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Care for your rechargeable batteries High current Battery Balancer Our new High Current Battery Balancer, introduced last month, is an advanced design which provides high efficiency and fast balancing by efficiently transferring charge between the connected cells or batteries. It can handle cells or batteries up to 16V, and two units can be combined for larger installations. This second and final article describes the assembly and testing steps, and how to use it. W e put considerable effort into keeping this design as simple as possible, while still providing excellent performance and many useful features. As a result, the parts count is not especially high. However, we have had to use mostly SMD parts to keep the size reasonable, and also because many of the best part choices were not available in through-hole packages at all. While the board assembly is not overly difficult, it is not suitable for beginners. Some SMD soldering experience is desirable. You will need a decent temperature-controlled soldering station (and ideally a reflow oven or hot air rework station), a syringe of flux paste, some solder wick, fine-tipped tweezers, a magnifier and a strong light source. None of the SMD parts are especially difficult to handle, although the smaller six-pin parts in SOT-363 packages are on the tricker side, along with QSOP-16 ICs, which have pins that are fairly close together. Finally, the transformers can present a bit of a challenge in making good solder joints due to their high thermal mass. But with a little care, the PCB can be built by hand. Refer to the PCB overlay diagrams, Figs.4(a) & 4(b) overleaf, for details on which parts go where. We suggest you start construction by populating the surface-mount components on the board’s underside, followed by the SMDs on the top side, then finally, the through-hole parts. As touched on earlier, you can use various assembly methods, including reflow soldering or hand-soldering. We will describe the hand-soldering method as it requires the fewest specialised tools, listed above. The general procedure is to place each part (with the correct orientation for polarised parts, which is pretty much all ICs, diodes & Mosfets) and tack down one pin. You then check the alignment of the other pins and re-position the part by melting the tack solder and gently nudging the part if it is not perfectly aligned with its pads. Once aligned, it is a good idea to add flux paste to all the pins, as that greatly reduces the chance of solder not adhering. You then solder the remaining pins, refresh the initially tacked pin (if you have added flux paste then all you need to do is touch it with the tip of the iron), then use solder wick and flux to clean up any bridges which might have formed. The order in which components are placed is not critiConstruction cal, but we think it is best to place the most difficult parts The High-efficiency Battery Balancer is built on on each side first, so that you do not have to deal with ina four-layer PCB coded terfering adjacent compo14102211 which measures nents. The following proPart 2 – Construction – by Duraid Madina cedure uses that method. 108 x 80mm. 76 Silicon Chip Australia’s electronics magazine siliconchip.com.au Note that SMD resistors are typically marked with a tiny code on the top that indicates the value (eg, 47kΩ = 473 [47 x 103] or 4702 [470 x 102]), which you probably need a magnifier to see. SMD ceramic capacitors are usually unmarked. Finally, note that most of the semiconductor devices used are sensitive to electrostatic discharge (ESD) – particularly those in the smaller packages. Therefore, when handling these devices, try and avoid touching their pins. A grounded anti-static wrist strap will usually ensure you can’t damage any parts, but there are many other ways of ensuring ESD safety. Assembly details Start by fitting the eight 1Ω gate drive resistors, because they are the smallest passive components on the board and are generally out of the way of other parts. Next, fit the eight gate drive NMOS/PMOS FET pairs: Q27, Q28, Q22, Q23, Q16, Q17, Q11 and Q12. These are relatively large as six-pin SMDs go, so they should not give you too much trouble, but watch the orientation! You might need a magnifier to find the pin 1 dot on the top of each device, which in each case goes in the bottom right corner, as shown in Figs.4(a) & 4(b). Next, mount the eight 4.7µF capacitors which are adjacent to these Mosfet pairs. Follow with the five 330Ω resistors on this side of the board, plus the four 20Ω resistors, then the eight 10µF capacitors alongside the mounting pads for Mosfets Q1-Q4. The components labelled “Rsnub” and “Csnub” are required if you are balancing 12V batteries, but are not needed for lower voltage balancing such as Li-ion/LiPo/LiFePO4 cells. If you need them, fit them now, using the values suggested in the parts list published last month (30Ω & 470pF). Now install Mosfets Q1-Q5. These are in LFPAK56 SMD packages, which are similar to 8-pin SOIC devices, but with a tab replacing four of the