Design Concerns of 3KW Bidirectional Converter

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As folks pursue a wholesome and close-to-nature life-style, the demand for out of doors transportable energy provide will increase. In accordance with the commercial improvement report of China Industrial Affiliation of Energy Sources, solely 52,000 transportable vitality storage units have been shipped worldwide in 2016. It’s estimated that the cargo will attain 4.83 million items in 2021, with the compound annual development fee of 148%. On the similar time, with the change of utility events in transportable energy provide, the proportion of large-capacity batteries will improve 12 months by 12 months, and with it, the demand for matched energy provide design may also rise from tons of of watts to kilowatts.

This paper introduces our design issues of a 3KW bidirectional converter that meet the demand for high-power transportable vitality storage merchandise. Firstly, we evaluate the efficiency of a number of energy units within the design and work out that SiC units can enhance the efficiency of bidirectional converters due to their wonderful switching traits and minimal reverse restoration efficiency of their physique diodes. Subsequent, we analyze the design challenges of the entire system, and work out that assembly the achieve necessities of the charging and inverter bidirectional DC/DC is a serious issue. And subsequent, we design an answer, and construct a bidirectional LLC resonant community by including a resonant inductor to satisfy the achieve necessities of the bidirectional LLC. Ultimately, the take a look at outcomes and information are given.

Desk 1 reveals the design specs of this design, and Fig. 1 reveals the facility circuit of this design. When working within the ahead route, it fees the battery. On the similar time of charging, the output AC might be linked to the inverter output by way of the relay, the inverter port takes energy from the enter AC; Within the case of inversion, the system first boosts the battery voltage to about 400V, after which inverts 400V DC into 220Vac sine wave AC for the load.

Desk 1: Design Specs of 3kW Bidirectional ConverterA

Theoretical evaluation of totem pole PFC

When the system works within the charging mode, the enter AC voltage must be transformed to DC400V by the entrance stage PFC circuit, after which the rear stage DC/DC converts the 400V DC voltage to the fixed present supply to cost the output battery.

The theoretical evaluation of totem PFC may be very mature. Let’s briefly evaluation it right here.

Fig. 2~ Fig. 5 reveals the working mode of totem pole PFC. Within the optimistic half cycle of AC, Q2 is the principle swap and Q1 is the freewheeling swap, This fall is at all times on and Q3 is at all times off. Within the AC auxiliary half-cycle, Q1 is used as the principle swap and Q2 because the freewheeling swap. On the similar time, Q3 is at all times on and This fall is at all times off.

It may be seen that:

The working frequencies of two bridge arms of totem PFC are totally different; Q1&Q2 represent a high-frequency bridge arm, and its working frequency is the switching frequency set by us. The bridge arm composed of Q3&This fall works in a low-frequency state, and its working frequency is the enter AC frequency.

Inside the optimistic and unfavourable half of the totem PFC, the principle swap and freewheeling swap want to modify.

Totem PFC works as a synchronous rectifier enhance circuit, and it’s often designed within the state of CCM inductor present in high-power purposes, which places ahead larger necessities for the efficiency of the system physique diode at this place.

Energy system choice i. MOSFET

Mosfet has very low on resistance, good switching traits and low switching loss. Nonetheless, within the design of totem pole PFC, since totem pole PFC might be equal to synchronous rectification enhance circuit, when the swap works as a freewheeling swap, to be able to forestall the higher and decrease MOS from going quick by way of, the inductor present will inevitably circulation by way of the physique diode, so the efficiency of the physique diode will probably be an essential consideration on this utility design.

Fig. 6 selects the physique diode parameters of the very best Mosfet in the marketplace at current. Its Qrr reaches 1.2uC. If the bus voltage of 400V and the switching frequency of 45KHz are thought of, the loss brought on by Qrr reaches 21.6W by way of calculation, which brings nice effectivity discount. Extra importantly, the issue of system temperature rise will change into extra severe. Due to this fact, the high-frequency switching MOS mustn’t solely have extraordinarily low conduction and switching losses, but in addition have excellent reverse restoration traits of its physique diode.

