APPLICATION NOTE │EM8500 EM MICROELECTRONIC - MARIN SA Subject to change without notice 608002, Version 1.0, 21-August-2015 Copyright @ 2015, www.emmicroelectronic.com Title: CONFIGURATION OF THE HARVESTING PARAMETERS Product Family: EM850X Part Number: EM8500 Keywords: Harvesting, Solar, TEG, MPPT, Configuration, Setup ABSTRACT The EM8500 offers a NVM containing all the configuration parameters. This document describes how to setup the registers in NVM linked to the harvester: • Type of harvester: Thermal Electrical Generator or Solar cell. • MPPT algorithm configuration • Timings and levels related to the harvester power capability ABBREVIATIONS NVM MCU STS LTS HRV TEG MPP Vmpp Vmpp_min Impp Pmpp Vov Vov_min BAT_LOW HRV_LOW Vhrv_scv Vlvl NVM MCU STS LTS HRV Non-Volatile-Memory Microcontroller Unit Short term storage element (capacitor connected to VDD_STS) Long term storage element (rechargeable battery connected to VDD_LTS) Harvester, main source of energy (solar or TEG) Thermal Electrical Generator (Maximum Power Point) This operating point is reached when the harvester delivers the maximum power (Pmpp) in a given condition HRV output voltage when at MPP Vmpp limit under which the HRV_LOW mode shall be activated HRV output current when at MPP HRV output power when at MPP HRV open voltage (when the EM8500 DCDC converter is disabled) HRV open voltage corresponding to Vmpp_min Flag indicating that the battery is in under-voltage condition Flag indicating that the HRV is under the minimum power level (HRV low mode when at 1) HRV voltage reference for short-circuit current measurement (70mV) Voltage level detector LSB (73 mV) Non-Volatile-Memory Microcontroller Unit Short term storage element (capacitor connected to VDD_STS) Long term storage element (rechargeable battery connected to VDD_LTS) Harvester, main source of energy (solar or TEG) 1 APPLICATION NOTE │EM8500 Subject to change without notice 608002, Version 1.0, 21-August-2015 Copyright @ 2015, www.emmicroelectronic.com 1 SCOPE The EM8500 addresses two main types of HRV: 1. Solar cells 2. TEG (Thermal Electrical Generator) The EM8500 has several parameters to setup the HRV depending on its type, its power range and its voltage range. Two main functions shall be setup: 1. Harvest energy at the MPP (Maximum Power Point) 2. Stop harvesting energy when the power available is too low. The following registers are involved for that action: Register name Address Description Define the period between two HRV checks when DCDC is operating reg_t_hrv_period 0x00 t_hrv_low_period: Define the period between two HRV checks in HRV low mode Define the duration of the HRV sampling phase reg_t_hrv_low_cfg 0x17 reg_t_hrv_meas 0x01 reg_v_hrv_cfg 0x04 reg_hrv_check_lvl 0x05 hrv_check_vld: Set the type of HRV (1 for a TEG ; 0 for a solar cell) v_hrv_min: set the minimum open voltage level (used for TEG in operating mode and Solar Cell in HRV low mode) Set the minimum short-circuit current level (used for Solar Only) reg_mppt_ratio 0x12 Set the HRV voltage ratio between open and loaded to operate @MPP Table 1: List of Registers Related to Harvesting Parameters The default value after reset or start-up of the registers listed in Table 1 is contained in a NVM memory at the following related addresses: Register name Register Related address in NVM Address eeprom0 0x40 reg_t_hrv_period 0x00 reg_t_hrv_low_cfg 0x17 eeprom23 0x57 reg_t_hrv_meas 0x01 eeprom1 0x41 reg_v_hrv_cfg 0x04 eeprom4 0x44 reg_hrv_check_lvl 0x05 eeprom5 0x45 reg_mppt_ratio 0x12 eeprom18 0x52 Table 2: Relation between Register and Corresponding NVM Address Note: offset between the register addresses and related address in NVM is 0x40 2 APPLICATION NOTE │EM8500 Subject to change without notice 608002, Version 1.0, 21-August-2015 Copyright @ 2015, www.emmicroelectronic.com 2 HRV CONFIGURATION SEQUENCE 2.1 MPPT setting The MPPT is the algorithm to find the point