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F ∎ Switch-mode buck-boost current/voltage driver module ∎ Suitable for driving a variety of 12V LED panels ∎ Adjustable current and voltage settings using trimpots ∎ Alternative fixed voltage/current settings with fixed resistors ∎ Lower-cost 5A option by omitting some parts ∎ Input voltage range: 11.3V-35V ∎ Output voltage range: 7-34V ∎ Maximum output current: 8A ∎ Maximum input current: 10A ∎ Other uses include charging a 12V battery from another 12V battery or other DC source ∎ Can also be used as a 12 ➿ 24V DC or 24 ➿ 12V DC converter or under $20, you can buy some impressive LED panels from AliExpress (eg, www.aliexpress. com/item/4001275542304.html). They measure about 22cm by 11cm with an active area of 20cm by 10cm. They’re also available from other online sellers such as on eBay or Banggood. The panels are based on an aluminium PCB and have a silicone gel coating over the LED array. They are specified as drawing 70W at 12V DC, and they simply expose two solder pads for the power source. There are several other modules with different sizes and power ratings, although we haven’t tested any of those alternatives. Having received some samples of these LED panels, we ran some tests using our 45V Linear Bench Supply (October-December 2019; siliconchip. com.au/Series/339) and produced the current/voltage curve seen in Fig.1. This is consistent with four groups of LEDs arranged in series, each with a voltage drop of around 3V, giving a forward voltage of about 12V. Running the panel at 50W (close to 4A) for a while, it got pretty hot and was way too bright to look at directly. So we expect that these panels can be run at lower power levels than that and still be very useful. Running them cooler should also extend their working life. When supplied with a small amount of current, the individual LEDs can be seen, and there are 336 of them, arranged in 28 rows of 12 (see the photo at the end of the article). Each group of LEDs connected in parallel corresponds to seven rows. YouTuber Big Clive ran some tests on similar modules, and even tore back the gel coating to see what lies beneath. You can see his video at https://youtu. be/uIspnsBp3o4 He found that each group of LEDs is simply wired in parallel, meaning that the panel is mostly unaffected if one LED fails open-circuit. A short-circuit failure would tend to shunt the entire panel current through a single LED, quickly turning it into an open circuit! It also appears that the LEDs are actually blue, and the gel is a phosphor coating. It’s an interesting construction that is quite robust, but simple and clearly cheap to manufacture. As LEDs are often touted as being around eight times more efficient (in terms of lumens per watt) than Australia's electronics magazine siliconchip.com.au High-Power Buck-Boost LED Driver Since we saw some ridiculously bright, low-cost LED panels for sale, we’ve been trying to figure out the best way to drive them. This Driver is the result; it is very flexible and useful for many other purposes, such as charging batteries from a DC source or converting between 12V DC and 24V DC. By Tim Blythman Background Source: https:// unsplash.com/photos/k4KZVfAXvSg Features & Specifications 40 Silicon Chip incandescent globes, 70W of LED light is equivalent to several hundred watts of incandescent light; easily enough to illuminate a large room very brightly. Fig.1: like any semiconductor diode, the current through these LED panels changes sharply with changes in voltage. As such, it’s not practical to regulate the panel brightness by controlling the voltage. We must instead control the current, one of the features of the LED Driver PCB. Limitations It’s evident from the current/voltage curve that applying much more than 13V will put the panel over its nominal 70W limit. So directly connecting a 12V battery, which could supply as much as 14V or higher, is not a feasible way to drive these panels. A 12V battery that’s nearly flat might only produce around 11.5V, so a resistive voltage dropper is not suitable for powering these panels over a battery’s useful charge range. We also expect the current/voltage curve to change depending on the panel temperature. That will change during operation as the panel selfheats due to its own dissipation. Like most LEDs or LED arrays, a current-controlled or current-limited supply is the best choice for driving this one. While the voltage may drift slightly under constant current conditions, it’s a much more stable arrangement. Thus our Driver incorporates current-control circuitry. The LED Driver Given that a common use case would be running these LED panels from a 12V battery or DC supply, we need a few specific features. The LED panel