Half-Bridge Inverter Design: Sizing a 24V LiFePO4 System for 800W Loads

A half-bridge inverter converts DC to AC using two switching devices and a split capacitor DC bus, outputting a peak AC voltage equal to half the DC bus voltage. For off-grid power and UPS applications under 1000W, it is the most cost-effective and thermally manageable topology. When paired with a 24V LiFePO4 battery bank, a half-bridge inverter comfortably handles an 800W continuous load without the complex isolated gate-drive circuitry required by full-bridge designs.

This guide walks through the exact system block architecture, battery sizing math (including Peukert losses), and component selection to build a reliable 800W 24V power system.

The Half-Bridge Topology and System Block Architecture

Unlike a full-bridge that uses four switches to swing the full DC bus voltage across the transformer primary, a half-bridge uses two switches and a capacitive voltage divider. This means the transformer primary only sees V_dc / 2. For a 24V nominal battery system (operating between 20V and 29.2V), the primary sees roughly 10V to 14.6V. This requires a transformer with a lower primary turn ratio but allows the use of standard, non-isolated bootstrap gate drivers for the high-side switch.

Because the output voltage is half the DC bus, the primary winding must be designed for lower voltage and higher current compared to a full-bridge. However, the reduction in component count—specifically eliminating two high-current MOSFETs and their associated isolated gate drive power supplies—makes the half-bridge the superior choice for sub-1kW benchmarks.

Sizing the 24V LiFePO4 Source: Series, Parallel, and C-Rates

To support an 800W AC load, we must first calculate the DC current draw, accounting for inverter efficiency and battery chemistry characteristics.

Sizing Math and Peukert's Law

Assume an 800W continuous AC load and a conservative half-bridge high-frequency inverter efficiency of 88%.

• DC Input Power: 800W / 0.88 = 909W
• Continuous Current (at 24V nominal): 909W / 24V = 37.8A
• Peak Current (at 20V BMS cutoff): 909W / 20V = 45.4A

We must also account for Peukert's Law, which describes how battery capacity decreases as the discharge rate increases. While lead-acid batteries suffer heavily from this (Peukert exponent $k \approx 1.3$), LiFePO4 chemistry is highly linear ($k \approx 1.04$). Drawing 45A from a 100Ah LiFePO4 bank yields roughly 94Ah of usable capacity. To ensure we don't hit the low-voltage cutoff during a 2-hour runtime (requiring ~90Ah), a 100Ah 24V bank is the exact minimum specification.

Series vs. Parallel Consequences

Building a 24V 100Ah bank from standard 12V 100Ah LiFePO4 drop-in batteries requires understanding the electrical consequences of your wiring topology:

• Series (2x 12V 100Ah): Voltage doubles to 24V. Capacity remains 100Ah. Total energy = 2560Wh. This is the preferred method.
• Parallel (2x 24V 50Ah): Voltage remains 24V. Capacity adds to 100Ah. Total energy = 2560Wh.

Charge and Discharge Limits

For maximum cycle life (4000+ cycles to 80% Depth of Discharge), adhere to these C-rate limits for your 100Ah bank:

• Continuous Discharge: 0.5C (50A). Our 45.4A peak draw falls safely within this limit.
• Peak Discharge: 1C (100A) for surges lasting less than 30 seconds.
• Charge Rate: 0.5C (50A) standard. Absorption voltage must be strictly set to 28.4V - 29.2V (3.55V - 3.65V per cell) with no float stage or equalization, as noted in standard Li-ion charging protocols.

Inverter and Charger Sizing for the Stated Load

With the battery bank defined at 24V 100Ah, we must size the switching components and the charging hardware to match the 800W load profile.

Half-Bridge Switching Components

Because the DC bus is low voltage (24V nominal) but high current, we need MOSFETs with a very low $R_{DS(on)}$ to minimize conduction losses, and a voltage rating ($V_{DSS}$) that can handle inductive kickback spikes. A 100V rating is standard for 24V systems.

The IRFP4468 is chosen over the common IRFP460 because its on-resistance is roughly one-tenth, drastically reducing heat sink requirements at 45A continuous. At 45A, conduction loss per MOSFET is $I^2 \times R_{DS(on)} = 45^2 \times 0.0026 = 5.2W$, which is easily managed with a standard extruded aluminum heatsink.

Charger Sizing

To recharge a 100Ah bank at the optimal 0.2C to 0.4C rate without stressing the cells, you need a 24V DC charger rated for 20A to 40A. Ensure the charger has a dedicated "LiFePO4" profile that disables the desulfation/equalization pulse and terminates cleanly after the absorption phase. A 24V 30A smart charger (like the Victron Blue Smart IP22) will recharge the bank from 20% to 90% State of Charge in roughly 2.5 hours.

Decision Matrix: Half-Bridge vs. Full-Bridge Topology

When designing a DC-to-AC inverter, the choice between half-bridge and full-bridge dictates your component cost, gate-drive complexity, and transformer sizing. Below is the decision framework for 24V battery systems.

The Final Decision Path

Use this logic tree to finalize your inverter topology and component selection:

• IF your continuous AC load is under 1000W AND your DC bus is 24V or 48V:
  → Choose the Half-Bridge. The reduced component count and simpler IR2110 gate drive save bench time and PCB space. The primary current is manageable with TO-247 package MOSFETs.
• IF your continuous AC load is between 1000W and 2000W AND your DC bus is 24V:
  → Upgrade to 48V if possible. If locked to 24V, you must use a Full-Bridge to keep per-switch current under 60A.
• IF your continuous AC load is over 2000W:
  → Choose the Full-Bridge and design for a 48V DC bus to prevent catastrophic $I^2R$ copper losses in your battery cabling and busbars.

By matching the half-bridge topology to its optimal power band and respecting the C-rate limits of LiFePO4 chemistry, you eliminate the most common failure points in DIY power systems: melted busbars, saturated transformer cores, and bricked gate drivers.