Energy system choice ii. Hybrid IGBT

IGBT units are broadly utilized in high-power design due to their low saturation conduction voltage drop and aggressive value benefits. Nonetheless, IGBT additionally has its disadvantages. First, IGBT itself can’t work within the third quadrant like Mosfet, so when it’s used as a freewheeling swap, there is no such thing as a freewheeling channel, and the inductor present can solely freewheel by way of the anti-parallel diode; Secondly, the present tailing impact of IGBT will increase the switching lack of IGBT and limits the switching frequency of IGBT. With a view to overcome the above shortcomings, the hybrid IGBT of LUXIN Semiconductor is chosen on this design. LUXIN Semiconductor supplies optimized IGBT efficiency and integrates anti-parallel SiC diodes inside. The switching frequency of totem pole PFC on this design might be elevated from 20KHz to 45KHz.

Energy system choice iii. SiC

Because the third-generation semiconductor, SiC Mosfet supplies wonderful efficiency with extraordinarily low on-resistance and switching loss. On the similar time, it might probably work within the 1st or third quadrant. When freewheeling, the inductor present flows by way of the conductive channel, realizing actual synchronous rectification. The physique diode solely works within the lifeless time of the higher and decrease transistors, which drastically reduces the system loss. On the similar time, the lack of reverse restoration of the physique diode of SiC mosfet talked about above can also be very small. Due to this fact, SiC units are very appropriate for the high-frequency bridge arm of totem pole PFC.

Fig. 7 compares the ahead voltage drop of a 50A IGBT and a 40 mω SiC Mosfet. It may be seen from the determine that the ahead voltage drop of SiC units is linear, which might be calculated by Rdson*Ids. On the similar time, with the rise of temperature, the turn-on voltage drop may also improve due to the rise of Rdson. The voltage drop curve of IGBT is a nonlinear curve. It may be seen from the determine that inside Vce voltage drop of 1.2V, SiC’s on-voltage drop is superior to IGBT’s. On the similar time, IGBT has the bottom threshold voltage of 0.8V. It may be anticipated that the effectivity benefit of SiC units is extra apparent below gentle load.

Fig. 8 and Fig. 9 present the present loops of SiC and IGBT in freewheeling state. When IGBT works in freewheeling mode, the inductor present can solely circulation by way of its anti-parallel diode, as a result of IGBT doesn’t have the power to reverse present. Nonetheless, when the SiC system is used within the freewheeling state, the inductor present can circulation by way of the conductive channel of SiC. As a result of the Rdson of SiC units is low sufficient, the voltage drop brought on by present flowing by way of Rdson will probably be a lot decrease than that of IGBT’s anti-parallel diode, which is able to additional enhance the effectivity of SiC circuits.

Theoretical evaluation of bidirectional LLC

As talked about above, when working within the ahead route, totem pole PFC converts AC voltage into 400V DC bus, and the second stage DC/DC converts 400V bus voltage into fixed present supply to cost the battery. When working in reverse, the battery voltage is boosted by LLC, and the totem pole PFC circuit will work in an inverter state, changing the DC voltage into 220V AC voltage.

Totally different from the design of totem pole PFC, LLC’s system choice will probably be a lot less complicated, as a result of LLC can understand ZVS of units and ZCS of rectifier diodes in a full vary. The design issue of LLC lies in the best way to meet the system necessities of voltage achieve in two instructions. Desk 2 reveals the system achieve necessities in two modes.

Desk 2: Achieve Design Necessities of LLC

In accordance with the achieve necessities in Desk 2, the ahead and reverse achieve curves of LLC are drawn as follows.

Fig. 10 reveals the achieve curve of LLC in charging mode. It may be seen that the achieve curve of LLC can meet the system design necessities below the situation of ahead operation;

Fig. 11 reveals the achieve curve of LLC in Inverter mode. It may be seen from the determine that the achieve of LLC is at all times lower than 1 in inverter state, which may’t meet the achieve necessities of the system.

With a view to meet the bidirectional system achieve necessities, there are a number of totally different options. The primary technique is to make use of roof of the world DC/DC converter, and add a synchronous rectification bidirectional buck enhance circuit between PFC and LLC. By means of the buck enhance converter, the required bus voltage might be flexibly adjusted when working in each instructions to assist the design necessities in each instructions, and LLC can even work at a greater frequency level to appreciate the optimum LLC design. Nonetheless, the rise of the third buck enhance circuit will make the system and management extra difficult and the price larger.

