This is only a preview of the November 2024 issue of Practical Electronics. You can view 0 of the 80 pages in the full issue. Articles in this series:
Articles in this series:
Articles in this series:
Articles in this series:
Items relevant to "Multi-Channel Volume Control, part one":
Articles in this series:
Articles in this series:
Articles in this series:
Articles in this series:
|
Constructional Project
Ideal Diode
Bridge Rectifiers
By Phil Prosser
Rectifiers have evolved a lot over the last century, from selenium piles and
mercury arc rectifiers to vacuum tube diodes, then germanium and silicon diodes. Now, active
rectifiers offer much greater efficiency than silicon diodes, running much cooler. We show you
how to make up to six different Bridge Rectifiers depending on how much power you want.
n the simplest terms, an ‘ideal diode’ Power Supply (to be published in
By comparison, if we use an LT4320
Icircuit
uses a power Mosfet with a control an upcoming issue), I wished I had
to replace a rectifier diode. the time to delve into these active ‘ideal bridge’ controller and TK6R-
Pros and cons
Combining four such devices gives
you an ‘ideal bridge rectifier’.
While they are not truly ideal,
they are much closer than a regular
diode, with a forward voltage (and
thus power loss and heat dissipation) typically around 1 /10 that of a
normal diode.
This idea caught my attention because I realised it would allow us to
build devices like power amplifiers
or power supplies that operate more
efficiently and deliver more power,
as less is lost in the bridge.
Bridge rectifiers used in large power
amplifiers need a lot of heatsinking!
They can dissipate tens of watts under
heavy load.
That all changes with this design,
which is a drop-in replacement for
many existing bridge rectifiers.
When designing my Dual Hybrid
bridges, as the power loss in a highcurrent DC power supply bridge is
also significant. For example:
● The PB1004 10A bridge rectifier
has a forward voltage drop of over 1V
at 5A, or 2V across the bridge. This
means it is dissipating 10W at 5A.
● The KBPC3510 35A bridge rectifier drops 1V at 10A, resulting in a
2V loss and 20W dissipation at 10A.
The 2V drop is manageable, if annoying, by increasing the transformer voltage. However, transformers
often come in 5V steps, meaning you
might be wasting a lot of power to
compensate for that relatively small
voltage loss.
On the other hand, that 10-20W
dissipation is troublesome, as it demands a substantial heatsink and
forces physical layout decisions to
enable this heat to be dissipated.
Lessons learned during the design process
The design of these modules served as a reminder on the need for attention to
detail and the value of peer review. I did the bulk of the PCB layout while I was on
holidays, and since there were only seven parts, what could go wrong? Plenty.
When I was making the CAD library for the LT4320 IC, I stuck the ‘pin’ that
denotes the thermal pad for the IC in the wrong spot. This led me to assume it
connected to the positive pin rather than the negative, where it belonged. I then
laid out seven variants of this board from the schematic, all with the pad connected to the wrong output.
I now know that the LT4320 will work for several minutes with the thermal pad
tied to the wrong pin, but after that, it will blow up, take out your Mosfets and
short your transformer! I found the bug after blowing many fuses, £50 worth of
bits, wasting a couple of days, and my whole budget of four-letter words.
To add insult to injury, I had to respin all the different prototype boards, another
£50 lesson. Ouch! All for about 2mm of misplaced PCB trace.
8
9P08QM power Mosfets, we will see
70mV maximum drop per device at
10A, which is a total of 1.4W or about
1/10th of the heat you get from a standard bridge rectifier!
So what is the catch, and why aren’t
these used everywhere? I suspect
there are a few reasons:
1. One of the complications that
needs to be dealt with is generating the Vgs drive for the N-channel
Mosfet, which requires a boost circuit to drive the gates well above the
source voltages.
2. For a bridge, you need four power
Mosfets and a controller, which increases parts count and cost.
3. The real benefits are accrued
when rectifying lower voltages at
high currents or if you cannot afford
losses in your system (or when high
efficiency is essential).
4. Because of how the control and
switching works, for the simplest off
the shelf solution, a dual-rail power
supply (such as for a power amplifier) needs two bridges, each fed by
one of the two secondary windings.
5. Your rectified output voltage rail
needs to stay above 9V, or bad things
happen (more on that later).
The best use cases for an ideal
diode bridge rectifier are where space
and capacity to dissipate power are
limited, where voltage drop from
the transformer is undesirable and
where lower voltages at higher currents need to be rectified.
In terms of using Mosfets to replace
Practical Electronics | November | 2024
Ideal Bridge Rectifiers
One of our Ideal Bridge Rectifiers
on a Dual Hybrid Power Supply board.
This increases the maximum output voltage by about
2V at full load while increasing efficiency and allowing it to run
much cooler under load!
diodes, it is interesting to note the
growing use of ‘synchronous’ switchmode converters.
In synchronous converters, the
usual schottky diodes are replaced
with power Mosfets. Many synchronous switch-mode controllers include
an output to drive the diode replacement Mosfets, resulting in increased
efficiency.