operating point might be above or below the battery voltage, so we need to be able to increase or decrease the incoming supply. And to provide a consistent level of lighting, we also need to regulate the output current. For efficiency, we need to use a switchmode circuit. For this to both increase and decrease the voltage, it needs to be able to either buck (reduce) or boost (increase) the incoming voltage. Some circuits do this by having two separate stages; for example, first by decreasing the input voltage as needed and then using a second stage to boost the output from the first stage. The design of such circuits can be complex; more so when current limiting or regulation is needed. But chips exist that can work in boost or buck mode as needed. That includes the LM5118, a device we used in the Hybrid Bench Supply from April-June 2014 (siliconchip.com.au/ Series/241). siliconchip.com.au The LM5118 handles the transition from boost to buck mode by using a hybrid mode that is somewhere in between at intermediate voltages, ensuring that the output remains stable at all times. It does provide current limiting, but only to protect the inductor that is used to store energy during the boost and buck phases. So we needed to add some parts to the design to provide independent, adjustable output current limiting. Circuit details Fig.2 shows the circuit that we have designed incorporating all these features. Parts of it look similar to the Hybrid Bench Supply because of the common external parts needed for the LM5118 to operate. Power comes in through a two-way barrier terminal, CON1, with the positive supply passing through 10A fuse F1. The 10A limit was chosen as a convenient level above the 7A limit of the LED panel. A bank of paralleled ceramic 10μF capacitors provides bulk supply bypassing to the power section of the circuit, while a 100nF capacitor is placed close to IC1, the LM5118, to stabilise its supply. The VIN supply feeds into pin 1 of IC1 with grounds at pins 6 and 14. An 82kW/10kW divider across this supply to IC1’s pin 2 UVLO (under-voltage lock-out) exceeds its threshold of 1.23V When the panel is off, you can just make out the numerous small LED chips that provide the light output under the phosphor gel coating (although they are a bit hard to see in this photo). when VIN is around 11.3V. This way, if a battery is used to feed the circuit, it will be prevented from discharging below 11.3V, a fairly conservative level for most lead-acid batteries. The 15kW resistor between pin 3 of IC1 and ground sets the boost/buck oscillator frequency to around 400kHz, which gives decent efficiency and low voltage ripple at the output. IC1’s pin 4 (EN) is pulled to ground by a 100kW resistor, but can be pulled up to VIN by shorting the pins of JP1. Thus, JP1 can be closed with a jumper to provide ‘always on’ operation, or connected to an external low-current switch to give a simple on/off control. The capacitors on pins 5 and 7 (RAMP and SS) set the ramp and softstart characteristics of IC1 to be suitable for our application. IC1’s pin 8 FB (feedback) input is used to set the output voltage. The divider formed by potentiometer VR1 and its two series ‘padder’ resistors feeds that pin with a fraction of the output voltage that is compared with a 1.23V reference within IC1. This adjustment gives a nominal output range between 6.8V and 34.7V. The 34.7V upper limit is chosen to stay well clear of the 60V Mosfet Vds limit for Q2 while maintaining a useful range for 24V systems. The 1kW resistor between the divider and the FB pin reduces the interaction between the voltage control and current limiting, which we will explain shortly. The 2.2nF capacitor, 4.7nF capacitor and 10kW resistor between pins 8 and 9 are a compensation network that forms part of the feedback loop that controls IC1’s duty cycle. IC1’s pins 12 and 13 connect across a pair of current measuring shunts to monitor the current through D3 and D4, thus limiting the current through L1 and L2. This works whether the circuit is operating in boost or buck mode. Pins 19 (HO) and 15 (LO) drive the external high-side (Q1) and low-side (Q2) Mosfets, respectively. Pin 16 is connected to an internal regulator that provides around 7V with an external 1μF capacitor to stabilise this. The 7V supply is used to drive the Mosfet gates and is a good compromise between turning them on fully while maintaining fast switching. It also powers shunt monitor IC2 which we’ll get to shortly. Pins 18 (HB) and 20 (HS) are connected to either end of a 100nF capacitor, which is charged and then used to drive the HO pin above the supply voltage. This ‘floating’ gate supply is needed to switch on the high-side N-channel Mosfet as its source terminal can be at or near the