STEVAL-ISV006V1

AN3319 Application note STEVAL-ISV006V1: solar battery charger using the SPV1040 Introduction The SPV1040 is a high efficiency, low power and low voltage DC-DC converter that provides a single output voltage up to 5.5 V. Startup is guaranteed at 0.3 V and the device operates down to 0.45 V when coming out from MPPT mode. It is a 100 kHz fixed frequency PWM step-up (or boost) converter able to maximize the energy generated even by one single solar cell (such as a polycrystalline or amorphous PV cells). The duty cycle is controlled by an embedded unit running an MPPT with the goal of maximizing the power generated from the panel by continuously tracking its output voltage and current. The SPV1040 guarantees the safety of the application device or of the converter itself by stopping the PWM switching in the case of an overcurrent or overtemperature condition. The IC integrates an 80 mΩ N-channel MOSFET power switch and a 120 mΩ P-channel MOSFET synchronous rectifier. February 2011 Doc ID 18265 Rev 1 1/24 www.st.com Contents AN3319 Contents 1 Application overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 Boost switching application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 SPV1040 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4 Application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5 Schematic and bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6 External component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 7 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Appendix A SPV1040 parallel and series connection . . . . . . . . . . . . . . . . . . . . . 19 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2/24 Doc ID 18265 Rev 1 AN3319 List of figures List of figures Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Boost application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 PV cell curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Inductor current in continuous mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Inductor current in discontinuous mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Typical application schematic using the SPV1040 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 SPV1040 equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 MPPT working principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 SPV1040 internal block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 STEVAL-ISV006V1 top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 STEVAL-ISV006V1 bottom view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 STEVAL-ISV006V1 schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 STEVAL-ISV006V1 Iout filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 STEVAL-ISV006V1 PCB top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 STEVAL-ISV006V1 PCB bottom view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 SPV1040 output parallel connection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 SPV1040 output series connection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Doc ID 18265 Rev 1 3/24 Application overview 1 AN3319 Application overview Figure 1 shows the typical architecture of a boost converter based solar battery charger: Figure 1. Boost application schematic 39SDQHO 9RXW 6ZLWFK FRQWUROOHU !-V The SPV1040 adapts the characteristics of load to those of panel. In fact, a PV panel is made up of a series of PV cells. Each PV cell provides voltage and current which depend on the PV cell size, on its technology, and on the light irradiation power. The main electrical parameters of a PV panel (typically provided at light irradiation of 1000 W/m2, Tamb=25 °C) are: ● Voc (open circuit voltage) ● Vmp (voltage at maximum power point) ● Isc (short-circuit current) ● Imp (current at maximum power point) Figure 2 shows the typical characteristics of a PV cell: Figure 2. PV cell curve 3RZ ZHU>:@ &XUUUHQW>$@ ,6& ,03  9ROWDJH J >>9@@ 903 92& !-V MPP (maximum power point) is the working point of the PV cell at which the product of the extracted voltage and current provides the maximum power. 