INTERLEAVED , MULTI-SWITCH , MULTI-PHASE BOOST CONVERTER FOR BATTERY DISCHARGE REGULATORS

Non-isolated step-up voltage converters are commonly required as Battery Discharge Regulators (BDR’s). The Weinberg and boost derived topologies are traditionally employed in modules up to one or two kilowatts. The Interleaved Boost Converter (IBC) is a distributed power processing approach that provides some advantages: reduced power processing (current) per phase, distributed losses (power semiconductors and magnetics), input and output current ripple cancellation, improved dynamic response and better modularity, among others. The IBC converter as BDR has been previously treated and advantages and drawbacks discussed. In this paper a new approach is proposed, a multi-switch, multi-phase interleaved boost converter (named Multi Interleaved Boost Converter, MIBC) that provides inductor and switching frequency decoupling, which in turn affects in different aspects, such as, magnetics, input and output current ripples or part count. This paper presents and describes the MIBC, the potential benefits compared to the IBC and its application as BDR.


INTRODUCTION
This paper is meant to be a general description of a PWM interleaved method for DC/DC converters.This modulation scheme is aimed to increase converter power density by combining number of modules (phases) in parallel and number of switches per module, please refer to figure 1.

MIBC DESCRIPTION
The MIBC has been devised using several multi-switch boost modules, which are out in phase [1].Traditional multi-switch approach considers the same driving signal for all switches in order to divide the current into several power semiconductors being the reasons for that insufficient current handling capability, excessive power dissipation, complex thermal management or switch redundancy.In the MIBC, the driving signal of each power switch keeps the same switching frequency but the on-time is reduced by a factor m and they are displaced each other in phase over 2pi/m radians.This driving scheme decouples switching frequency and inductor frequency, which actually increases by a factor m. Also the common point, mosfet drain, has an effective switching frequency m times higher than switching frequency.Another interesting feature is that only one switch conducts at a given time, so average current sharing could be adjusted by increasing the number of switches and decreasing the on time.
On the other hand, phase shifted paralleled modules, socalled interleaved converters, also offer reduction of input and output current ripple, improved dynamic response or power losses distribution, among other advantages.For the MIBC, modules will be displaced 2pi/(nm) radians.

DC equations
The MIBC DC analysis has been performed for the general (n x m) configuration.
Inductor frequency Diode RMS current Table 1.DC equations of the general (n x m) MIBC converter

Small-signal transfer functions
Neglecting all parasitic elements, the small-signal transfer functions of the general (n x m) MIBC converter have also been obtained and listed in table 2.
Descrip.Equation nb duty cycleto-output voltage Table 2. Small-signal equations of the general (n x m) MIBC converter

COMPARATIVE STUDY: 8-IBC vs 4-2-MIBC vs 2-4-MIBC
In order to evaluate and compare the MIBC and the IBC converters used as BDRs, a theoretical study has been carried out using [2] as the baseline specifications, please refer to table 3. Table 3.
All have the same number of MOSFETs and consider the same parameters, listed in table 3.

Input & output current ripple
Substituting values in the equations ( 16) and ( 17) result in the input and output current ripples represented in figures 3 and 4.  From the point of view of the output current, the 2-4-MIBC has the highest RMS value in the whole range, which is a key parameter for output capacitor losses and voltage ripple.Thus, low ESR capacitors are mandatory to not impair MIBC performance.

Inductor design
Inductor design, using Magnetics toroid MPP cores, has been considered to evaluate size and volume reduction using MIBC.The inductor design considers Dmax and Po max, the same inductance value and switching frequency for all three configurations.The design procedure has been tuned to achieve similar inductor losses (global losses considering the sum of all inductors) and temperature increment.

CCM/DCM boundary
Another interesting feature concerns to CCM and DCM limits when MIBC is considered.As the number of phases and inductor frequency changes, the minimum inductor current also varies.Since boost transfer functions depend on conduction mode, the wider the range in a particular mode, the better from the control point of view.Analysing the three configurations, please refer to table 5, one realises that 2-4-MIBC requires the lowest output power to remain in CCM.

Power semiconductor stress and impact on losses
An important concern regards to part electrical rating, subsequent part selection and converter losses.One of the most significant differences between the three configurations relates to conduction time and maximum current per switch.Regarding the power mosfet, reduction of both, phases and conduction time per mosfet, implies identical average current value, similar RMS value but higher peak current.Two important considerations arise from this fact, higher peak current capacity will be required and switching losses could increment substantially.Since all configurations will exhibit similar conduction losses, soft-switching techniques can be explored to keep similar performance.
Regarding power diodes, one diode per phase is only required, but working at higher current level and m times switching frequency.Parallel diodes could be also considered to split power losses.Table 6.MIBC electrical rating

Small-signal transfer functions
The MIBC small-signal transfer functions depend on both, n and m parameters.The number of phases, n, determines the equivalent inductance whereas the number of devices, m, has impact on the duty cycle.Input current ripple at the minimum input voltage and maximum power for each configuration is represented in figure 8.The lowest input current ripple corresponds to the n=2; m=4, according to ( 16) and figure 3. Output capacitor current at the minimum input voltage and maximum power for each configuration is represented in figure 9.The lowest RMS value corresponds to the n=8; m=1, according to (18) and figure 4.  Mosfet current is depicted in figure 11.As it can be observed, the average value is the same for all configurations but the peak value and RMS value is higher as n decreases.Figure 14 represents the PWM signal (d1,1) and the sawtooth (SAW1,1) of one power MOSFET using the second method with the commercial multiphase integrated oscillator LTC6909 and a constant current source that charges a capacitor.

MIBC: DIGITAL (FPGA) PWM GENERATOR
Analog implementation of the PWM generator offers simplicity as the main advantage.On the contrary, digital approach, FPGA is considered here, is more complex but has some benefits that should be considered.To highlight some, high time resolution to achieve very low duty cycles, synchronous signals with adjustable dead time for bidirectional conversion or soft-switching, hot reconfiguration for redundancy, protection or efficiency purposes.
In this paper a digital PWM adapted from analog method option 1 is presented.Figure 15 shows the four gate signals of one phase switching at 96kHz.

CONCLUSIONS
This paper presents an interleaving technique that is suitable for DC/DC converters, BDR's is one possible application, as studied here, but other power conditioning functions could be explored.Design equations, comparative study and analog and digital (FPGA) practical implementation of the PWM generator system is presented.

Figure 15 .
Figure 15.Digital PWM signals for one phase: adapted from circuit option 1.
Main equations are listed in table 1.

Table 4 .
Inductor comparisonFrom table 4, 4-2-MIBC exhibits the best outputs in terms of total area and mass, 2-4-MIBC has the lowest losses and 8-IBC gives the poorest results in terms of losses and required size, bringing to light that MIBC offers less input ripple, less losses and less area and mass if compared to the traditional IBC.
Figures 5 and 6 represent the Bode plot of the smallsignal transfer functions Vo/d and iL/d obtained by the PSIM simulator for the configurations specified in the table 7.