where the power is maximum along a V=f(I) curve. For each type of HRV, there is a ratio between the loaded (Vmpp) and open voltage (Vov) where the power is maximum (MPP). The EM8500 DCDC regulates the voltage VDD_HRV to be equal to Vmpp. This ratio is set by the register reg_mppt_ratio as follows: reg_mppt_ratio [hex] MPPT ratio [%] 0x00 50 0x01 60 0x02 67 0x03 71 0x04 75 0x05 78 0x06 80 0x07 82 0x08 83 0x09 85 0x0A 86 0x0B 87 0x0C to 0x0F 88 Table 3: MPPT Ratio Selection The EM8500 samples Vov * MPPT ratio and holds that value in an internal capacitor. The DCDC converter regulates VDD_HRV to be equal to the hold value. When this condition is fulfilled, the maximum power is transferred into the charge. 2.1.1 MPPT for a TEG The TEG has a pure ohmic impedance and therefore a linear V=f(I) curve as follows: Figure 1: V=f(I) Curve of a TEG 3 APPLICATION NOTE │EM8500 Subject to change without notice 608002, Version 1.0, 21-August-2015 Copyright @ 2015, www.emmicroelectronic.com The MPP is reached when the loaded voltage is 50% of the open voltage (Vteg = Vmpp). The register reg_mppt_ratio shall be set to 0x00 when EM8500 harvests energy from a TEG. 2.1.2 MPPT for a solar cell The solar cell has a non-linear V=f(I) curve as follows: Figure 2: V=f(I) Curve of a Solar Cell The MPP is reached when the loaded voltage is around 80% of the open voltage (Vsol = Vmpp). Depending on the type of solar cell, the power range or the temperature, Vmpp can be slightly different than 80%. The EM8500 offers other ratios around 80% as shown in the Table 3. Typically, the register reg_mppt_ratio shall be set to 0x06 when the EM8500 harvests energy from a solar cell. 4 APPLICATION NOTE │EM8500 Subject to change without notice 608002, Version 1.0, 21-August-2015 Copyright @ 2015, www.emmicroelectronic.com 2.2 HRV check settings The EM8500 regularly checks if the HRV can generate energy. When this is not the case the EM8500 stops the DCDC converter and sets the flag HRV_LOW to ‘1’. There 2 ways to check the HRV capability: Measure the open voltage of the HRV: reg_v_hrv_cfg.hrv_check_vld = 1 (preferably for TEG) Measure the short-cut current of the HRV: reg_v_hrv_cfg.hrv_check_vld = 0 (preferably for Solar) 2.2.1 HRV open voltage measurement The HRV open voltage measurement method is active when: reg_v_hrv_cfg.hrv_check_vld = 1 This technique is used mainly for harvesters having a high voltage swing from worst to best conditions. It is clearly the case for a TEG. The open voltage grows linearly with the temperature proportionally to the Seebeck coefficient: Figure 3: V=f(I) Curve of a MPG-D655 for Different Delta Temperatures The maximum power that a TEG can deliver, at a given open voltage, depends on its internal resistivity. It can be calculated using the following equation: 2 V Pmpp_max ov 4 Rteg Equation 1: Maximum Power Delivered by a TEG over Vov and Rteg 5 APPLICATION NOTE │EM8500 Subject to change without notice 608002, Version 1.0, 21-August-2015 Copyright @ 2015, www.emmicroelectronic.com The minimum power the EM8500 can harvest depends on the input voltage delivered by the HRV and the output voltage that the DCDC converter shall deliver to VDD_LTS. Then, there is a relation between the internal resistivity of the TEG, the minimum Vov to harvest and the DCDC converter output voltage range as shown in the following table: Maximum TEG internal resistivity v_hrv_min = 0x02 Vout range 1.5V to 2V Vout range 2V to 3V Vout range 3V to 3.6V 830 Ohm 410 Ohm 80 Ohm 2980 Ohm 1490 Ohm 290 Ohm 7770 Ohm 3880 Ohm 1160 Ohm 16780 Ohm 8390 Ohm 3350 Ohm Open voltage level 216mV v_hrv_min = 0x03 Open voltage level 288mV v_hrv_min = 0x04 Open voltage level 359mV v_hrv_min = 0x05 Open voltage level 431mV Table 4: Maximum TEG Resistivity Related to v_hrv_min and Vov Range According to the Table 4, if the TEG MPG-D655 with a resistivity of 210 ohm is used, v_hrv_min shall be set to 0x02 if the maximum battery voltage is in the range 1.5V to 2V or 2V to 3V. If the maximum battery voltage is in the range 3V to 3.6V, v_hrv_min shall be set to 0x03. 