Design approach
Given the desire to investigate this
technology, our efforts turned to an
integrated solution. We wanted an
option that could be used in a range
of projects and showcase the potential
of this technology, without making
construction too tricky or the device
too expensive.
A survey of ideal diode controller
ICs shows that many are intended
for hot-swap and redundant power
supply applications. In this case,
multiple power supplies are combined in an ‘OR’ function so that if
one supply fails, the other picks up
the load. Supply currents can be very
high in a server application, so reducing diode losses is critical.
We also found several controllers
for automotive applications, in alternators and circuit protection. These
are generally intended for singlerail applications and are not suited
to more general AC rectification. In
particular, most utilise the diode to
operate the circuit itself. This limits
their application as generic diode replacements.
The range of available parts in this
Practical Electronics | November | 2024
field is growing, so new ICs that are
useful in a range of applications are
coming on the market. In this project,
we show how to use the most available controller IC and build a range of
‘ideal diode bridge rectifiers’ that can
replace conventional diode bridges
in various projects.
The controller we have selected is
the LT4320, as this allows simple and
compact boards to be built, ranging
from tiny SOT-23 Mosfet based bridges through DPAK (TO-252) to very
high current TO-220 based throughhole versions. Where might each of
these be used?
● The SOT-23 Mosfet based bridge
PCB is only 9 × 15mm and can be
used inline on the DC power supply
lead to a device or soldered in place
of a small bridge. This can make the
power lead for your device polarity
agnostic without affecting its operation noticeably.
● Our boards using DPAK SMD
Mosfets can replace the common 5mm
pitch 19mm SIL bridge or rectangular bridges with corner pins or spade
connectors (see the photo above) and
handle high currents.
● There are also two ‘standalone’
versions that are basically just small
boards you can mount in a chassis
to provide the rectification function.
One uses TO-220 Mosfets and other
through-hole parts and can handle
very high currents, limited mainly
by the PCB itself!
There are a few limitations or requirements we need to work with
that initially may sound onerous.
However, in a real-world application, the following are not that hard
to meet:
● The LT4320 works in a ‘single-
rail’ configuration only.
● For an audio amplifier, you need
to rectify the outputs of the two secondary windings independently. You
then connect the negative output from
one bridge to the positive output
from the second bridge to get your
split supply, usually at the main capacitor bank.
● We have achieved pin compatibility for all the larger bridge types.
But DIP-8 and W02/W04 type bridges are a bit small for us to match, so
if replacing one of those, you will
need to mount the SOT-23 version
on leads.
● The minimum output voltage
allowed is 9V DC, while the maximum is 72V peak. This means that
we should limit the AC input to 40V
RMS to provide reasonable safety margins. We must ensure that the rectified output’s minimum voltage does
not drop below 9V during operation.
How it works
Its operation is similar to a diode
bridge but with a controller IC that
turns the Mosfets on when required
to minimise losses. Fig.1 is the circuit diagram while Fig.2 shows how
current flows during the two main
phases when the bridge is conducting.
The Mosfets are arranged so the current flows from their source to drain
terminals in regular operation, the opposite to a standard common-source
Mosfet switch application. This is so
that the current flows through the
Mosfet body diodes in the forward
direction.
Therefore, in the absence of the controller, current would flow through
those body diodes. However, there
would be a high typical 1V forward
drop at high currents, similar to a
silicon power diode.
Silicon Chip Ideal Rectifier Kits
SC6850 (~£18) 28mm spade version
SC6851 (~£18) 21mm square PCB
pin version
SC6852 (~£18) 5mm pitch SIL version
SC6853 (~£15) mini SOT-23 version
SC6854 (~£22) standalone D2PAK
SMD version
SC6855 (~£28) standalone TO-220
through-hole version
9
Constructional Project
During operation, the LT4320 determines which of the input voltages
(IN1 & IN2) is lower and switches on
either Q3 or Q4 full to connect the
input terminal with the lower voltage to the negative rail and hence the
negative output.
The controller switches Mosfet Q1
or Q2 on when current flows through
them, reducing the effective forward
voltage to about 20mV. The drop is set
by the controller; if the LT4320 detects a differential greater than 20mV
between the highest AC input voltage
and the output terminal, it switches
the respective Mosfet on harder.
If the Mosfets have a relatively
high Rds(on) figure resulting in more
than 20mV across the Mosfet, it will
be switched on fully, and the input/
Fig.1: the circuit is slightly more complex than a conventional bridge
rectifier. Pin numbers in black are for the MSOP-12 package while those
in brackets in cyan are for DIP-8. Dashes in parentheses indicate pins that
don’t exist on the DIP-8 package.
output differential will be higher
than 20mV.
The gate drive to the Mosfets is not
very ‘strong’ in that a fairly low current is supplied. This reflects the application for this IC in low-frequency
(50/60Hz mains) or for the LT4320-1
(to 600Hz) operation.
With a 9V DC output voltage and
the top Mosfet (Q1 or Q2) Vgs at 2V,
the pullup current is only 500μA. Our
recommended DPAK SMD Mosfet, the
TK6R9P08QM, has an input capacitance of 2.7nF. So the gate voltage
will change at a rate of 180mV/μs.
