AN3971 STMICROELECTRONICS | Alldatasheet

Document overview

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Technical content

Datasheet sections

  • 1 Application overview
  • 2 Optimizing the energy from the panel
  • 2.1 Regulations, protection, and features
  • 3 Charging a lead acid battery
  • 3.1 Constant current – constant voltage control
  • 3.2 SEA05 features
  • 3.3 Interaction between the SPV1020 and SEA05
  • 4 External component selection
  • 4.1 Output current regulation
  • 4.2 Battery voltage control
  • 5 STEVAL-ISV005V2 schematic
  • 6 Bill of materials
  • 7 Layout guidelines
  • 8 Application connection example
  • 9 Experimental results
  • 10 Conclusion
  • 11 References
  • 12 Revision history

efficient solar energy harvesting system are required. PV installations) must optimize both the charging time and the lifetime of the battery. conversion unit which performs an MPPT (max. power point tracking) algorithm. converter, with interleaved topology (IL4) and implementing MPPT. constant current-constant voltage) IC. Figure 1. STEVAL-ISV005V2 demonstration board

1 Application overview

emergency telephones, isolated navigational buoys, traffic signs, boats, camper vans, etc. They are most suitable for users with a limited power need. the load in case of low renewable energy production. Figure 2. Typical stand-alone PV systems

The primary function of a charge controller in a standalone PV system is to maintain the battery at the highest possible state of charge, and to protect it from overcharge by the array and from over-discharge by the loads. Although some PV systems can be effectively designed without the use of charge control, any system that has unpredictable loads, user intervention, optimized or undersized battery storage (to minimize initial cost), typically requires a battery charge controller. The algorithm or control strategy of a battery charge controller determines the effectiveness of battery charging and PV array utilization, and ultimately the ability of the system to meet the load demands. Important functions of battery charge controllers and system controls are:

  • To prevent battery overcharge: to limit the energy supplied to the battery by the PV array when the battery becomes fully charged
  • To prevent battery over-discharge: to disconnect the battery from electrical loads when the battery reaches a low state of charge
  • To provide load control functions: to automatically connect and disconnect an electrical load at a specified time, for example, operating a lighting load from sunset to sunrise. The most common battery type used is the valve regulated lead acid (VRLA) battery, because of its low cost, maintenance-free operation and high efficiency characteristics. Although the battery installation cost is relatively low compared to that of PV systems, the lifetime cost of the battery is greatly increased because of the limited service time. The lifetime parameter is reduced if there is low PV energy availability for prolonged periods or improper charging control, both resulting in low battery state of charge (SOC) levels for long time periods. An increase in the lifetime of the battery results in improved reliability of the system and a significant reduction in operating costs. The life of a lead acid battery can be extended by avoiding critical operating conditions such as overvoltage and overcurrent during the charge.

2 Optimizing the energy from the panel

of the load to the dynamic output impedance of the panel. Figure 3. SPV1020 equivalent circuit the load which must match Zm to guarantee maximum power is extracted from the source. in = Vin * Iin) is maximum (Pmpp = Vmpp* Impp). current of a photovoltaic panel. Figure 4. I/V panel electrical curve

Figure 6. Input voltage partitioning sample circuit time necessary to reach the total battery SOC.

2.1 Regulations, protection, and features

  • Overtemperature protection
  • Output overvoltage regulation
  • Output overvoltage protection: input overcurrent protection
  • Current balance
  • Input MPPT settings. For details regarding the above list of protection and functionalities, please refer to the SPV1020 datasheet and its basic application STEVAL-ISV009V1. In addition to the above list, the STEVAL-ISV005V2 also implements an output overcurrent protection through the SEA05 IC (details in Section 3.2 of this document). Figure 7 offers a brief description of the architecture implemented. 6IN 0ANEL 4O)NDUCTORS 4O3063UPPLY 6IN 4O3066OLTAGE3ENSE 6IN?SNS # 6IN?SNS

3 Charging a lead acid battery

A proper charging profile is important to guarantee a long battery lifetime. Figure 9. Typical SLA battery charging curve

  1. Starting from a battery discharge, the maximum current must be lower than a value of
  2. In any charging step, the voltage applied must not be greater than the gassing voltage
  3. During the recharge and up to 100% of the previous discharge capacity, the current

charge time, this voltage can be just below the gassing voltage.

  • Constant current to perform a bulk charge, when the battery is charged using a current regulation to I_batt_max, up to 70% SOC, in about 4 hours, and the battery voltage slowly increases up to the nominal value (equal to 2 V per cell)
  • Constant voltage to perform a floating charge, when the battery is charged using a voltage regulation to V_batt_max, up to the remaining 30% SOC, with a slow current decrease down to C/10 or C/100 values. This stage lasts 6 hours and is essential for the battery lifetime
  • Constant current to perform a trickle charge, which compensates the self-discharge of the battery, even after it has been fully charged. Normally the charging current is less than C/100, and even if the battery is not completely saturated, the SLA can eventually lose its ability to accept a full charge and its performance is reduced.

3.1 Constant current – constant voltage control

bulk charge, and the max. voltage during the floating charge, the SEA05 IC has been used. The lead acid battery chemistry and physics behavior affect the charging strategy itself.

  • During the floating charge, the sink current decreases slowly because of the battery features keeping a constant voltage charge at the maximum value
  • During the trickle charge (which is normally necessary just after the natural battery discharging), in order to keep the battery at a maximum SOC. In this case, no control acts, just to allow the energy flow directly from the panel towards the battery, without any constraints or voltage/current limitation.

3.2 SEA05 features

two operational amplifiers, and a low-side current sensing circuit. Figure 10. SEA05 internal architecture

current sensing circuit and the other op amp make up the current control loop.