6 APPLICATION NOTE │EM8500 Subject to change without notice 608002, Version 1.0, 21-August-2015 Copyright @ 2015, www.emmicroelectronic.com 2.2.2 HRV short-cut current measurement The HRV short-cut measurement method is active when: reg_v_hrv_cfg.hrv_check_vld = 0 As shown in the Figure 4, the light strength affects the current capability more than the open voltage of a solar cell. The output current capability of a solar cell is more or less linear with the light strength. Figure 4: V=f(I) Solar Cell Curves for Different Light Strengths The EM8500 uses the function “HRV check” to determine if the solar cell current capability is above or below a certain level. It loads the HRV with a resistor and compares the loaded voltage Vsol with a typical reference value, Vhrv_scv (70mV). If Vsol is lower than this reference, the EM8500 considers the HRV too weak to deliver energy, stops the DCDC converter and sets the flag HRV_LOW to ‘1’. In HRV low mode (HRV_LOW = 1), in order to save energy the voltage reference Vhrv_scv is not available. The reference of the voltage level detector measuring the HRV open voltage is used instead. Therefore the register v_hrv_min shall be set 0x01 (Vlvl = 146mV) when the short-cut measurement method is set. 7 APPLICATION NOTE │EM8500 Subject to change without notice 608002, Version 1.0, 21-August-2015 Copyright @ 2015, www.emmicroelectronic.com The minimum power the EM8500 can harvest depends on the input voltage delivered by the HRV and the output voltage that the DCDC converter shall deliver to VDD_LTS. Then, there is a relation between the output current capability of the solar cell, the minimum Vmpp and the DCDC converter output voltage range as shown in the following table: reg_hrv_check_lvl Vout range 1.5V to 2V Vmpp_min = 0.2 0x0A (Vov_min = 0.25 @80%) Ihrv_lim = 11uA Vout range 2V to 3V Vout range 3V to 3.6V N.A. N.A. Vmpp_min = 0.3 0x03 0x07 0x0E (Vov_min = 0.38 @80%) Ihrv_lim = 4uA Ihrv_lim = 8uA Ihrv_lim = 15uA Vmpp_min = 0.4 0x02 0x05 0x08 (Vov_min = 0.50 @80%) Ihrv_lim = 3uA Ihrv_lim = 6uA Ihrv_lim = 9uA Vmpp_min = 0.5 0x02 0x02 0x04 Ihrv_lim = 5uA (Vov_min = 0.63 @80%) Ihrv_lim = 3uA Ihrv_lim = 3uA Vmpp_min = 0.6 0x01 0x01 0x03 (Vov_min = 0.75 @80%) Ihrv_lim = 2uA Ihrv_lim = 2uA Ihrv_lim = 4uA Vmpp_min = 0.7 0x01 0x01 0x03 (Vov_min = 0.88 @80%) Ihrv_lim = 2uA Ihrv_lim = 2uA Ihrv_lim = 4uA Vmpp_min = 0.8 0x01 0x01 0x02 (Vov_min = 1.00 @80%) Ihrv_lim = 2uA Ihrv_lim = 2uA Ihrv_lim = 3uA Vmpp_min = 0.9 0x01 0x01 0x02 (Vov_min = 1.13 @80%) Ihrv_lim = 2uA Ihrv_lim = 2uA Ihrv_lim = 3uA Vmpp_min = 1.0 0x01 0x01 0x02 (Vov_min = 1.25 @80%) Ihrv_lim = 2uA Ihrv_lim = 2uA Ihrv_lim = 3uA Vmpp_min = 1.1 0x00 0x00 0x01 (Vov_min = 1.38 @80%) Ihrv_lim = 1uA Ihrv_lim = 1uA Ihrv_lim = 2uA Vmpp_min = 1.2 0x00 0x00 0x01 (Vov_min = 1.50 @80%) Ihrv_lim = 1uA Ihrv_lim = 1uA Ihrv_lim = 2uA Vmpp_min = 1.3 0x00 0x00 0x01 (Vov_min = 1.63 @80%) Ihrv_lim = 1uA Ihrv_lim = 1uA Ihrv_lim = 2uA Vmpp_min = 1.4 0x00 0x00 0x01 (Vov_min = 1.75 @80%) Ihrv_lim = 1uA Ihrv_lim = 1uA Ihrv_lim = 2uA Vmpp_min = 1.45 0x00 0x00 0x01 (Vov_min = 1.80 @80%) Ihrv_lim = 1uA Ihrv_lim = 1uA Ihrv_lim = 2uA Table 5: HRV Check Level Selection Related to Minimum Vmpp and Output Voltage Range Vmpp_min means Vmpp at lower light condition. For instance, the solar cell in Figure 4 at lower light condition, Vmpp is around 0.4V. If the battery voltage connected to VDD_LTS is within the range 3V to 3.6V, the register reg_hrv_check_lvl shall be set to at least 0x08. The following equation defines the relation between Ihrv_lim and the register reg_hrv_check_lvl: I hrv _ lim 1A reg _ hrv _ check _ lvl 1 Equation 2: Ihrv_lim over Register reg_hrv_check_lvl 2.2.3 HRV check timing settings The user can configure the period of the HRV