How to use an ideal diode controller in a bypass circuit for solar applications and extend its input voltage range

introduction

In solar photovoltaic (PV) systems, module-level power electronics (MLPE) can improve power generation performance under certain conditions, especially shaded conditions. Once considered a specialty application with a higher cost, MLPE is now one of the fastest growing market segments in the solar industry. A solar power optimizer is a type of MLPE used to optimize the power output of a PV panel and improve efficiency .

Traditional solar power optimizers use PN junction diodes or Schottky diodes as bypass circuits. When a large current flows through the diode, the high power dissipation caused by the relatively high forward voltage drop of the diode can cause severe thermal problems. An improved method is to use metal oxide semiconductor field effect transistors (MOSFETs) with lower voltage drop than diodes to overcome the high power loss problem.

In addition, solar optimizers can now support higher input voltages (up to 150V transient voltage for two PV panels in series) due to improved efficiency and lower system cost from lower conduction losses at a given power level. In this article, we will discuss a scalable input bypass circuit solution using a floating gate ideal diode controller. This circuit solves the challenges associated with wide voltage support for bypass switches in solar power applications such as solar power optimizers, fast shutdown, and PV junction boxes.

What is a Solar Power Optimizer?

Figure 1 shows a PV system where a solar power optimizer is mounted on a single PV panel.

Think of a power optimizer as a compromise between a microinverter and a string inverter. Power optimizers are installed on individual solar panels like microinverters, but their function has nothing to do with converting DC power to AC power. Power optimizers track the maximum power of each solar panel in real time and adjust the output voltage before transmitting it to the inverter. As a result, the inverter can handle more power, optimizing the power generation performance of each solar panel, regardless of the angle at which the panel faces the sun, the shading situation, or even if one or more panels are damaged. Solar systems with power optimizers installed on each PV panel can be 20% to 30% more efficient than solar systems without separate panel-level optimizers.

Output Bypass Function of Solar Power Optimizer

For high-power PV inverters, connecting multiple PV panels in series allows for high DC input voltages into the inverter input. Superior efficiency can be achieved by deploying optimizers to the corresponding PV panels, as shown in Figure 2. The PV panel strings are actually connected to each other through the output of the optimizer. Since all PV boards are connected in series, if any one solar panel fails, the voltage of the PV panel string collapses. The output bypass circuit provides a parallel path for the string current around the damaged optimizer. Figure 2 shows how the bypass function works when one of the PV boards is disconnected.

Output Bypass Circuit Solutions

There are usually two solutions for bypass circuits. A common way to implement the bypass function is to use a PN junction diode or a Schottky diode, as shown in Figure 3. This method is low cost, easy to use, and can achieve very high reverse voltages depending on the selected diode. However, there are some disadvantages, such as high forward voltage drop (0.5V to 1V), which leads to higher power dissipation and requires a larger printed circuit board. To overcome the disadvantages of the bypass diode solution, an N-channel MOSFET can be used, which has a lower voltage drop and lower power loss (due to low R _DS(on) ). However, this method also has the following disadvantages:

• The MOSFET is not a standalone solution, it needs to be used as a switch under the control of a control circuit, usually a microcontroller (MCU) with a discrete MOSFET driver circuit.

• The MCU needs to be powered by the PV board. If the PV board is severely damaged or completely covered by shadows or shielding, the MCU will not work and the MOSFET will not turn on.

• In the event of a malfunction in the MCU, the MOSFET cannot conduct and the bypass path will be through the body diode of the MOSFET. However, the body diode of the MOSFET cannot withstand large currents and will generate high temperatures due to heat accumulation, posing a fire risk.

To overcome the shortcomings of MCU-based on/off control schemes, a smart approach is to use an independent MOSFET controller that can operate autonomously without any external intervention. Texas Instruments' LM74610-Q1 series of floating-gate ideal diode controllers provide an independent low-loss bypass switch solution by controlling an external N-channel MOSFET to emulate the behavior of a series diode. These controllers have a floating-gate drive architecture that can operate with input voltages as low as the forward voltage drop of the MOSFET body diode (approximately 0.5V).

However, as PV inverter power levels increase and higher voltage PV panel applications increase, the bypass circuit needs to meet certain requirements to be superior to traditional solutions. It needs to work with PV panels ranging from 20V to 150V, be scalable across multiple platforms, and it should be independent of other circuits.