supply voltage when it is switched on. Mosfets Q1 & Q2, inductors L1 & L2 and diodes D1-D4 are arranged in a bridge-like configuration that can be driven in either boost or buck switching modes. Fig.3 shows how such a bridge can work in both modes. The circuit works as a buck switcher for low output voltages (compared to the input voltage). When Q1 is on, current flows through L1 and L2 and then D1 and D2 towards the load. When Q1 switches off, the current continues to circulate via D3 and D4. Above 75% duty cycle on Q1, IC1 operates in the hybrid boost-buck mode. Q2 starts to switch on with a duty cycle that overlaps with Q1’s on-time. This increases the current through the inductors during the on-time, and this extra energy gets fed to the output during the Mosfet off-time, increasing the output voltage. A simple implementation of the boost mode would have Q1 on all the time boost mode is active, but this is not possible with the LM5118, so it is switched on and off in synchrony with Q2. This is necessary because the Fig.2: the circuit is based around IC1, an LM5118 buck-boost controller. It drives the H-bridge made from Mosfets Q1 & Q2, diodes D1-D4 and inductors L1 & L2. These allow it to step down the incoming voltage (by pulsing Q1 on) or step it up (by pulsing Q1 & Q2 on simultaneously). Varying the duty cycle/on-time allows it to change the output-to-input voltage ratio. We’ve added IC2 and some other components to provide an adjustable current limit. 42 Silicon Chip Australia's electronics magazine siliconchip.com.au bootstrap capacitor needs to be periodically refreshed to maintain the gate voltage, which can only happen while Q1 is off. All this is done transparently by the controller inside the LM5118. Current limiting The voltage at the cathodes of D1 and D2 is smoothed by a bank of five 10μF capacitors accompanied by a 100nF capacitor. From there, it passes through another 15mW current sensing shunt, then through fuse F2 to output connector CON2. We can keep the grounds common between the input and output by placing the current shunt in series with the positive output. This has several advantages, one of which is that you don’t need to have the ground current pass through this module; it can go straight from the load to the power source, possibly simplifying the wiring and reducing wire-related power loss. The voltage across the shunt is measured by IC2’s pins 1 and 8 and amplified with a gain of 50. IC2 is an INA282 current shunt monitor, and it takes its supply on pin 6 from IC1’s internal 7V regulator. It also has its own 100nF supply bypass capacitor. IC2’s pins 3 and 7 are both connected to ground, so the output voltage from pin 5 is relative to ground. The voltage at pin 5 is divided and smoothed by the network consisting of the 100W resistor, 5kW trimpot VR2, 1kW resistor and 10μF capacitor. The smoothing is necessary to eliminate instability which would cause LED flickering due to oscillations in the output voltage. The resulting voltage is fed into IC1’s FB pin via schottky diode D5. Thus, as the output current increases beyond a certain threshold, the voltage at the FB pin increases similarly to the situation where the output voltage is too high. IC1 attempts to control this by reducing its output voltage, thus reducing the current. The diode ensures that an output current below the limit does not drag down the reference. If the target current is not met, the control loop is based only on the output voltage. The result is not a ‘brick wall’ current limit; it allows higher currents at lower output voltages. This is because a higher voltage is needed at D5 to maintain balance at the FB pin as the siliconchip.com.au Fig.3: an illustration of how the LM5118 works, in buck mode (diagrams at left) and boost mode (at right). The mode of operation is determined by whether S2 (actually a Mosfet) is switched with S1 or just left open (ie, off). In buck mode, as the duty cycle approaches 100%, the output voltage approaches the input voltage while in buck/boost mode, a 50% duty cycle gives an output voltage equal to the input with higher duty cycles boosting the output voltage above the input, approximately doubling it at 75% duty, quadrupling it at 87.5% and so on. The LED Driver is designed to mount directly to the 70W LED panels, with just two flying leads between the two. As it has many other potential uses, you can mount it in just about any kind of box using tapped spacers. Australia's electronics magazine June 2022  43 output voltage drops further below its setpoint. The 1kW resistor between VR1 and the FB pin helps maintain this balance and limit the extent to which the two parts of the circuit interact. With VR2 at its minimum setting, an output current of 1.8A will induce 27mV