check. If the HRV open voltage measurement method is used, the sample & hold required for MPPT measurement is done in the same time as the HRV check. Otherwise, when the short-cut current method is used, the sample & hold is done alternatively with the HRV check; the sample & hold is done on Vov but the current measurement is done on short-cut voltage. The sample & hold and the open voltage measurement last a certain duration to allow Chrv to charge up to Vov. This duration is configurable with the register reg_t_hrv_meas. When a TEG is used, we advise setting a duration in accordance with the Equation 3: Thrv _ meas 5 Rteg Chrv Equation 3: Min Thrv_meas Calculation for a TEG 8 APPLICATION NOTE │EM8500 Subject to change without notice 608002, Version 1.0, 21-August-2015 Copyright @ 2015, www.emmicroelectronic.com For the TEG this duration does not depends on Vov, but only on the internal resistivity and Chrv. Concerning the solar cell, we consider that in the worst case condition Chrv is charged up to Vov (Vmpp / 0.8) with the current set by the register reg_hrv_check_lvl in Table 5 multiplied by 2.5 as margin. This margin is required to compensate the fact that the current Isol is not constant all along Vsol: Thrv _ meas 2.5 (Vov _ min V pp _ min ) Chrv I hrv _ lim Equation 4: Min Thrv_meas Calculation for a Solar Cell When the HRV check is done by the short-cut measurement method, the measurement duration is constant: 64ms. The HRV measurement and the sampling & hold impact the global efficiency of the transfer of energy. To reduce this impact, it is important to have a duty-cycle between the HRV measurement phase and the harvesting phase low enough. During the sampling phase or the HRV check measurement, no energy is transferred into the charge. The loss of efficiency is therefore: Effloss Thrv _ meas Thrv _ period Equation 5: Efficiency Loss Due to HRV Check & Sampling Phase On other hand, if Thrv_period is too long, the MPPT could be detuned if the harvesting conditions quickly change. Considering a solar application, if the light is blinking with a period shorter than Thrv_period, the MPPT reference will not be adapted all the time. When the conditions change slowly we advise selecting the longest Thrv_meas that the sample & hold can afford (i.e. 32s). If the conditions quickly change, the shortest Thrv_period with an acceptable efficiency loss shall be selected. If a period of 2048ms is selected with 64ms sampling phase, the efficiency loss is 3%. 2.2.4 HRV check in HRV low mode In HRV low mode (HRV_LOW = 1), there is no sampling and hold done but only the HRV check using the voltage level detector. In this condition, Thrv_period is defined by the register reg_t_hrv_low_cfg.t_hrv_low_period instead of the register reg_t_hrv_period. The selection of Thrv_meas and Thrv_period is done with the related register t_hrv_meas, t_hrv_period and reg_t_hrv_low_cfg.t_hrv_low_period as follows: Thrv_meas Register value Thrv_period (register : t_hrv_meas) (register : t_hrv_period ; HRV_LOW = 0) (register : t_hrv_low_period ; HRV_LOW = 1) 000 16ms 256ms 001 32ms 512ms 010 64ms 1s 011 128ms 2s 100 256ms 4s 101 512ms 8s 110 1s 16s 111 2s 32s Table 6: Thrv_meas and Thrv_period Related Registers 9 APPLICATION NOTE │EM8500 Subject to change without notice 608002, Version 1.0, 21-August-2015 Copyright @ 2015, www.emmicroelectronic.com TABLE OF CONTENTS Configuration of the harvesting parameters ................................................................................. 1 ABSTRACT ..........................................................................................................................................1 ABBREVIATIONS.................................................................................................................................1 1 2 Scope ..................................................................................................................................... 