High-Power
Buck-Boost
LED Driver
There are some very bright, low-cost LED
panels for sale, and we’ve been figuring out the
best way to drive them. This Driver is the result;
it is 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
F
or around a fiver, you can
buy impressive LED panels from
AliExpress (eg, www.aliexpress.
com/item/4001275542304.html). They
measure about 22cm by 11cm with an
Background Source: https://
unsplash.com/photos/k4KZVfAXvSg
active area of 20cm by 10cm. They’re
also available from other online sellers such as eBay or Banggood.
The panels are based on an aluminium PCB and have a silicone gel
Features and Specifications
∎ 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
32
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 2020) 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. 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 watch 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 incandescent globes, 70W of LED light is
equivalent to several hundred watts
of incandescent light; easily enough to
illuminate a large room very brightly.
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.
Practical Electronics | June | 2023
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
switch-mode 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. However, chips exist
that can work in boost or buck mode
as needed; for example, the LM5118.
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 twoway 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
Practical Electronics | June | 2023
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.
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 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
These panels are incredibly bright,
too bright to look at directly when
set to anything but the lowest setting.
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.
33
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 and Q2, inductors L1
and 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 offtime, 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 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
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.
Buck-boost LED Supply
Fig.2: the circuit is based around IC1, an LM5118 buck-boost controller. It drives the H-bridge made from MOSFETs Q1
and Q2, diodes D1-D4 and inductors L1 and L2. These allow it to step down the incoming voltage (by pulsing Q1 on) or
step it up (by pulsing Q1 and Q2 on simultaneously). Varying the duty cycle/on-time allows it to change the output-toinput voltage ratio. We’ve added IC2 and some other components to provide an adjustable current limit.
34
Practical Electronics | June | 2023
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 and L2,
D1-D4 and the 15mW current shunts
connected to D3 and 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 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
Practical Electronics | June | 2023
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.
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
The LED Driver is designed to mount directly to the 70W
use cases is left as an LED panels, with just two flying leads between the two.
exercise for the reader. As it has many other potential uses, you can mount it in
just about any kind of box using tapped spacers.
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
35
point that current limiting first kicks
in. Similarly, exact and nearby E24
series values are given, and the correlation is relatively linear.
Battery charging
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
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. Note
that 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 available
from the PE PCB Service. It 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.
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
Table 2: resistor values for fixed output currents
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
36
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
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 increase your iron’s 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.
Practical Electronics | June | 2023
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
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 orienting 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
they are reversed, they will not operate correctly.
Practical Electronics | June | 2023
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
orienting.
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
Reproduced by arrangement with
6 100nF
SILICON CHIP magazine 2023.
1 4.7nF
www.siliconchip.com.au
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
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
37
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 16 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.
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 and 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
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38
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 over 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 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 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 applications.
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.
Practical Electronics | June | 2023