across the shunt or 1.35V at pin 5, which corresponds to 1.23V at the divider output, meaning that this is the point that current limiting begins. With VR2 set higher, a smaller fraction of the pin 5 voltage is sampled, and thus a higher output current is allowed. In practice, since IC2’s supply is around 7V, the maximum current setting is around 8A. So setting VR2 above around 3/4 of its travel will effectively disable the current limiting. Lower output current settings can be achieved by increasing the shunt resistance, although that would arguably be a poor use of a circuit capable of 8A. That the current limit tapers off is actually an advantage as it tends to put the system closer to constant-power operation. For the LED panels, the operating voltage range will be quite narrow in any case. Pairs of parts You might notice from the schematic that a few parts are duplicated and paralleled. These include L1 & L2, D1-D4 and the 15mW current shunts connected to D3 & D4. The circuit has been designed with these extra parts to handle up to 8A, by splitting the current between the pairs of components and thus moderating the heating of any single part. For operation up to 5A, L2, D2, D4 and one of the shunts can be omitted. The input and output fuses should also be changed to suit 5A operation. All other components can work happily up to the 8A limit. While the shunt resistors do not dissipate any significant amount of power, they are used by IC1 to monitor the current through the inductors. Whether one inductor and one shunt or two inductors and two shunts are present, the current limit through each inductor is the same. Extra parts There are a few component locations that are usually left empty. These are shown in red on the circuit and PCB overlay diagram. We’ve incorporated these in the design as they are Table 1: resistor values for fixed output voltages Target voltage Calculated resistance E24 resistor value Resulting voltage 8V 210W 220W 8.05V 10V 568W 560W 9.95V 12V 926W 910W 11.91V 14V 1284W 1300W 14.09V 15V 1462W 1500W 15.21V 20V 2357W 2400W 20.24V 24V 3072W 3000W 23.59V 28V 3788W 3900W 28.63V 30V 4145W 4300W 30.86V Target current Calculated resistance E24 resistor value Resulting current 2A 119W 120W 1.98A 3A 729W 680W 2.92A 4A 1339W 1300W 3.93A 5A 1949W 2000W 5.08A 6A 2558W 2700W 6.23A 7A 3168W 3000W 6.72A 8A 3778W 3600W 7.71A Silicon Chip Options R13, adjacent to VR2, is a different case. This fixed resistor is intended to replace VR2 for a fixed setpoint. Alternatively, you can replace either VR1 or VR2 with a fixed resistor between their two leftmost terminals, as they are simply wired as variable resistors (rheostats). Table 1 shows typical resistor values for fixed output voltages, including the exact and nearest E24 series values. The values are linear across the range, so you can interpolate them to find intermediate values if necessary. Table 2 does the same for current, with the listed values being at the point that current limiting first kicks in. Similarly, exact and nearby E24 series values are given, and the correlation is relatively linear. Battery charging Table 2: resistor values for fixed output currents 44 shown in the application notes for the LM5118, and are useful in certain situations. We were initially unsure whether these parts were needed for stable operation, but it turned out they were not. Some enthusiastic readers might be tempted to experiment with the design and use these component locations, as shown in the LM5118 data sheet. The optional parts include an RC snubber for the switching node and components to disable IC1’s internal regulator if the input supply voltage will always be within a suitable range (about 5-15V). Since the LM5118 can operate up to 76V (with some parts changes needed in our design to achieve that), this board would have many potential applications. Some configurations may not be as stable as the one presented here, so figuring out what components are needed in different use cases is left as an exercise for the reader. Australia's electronics magazine Although we have not done thorough testing with this configuration, the Driver is well-suited for charging a 12V battery from another 12V battery. This might seem like an unusual requirement, but it often crops up in situations involving a caravan or similar that has a ‘house’ battery, usually a deep-cycle type. Such a battery is typically charged from the 12V system of a towing vehicle while the vehicle is charging its siliconchip.com.au starter battery. Due to voltage drops over long cables and the tendency of modern vehicles not to fully charge their starter battery, there may not be enough volts available to fully charge such a house battery via a direct connection. The Driver can