2 HRV configuration sequence ................................................................................................. 3 2.1 MPPT setting..............................................................................................................................3 2.1.1 MPPT for a TEG .......................................................................................................................................3 2.1.2 MPPT for a solar cell.................................................................................................................................4 2.2 HRV check settings ....................................................................................................................5 2.2.1 HRV open voltage measurement ..............................................................................................................5 2.2.2 HRV short-cut current measurement ........................................................................................................7 2.2.3 HRV check timing settings ........................................................................................................................8 2.2.4 HRV check in HRV low mode ...................................................................................................................9 10 APPLICATION NOTE │EM8500 Subject to change without notice 608002, Version 1.0, 21-August-2015 Copyright @ 2015, www.emmicroelectronic.com LIST OF TABLES Table 1: List of Registers Related to Harvesting Parameters ............................................................................... 2 Table 2: Relation Between Register and Corresponding NVM Address ............................................................... 2 Table 3: MPPT Ratio Selection.............................................................................................................................. 3 Table 4: Maximum TEG Resistivity Related to v_hrv_min and Vov Range .......................................................... 6 Table 5: HRV Check Level Selection Related to Minimum Vmpp And Output Voltage Range ............................ 8 Table 6: Thrv_meas and Thrv_period Related Registers ...................................................................................... 9 LIST OF FIGURES Figure 1: V=f(I) Curve of a TEG ............................................................................................................................. 3 Figure 2: V=f(I) Curve of a Solar Cell .................................................................................................................... 4 Figure 3: V=f(I) Curve of a MPG-D655 for Different Delta Temperatures ............................................................. 5 Figure 4: V=f(I) Solar Cell Curves for Different Light Strengths ............................................................................ 7 LIST OF EQUATIONS Equation 1: Maximum Power Delivered by a TEG over Vov and Rteg ................................................................. 5 Equation 2: Ihrv_lim over Register reg_hrv_check_lvl .......................................................................................... 8 Equation 3: Min Thrv_meas Calculation for a TEG ............................................................................................... 8 Equation 4: Min Thrv_meas Calculation for a Solar Cell ....................................................................................... 9 Equation 5: Efficiency Loss Due to HRV Check & Sampling Phase ..................................................................... 9 EM Microelectronic-Marin SA (“EM”) makes no warranties for the use of EM products, other than those expressly contained in EM's applicable General Terms of Sale, located at http://www.emmicroelectronic.com. 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