4/24 Doc ID 18265 Rev 1 AN3319 2 Boost switching application Boost switching application A step-up (or boost) converter is a switching DC-DC converter able to generate an output voltage higher than (or at least equal to) the input voltage. Referring to Figure 1, the switching element (Sw) is typically driven by a fixed frequency square waveform generated by a PWM controller. When Sw is closed (Ton) the inductor stores energy and its current increases with a slope depending on the voltage across the inductor and its inductance value. During this time the output voltage is sustained by Cout and the diode does not allow any charge transfer from the output to input stage. When Sw is open (Toff), the current in the inductor is forced, flowing toward the output until voltage at the input is higher than the output voltage. During this phase the current in the inductor decreases while the output voltage increases. Figure 3 shows the behavior of inductor current. Figure 3. Inductor current in continuous mode ,/ ,SN 9RXW  9LQ / 9LQ / ,PLQ ( (RQRQ ( RII (RII 7RQ 7RII WLPH !-V The energy stored in the inductor during Ton is ideally equal to the energy released during Toff, therefore the relation between Ton and Toff can be written as follows: Ton D = -------------------------------( Ton + Toff ) where “D” is the duty cycle of the square waveform driving the switching element. Boost applications can work in two different modes depending on the minimum inductor current within the switching period, that is if it is not null or null respectively: ● Continuous mode (CM) ● Discontinuous mode (DCM) Doc ID 18265 Rev 1 5/24 Boost switching application Figure 4. AN3319 Inductor current in discontinuous mode ,/ ,SN 9LQ / (RQ 7RQ 9RXW  9LQ / (RII 7RII 7LGOH !-V Obviously the efficiency is normally higher in CM. Inductance and switching frequency (Fsw) impact the working mode. In fact, in order to have the system working in CM, the rule below should be followed:   L> Vout 2 (D * (1 − D))2 ∗ Pin 2 * Fsw According to the above, L is minimum for D = 50 %. 6/24 Doc ID 18265 Rev 1 AN3319 SPV1040 description 3 SPV1040 description The following is a quick overview of SPV1040 functions, features, and operating modes. Figure 5. Typical application schematic using the SPV1040 COUT L Lx XSHUT VPV R3 GND MPP SET MPP-SET CIN R4 CINsns RS VOUT RF1 ICTRL_PLUS ICTRL_MINUS VBATT CF RF2 R1 VCTRL DOUT R2 COUTsns AM06700v1 The SPV1040 acts as an impedance adapter between the input source and output load which is: Figure 6. SPV1040 equivalent circuit 639 ,LQ 9LQ 39 3DQHO JP9LQ &LQ ,RXW &RXW 5RXW 9RXW ='& !-V Through the MPPT algorithm, it sets up the DC working point properly by guaranteeing Zin = Zm (assuming Zm is the impedance of the supply source). In this way, the power extracted from the supply source (Pin = Vin * Iin) is maximum (PM = VM * IM). The voltage-current curve shows all the available working points of the PV panel at a given solar irradiation. The voltage-power curve is derived from the voltage-current curve by plotting the product V*I for each voltage generated. Doc ID 18265 Rev 1 7/24 SPV1040 description AN3319 Figure 7. MPPT working principle 30$; RZHU >:@ 3R &XUUHQWW>$@ ,03  9ROWDJH>9@ 903 92& !-V Figure 7 shows the logical sequence followed by the device which proceeds for successive approximations in the search for the MPP. This method is called “Perturb and Observe”. The diagram shows that a comparison is made between the digital value of the power Pn generated by the solar cells and sampled at instant n, and the value acquired at the previous sampling period Pn-1. This allows the MPPT algorithm to determine the sign of duty cycle and to increment or decrement it by a predefined amount. In particular, the direction of adjustment (increment or decrement of duty cycle) remains unchanged until condition Pn≥Pn-1 occurs, that is, for as long as it registers an increase of the instantaneous power extracted from the cells string. On the contrary, when it registers a decrease of the power Pn 1 Vmp * 9μs 1 Vmp * 9μs * = * 2 2 − IL1 (rms) 2 2 − Imp A safer choice is to replace Vmp with Voc. Usually, inductances ranging between 10 µH to 100 µH satisfy most application requirements. Other critical parameters for the inductor choice are Irms, saturation current, and size. 14/24 Doc ID 18265 Rev 1 AN3319 External component selection Irms is the self rising temperature of