First of all, understand what is depletion MOSFET. Distinguishing from the common used enhanced MOSFET that normally turns ON with applying some voltage to the gate (i.g, V_GS > V_{GS (th)} for N-channel MOSFET), a depletion mode MOSFET normally turns ON without applying any gate voltage, which means the MOSFET can turn on when V_GS > 0, V_GS = 0, or V_GS < 0. Because the depletion MOSFET is based on the enhanced MOSFET with positive ions being injected into the insulation layer of the FET. So there naturally exists a conductive channel.

For depleted N-channel MOSFET, the original positive ion field can only be offset once V_GS (negative value) is negatively increased further to reach the pinch-off mode at V_{GS (OFF)}, or is called V_{GS (th)}. Then the conductive channel can be closed.

Figure 3-3 shows the modes of operation with adding the depletion MOSFET Q_d to the existing ideal diode controller circuit.

When V_CATHODE <= V_ANODE, the normal operation that V_IN >= V_OUT. At this time, the bypass circuit of the solar power optimizer works. MOSFETs Q₁ and Q_d are turned ON, V_CATHODE ≌ V_ANODE.

V_CATHODE > V_ANODE during reverse polarity or reverse current protection operation that V_IN < V_OUT. At this time, the bypass circuit of the solar power optimizer does not operate. MOSFET Q₁ is turned off, MOSFET Q_d is in regulation mode as a source follower, maintaining V_CATHODE above V_ANODE, V_CATHODE = V_IN(VANODE)+ Abs (V_{GS (th)}). So, voltage across V_CATHODE to V_ANODE is within absolute maximum rating V_{GS (th)} of Q_d (< 5V. typ) which is far less than the maximum reverse voltage 45V_MAX (transient) of LM74610-Q1. Now, the high reverse voltage (V_OUT – V_IN) is sustained by the drain-source voltage V_DS of Q_d.

How to choose the right depletion MOSFET mainly depends on two factors.

• Choose a V_DS rating of Q_d ≥ V_IN(max) (maximum peak input voltage).
• R_DS(on) can be in the hundreds of ohms range (LM746x0-Q1 is floating gate drive architecture with large impedance of CATHODE pin to ground, I_CATHODE of the controller is in µA range).

Figure 3-3 Modes of Operation With Adding Depletion MOSFET (V_CATHODE ≤ V_ANODE)

Figure 3-4 Modes of Operation With Adding Depletion MOSFET (V_CATHODE > V_ANODE)

Figure 3-5 and Figure 3-6 shows the simulation circuit and results of the bypass circuit with LM74610-Q1 and depletion MOSFET. From the simulation results, this can be seen that using depletion MOSFET is an effective method to extent the reverse voltage range of the current low voltage ideal diode controller, which solves the challenge of very large input voltage range of PV panel or string.

With this design, there also have system benefits to the solar power optimizer system as the following.

• Solve hot-spot issue due to very low power loss.
• Make solar equipment be safer, reduce the fire risk.
• Improve solar power generation efficiency.

Figure 3-5 Simulation Circuit of the Bypass Circuit With LM74610-Q1 and Depletion MOSFET

Figure 3-6 Simulation Results of the Bypass Circuit With LM74610-Q1 and Depletion MOSFET