pins on one side. As such, it should be obvious which way round they go, but don’t get the BUK9Y8R5-80E used for Q5 mixed up with the similar BUK9Y4R8-60Es used for Q1-Q4. In each case, spread a little flux paste on the tab pad before tacking one of the small pins, then solder the remaining three small pins before the tab. You might need to crank your iron temperature up to solder the tabs as they have a lot of thermal mass. The flux paste you added earlier should help draw the solder you feed in under the tab for a good thermal and electrical connection. With those in place, fit the eight remaining Mosfets on this side of the board using the same technique. They are all BUK9Y14-80Es (a different type again from Q1-Q4 & Q5). Now fit the four SMB TVS diodes, ZD1-ZD4, ensuring that their cathode strips are oriented as shown in Fig.4. Note that the voltage rating of these parts varies depending on what type of cells or batteries you are balancing (see the parts list last month). Solder them similarly to the passives, but being larger, they take a bit more heat. Their leads wrap around the sides, so make sure the solder adheres to both the PCB and the device leads (flux paste makes this much easier to achieve). siliconchip.com.au The next job is to solder the four 3A SMD fuses, which mount similarly to resistors (they are not polarised). That just leaves two small resistors: one 100kΩ 0.1% resistor, and another 0.1% resistor, the value of which varies depending on your application. Make sure you don’t get them mixed up. Top-side SMDs Flip the board over and continue assembly by fitting the four larger, 3.2 x 2.6mm (M3226 or 1210) sized ceramic capacitors near the transformer T1-T4 footprints. We used 4.7µF 100V capacitors (TDK CNA6P1X7R2A475K250AE) but you can more than double the capacitance by using 10µF 75V capacitors which cost only a little bit more (TDK CGA6P1X7R1N106K250AC). Follow by fitting the five small dual Mosfets, Q8, Q18, Q13, Q19 and Q24. In each case, make sure that the pin 1 dot is lined up correctly first. These are in smaller packages than the ones you mounted on the bottom of the PCB, with more closely spaced pins, so they might be a little bit trickier. But they aren’t too hard as long as you remember to carefully check for bridges between pins using a magnifier and fix any bridges you find using flux paste and solder wick. Mosfet Q7, in the bottom-right corner, is in the same package as those five but it is a slightly different device so don’t get it mixed up. Again, check its orientation carefully before soldering it in place. Now is a good time to mount the microcontroller, IC2. It should be relatively easy compared to the devices you have already soldered, but make sure that the pins on all four sides are lined up before soldering more than one pin, and as usual, be careful to get its pin 1 in the correct location. Follow with the four isolators: IC4, IC6, IC8 & IC10. In each case, pin 1 is at upper left. These have a similar pin pitch to the small dual Mosfets you already mounted, so should not be any harder to install. Next, fit the eight 470nF capacitors, followed by the five regulators. For the regulators, spread a little flux paste on the large pad before taking one of the smaller pins, then Australia’s electronics magazine April 2021  77 solder the remaining small pins before tacking the tabs. You might need to turn your iron temperature up a bit when soldering the tabs. With those in place, now you can fit the six 1µF SMD capacitors, the two ferrite beads, plus the 680Ω and 100Ω SMD resistors. Then mount the two ESD protection arrays, which are in four-pin packages with one larger than the others. Check Fig.4(a) and to verify their orientation if you are not sure. Now install the eight 10kΩ resistors and then the five 1nF, three 100nF and three 10µF capacitors. Follow by fitting the remaining TVS (the higher voltage one, ZD5). Make sure it is orientated correctly. Then mount the two fuses, with the lower-current (0.75A) fuse being F7, near 8-pin header CON15, and the higher-current (3A) fuse near CON2 at upper left In terms of passives, that just leaves the sole 20Ω resistor near CON10, plus the eight 0.1% resistors. As mentioned last month, the lower value 0.1% resistor values need to be changed depending on your battery voltages. The upper resistor in each pair is 100kΩ. Ensure that the lower resistor is either 6.8kΩ, for a total stack up to about 24V, or 2.2kΩ for higher stack voltages. Fig.4(a): top-side PCB component overlay, with matching photo below. Transformer mounting Due to the significant thermal mass of the transformers and the large power planes they connect to, we recommend avoiding the use of solder paste for mounting these parts, unless you have a very high-quality reflow oven. Instead, we suggest placing them as accurately as possible, holding them in place with Kapton tape, then soldering their four tabs with a hot iron and flux-cored wire solder. Once the transformers are fitted, it is essential to ensure that all flux residues are removed. This can be challenging as most residues will