How to Build an Energy-Storage Bidirectional Inverter Board: Topologies and Power Device Selection for Bidirectional Charge / Discharge
Bottom line: an energy-storage bidirectional inverter board puts battery charging and battery discharging to the load onto the same power platform. Its core is bidirectional DC/DC + DC bus + bidirectional DC/AC (rectifier / inverter reused) + on-grid / off-grid switchover. A low-voltage 48V battery platform usually uses an isolated high-frequency bidirectional DC/DC to step up to a ~400V bus, then uses 600/650V-class devices for bidirectional DC/AC. The selection order is fix the battery voltage platform and isolation method first, then the bus voltage, then pick devices for the three positions — battery side / bus side / AC side — instead of starting with the question "IGBT or MOSFET?"
What Really Differs Between a Bidirectional Inverter and a Conventional Inverter
A conventional inverter only does one-way conversion: DC (battery / PV) → AC, feeding the load. An energy-storage bidirectional inverter board (also called a bidirectional energy-storage converter, or the single-phase / three-phase board of a hybrid inverter / PCS) must move energy in both directions:
- Discharge direction (battery → AC): battery DC is stepped up to the bus by the DC/DC stage, then inverted to AC by the DC/AC stage — following grid phase when on-grid, building voltage autonomously to carry the load when off-grid. This step is identical to a conventional inverter;
- Charge direction (AC → battery): the grid (or an AC source inside a solar-storage system) is fed back into the bus through the DC/AC stage running in rectifier (PFC) mode, then stepped down by the bidirectional DC/DC to charge the battery — a path a conventional inverter does not have.
When a customer builds a residential solar-storage all-in-one unit, a portable power station, or a solar-storage-charging microgrid, this bidirectional board is exactly what they need: charge the battery when PV is abundant or during off-peak tariff hours, and discharge the battery to feed loads when the grid goes down. That is the essential difference from a conventional inverter. The front-end PV input is handled by an MPPT controller — see How to Build an MPPT Controller: Topologies and Power Device Selection for PV Charge Controllers.
Design Flow: Lay Out the Architecture First, Then Select Devices
flowchart LR
A["Requirement definition<br/>(power / battery platform / on-grid & off-grid)"] --> B["Battery voltage platform<br/>48V low-voltage or high-voltage battery"]
B --> C["Architecture selection<br/>isolated or non-isolated / single-stage or two-stage"]
C --> D["Bus voltage and<br/>device voltage class"]
D --> E["Device selection by position<br/>battery side / bus side / AC side"]
E --> F["Control / drive / protection<br/>sampling / switchover / surge"]
F --> G["System-level testing<br/>charge-discharge efficiency / on-grid & off-grid switchover / certification"]Step 1: The Battery Voltage Platform Decides the Architecture
The battery voltage platform is the "anchor" of the whole board — it directly determines current magnitude, isolation method and device voltage class:
- 48V low-voltage platform (traditional low-voltage residential-storage batteries, portable power stations): battery voltage is low but current is large — taking 5kW as an example, battery-side current is about 100A (5000W ÷ 48V, excluding efficiency losses, so higher in practice), requiring parallel MOSFETs, high-current traces and enhanced thermal design. To lift 48V to the ~400V bus needed to invert 220V, an isolated bidirectional DC/DC with a high-frequency transformer is widely used (low-voltage MOSFETs in parallel on the primary side), which also solves galvanic isolation between the battery side and the AC side; two-phase interleaved non-isolated solutions also exist, but device stress and safety isolation must be evaluated separately.
- High-voltage battery platform (battery packs of several hundred volts, voltage depending on series cell count): current drops significantly, harness and paralleling pressure are lower, and a non-isolated bidirectional conversion can save one stage of isolation loss and transformer cost; whether to keep isolation depends on the overall safety requirements and the customer corporate standard.
- Three-phase high-power PCS / solar-storage-charging: bus voltage is higher, the power stage moves to IGBT modules or three-level topologies, and device voltage class rises to the 1200V level.
Mainstream Architecture Comparison
| Architecture | Typical application | Advantages | What you pay for |
|---|---|---|---|
| Two-stage · high-frequency isolation (isolated DC/DC on battery side + bidirectional DC/AC) | 48V low-voltage residential storage / portable power stations | Battery and AC are galvanically isolated, clear safety path, low-voltage MOSFETs usable on the primary side | One extra high-frequency conversion stage, slight penalty in efficiency and cost |
| Two-stage · non-isolated (bidirectional DC/DC + bidirectional DC/AC) | High-voltage-battery residential storage, systems where bus voltage matches the battery well | Saves the transformer, higher efficiency, smaller size, better cost | Must satisfy overall isolation / safety requirements on your own |
| Single-stage bidirectional DC/AC | Special cases where battery voltage is close to bus voltage | Simplest structure | Poor versatility, rarely seen in residential storage |
| AC-side line-frequency transformer isolation | Legacy off-grid inverters, heavy-load applications | Rugged, surge-tolerant, easy to maintain | Bulky transformer, high cost, low efficiency |
For the bidirectional DC/AC section: single-phase 220V systems mostly use a full-bridge (H-bridge) two-level topology; a T-type three-level can be used where higher efficiency and lower harmonics are wanted; three-phase systems use a three-phase full bridge or three-level. Do not lock devices before the topology is fixed.