On this design, a brand new technique is adopted. Firstly, the bus voltage will probably be adjusted with the battery voltage. With a view to additional widen the adjustment vary, 500V electrolytic capacitor is adopted. Within the ahead route, the PFC voltage will probably be adjusted with the battery voltage within the vary of 380V to 460V; When working within the reverse route, the bus voltage will probably be adjusted within the vary of 360V to 460V V. Secondly, to be able to obtain the design purpose of LLC reverse achieve better than 1, we add an inductor between the midpoints of the high-voltage bridge arms. This inductor, resonant inductor and resonant capacitor kind a brand new resonant community. The precise circuit is proven in Fig. 12, when working within the ahead route, Lr,Cr and the excitation inductance Lm of the transformer kind a ahead LLC resonant community; In reverse, the elevated inductances L2,Lr and Cr represent a reverse LLC resonant community. By growing the exterior inductance, the design purpose of reverse enhance of LLC converter is realized.

Fig. 13 reveals the achieve curve of LLC in reverse inverter mode after including exterior inductance. It may be seen from the achieve curve that the improved achieve curve can meet the achieve necessities of the system.

System management

Fig. 14 reveals the block diagram of the entire system. The management core of the entire system is a MCU, SPC1168. SPC1168 is a floating-point MCU with Arm4 core, which has considerable peripheral sources to appreciate most digital energy management. The MCU is positioned on the battery facet of the system to facilitate communication with BMS administration. Crisscross facet sampling, together with AC enter voltage, inverter output voltage and PFC bus voltage, all understand remoted voltage sampling by way of the remoted voltage sampling chip NSI1311 of Novosense; AC enter present and inverter output present are sampled by MCA1101 collection present sensor of Aceinna. As well as, LLC main resonance present is remoted and sampled by CT transformer, and all isolation sampling units meet the necessities of strengthened insulation of main and secondary sides. DC battery facet sampling contains sampling of output voltage and output present. As a result of the output voltage is shared with MCU, the output voltage is sampled by a easy voltage divider resistor. The output present passes by way of the present sampling resistor, and the differential operation amplifier circuit is adopted to appreciate the correct sampling of the output present.

When working within the ahead route, the MCU samples the enter AC voltage, the enter inductor present and the PFC bus voltage, and obtains the frequency of the enter AC voltage by section locked loop, thus finishing the PFC management; On the similar time, the output voltage and present data are sampled, and in keeping with the output voltage data, the setting of PFC bus voltage is adjusted in actual time, in order that LLC works as shut as doable to the resonant working level, and the output fixed voltage and fixed present management is accomplished. When working within the reverse route, the MCU controls the inverter output, and the inverter output adopts double closed-loop management of output voltage and present, the present loop adopts PI regulation, and the voltage loop adopts PR regulator, which realizes excessive achieve at attribute frequency and no static error regulation of the inverter output voltage.

The profitable use of MCU on this design has changed the normal twin DSP scheme, diminished the system value, and on the similar time diminished the consumer’s design dependence on DSP, which not solely has apparent value benefit, but in addition has extra steady provide, and eased the shopper’s worries about DSP provide.

Experimental consequence

Primarily based on the above evaluation outcomes, a 3KW prototype was made in our laboratory. The effectivity of SiC system and hybrid IGBT in totem pole PFC and the entire system is in contrast. A part of LLC adopts IXFH34N65X2 of Littelfuse, and the secondary rectifier samples the 100V system SRT10N047H of Sanrise. A MCU SPC1168 with Arm4 core of Spintrol completes the charging and inverter management of the entire system. On the similar time, to be able to meet the security necessities of the system, the remoted sampling chip NSI1311 and remoted driving chip NSI6602B from Novosense, the 65A present sensor MCA1101 of Aceinna, and many others. meet the necessities of strengthening the insulation of the system.

The take a look at information and experimental waveforms are as follows. Figs. 15-16 present the effectivity curves of PFC stage and system stage when SiC units and IGBT units are used respectively. It may be seen from the determine that the totem pole PFC with SiC is 3.5% larger than the totem pole PFC with IGBT at gentle load and 0.6% larger at full load. Underneath gentle load (250W), the effectivity of the entire machine is improved by 3%, and below full load, it’s improved by 0.5%.

From the take a look at outcomes, the effectivity enchancment is apparent when SiC system is used below gentle load, which is critical for out of doors vitality storage system. The three% effectivity enchancment implies that the 20W charger can final 270 minutes longer, and the 65W charger can final 83 minutes longer.