be hidden between the underside of the transformers and the PCB. If flux residues are allowed to remain, the idle current can increase by orders of magnitude (beyond 1mA). The flux can break down at higher voltages, resulting in erratic behaviour and even arcing through the residues. Here, an ounce of prevention is worth a pound of cure, so try to limit 78 Silicon Chip the build-up of flux residue by not allowing too much to accumulate in the first place. If you have the choice, try to use a “no-clean” flux. However, if the flux you are using requires cleaning, make sure to wash the transformers thoroughly with a high-quality flux remover and wipe off any visible residue. Through-hole components Fit tactile switch S1 now. It has a standard footprint, so switches with various actuator heights are available. If you will be frequently adjusting the unit, you might consider chassis-mounting a switch and wiring it back to the pads. If doing that, make Australia’s electronics magazine sure the wires connect to one of the upper pair and one of the lower pair (which is GND). Then you can fit terminal blades for battery/cell connection as required. Most 5.08mm-pitch two-terminal types will work, but check to make sure your intended spade connector will fit. An example blade is Wurth Elektronik 7471286, or use the Altronics parts suggested in the parts list last month. It may be preferable to solder wires instead of spade lugs for some installations, perhaps to reach panel-mounted connectors. However, the Balancer should not be directly soldered to batteries. A failure siliconchip.com.au Fig.4(b): and here’s the underside of the board, again with matching photo below. in either the Balancer or the batteries will be more difficult to resolve if the two are permanently connected. There is no need to use particularly heavy gauge wire as balancing currents are modest, but as a rough guide, they should be able to carry 2A with a negligible temperature rise. 0.8mm diameter copper wire (20AWG) is a reasonable option. If using spade lugs, ensure that no part of the blade, lug, or wire can contact other nearby components. Insulated spade quick connectors are available, and it’s a good idea to use them. For some installations, you might want to mount the board inverted and siliconchip.com.au have terminals or wires exiting from the rear of the board. You can also fit a 5-position 2.54mm header (either vertical or right angle) at CON13 for lower-power applications such as balancing smaller lithium-polymer (LiPo) batteries. CON13 is conveniently located at the edge of the board. If the board is mounted right at the edge of a case with a cut-out in the side, you can plug a standard balance connector straight in. Watch the polarity, though! Now is a good time to fit the 2x4-pin header for JP1. For some installations, where the batteries are of a fixed type, this could be replaced with a soldered Australia’s electronics magazine wire link if desired. Follow with trimpot VR1, ensuring its adjustment screw is located as shown. If you will be adjusting the balancing voltage frequently, you could instead use a chassis-mount 100kΩ potentiometer and run flying leads back to VR1’s pads, possibly plugging into a pin header. Neither the potentiometer’s accuracy nor power dissipation are critical, but we suggest using a sealed design for greater long-term reliability. Follow with the four LEDs, ensuring that the cathodes go towards the top as shown. If mounting the LEDs on the PCB, you will need to use 3mm types. Still, you could instead fit pin headers or flying leads and mount them in a location that will be externally visible (eg, mounted onto a panel or case side using bezels), in which case you could use 5mm LEDs or virtually any other types. For some colours, a different value of current-limiting resistor from the 680Ω specified could be desired to increase the brightness or decrease power consumption. As the drive voltage is 3.3V, blue or white LEDs are not recommended, although you might find that such types give adequate light given their high efficiency. Pin headers CON14 & CON15 are optional. CON14 is only required if you need access to the serial port, such as for debugging or connecting two Balancers to work together (via an isolator) on an 8-cell battery. CON15 is only needed if you have fitted a blank microcontroller and will need to program it on-board, or wish to reprogram it later. That just leaves the six capacitors. Don’t get the two different types mixed up, and make sure to insert the longer leads into the holes marked with + signs. We have specified organic polymer capacitors, not ordinary electros, for their much superior performance characteristics. Programming On the topic of programming IC2, as mentioned last month, it can be done with a PICkit 4 plugged into CON15 (pin 1 to pin 1). This can be done using the MPLAB X IPE software, which comes with Microchip’s free MPLAB X IDE. (Or simply use a pre-programmed chip from our Online Shop). April 2021  79 Safety notes Working with batteries presents some hazards. The most important thing to do