overcome this and comfortably deal with batteries in all charge states due to the current limiting feature. The Driver is set to provide a voltage that suits the desired house battery’s fully charged level, with the current limit set to a safe level for the batteries and wiring. A diode or VSR (voltage sensitive relay) on the Driver’s output may be necessary to prevent the house battery from draining through the Driver’s voltage sense divider. The Driver should be located close to the house battery so that cable resistance does not affect sensing the house battery voltage. Construction The LED Driver is built on a double-­ sided PCB coded 16103221 that measures 85mm x 80mm. Fig.4 shows where all the parts go on the board. This design uses almost exclusively surface-mounted parts of varying sizes, so you will need the usual set of surface mount gear. A temperature-adjustable iron will help greatly in dealing with the wide range of part sizes that are used. Several of the components connect to solid copper pours (for current and thermal handling) and will likely require the iron to be turned up to a higher temperature to make the joints. Tweezers, flux, solder wicking braid, magnifying lenses and fume extraction are all important requirements for assembly. Also, since you’ll need to keep the iron’s tip clean, have a tip cleaner on hand. Begin construction with the two ICs. IC1 has the finest-pitch leads, so start with it. Apply flux to its pads, then align the part with the pin 1 marker and tack one lead in place. Use a magnifier to confirm that the part is aligned with the pads and flat against the PCB, then tack the diagonally opposite lead and re-check its position. Solder the remaining leads one at a time, or by gently dragging the iron tip loaded with solder along the edges of the pins. These techniques depend on loading a small amount of solder onto siliconchip.com.au Fig.4: most of the components on the board are SMDs, but only IC1 has closely-spaced leads. Having said that, some of the other components can be somewhat challenging simply due to the combined thermal mass of those parts and the PCB copper. Most components are not polarised or only fit one way; it’s mainly the ICs and trimpots that you have to be careful orientating. Parts List – Buck-Boost LED Driver 1 double-sided PCB coded 16103221, 85mm x 80mm 2 2-way 10A barrier terminals, (CON1, CON2) [Altronics P2101] 1 2-way pin header, 2.54mm pitch, with jumper shunt (JP1) 2 10A 10μH SMD inductors, 14 x 14mm (L1, L2) [SCIHP1367-100M] 4 M205 fuse clips (F1, F2) 2 10A M205 fast-blow fuses (F1, F2) 6 M3 x 10mm tapped spacers (to mount to LED panel) 10 M3 x 6mm panhead machine screws (to mount to LED panel) 2 5kW 25-turn vertical top-adjust trimpots (VR1, VR2) [Jaycar RT4648 or Altronics R2380A] Semiconductors 1 LM5118MH buck-boost regulator, SSOP-20 (IC1) 1 INA282AIDR current shunt monitor, SOIC-8 (IC2) 4 SBRT15U50SP5 schottky diodes, POWERDI5 package (D1-D4) 2 PSMN4R0-60YS or BUK9Y4R8-60E N-channel Mosfets, LFPAK56/SOT669 (Q1, Q2) 1 BAT54, BAT54S or BAT54C schottky diode, SOT-23 (D5) Capacitors (SMD M3216/1206-size SMD X7R ceramics, 35V or higher rating) 16 10μF 1 1μF 6 100nF 1 4.7nF 1 2.2nF 1 330pF Resistors (all SMD M3216/1206-size 1/8W 1% except as noted) 1 100kW 1 82kW 1 15kW 2 10kW 3 1kW 1 220W 1 100W 3 15mW 3W M6332/2512 A complete kit (Cat SC6292; siliconchip.com.au/Shop/20/6292 siliconchip.com.au/Shop/20/6292) is available for $80. It includes everything in the parts list above. We can supply the LED panels, cool white (~6000K, SC6307) or warm white (~3000K, SC6308) for $19.50 each. Australia's electronics magazine June 2022  45 the iron’s tip. Practice is the only way to get this right. Once finished, carefully inspect the leads for solder bridges. If you see any, add some extra flux paste and then use solder wick to gently remove the excess solder. Finally, clean away the flux residue with a flux cleaner (or pure alcohol if you don’t have one) and a lint-free cloth, then check again with a magnifier to ensure all the pins are correctly soldered, and no bridges are left. Use a similar technique to fit IC2 to the board. Then mount the smaller passive SMDs (except for the shunt resistors) using a similar approach; their larger pads are a bit more forgiving. Remember that some of these parts are not needed (they’re labelled in red in Fig.4). The main trick here is to avoid touching the iron to one side of the part until you are sure the solder on the other side has solidified, or it might shift out of place. The SMD capacitors are unmarked, so be careful not to mix them up. It’s best to unpack and fit all the capacitors of one value at a time. As some of the capacitors (particularly the 10μF parts) are across ground planes, you