Inversion circuit of a transformerless PV inverter: H4, H5 and HERIC bridges and the leakage current each lets through
Transformerless PV inverters · The DC-to-AC bridge stage: H4, H5 and HERIC switching, common-mode voltage and leakage current
Inside a grid-tied PV inverter, after any boost stage and the DC-link capacitors, the inversion circuit is the bridge of power switches, freewheeling diodes and output inductors that chops the DC into a sine-weighted current for the grid. In transformerless designs the panel array stays galvanically connected to the grid, so the bridge's switching pattern decides how much the array's potential to earth jumps at switching frequency, and therefore how much current leaks through the array's capacitance to earth. Topology is chosen to keep that common-mode voltage steady while still giving a three-level output for low distortion.
The DC-to-AC bridge stage of single-phase transformerless PV inverters: full-bridge (H4) with bipolar or unipolar modulation, the H5 bridge with a DC-side decoupling switch, and the HERIC bridge with an AC-side bypass, with their switching patterns, freewheeling paths and leakage-current behaviour. Complete PV inverters are covered on the Solar inverter page and general inverters on the Power inverter page; liquid-cooled power-stage assemblies for drives on the Power electronics board page and welding inverter stages on the Welding inverter module page.
CNFX editorial research · Sources checked October 17, 2026 · 3 sources
Common-mode voltage swing is what drives leakage through the array
Shrestha et al. explain that a transformerless inverter forms a loop from grid to panel through the parasitic capacitance between the PV array and ground; high-frequency switching makes the array-to-ground voltage fluctuate, and the coupling of that fluctuating common-mode voltage (CMV) with the capacitance produces leakage current. They cite VDE 0126-1-1 as limiting leakage current to 300 mA. [3]
The simulation used a 400 V input, 3,000 µF input capacitor, 4 mH filter inductors and a 24 nF parasitic array capacitance, analysing 40 cycles of 50 Hz grid current up to the third harmonic. Its authors conclude that H5 gives the best balance of low leakage and low THD among the three, and attribute HERIC's higher THD to its extra high-frequency switch. These are simulation results from one study, not limits for a product. [3]
Editorial calculation: the unipolar H4 leakage of 0.2697 A is about 180 times the H5 figure of 1.495 mA (0.2697 / 0.001495 ≈ 180), both from the same simulated circuit. [3]
Ask the inverter or power-stage supplier which bridge topology and modulation is used, the measured leakage current at the array capacitance it was tested with, and the THD at rated power.
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H5: one more switch on the DC side, opened during freewheeling
SMA's patent US7411802 adds a fifth switch, V5, to an H-bridge of four switches V1–V4 with freewheeling diodes D1–D5, between the DC circuit formed by the solar generator and its capacitor and the bridge. During freewheeling phases the AC circuit is decoupled from the DC circuit by this switch, avoiding high-frequency voltages at the generator terminals; the additional switch is clocked simultaneously with only one bridge switch. [1]
The patent argues that, because forward losses matter less than switching losses, efficiency with the extra switch is almost the same as with an H-bridge that adds connection paths between the bridge outputs, while using fewer components; it gives 50 Hz as an example output frequency. [1] In the simulation study, H5's two line-frequency switches (Q1, Q3) and its freewheeling through Q3, D1 and a clamping diode give the constant CMV of VPV/2 listed in Table 3 of that paper. [3]
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AC-side decoupling: the HERIC bridge and Fraunhofer's bypass circuit
Shrestha et al. describe HERIC as a six-switch bridge in which Q5 and Q6, connected anti-series between the AC terminals, decouple the AC and DC sides during freewheeling ("AC decoupling", against H5's DC decoupling), with clamping diodes D7 and D8 holding the CMV at VPV/2; Q1 to Q4 switch at high frequency and Q5 and Q6 at line frequency. Elsewhere the same paper attributes HERIC's higher THD to an additional switch "switching at high frequency", which does not match that description. [3]
Fraunhofer's patent US7046534 describes a bypass bridge of this AC-side kind: a full bridge of switch units A–D with recovery diodes, a buffer capacitor and inductors L1 and L2 to the AC terminals, plus two separate connecting paths each with a switch (E or F) and a series diode. It states the switch units are high-frequency switches "with frequencies of up to several 100 kHz", preferably MOSFETs or IGBTs. [2]
The patent explains the gain: in the zero-voltage period with unsymmetrical timing, the inductor current does not commutate via the recovery diodes to the buffer capacitor, which it says decisively improves efficiency over symmetrical timing, and only the grid voltage, not grid plus generator voltage, is applied to the inductors, reducing current ripple. It contrasts this with the plain transformerless full bridge, which it credits with high conversion efficiency and low volume but poor input-side EMC behaviour. [2]
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Bridge-stage questions for a transformerless PV inverter
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Topology: [H4 bipolar/unipolar; H5; HERIC; NPC]
DC link: [voltage range; capacitance]
Switches: [Si IGBT/MOSFET or SiC; switching frequency]
Leakage: [measured RMS at stated array capacitance]
Output quality: [THD at rated power]
Compliance: [residual-current and grid-code standards]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 17 October 2026
Buyer questions
Why does a transformerless PV inverter have leakage current?
The array has capacitance to earth, and a bridge that makes the common-mode voltage jump at switching frequency drives current through it. [3]
What does the fifth switch in an H5 inverter do?
It disconnects the bridge from the DC circuit during freewheeling so the panels see no high-frequency voltage. [1]
How does HERIC differ from H5?
H5 decouples the panels on the DC side with one extra switch; HERIC uses two anti-series switches between the AC terminals, so it decouples on the AC side. [3]
Sources and what still needs a direct check
Sources checked 2026-10-17. Circuit descriptions are from SMA's H5 patent and Fraunhofer-Gesellschaft's patent for the bypass bridge; the leakage and THD comparison is a 2025 simulation study from Tribhuvan University posted on arXiv.