is to be thoroughly familiar with your particular batteries’ safety requirements. In general, having fuses close to the terminals of all larger batteries is a good idea to prevent cables catching fire. You can buy fuses that connect directly to the terminals, with provision to attach thick wires at the other end. You can also use inline fuses, but you should ideally keep the section of wire between the terminal and fuse short. There are a few other things to keep in mind when using the Balancer: • Always check that the Balancer is working as intended before attaching it to batteries or other power sources. Ideally, this is done with current-limited power supplies, as described in the main text. • Don’t leave the Balancer unattended until you are satisfied that it works reliably for your particular application. Take particular care if setting a lengthy timeout period. • Keep the Balancer physically separate from the batteries. If they are too close, heat from the Balancer could degrade the batteries, or lead to a hazardous situation. • Ensure that the Balancer is kept clean and dry at all times. • Don’t permanently attach the Balancer to batteries or other power sources; if a hazardous situation arises, it is good to have the ability to quickly disconnect the Balancer. • Periodically check that your batteries are healthy: if the Balancer is constantly balancing one cell, or if you notice that your batteries are losing their ability to store charge, be sure to test and replace any failing cells. • Remember that the Balancer can’t stop a battery from being charged or discharged by external circuitry: over-charging and over-discharging cells can not only damage them, but can lead to hazardous situations. • Note that in higher voltage applications, some of the voltages present on the Balancer could be dangerous (although its maximum rating of 60V total is well within the extra-low-voltage or ELV domain) and so the Balancer should not be touched. Additionally, some components on the Balancer can get hot during operation. It’s very important not to program the device while attached to any kind of power source (cell/battery or otherwise), so enable the “power target from PICkit” option. Test the device in low power/current limited situations after programming, as described below, in case there’s an error with the newly programmed software. Testing Before connecting the Balancer to batteries, it’s essential to test it to ensure that nothing has gone wrong with the assembly that could affect safety or reliability. The easiest way to do this is with a pair of isolated, current-limited power supplies. Set their output voltages to be the same (eg, 4V each) and their current limits to around 500mA. Connect one supply between STACK- (CON7) and CELL1 (CON6), with the positive terminal to CON6. Connect the other between CELL1 (CON6) and CELL2 (CON5), with the positive terminal to CON5. Ensure that a jumper is installed so that the control block is powered from one of these two points, ie, at the positions marked 1 or 2 for JP1 (across pins 80 Silicon Chip 1 & 2 or pins 3 & 4). With an oscilloscope, check to see periodic pulses on the SENSE_EN and SAMPLE lines (pins 19 & 20 of IC2 respectively). If these are absent, there is a fault in or around the microcontroller, or it is not receiving power. If you don’t have a scope, you might be able to pick up the pulses using the frequency counter mode on a DMM, or even an analog voltmeter. If the microcontroller is functional, tie the top-most cell to the stack voltage rail (connecting CON5 [CELL2] to CON2 [STACK+]), and slowly make a small change to the voltage of one of the cells. You should see that the voltage on the power supply with the lower output voltage increases. If this is difficult to observe, you can use an oscilloscope to check the CSPWM/SSPWM lines on the corresponding cell (pins 11 & 17 for the lowest cell or pins 12 & 18 for the second-lowest). You should see narrow, square pulses on these lines. If this test is successful, check the third and fourth cell sections, but note that cells must always be populated in-order from ground; none can be left empty except at the top. Australia’s electronics magazine If you are considering higher-voltage applications, test these carefully, taking great care to use appropriate current limits, and ensuring that the control logic section is powered from only the lowest possible cell. This avoids wasted power in the control regulator (REG1) and potential damage if its maximum input voltage is exceeded. In general, if your lowest expected cell or battery voltage is above 3.6V, then you should always leave JP1 in position 1, so the control circuitry runs off the lowest cell. If your lowest expected cell voltage is lower than this, down to the minimum supported of 2.5V, then it should always be safe to run the control circuitry off the second cell (position 2 on JP1). Higher positions are only useful if you need to ensure that the small current which powers the control section comes from the whole stack, which would be unusual. Final assembly Once you’ve tested