the inductor, affecting the nominal inductance value. In particular, the inductance decreases with Irms and the temperature increases. As a consequence the inductor current peak can reach or surpass 1.7 A. Inductor size also affects the maximum current deliverable to the load. In any case, the saturation current of the choke should be higher than the peak current limit of the input source. Hence, the suggested saturation current must be > 1.7 A. At the same size, small inductance values guarantee both faster response to load transients and higher efficiency. Inductors with low series resistance are suggested in order to guarantee high efficiency. Output voltage capacitor A minimum output capacitance must be added at the output in order to reduce the voltage ripple. Critical parameters for capacitors are: capacitance, maximum voltage, and ESR. According to the maximum current (Isc) provided by the PV panel connected at the input, the following formula can be used to select the proper capacitance value (Cout1) for a specified maximum output voltage ripple (Vout_rp_max):   Cout 1 ≥ Iout Fsw * Vout _ rp _ max Maximum voltage of this capacitor is strictly dependent on the output voltage range. SPV1040 can support up to 5.5 V, so the suggested maximum voltage for these capacitors is 10 V. Low-ESR capacitors are a good choice to increase the whole system efficiency. Output voltage partitioning R1 and R2 are the two resistors used for partitioning the output voltage. The said VOUT_MAX the maximum output voltage of the battery, R1 and R2 must be selected according to the following rule:  R1 R2 = Vout _ max −1 1.25 Also, in order to optimize the efficiency of the whole system, when selecting R1 and R2, their power dissipation must be taken into account. Assuming a negligible current flowing into the Vctrl pin, maximum power dissipation on the series R1+R2 is:   Pvout _ sns = (Vout _ max)2 R1 + R2 Doc ID 18265 Rev 1 15/24 External component selection AN3319 As an empirical rule, R1 and R2 should be selected to get:  P vout _ sns 0.5 A is connected when very low load is connected. In fact, SPV1040 is supplied by the Vout pin, so in the above condition the device is still OFF when the PV cell is connected and a voltage spike can occur damaging the converter and the battery. In order to guarantee the best system performance and reliability, DOUT should be selected as follows: VBR > 5.2 V and VCL < 7 V Dout must be able to dissipate the following maximum power: Pmax = Isc*VCL XSHUT resistor The XSHUT pin controls SPV1040 turn-on (0.3 V ≤ XSHUT ≤ 5.5 V) or turn-off (XSHUT < 0.3 V). R5 is a 0 Ω pull-up resistor shorting the XSHUT and MPP-SET pins. Removing R5 enables the external control of the XSHUT pin to turn the SPV1040 on/off. Doc ID 18265 Rev 1 17/24 Layout 7 AN3319 Layout Figure 13. STEVAL-ISV006V1 PCB top view Figure 14. STEVAL-ISV006V1 PCB bottom view Layout guidelines PCB layout is very important in order to minimize voltage and current ripple, high frequency resonance problems, and electromagnetic interference. It is essential to keep the paths where the high switching current circulates as small as possible in order to reduce radiation and resonance problems. Large traces for high current paths and an extended ground plane reduce noise and increase efficiency. The output and input capacitors should be placed as close as possible to the device. The external resistor dividers, if used, should be as close as possible to the VMPP-SET and Vctrl pins of the device, and as far as possible from the high current circulating paths, in order to avoid picking up noise. 18/24 Doc ID 18265 Rev 1 AN3319 SPV1040 parallel and series connection Appendix A SPV1040 parallel and series connection Output pins of many SPV1040s can be connected either in parallel or in series. In both cases the output power (Pout) depends on light irradiation of each panel, on application efficiency, and on the specific constraints of the selected topology. The objective of this section is to explain how the output power is impacted by the selected topology. An example with 3 PV panels (Panel1, Panel2, Panel3) is presented, but the conclusion can be extended to a larger number of PV panels. If the panel is lighted and the SPV1040 