Step 2: Calculate Bus Voltage and Device Voltage Class First
The AC-side voltage determines how high the DC bus must be:
- Single-phase 220V: the bus must be higher than the grid peak voltage (220 × √2 ≈ 311V); a typical bus is 380~400V, and AC-side and bus-side devices use the 600/650V class (IGBT / superjunction MOSFET / SiC) with sufficient margin for turn-off spikes;
- Three-phase 380V: the bus is higher and devices generally move to the 1200V class (IGBT modules or SiC MOSFETs).
The general principle for device voltage class and the way to keep margin are the same as in How to Select Power Devices (IGBT / MOSFET) for Residential Storage and Inverters: allow more than 20% margin over the maximum operating voltage plus turn-off spike, and remember that low temperature, full load and upper input-voltage limit stacking together is the worst case.
Step 3: Select Devices by "Position + Stress"
Devices on a bidirectional board are not "one transistor for everything" — each position picks its own. The table below is a reference for typical ranges; actual voltage rating, current and package depend on your topology calculation and the manufacturer datasheet:
| Position | Typical stress (example) | Recommended device category | Selection points |
|---|---|---|---|
| Battery side · isolated DC/DC primary (48V platform) | Tens of volts, tens to hundreds of amps | Low-voltage MOSFET (75~150V class) | Paralleling and current sharing, low RDS(on), Qg matched to drive capability, body-diode reverse recovery |
| Battery side · non-isolated conversion (high-voltage battery) | Covers maximum battery voltage + margin | High-voltage MOSFET / superjunction (per calculation) | Duty-cycle range, bus-reflected stress, low-side current capability |
| Bus side · synchronous rectification / bus conversion | ~400V bus | 650V superjunction MOSFET / 650V SiC | Synchronous rectification efficiency, switching loss, reverse recovery |
| AC side · single-phase DC/AC (220V) | ~400V bus | 600/650V IGBT or superjunction MOS / SiC | Higher switching frequency tilts toward MOS / SiC; off-grid overload current capability |
| AC side · three-phase (380V grid-tied) | Higher bus | 1200V IGBT (discrete / module) or 1200V SiC | Short-circuit withstand, thermal design, isolated drive and negative-voltage turn-off |
| IGBT companion freewheeling | Anti-parallel diode position | Fast recovery diode (FRD) | Reverse recovery matched to the IGBT; MOSFET solutions use synchronous rectification instead |
| Sampling / communication / control line protection | Sensitive to surge residual voltage | Low-clamping (foldback) TVS | Look at clamping voltage first, power second — same approach as BMS sense lines |
Two extra device considerations specific to bidirectional boards:
- The reverse path cannot be left bare: MOSFETs carry reverse current through the body diode plus synchronous rectification, so body-diode reverse recovery and thermal behavior are key assessment points; IGBTs have no internal anti-parallel diode and must be paired with an external FRD — this is the biggest difference in the bill of materials between a "bidirectional" and a unidirectional inverter.
- Off-grid loading stresses devices more than on-grid: in off-grid mode the board must directly start inductive / high-inrush loads such as air conditioners and water pumps, so device short-circuit current capability, overcurrent protection response time and bus capacitor support must be evaluated together — many boards are destroyed at the instant of off-grid startup.
Control, Drive and Protection: The Other Half of a Bidirectional Board
- Control loops: battery-side current loop + bus voltage loop, plus grid synchronization / phase lock for on-grid, plus output voltage loop for off-grid. The real difficulty is mode switchover (charge ↔ discharge, on-grid ↔ off-grid) without letting the bus collapse or current overshoot; switchover time and load compatibility are common items in customer corporate standards. Sampling channels must be multi-channel, synchronized and accurate, and the main controller needs floating-point capability and advanced PWM; for controller selection see Which MCU for Energy-Storage BMS? A GD32 Selection Guide.
- Drive: for IGBT / SiC, use isolated drive + negative-voltage turn-off + desaturation (desat) short-circuit protection; for low-voltage MOSFETs, watch gate-loop inductance and the Miller plateau, and tune the gate resistor together with switching loss.