your Balancer board, it should be enclosed to protect it from dust and other contaminants. siliconchip.com.au You can use just about any box that’s large enough to fit the PCB module, and which that allows cables to be fed through. Ideally, it should offer some method of exposing the LEDs (eg, a clear lid), potentiometer and pushbutton (possibly via a screwdriver through small holes in the lid). Mount the PCB to the bottom of the case using standoffs so that the board does not flex, and take care that all of the components have adequate clearance from the case walls as it can dissipate some heat. Four mounting holes are provided to suit M3 machine screws, and plastic or metal spacers can be used. Just be careful if using metal spacers that they fit within the copper areas provided around the holes. Heatsinking is not usually required on any of the components, but allowing even a modest amount of airflow will go a long way towards keeping the Balancer cool, prolonging its life. In harsh environments, a small temperature-switched fan could be used (eg, with the thermal switch glued to transformer T1). But in most cases, passive airflow will be adequate, with a few vents or holes drilled in the bottom and the top, or the sides of the case, being sufficient for convection to remove the heat. Using it Now that you’ve built and tested your Balancer, how can you use it? Before connecting it to a battery, run through the following checklist to make sure it’s correctly configured: 1) Configure the source of control power. As described above, if balancing 12V batteries, ensure that the control power source select jumper is securely installed in the right-most position (marked 1), so that the lowest cell is providing control power. If balancing ~3.6V cell (eg, Li-ion, LiPo or LiFePO4), you will probably want the power source select jumper in the second-rightmost position, so that the lowest pair of cells are providing control power. 2) Connect the battery leads to their respective terminals. We suggest connecting them either sequentially (CELL1, CELL2…) or simultaneously (if using an external connector). Plug spade quick connects onto CON8-CON12 for higher-current applications, or a plug designed to mate with 2.54mm-pitch header pins to siliconchip.com.au Screen1: sample serial output. CON13 for balancing up to 1A. If using CON13, make sure the plug orientation is correct, with the negative-most terminal to pin 1! There might be small sparks when connecting battery leads, but these should be momentary. 3) Finally, connect the stack leads (STACK- to CON7 and STACK+ to CON2). If balancing, you can simply bridge the positive stack voltage terminal to the top-most cell terminal. For charging, connect the negative stack terminal to the negative end of your power source, and the positive stack terminal to the positive end. If available, we recommend setting a reasonably low current limit on your power source, to help prevent damage to batteries in case of malfunction. Making adjustments Operation is essentially automatic, with the Balancer simply transferring charge based on the differences it senses in voltage across the batteries or cells. However, there are some options you can set, either using trimpot VR1 and pushbutton S1, or via the serial interface. The options include the minimum difference between battery/cell voltag- es for balancing to start, the maximum balancing current and the minimum and maximum battery/cell voltages outside which balancing will cease. The defaults are for the maximum possible balancing current (about 2.5A), to begin balancing with a 50mV imbalance for 12V lead-acid batteries or a 10mV imbalance for li-ion cells, and for an operating cell voltage range of 2.5-4.3V for li-ion applications and 10-14.8V for nominally 12V batteries. You can change most of these settings using trimpot VR1 and pushbutton switch S1, although a larger range of configuration and calibration settings are available via the serial/USB interface. Table 1 shows the various commands which can be issued by pressing pushbutton S1 in various ways – either a single, long press or with several short presses in a row. Some of these control the unit while others adjust settings in combination with the current rotation of trimpot VR1. Unfortunately, making settings changes this way is a bit imprecise. You can measure the voltage at the wiper of VR1, either by probing its centre pin on the bottom of the board with a DVM or by probing pin 3 of nearby Mosfet Function Check that unit is powered up Pause/resume balancing Switch between li-ion and lead-acid presets Set allowable voltage delta (0-300mV/0-1V) Set maximum balancing current (0-2.5A) Set minimum battery/cell voltage (0-5/0-15V) Set maximum battery/cell voltage (0-5/0-15V) Number of S1 presses One short (<500ms) One medium (1-2s) One long (5s+) Two short Three short Four short Five short Table 1: functions accessible by pressing pushbutton S1 Australia’s electronics magazine April 2021  81 Example Result p r t 600 l 3000 h 4300 d 50 i2 50 o3 25 c2 100000 6790 st 100000 6812 v 3280 Pauses automatic