is on (it means that light irradiation intensity is such that VMPP-SET ≥ 0.3 V):  Poutx = η Pinx  [x = 1..3] If the panel is completely shaded: Poutx=0 SPV1040 parallel connection This topology guarantees the desired output voltage even when only one panel is irradiated. The obvious constraint of this topology is that Vout is limited to the SPV1040 maximum output voltage. Figure 15 shows the parallel connection topology: Figure 15. SPV1040 output parallel connection Vout+ PV3+ PV3 PV2 PV1 Vo3+ SPV1040 PV3- Vo3- PV2+ Vo2+ SPV1040 PV2 PV2- Vo2- PV1+ Vo1+ SPV1040 PV1- Vo1Vout- AM06711v1 The output partitioning (R1/R2) of each SPV1040 must be coherent with the desired Voutx. According to the topology: Vout=Vout1=Vout2=Vout3 Iout=Iout1+Iout2+Iout3 Doc ID 18265 Rev 1 19/24 SPV1040 parallel and series connection AN3319 According to the light irradiation on each panel and to the system efficiency (η), the output power results:  Pout = Pout1 + Pout2 + Pout 3  Poutx = Voutx * Ioutx  [x = 1..3]  [x = 1..3]  Pinx = Vinx * Iinx Therefore: Pout = Vout (Iout1 + Iout2 + Iout 3) = η Pin1 + ηPin2 + ηPin3 Each SPV1040 contributes to the output power providing Ioutx. Finally, the desired Vout is guaranteed if at least one of the 3 PV panels provides enough power to turn on the SPV1040 relating to it. SPV1040 series connection This topology provides an output voltage that is the sum of the output voltages of the SPV1040 connected in series. The objective of this section is to explain how the output power is impacted by the selected topology. Figure 16 shows the series connection topology: Figure 16. SPV1040 output series connection Vout+ PV3+ PV3 Vo3+ SPV1040 PV3- Vo3- PV2+ Vo2+ PV2 SPV1040 PV2 PV2- Vo2- PV1+ Vo1+ PV1 SPV1040 PV1- Vo1Vout- In this case, the topology imposes:  Iout = Iout1 = Iout 2 = Iout 3  Vout = Vout1 + Vout 2 + Vout 3 In case irradiation is the same for each panel:  Pin1 = Pin2 = Pin3  Pout = 3 * Poutx   Poutx =  [x = 1..3] 1 Pout 3 Poutx = Voutx * Ioutx = Vout1* Iout 20/24 Doc ID 18265 Rev 1 AM06710v1 AN3319 SPV1040 parallel and series connection Therefore:   1 Vout 3 Voutx = For example, assuming Pout = 3 W and Vout = 12 V, then Voutx = 4 V. Lower irradiation for one panel, for example on panel 2, causes lower output power, so lower Vout2 due to the Iout imposed by the topology:   Poutx Iout The output voltage required by the load can be provided by the 1st and the 3rd SPV1040 but only up to the limit imposed by each of their R1/R2 partitionings. Voutx = Some examples can help in understanding the various scenarios assuming that each R1/R2 limits Voutx to 4.8 V. Example 1: Panel 2 has 75 % irradiation of panels 1 and 3:   Vout2 = 3 3 * Vout1 = * Vout3 4 4  Pout1 = Pout 3 = 1W Pout2 = 3 Pout1 = 0.75W 4  Pout = Pout1+ Pout 2 + Pout 3 = 2.75 W   Iout = Pout 2.75 = = 0.23A Vout 12   Vout1 = Vout3 =   Vout2 = 1 = 4.35V 0.23 0.75 = 3.26V 0.23 Two SPV1040s (1st and 3rd) supply the voltage drop caused by the lower irradiation on panel 2. Warning: SPV1040 is a boost controller, so Voutx must be higher than Vinx, otherwise the SPV1040 turns off and the input power is transferred to the output stage through the integrated Pchannel MOS without entering the switching mode. Doc ID 18265 Rev 1 21/24 SPV1040 parallel and series connection AN3319 Example 2: Panel 2 has 50 % irradiation of panels 1 and 3:   1 1 Pout2 = 2 * Pout1 = 2 * Pout3  Pout1 = Pout 3 = 1W   1 Pout2 = Pout1 = 0.5W 2  Pout = Pout1+ Pout 2 + Pout 3 = 2.5W   Iout = Pout 2.5 = = 0.21A Vout 12   Vout1 = Vout3 =   Vout2 = 1 = 4.76V 0.21 0.5 = 2.38V 0.21 In this case the system is close to its maximum voltage limit, in fact, a lower irradiation on panel 2 impacts Vout1 and/or Vout3 which are very close to the maximum output voltage threshold (4.8 V) imposed by R1/R2 partitioning. Example 3: Panel 2 completely shaded. In this case the maximum Vout can be 9.6 V (Vout1+Vout3). The current flow is guaranteed by the body diodes of the power MOSFETs integrated in the SPV1040 (or by the bypass diodes, if any, placed between Vout- and Vout+). 22/24 Doc ID 18265 Rev 1 AN3319 Revision history Revision history Table 2. Document revision history Date Revision 02-Feb-2011 1 Changes Initial release. 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