Figs. 17~18 present the take a look at information of PF worth of ahead PFC and voltage THD of reverse inverter. At full load, it reaches the PF worth of 0.995. At reverse inverter output and resistive load, the voltage THD is inside 2% in the entire load vary. Underneath RC and RL hundreds, the AC output voltage THD can be saved inside 3%.

Figs 19~20 present the working waveforms of every a part of the system. Fig. 18 reveals the waveforms of enter voltage and enter present when working within the ahead route. The enter present tracks the output voltage, and good PFC impact is achieved. Fig. 19 reveals the working waveform of LLC. LLC works in enhance mode at the moment, and the driving sign of synchronous rectifier is turned off prematurely to stop the present from flowing within the reverse route.

Figs 21~22 present the take a look at waveforms throughout inversion. Fig. 20 is the startup waveform within the inverter mode. It may be seen from the waveform that the output 220V AC voltage is progressively established. Fig. 21 reveals the output voltage and present waveforms at resistive full load. It may be seen that when the resistance is totally loaded, the output voltage has no distortion, and a very good management impact is achieved.

Moveable vitality storage system

With a view to meet the wants of the entire transportable vitality storage energy provide, in addition to the 3KW bidirectional converter, we additionally designed a 1200W photo voltaic MPPT energy board. Fig. 22 reveals the photograph of the prototype, with a bidirectional conversion measurement of 200mm*320mm*55mm, which is designed in keeping with the scale necessities of the battery pack. MPPT board helps the utmost charging energy of 1200W, which is handy for customers to make use of photo voltaic panels to shortly replenish batteries in actual time when outside. Desk 3 reveals the design specs of MPPT energy board. MPPT board helps a large voltage vary of 10~150V, and customers can flexibly configure the collection and parallel connection of photo voltaic panels. With a view to understand a variety of enter and output voltage circumstances, MPPT design adopts 4-switches buck-boost circuit, which converts the voltage of photo voltaic panel into a continuing present supply of 20A to cost the battery. The MPPT management algorithm can also be realized by MCU SPC1168 of Spintrol, and the MPPT effectivity reaches 99%.

Desk 3:MPPT design specs

The MPPT board additionally integrates a 300W buck converter. The voltage of the battery is transformed to 24V output by the step-down converter, which provides energy to the PD a part of the system. PD board supplies 4 outputs, 2 of port A and a pair of of port C. The utmost output energy of port A is 20W, and that of port C is 100W. Desk 4 reveals the design specs of PD board.

Due to this fact, a transportable vitality storage system wants to incorporate battery BMS, bidirectional converter, photo voltaic charging MPPT circuit and PD port. Our system design is confirmed as Fig. 23. We’ve accomplished the facility design required within the system, and BMS system is offered by battery producers typically.

Desk 4: Design Specs of PD Board

Abstract

This paper introduces some design issues of 3KW bidirectional converter in transportable vitality storage purposes. The benefits and downsides of conventional Mosfet, IGBT and SiC Mosfet in totem pole PFC utility are in contrast. It’s identified that SiC system is probably the most appropriate energy system for totem pole PFC utility due to its wonderful efficiency and diode traits.

It analyzes the achieve demand of LLC half below charging and inverter circumstances, and several other strategies to satisfy the achieve demand. It’s concluded that including an exterior inductor is the only strategy to meet the bidirectional achieve of LLC. By including an exterior inductor between the bridge arms, the achieve necessities of LLC below each charging and inverter circumstances are met.

The system sampling and management of bidirectional converter are launched. The appliance of MCU in bidirectional converter reduces the system value, in addition to the consumer’s design dependence on DSP and provide worries. It brings nice reference worth to customers.

Ultimately, in keeping with these analyses, a 3KW bidirectional converter prototype is made within the laboratory, and the take a look at outcomes are given. The results of SiC units and IGBT units on the effectivity of the entire system are in contrast. SiC system can drastically enhance the light-load effectivity of the system, with a rise of three%. By means of the effectivity improve of three%, 3KW transportable vitality storage can present an extra charging time of almost 4 hours for the 20W cell phone charger. Present an extra 1 hour and 20 minutes for the 65W laptop adapter.

Along with 3KW bidirectional converter, we have now additionally made 1200W MPPT board and 300W PD board. Clients can simply use these mature reference designs to finish the entire transportable vitality storage system design. For extra schemes and detailed design data, please go to www.apl-power.com or contact gross sales@szapl.com.