balancing Resumes automatic balancing Set balancing timeout to 600s; if balance not reached in this time, shut down Set low battery/cell threshold to 3V (3000mV); below this, it shuts down Set high battery/cell threshold to 4.3V (4300mV); above this, it shuts down Batteries/cells can vary by up to 50mV before balancing starts Move charge into battery/cell #2 (1-4) at 50% of maximum rate (1-100) Move charge out of battery/cell #3 (1-4) at 25% of maximum rate (1-100) Calibration – set battery/cell divider #2 to have a voltage division ratio of 100kΩ:6.79kΩ Calibration – set stack divider to have a voltage division ratio of 100kΩ:6.812kΩ Calibrate – set the typical supply voltage to 3.28V (3280mV) Table 2: Serial Commands Q7 relative to GND. You then need to divide that reading by 1.65V (or better, the actual measured 1.65V ADC reference voltage) and then multiply by the range given in Table 1. If you can hook up the serial interface, you are much better off making changes that way as they will be exact, and you can also calibrate the unit properly that way. Read on for further details on the serial interface. Monitoring its operation The simplest way to do this is visually. One of the four LEDs on the board will flash to indicate when balancing is occurring, with the right-most LED (LED7) corresponding to the bottom-most cell, LED8 to the next cell up in the stack, etc. They blink slowly if a battery/cell is being charged, or rapidly if a battery/ cell is being discharged. If no balancing/charging is occurring, LED7 will occasionally flicker very lightly, just to let you know that the circuit is ‘alive’, while consuming as little power as possible. If there is an over-voltage error, all four LEDs will flash simultaneously at 1Hz, with a 50% duty cycle. If an under-voltage error is detected, the unit simply shuts down and does not flash the LEDs at all (not even a heartbeat). If you are paying attention, the lack of heartbeat will tell you something is wrong, and by leaving the LEDs off, we don’t risk discharging an already over-discharged cell or battery. If you want more details of the unit’s operation and be sure that it is doing its job, you can monitor the serial port at CON14. Ideally, this should be con82 Silicon Chip nected to your computer via an isolating interface (a good one is described below). You can then wire the output of that isolating interface to a USB/serial adaptor. Set a terminal emulator to 38,400 baud N,8,1 and you should see a stream of information, like that shown in Screen1. This shows you the measured voltage at each input, plus the whole stack, whether it is currently moving any charge into or out of a battery/cell, and how fast it is doing so (0-100%). The data is both human-readable and machine-readable, so it would be quite easy to create software to parse the information and display it differently, or take actions depending on the results. As shown in Table 2, you can also send commands to pause or resume balancing, change the settings, or even force it to transfer charge into or out of a given battery/cell. This means that you could centralise the control via a computer program if you are using several Balancer boards. Combining multiple balancers You can use two Balancer boards to balance up to eight batteries or cells, as long as the total stack voltage is still within the 60V DC maximum rating. The only extra hardware that you need to do this is an isolated serial link. Fortunately, we published just such a design in March 2021 (siliconchip. From last month’s SILICON CHIP, this isolated serial link is ideal to link together two Balancer boards together. Australia’s electronics magazine com.au/Article/14785), and PCBs are available. Build that board, but leave off the headers, and set both jumpers (JP1 & JP2) to the 5V position (they will actually be supplied with 3.3V, as that is the only low-voltage rail available on the Balancer boards). You can then solder pins 3-6 of either CON1 or CON2 directly to CON14 on one of the Battery Balancer boards, as the pinout is an exact match. Run a ribbon cable or similar from the other end of the board to CON14 on the other Balancer board. The wiring will be the same as the other end and you should have the TX pin on the Balancer connected to the TX pin on the Isolator board. Similarly, the RX pin on the Balancer connects to the RX pin on Isolator. The reversal is effected within the Isolator. Then, all you have to do is connect between one and four contiguous cells/ batteries in your stack to one balancer board, starting with the CELL1 connection, and join the remainder to the other. Connect both full stacks across the STACK- and STACK+ terminals on both boards. The two units will power up and negotiate over the serial link, automatically detecting that they are talking to each other. They will then balance as if they are one eight-input Balancer instead of two four-input balancers. Finally, there is an error in the parts list in last month’s part 1: on p27, several Mosfets (Q11,Q12…) are listed as “S6M4” types. The correct type code is QS6M4. SC siliconchip.com.au