- Protection: DC bus precharge (to prevent inrush at power-up), fast shutdown on overcurrent and short circuit, bus overvoltage, overtemperature derating, battery reverse-polarity protection, AC-side leakage / ground monitoring.
- Surge protection: use MOV / gas discharge tube / TVS in staged protection against lightning and switching surges at the AC inlet; sampling, communication and control lines are the worst-hit area for "downstream still fails even with a TVS installed" — apply the low-clamping TVS approach directly. The selection logic is the same as for BMS sense lines: see How to Select TVS for BMS Protection Boards: A Low-Clamping TVS (Foldback TVS) Guide and What to Do When a BMS Protection Board Blows Up?.
What Intek Technology Can Provide
The energy-storage bidirectional inverter board is one of the in-house system solutions of Intek Technology (see the solution matrix under Product Lines and Solution Capabilities). Around the bidirectional board, Intek Technology can provide:
- Full power-device chain: China Resources Micro (华润微) MOSFETs (from low-voltage high-current to superjunction), IGBTs and SiC — a selection comparison table by position for your topology; Intek Technology is an authorized distributor for China Resources Micro.
- Controller and protection devices: GigaDevice (兆易创新) GD32 MCU controller selection (Intek Technology is an authorized IDH for GigaDevice) plus AMSEMI (Anhui Anmei Semiconductor) low-clamping / foldback TVS for sense-line protection (Intek Technology is an authorized distributor for AMSEMI TVS) — one BOM covering controller + power + protection.
- Implementation support: full-board solution or key-component kitting, sample testing, and follow-up on on-grid / off-grid and surge test issues.
Send over the power level, battery platform and on-grid / off-grid requirements, and the device selection list will be produced directly for your project; specific part parameters are subject to the manufacturer datasheet.
FAQ
Q1: What is the difference between a bidirectional inverter and a conventional inverter?
A conventional inverter only converts battery / PV DC into AC (one-way discharge); a bidirectional inverter board can also run in reverse, converting grid AC into DC to charge the battery (rectifier / PFC mode) — hence "bidirectional". Architecturally it adds a bidirectional DC/DC and rectifier path, supports on-grid / off-grid switchover, and is the core power board of residential-storage all-in-one units and portable power stations.
Q2: For a 5kW bidirectional inverter board on a 48V battery, how large is the battery-side current, and how should MOSFETs be selected?
Battery-side current is roughly power divided by battery voltage: 5kW ÷ 48V ≈ 100A order of magnitude (excluding efficiency losses), plus ripple on top of that. So on a 48V platform the battery side commonly uses 75~150V-class low-voltage MOSFETs in parallel, comparing RDS(on), Qg and parallel current sharing, while laying out high-current traces and thermal design carefully; for higher power, consider interleaved paralleling or moving to a high-voltage battery platform. The exact number of paralleled devices is calculated from losses and junction temperature, subject to the manufacturer datasheet.
Q3: IGBT or MOSFET on a bidirectional inverter board, and when should SiC be adopted?
Look at position and switching frequency first: use MOSFETs on the low-voltage battery side and in high-frequency sections (synchronous rectification saves loss); near the 400V AC-side bus, an IGBT is more economical when switching frequency is not high, while superjunction MOSFETs or SiC suit high frequency, high efficiency and small size; three-phase high-power and EV-charger-type high-frequency high-voltage scenarios are where SiC really pays off — SiC costs more than IGBT, so the price gap must be earned back through high-frequency losses, heatsink volume and overall system efficiency.
Q4: What tests and certifications does an energy-storage bidirectional inverter board usually have to pass?
Usually three categories: safety and EMC (per the target market, e.g. the European CE system, the North American UL system, the domestic GB system, subject to the current requirements of the issuing body); on-grid / off-grid performance (grid-tied models must meet local grid codes and grid adaptability tests; off-grid models are judged on switchover time, load carrying and waveform quality per corporate standards); battery side (the companion battery pack may involve IEC 62619, GB/T 36276 and other energy-storage battery safety standards depending on the application, subject to customer requirements and the certification body). At component level, automotive-grade or harsh-environment applications can use AEC-Q101 qualified devices (such as the AMSEMI -N series TVS). Align the certification list with the customer and the certification body at project kickoff to avoid retrofitting later.
Contact Us
For topology evaluation of an energy-storage bidirectional inverter board, a position-by-position power device selection comparison table, or sample testing of China Resources Micro IGBT / MOSFET / SiC and AMSEMI low-clamping TVS, contact us.
Shenzhen Intek Technology Co., Ltd — electronic components distributor and system solutions provider Tel / WeChat: 136-3264-8484 Address: 4F, Building 2, Jingwei Center, No. 309 Pingan Avenue, Pinghu Street, Longgang District, Shenzhen, China Web: www.intek.vip