repaired, and the RESET signal must be reset to 0. Enables the system to switch modulation again and operate at optimal efficiency.

Figure 11 .
Figure 11.(a) FSBB has a load of 1A in buck mode(b) FSBB has a load of 2A in buck mode Finally, Virtuoso simulation platform is used to apply the design to a FSBB converter with peak current mode.The converter can achieve stable output of 2.8V~29V in a wide input voltage range of 3.3V~29V.The converter can change the operating frequency of the converter within 100kHz~1MHz according to the load condition through PWM/PFM hybrid modulation circuit.It has a maximum load capacity of 2A, as shown in Figure 11.It can be seen that the output voltage changes very little under heavy load and light load.Under typical conditions, the energy conversion efficiency of 91.99% can be achieved.The PWM/PFM hybrid modulation circuit in this study will enable FSBB converters to operate at high efficiency.