DC bias prevention in transformerless inverters

1. A method of operating a transformerless DC to AC converter comprising circuitry including a plurality of transistor switches and a controller operative to control the switches to commutate DC inputs onto AC outputs so as to generate an approximation of an AC waveform at a desired power line frequency, comprising:

monitoring a DC bias component on the AC output of the converter;

comparing the DC bias component to a predetermined threshold; and

only when the DC bias component is below the predetermined threshold, modifying, by the controller, the AC waveform generated, so as to reduce the DC bias component.

2. The method of claim 1 , further comprising:

if the DC bias component meets or exceeds the predetermined threshold, ceasing to generate the AC waveform.

3. The method of claim 2 , wherein ceasing to generate the AC waveform comprises:

ceasing to drive one or more switches used to commutate DC inputs onto AC outputs, thereby ceasing to generate the approximation of an AC waveform; and

opening one or more AC output relays to disconnect the DC to AC converter from any AC load.

4. The method of claim 1 , wherein modifying, by the controller, the AC waveform generated, so as to reduce the DC bias component, comprises:

determining whether the DC bias component is positive or negative;

if the DC bias component is positive, performing one or both of

reducing a volt-time integral of a positive half cycle of the AC waveform, and

increasing the volt-time integral of a negative half cycle of the AC waveform, so as to reduce a magnitude of the DC bias component; and

if the DC bias component is negative, performing one or both of increasing the volt-time integral of the positive half cycle of the AC waveform, and

reducing the volt-time integral of the negative half cycle of the AC waveform, so as to reduce the magnitude of the DC bias component.

5. The method of claim 1 , wherein modifying, by the controller, the AC waveform generated, so as to reduce the DC bias component, comprises:

determining whether the DC bias component is positive or negative;

if the DC bias component is positive, performing one or both of

delaying a timing of transitions from a lower voltage point on the AC waveform to a higher voltage point on the AC waveform, and

advancing a timing of transitions from a higher voltage point on the AC waveform to a lower voltage point on the AC waveform, so as to reduce a magnitude of the DC bias component; and

if the DC bias component is negative, performing one or both of

advancing the timing of transitions from a lower voltage point on the AC waveform to a higher voltage point on the AC waveform, and

delaying the timing of transitions from a higher voltage point on the AC waveform to a lower voltage point on the AC waveform, so as to reduce the magnitude of the DC bias component.

6. A transformerless DC to AC converter, comprising:

circuitry connected between DC inputs and AC output, the circuitry including a plurality of transistor switches;

a controller operative to control the switches to commutate the DC inputs onto the AC outputs so as to generate, at the AC outputs, an approximation of an AC waveform at a desired power line frequency;

a measuring circuit connected to the controller and operative to continuously measure a value of a DC bias component on the AC output of the converter; and

wherein the controller is further operative to compare a magnitude of the DC bias component to a predetermined threshold and only when the DC bias component is below the predetermined threshold, to modify the AC waveform generated, so as to reduce the DC bias component.

7. The transformerless DC to AC converter of claim 6 , wherein the controller is further operative to cease to generate the AC waveform if the magnitude of the DC bias component meets or exceeds the predetermined threshold.

8. The transformerless DC to AC converter of claim 7 , further comprising:

one or more AC output isolation relays interposed between the transistor switches and the AC outputs; and

wherein the controller is operative to cease to generate the AC waveform by:

ceasing to drive one or more switches to commutate the DC inputs onto the AC outputs, thereby ceasing to generate the approximation of an AC waveform; and

opening one or more of the AC output isolation relays to disconnect the DC to AC converter from any AC load.

9. The transformerless DC to AC converter of claim 8 , wherein, after opening one or more AC output isolation relays, the controller is operative to:

automatically restart the DC to AC converter; and

modify the AC waveform generated, so as to reduce the DC bias component, with the load disconnected.

10. The transformerless DC to AC converter of claim 6 , wherein the controller is operative to modify the AC waveform generated, so as to reduce the DC bias component, by: determining whether the DC bias component is positive or negative;

if the DC bias component is positive, performing one or both of

reducing a volt-time integral of a positive half cycle of the AC waveform, and

increasing the volt-time integral of a negative half cycle of the AC waveform, so as to reduce the magnitude of the DC bias component; and

if the DC bias component is negative, performing one or both of

increasing the volt-time integral of the positive half cycle of the AC waveform, and

reducing the volt-time integral of the negative half cycle of the AC waveform, so as to reduce the magnitude of the DC bias component.

11. The transformerless DC to AC converter of claim 10 , wherein the controller is operative to increase or decrease the volt-time integral of a half cycle of the AC waveform by modifying a timing of switching the transistor switches.

12. The transformerless DC to AC converter of claim 6 , wherein the controller is operative to modify the AC waveform generated, so as to reduce the DC bias component, by:

determining whether the DC bias component is positive or negative;

if the DC bias component is positive, performing one or both of

delaying a timing of transitions from a lower voltage point on the AC waveform to a higher voltage point on the AC waveform, and

advancing a timing of transitions from a higher voltage point on the AC waveform to a lower voltage point on the AC waveform, so as to reduce the magnitude of the DC bias component; and

if the DC bias component is negative, performing one or both of

advancing the timing of transitions from a lower voltage point on the AC waveform to a higher voltage point on the AC waveform, and

delaying the timing of transitions from a higher voltage point on the AC waveform to a lower voltage point on the AC waveform, so as to reduce the magnitude of the DC bias component.

13. The transformerless DC to AC converter of claim 6 , wherein said plurality of transistor switches are connected in at least one H-bridge configuration comprising two upper transistors having drain terminals connected together and two lower transistors having source terminals connected together, and further comprising:

first and second H-bridge driver circuits, each generating drive signals for one upper and one lower transistor; an H-bridge driver power supply circuit supplying a DC voltage to the H-bridge driver circuits for the lower transistors;

first and second bootstrap circuits, each applying a DC voltage offset to source terminals of respective upper transistors when the corresponding lower transistors are off, such that a drive signal applied to a drain terminal of an upper transistor by the respective H-bridge driver circuit provides a sufficient gate-source voltage to turn the upper transistors on;

wherein, when a lower transistor fails to switch, due to a fault condition, for greater than a predetermined duration, the associated bootstrap DC voltage offset decays to a low-voltage lockout level that causes the associated H-bridge driver circuit to prevent the upper transistor from being turned on.