Scalable Bypass Switch Solution Using Low Voltage Ideal Diode Controller

The bypass circuit solution uses an ideal diode controller with a floating gate drive architecture (such as the LM74610-Q1) to drive the external MOSFET and emulate an ideal diode as a bypass circuit, making it independent of other circuits. The floating gate drive architecture can achieve a universal input range because the gate drive is not referenced to ground. In addition, the unique advantage of this mechanism is that it is not referenced to ground, so the quiescent current is zero.

When the solar panel and solar device are operating normally, the bypass MOSFET is off and a reverse voltage equal to the maximum panel voltage appears from the cathode to anode pin of the ideal diode controller. However, the reverse voltage from the cathode to anode pin of the ideal diode controller (PV+ to PV-) can be very high and can reach the transient voltage of the PV panel and panel string. When using multiple PV panels with a large input voltage range in series, it can be challenging to design the maximum input voltage range for the bypass circuit. The maximum reverse voltage of the LM74610-Q1 is limited to 45V transient. Therefore, the currently available ideal diode controller devices are not suitable for solar panels with a rated input voltage of 80V or 125V.

This voltage level can be maintained for any range by extending the reverse voltage range of the ideal diode controller by adding a depletion-mode MOSFET QD in the sense path, as shown in Figure 4. _The drain of _QD is connected to the output PV+. The source and gate are connected to the cathode and anode of the ideal diode controller, respectively.

Figure 4: Scalable bypass switch solution

How the LM74610-Q1 reverse voltage range extension works

Depletion-mode MOSFETs are turned on by default when the MOSFET V _GS is 0V, unlike enhancement-mode MOSFETs, which require V _GS to be greater than the MOSFET threshold voltage to turn on. To turn off a depletion-mode MOSFET, V _GS needs to be less than 0V (typically in the range of –1V to –4V). To analyze the role of a depletion-mode MOSFET in the ideal diode detection path, let’s look at the device operation under the following conditions:

• When VPV– is greater than or equal to VPV+: The ideal diode controller is in forward conduction, keeping the power MOSFET Q1 _and the depletion FET QD _on . Under these operating conditions, you can calculate the output voltage V _OUT = V _IN – (I _{D _Q1} R _{DS(on)_Q1} ), which is approximately V _PV+ .

• When VPV– is less than VPV+: The ideal diode controller is in reverse current blocking mode and MOSFET Q1 is off. MOSFET Q _D is in regulation mode as a source follower, maintaining V _CATHODE above V _ANODE with V _CATHODE = V _IN (V _ANODE ) + (V _GSMAX ). Therefore, the voltage between V _CATHODE and V _ANODE is within the absolute maximum rating V _{GSMAX of Q D} (typically less than 5V), which is much less than the 45V maximum transient reverse voltage of the LM74610-Q1. The high reverse voltage (V _OUT – V _IN ) is maintained by the drain-source voltage (V _DS ) of Q _D and Q ₁ .

Choosing the right depletion mode MOSFET and power MOSFET depends on the following points:

• Q1 and QD are selected so that their V _DS ratings are greater than the maximum peak input voltage.

• When selecting RDS(on), ensure that ultra-low power dissipation can be achieved on the power path MOSFET. The drain current (ID ₎ of the FET should be higher than the maximum peak current required by the output load. As a starting point, choose a depletion-mode MOSFET that will drop 50mV to 100mV across the power MOSFET at full load current.

• R _DS(on) can be selected in the hundreds of ohms range (the floating gate drive architecture of the LM74610-Q1 has a large cathode pin to ground impedance and the controller's I _CATHODE is in the microamp range).

Figure 5: Test results of a 60V bypass circuit using the LM74610-Q1 and a depletion-mode MOSFET

Conclusion

If a series connected PV panel or solar device is damaged or fails, proper design must be used to avoid hot spots and/or voltage supply interruption. This responsibility is usually shouldered by a solar power optimizer or fast shutdown device. Although using standard rectifier diodes or Schottky diodes is the simplest solution to bypass the damaged panel, they are not preferred due to thermal inefficiency. Compared to the bypass switch solution, the floating gate ideal diode controller with N-channel MOSFET can achieve less independent losses, and further system solutions by adding depletion mode MOSFET can provide a fully scalable input range solution to cope with the wide input voltage range requirements of PV panels.