  • A resistor divider that senses the output of the power supply and fixes the voltage regulation set-point at the specified value
  • A sense resistor that feeds the current sensing circuit with a voltage proportional to the DC output current, setting the current regulation set-point (it must be adequately rated in terms of power dissipation)
  • The frequency compensation components (R-C networks) for both loops. Please refer to the SEA05 datasheet for further details.

3.3 Interaction between the SPV1020 and SEA05

even one of the protection or regulation thresholds is triggered. capacitor (pole-zero compensation) between the PZ_OUT pin and SGND pin.

  • To perform compensation loop to control the Vout_sns behavior
  • To force an imposed duty cycle proportional to its voltage on the pin.

Figure 11. Internal duty cycle reference algorithm, and the second one imposed by the PZ_OUT external voltage. battery voltage and current control. the internal duty cycle imposed is proportional too.

4 External component selection

The STEVAL-ISV005V2 is based on two ST key devices: the SPV1020 and SEA05. components of the SPV1020. Please refer to the SPV1020 datasheet.

  • Output resistor partitioning (R7/R8) according to the SEA05 internal voltage control threshold, to control the maximum overvoltage battery protection
  • Sensing resistor (Rsns: R9 and/or R10) in the PV-loop, to control the maximum overcurrent battery protection, according to the internal current threshold
  • The PV panel must be selected in order to guarantee the SPV1020 functionality; and so, in order to respect the SPV1020 step-up conditions, the Voc of the PV panel must be lower than Vbat_min (voltage when the battery is deeply discharged). The STEVAL-ISV005V2 application example has been developed for the following features: SLA battery: V batt_nom = 24 V & C = 4 Ah PV panel: Vmp = 18 V & Voc = 20 V, Imp = 1.6 A & Isc = 2 A So the SEA05 IC must limit at the following voltage and current: V batt_max = (24 * 1.2) V = 28.8 V Ibatt_max = (4/4) A = 1 A

Figure 12. System architecture SPV1020 + SEA05

External component selection AN3971 14/28 Doc ID 022119 Rev 2

4.1 Output current regulation

Output current regulation is implemented by the SEA05 current control loop. The voltage threshold related to the current control is equal to 50 mV. So to perform a current regulation, R sense must be selected by the following equation: For example, with Iomax = 1 A, Vcsth = 50 mV, then Rsense = 50 mΩ. Note that the Rsense resistor should be chosen taking into account the maximum power dissipation (Plim) through it during full load operation.

4.2 Battery voltage control

The voltage loop is controlled via a voltage divider R7, R8 directly inserted on the SPV1020 output voltage. It is possible to choose their values using the following equations: and where V O is the desired output voltage = V_batt_max. In the case of V_batt_max = 28.8 V, with Vctrl internally fixed by the SEA05 to 2.5 V, the values must be: R7 = 2.7 MΩ ; R8 = 255 kΩ. Rsense Iomax⋅ Vcsth= Rsense Vcsth Iomax Plim Vcsth Iomax⋅= VO Vcsth R1 R2+() R1 R2 VO Vctrl+() Vctrl

Figure 13. SEA05 schematic

5 STEVAL-ISV005V2 schematic

Figure 14. STEVAL-ISV005V2 schematic

5 N5 N

5 N5 N'

5 GQP5 GQP

6 Bill of materials

Table 1. Bill of materials

24 C1, C2,

Table 1. Bill of materials (continued)

1469782 Current sensing resistor

  1. R6 must be removed if R7 is soldered.
  2. Better performances can be obtained us ing part number B82477G4473M003 (DCR = 52m Ω)
  3. Default value to sense 50 mV @10 A max.

7 Layout guidelines

frequency resonance problems, and electromagnetic interference. damage and a bad tracking of the MPPT. circulates, to reduce peak voltages, radiation and resonance problems. layers may be required, and in this case thermal vias must connect the ground plane layers. application it is suggested to refer to the TN0054 technical note. Figure 15. STEVAL-ISV005V2 (top view)

Figure 16. STEVAL-ISV005V2 (bottom view)

8 Application connection example

Figure 17. STEVAL-ISV005V2 board connection

9 Experimental results

  • Two SLA batteries (12 V, 4 Ah), connected in series

Figure 18. SLA battery (12 V, 4 Ah)

  • A solar array simulator (SAS), that allows total emulation of a PV panel electrical behavior

Figure 19. Solar array simulator (SAS)

  • Four multimeters, to check the voltage and current values in the input and the output of the STEVAL-ISV005V2, to evaluate the MPP tracking efficiency, such as the power efficiency, and obviously to trace the battery charging curve.

Figure 22. SLA battery charging profile (24 V, 24 Ah)

10 Conclusion

In a standalone PV system, an SPV1020 with MPPT and step-up IL4 embedded architecture, used in an application field together with the constant voltage - constant current SEA05, allows the proper charging of an SLA battery, without any damage or lifetime reduction. The architecture, compared with a similar one implemented with a microcontroller, external discrete components and IL4 step-up, performs better in terms of power and MPPT efficiency. Furthermore, the distributed approach, directly applied on the panel, allows the management of any power reduction due to shadow, clouds, etc.; all features which are unsuitable to centralized architecture. The proposed solution is cost effective when compared with other systems, because of its MPPT algorithm, and IL4 architecture, which are fully integrated inside the SPV1020.

11 References

  • SPV1020 datasheet
  • SEA05 datasheet
  • TN0054 technical note
  • E. Koutroulis, K. Kalaitzakis, “A Novel Battery Charging Regulation System for

Table 2. Document revision history 15-Feb-2012 1 Initial release. Updated BOM list Table 2 item 29.