Circuits and methods for limiting open circuit voltage of photovoltaic strings

1. A photovoltaic string comprising:

a plurality of serially connected solar cells;

a positive lead and a negative lead on opposite ends of the photovoltaic string, one end of the plurality of serially connected solar cells being connected to the positive lead and an opposite end of the plurality of serially connected solar cells being connected to the negative lead;

an open circuit voltage limiter across a group of solar cells in the plurality of serially connected solar cells, the open circuit voltage limiter having a positive terminal connected to the negative lead and a negative terminal connected to a connection between adjacent solar cells in the plurality of solar cells, the open circuit voltage limiter having a limiter voltage less than an open circuit voltage of the group of solar cells for current flowing through the open circuit voltage limiter in one direction and allows current flow through the open circuit voltage limiter in another direction; and

a bypass diode having a cathode connected to the positive lead and an anode connected to the negative lead.

2. The photovoltaic string of claim 1 wherein the open circuit voltage limiter comprises a Zener diode and the limiter voltage is the Zener voltage of the Zener diode.

3. The photovoltaic string of claim 1 wherein the open circuit voltage limiter comprises:

a first diode having a cathode connected to the positive terminal of the open circuit voltage limiter and an anode connected to the negative terminal of the open circuit voltage limiter; and

a second diode having a cathode connected to the negative terminal of the open circuit voltage limiter and an anode connected to the positive terminal of the open circuit voltage limiter.

4. The photovoltaic string of claim 3 wherein the open circuit voltage limiter further comprises:

a third diode having a cathode connected to the positive terminal of the open circuit voltage limiter and an anode connected to the negative terminal of the open circuit voltage limiter.

5. The photovoltaic string of claim 1 wherein the open circuit voltage limiter comprises an electrical circuit.

6. A photovoltaic string comprising:

a plurality of serially connected solar cells;

a first open circuit voltage limiter across a first group of solar cells in the plurality of serially connected solar cells, the first open circuit voltage limiter having a first limiter voltage less than a maximum voltage of the first group of solar cells for current flowing through the first open circuit voltage limiter in one direction and having a first positive bypass voltage for current flowing through the first open circuit voltage limiter in another direction; and

a bypass diode connected across the plurality of serially connected solar cells.

7. The photovoltaic string of claim 6 wherein the first open circuit voltage limiter comprises a Zener diode and the limiter voltage comprises a Zener voltage of the Zener diode.

8. The photovoltaic string of claim 6 wherein the first open circuit voltage limiter comprises:

a first diode having a cathode connected to a positive terminal of the first open circuit voltage limiter and an anode connected to a negative terminal of the first open circuit voltage limiter; and

a second diode having a cathode connected to the negative terminal of the first open circuit voltage limiter and an anode connected to the positive terminal of the first open circuit voltage limiter.

9. The photovoltaic string of claim 8 wherein the first open circuit voltage limiter further comprises:

a third diode having a cathode connected to the positive terminal of the first open circuit voltage limiter and an anode connected to the negative terminal of the first open circuit voltage limiter.

10. The photovoltaic string of claim 6 wherein the first open circuit voltage limiter comprises an electrical circuit.

11. The photovoltaic string of claim 6 further comprising:

a second open circuit voltage limiter across a second group of solar cells in the plurality of serially connected solar cells, the second open circuit voltage limiter having a second limiter voltage less than a maximum voltage of the second group of solar cells for current flowing through the second open circuit voltage limiter in one direction and having a second positive bypass voltage for current flowing through the second open circuit voltage limiter in another direction.

12. A method of operating a photovoltaic string, the method comprising:

providing a first open circuit voltage limiter across a first group of serially connected solar cells of the photovoltaic string;

providing a bypass diode across the photovoltaic string;

limiting a maximum voltage across the first group of serially connected solar cells to a first limiter voltage across the first open circuit voltage limiter for current flowing in a first direction through the first open circuit voltage limiter; and

allowing current to flow through the first open circuit voltage limiter in a second direction opposite the first direction.

13. The method of claim 12 further comprising:

limiting a maximum voltage across a second group of serially connected solar cells to a second limiter voltage across a second open circuit voltage limiter for current flowing in a third direction through the second open circuit voltage limiter, the second group of serially connected solar cells being connected in series with the first group of serially connected solar cells; and

allowing current to flow through the first open circuit voltage limiter in a fourth direction opposite the third direction.