might need to turn your iron up to make good joints. Ensure the solder flows both onto the end of the part and onto the PCB pad below. The solitary SOT-23 part, D5, is a BAT54 schottky diode. With one lead on one side and two on the other, its orientation should be obvious. Just make sure its leads are flat on the board, not sticking up in the air, which would indicate that it’s upside-down. Note that you can substitute a dual BAT54S (series) or BAT54C (common cathode) diode as one of their two internal diodes connects between the same set of pads. The other diode in the package will be unconnected and unused. The remaining surface-mounting parts are larger, so you might like to raise your iron temperature before proceeding. Also, they are mostly arranged around the top half of the PCB. Solder the three larger 15mW shunt resistors, then the four power diodes. The diodes have two small leads on one end and a larger one on the other. In each case, the ends with two small leads go towards CON1 while the 46 Silicon Chip larger single lead is towards CON2. The pad arrangement on the PCB should make this clear. Solder these like the passives, but take extra care that the part is aligned correctly so that the large tab that runs under the part does not short onto the smaller pads. While the packages used for Mosfets Q1 and Q2 may look unusual, they are actually much the same as an 8-pin SOIC package IC, but with the leads along one side joined into one larger tab. This improves heat removal, lowers resistance and also makes correctly orientating them easier. Take care that the leads are aligned within their pads. The only real difference in soldering these compared to SOIC-8 parts is due to the greater thermal mass of the large metal tab and the copper areas on the PCB. Moving on to inductors L1 and L2, the thermal effect will be even more apparent here. They are not polarised, but you will need a good amount of heat to complete the soldering. It’s best to lay down some flux paste on one pad, add some solder to the other pad, slide and/or press down the part into place while heating that solder, then add solder to the opposite pad. Finally, refresh the first pad you soldered. Check that the solder fillets are joined to both the inductor and PCB pads before proceeding further. Now clean the PCB of excess flux and thoroughly inspect all the parts for bridges and dry joints; they will be easier to see and fix after cleaning. There are only a handful of throughhole parts remaining. You can mount fuse holders F1 and F2 by installing a fuse and slotting the whole assembly into the PCB. This ensures that the tabs are aligned correctly and spaced far enough apart to allow a fuse to be fitted. Like many of the parts, they may need more heat to let the solder take to the large copper areas. Next, mount the terminals for CON1 and CON2, ensuring that any connected wires can exit from the board (most barrier terminals allow wires to be inserted from either side, but there are exceptions). JP1 and its jumper can then be installed near CON1. Leave the jumper in place for testing. Finally, fit the two multi-turn trimpots, VR1 and VR2, near F2. Make sure their screws are to the left, as shown in the overlay and photos; if Australia's electronics magazine they are reversed, they will not operate correctly. Ensure that they are both wound to their minimums by turning their adjustment screws anti-clockwise by 25 turns or until you hear a clicking indicating that they have reached the end of their travel. There are seven test points on the board, but you do not need to fit PC pins; you can simply probe them with a standard set of DMM test leads. Testing You will need fuses installed for testing, but since initial testing is done with a multimeter, you can fit lower-­ rated (eg, 1A) fuses if you have them on hand. If you have a current-limited PSU, you can use that too. Connect a voltmeter across CON2 and apply a power source of around 12V DC (above 11.5V) to CON1. You should see about 6.6-7.0V at CON2. If you get a reading near the supply voltage instead, you could have a short circuit somewhere. In that case, switch off and check the PCB for faults before proceeding. Slowly turn VR1’s screw clockwise. After the trimpot’s mechanism re-­ engages, you should see the voltage on CON2 increase, rising to nearly 35V at its maximum setting. If so, wind it back down around 11V. If you can’t adjust the output voltage correctly, switch off and check for faults. If you have used low-value fuses, change these now to your nominal value; for the LED panels we described earlier, 10A each is a good choice. You can also test that the current limiting works if you have a suitable load such as a power resistor or test load (like the one described starting on page 48 of this issue). The minimum current limit when VR2 is set fully anti-clockwise is around 1.8A. You can easily monitor the output current at TP5 (near IC2) relative to TP3 (ground, at top left). This is the raw output from IC2, and it gives 0.75V per amp. So 1.5V at TP5 corresponds to 2A. Also, you can monitor the output voltage at TP6 (near CON2) relative to ground. Adjust your load until the current limiting kicks in. Reducing the load resistance should let the output voltage drop while the current stays mostly constant. siliconchip.com.au LED panel mounting The Driver is designed to mount on the back of the LED panel using the mounting holes near the power terminals, so you can use short flying leads to connect from CON2 to the panel’s inputs. While your iron is on, you can connect some leads to the LED panels. As you will know by now, soldering inductors L1 & L2 to the PCB requires much heat, but nowhere near as much as is needed for soldering to the aluminium-­cored PCB that forms the LED panel. You might even find that you need to preheat the panels with a hot air rework tool or similar before you can successfully solder those leads. We also suggest that you pre-tin the leads and have a generous amount of solder on the iron’s tip (to accumulate some thermal mass). To set up the Driver to work with LED panels, disconnect all loads, set the output voltage to around 13V and adjust the current limit fully anti-clockwise to 2A. The 13V setting is simply a failsafe in case the current limiting stops working. Keep in mind that the LED panels are very bright; even at 2A, it will likely be too bright to look at! We rested them on their edge during testing to aim them away from our faces. If you then connect the LED panel and power up the Driver, you should see the output voltage drop to approximately 12V as the Driver switches to its current-limited mode. If you don’t see the voltage drop, the current limiting may not be working. In that case, measure the voltage at TP7 neat VR1. This feedback voltage should always be around 1.23V when the Driver is operating correctly. Check that there is a slightly higher voltage at D5’s top right (anode) terminal; this means that the diode is feeding current into TP7 and controlling the output. If this is more than around 0.3V higher, D5 may be the wrong type or not injecting current correctly. If all is well, you can then permanently wire up CON2 to the LED panel and mount the Driver using tapped spacers. Use four tapped spacers with a screw at each end to mount the Driver PCB to the LED panel at its mounting holes. Then use two further tapped spacers mounted to the PCB only as standoffs siliconchip.com.au These panels are incredibly bright, and the two photographs above do not do them justice. They are too bright to look at directly when set to anything but the lowest setting (at left). to keep the PCB from moving, flexing and shorting against the aluminium back of the LED panel. See our photos for details of this arrangement. Adjust VR2 to provide a suitable current and thus brightness. If you get much above 5A, you might find that the current limiting no longer dominates, and the VR1 voltage setting may need to be increased above 13V. Keep in mind that both the Driver and LED panel will get quite warm during use, so they should be mounted to allow free air circulation. Suppose you see the LED panel rapidly flickering during operation. In that case, the supply voltage is probably dropping below the UVLO threshold, causing the Driver to cut out and then switch back on when the input voltage recovers. Check your supply and that the connections to CON1 do not have too much resistance. Driving two panels We briefly experimented with running two panels in series, as this is the Australia's electronics magazine easiest way to guarantee they operate at the same current. The main difference is that the voltage needs to be set to around 26V. This certainly seems to work fine, but the Driver is likely to be less efficient in this mode unless the input voltage is raised to about 24V. You can change the UVLO threshold to suit a 24V battery by changing the 82kW resistor to 160kW, and 10kW resistor to 9.1kW. This will set the threshold to approximately 22.8V. As noted in the Features panel, you can also use the Driver as a DC-­ powered battery charger, a 24V to 12V converter, or a 12V to 24V converter for many different purposes. For the 24V to 12V arrangement, the output limit can be set up to 8A, with a 10A fuse at F2, but with F1 reduced to 5A. In this case, you would also change the 82kW resistor to 180kW. For a 12V to 24V arrangement, F1 should be 10A and F2 should be 5A, with an appropriate current limit near 5A set using VR2. SC June 2022  47