That is terribly slow compared to
most Mosfet applications, but for
mains-frequency operations, if each
Mosfet is on for 10% of the cycle,
that’s 2ms. The switch-on time of
20μs or so is only 1% of that period.
The losses are minimal because this
switching is just as the mains cycle
crosses over.
The 1μF ceramic capacitor across
the OUTP and OUTN pins is important for the correct circuit operation
as it prevents the output voltage from
changing too rapidly. It should be kept
as close to the LT4320 as possible.
The Ideal Bridge Rectifier can operate from 9-72V. If the rectified output
goes below 9V, the LT4320 will not
drive the Mosfet gates, and rectification falls back to the body diodes in
the Mosfets. This is OK at startup, but
we must ensure the rectified rail remains above 9V afterwards. We will
come back to this later on.
During tests where we were hammering the bridge and applied a load
so severe that the output voltage
dropped below 9V, we found that
the Mosfets were getting hotter than
we expected. However, that’s a fairly
unusual situation for a real bridge
rectifier.
Parts selection
Fig.2: during part of the mains waveform, when the upper AC input voltage
is higher than the lower, IC1 switches on Q1 & Q4 and current flows via the
red paths. During the opposite part of the waveform, the upper AC input
voltage is lower, Q2 & Q3 are on and current flows via the blue paths.
10
There were a few things to keep in
mind when choosing the Mosfets for
this design. We have tested the devices specified in the parts list and
in the panel titled “Ideal Bridge Recitfier PCBs”, although there is no
doubt that many others would work.
Besides being in the correct package
for the board, they need sufficiently
high voltage and current ratings, low
on-resistances (for highest efficiency)
and a gate-source threshold voltage
in the correct range.
For the latter, the recommendation
Practical Electronics | November | 2024
Ideal Bridge Rectifiers
is that it should be more than 2V.
This is required to ensure that the
controller can switch the Mosfet off
quickly, to keep efficiency high.
Many modern Mosfets have a low
gate threshold to allow them to be
controlled by lower voltage circuits
(often called ‘logic-level’ Mosfets),
making them unsuitable. These can
sometimes be spotted as they tend
to have a lower maximum Vgs rating,
below the ±20V to ±30V that used to
be typical. However, there are still
logic-level Mosfets with a higher
Vgs rating, so you need to check the
data sheet.
As for the current rating, in a bridge
rectifier, the current usually only
flows while the reservoir capacitors
are charging. With very large capacitor banks and a low internal impedance transformer, this can be pretty
short, resulting in peak charging currents much greater than the average
(or “DC”) current being drawn from
the power supply.
The recommendation is that the
Mosfets have a DC rating triple the
average direct current. Therefore, we
have selected Mosfets with higher
current ratings than you might expect
are necessary.
However, we tried not to go overboard with this as ultra-high-current
Mosfets tend to have a high gate capacitance. The LT4320 does not have
a strong gate drive capability, so that
would slow switch-on and switch-off,
resulting in increased losses.
The Vds(MAX) rating should be well
above the voltage at which you want
to operate the bridge, with a solid
margin to allow for ringing and spikes.
We looked for a minimum rating of
80V, although our SOT-23 version is
limited to 40V.
Mosfet heating is primarily determined by the average current and their
Rds(on). For the TK6R9P08QM DPAK
Mosfet we use in many module versions, the typical Rds(on) is specified
as 5.5mW for Vgs > 10V.
The LT4320 delivers about 11V
to the gates for voltages greater than
13V. For an average current of 10A,
this results in 550mW dissipation in
each conducting Mosfet, or 275mW
per Mosfet for an AC input, which
is easily manageable, and the boards
only get warm.
For a current of 20A, this dissipation increases to about 1W per Mosfet,
making them very warm indeed, at
Practical Electronics | November | 2024
Ideal Bridge Rectifier PCBs
For maximum flexibility, we have produced six different PCBs that implement
essentially the same circuit, as follows:
#1 Square 28mm metal bridge using 6.3mm spade connectors
Compatible with KBPC3504
PCB code: 18101241 (28 × 28mm with a central mounting hole)
Current & voltage handling: 10A continuous (20A peak), 72V
Connectors: 6.3mm spade lugs, 18mm tall
IC1 package: MSOP-12 (SMD)
Mosfets: TK6R9P08QM,RQ (DPAK/TO-252 SMD)
Operates at 10A continuously and much higher currents intermittently but will get hot. In
a long-term 8A test, it reached 79°C in free air.
#2 Square 21mm plastic bridge with 13mm pitch pins
Compatible with PB1004
PCB code: 18101242 (22 × 22mm with a central mounting hole)
Current & voltage handling: 10A continuous (20A peak), 72V
Connectors: solder pins on a 14mm grid (can be bent to a
13mm grid)
IC1 package: MSOP-12 (SMD)
Mosfets: TK6R9P08QM,RQ (DPAK/TO-252 SMD)
A PB1004 leaded bridge replacement, typically capable of 5-10A. We used these to
upgrade our Dual Hybrid Power Supply module.
#3 5mm pitch SIL
Compatible with KBL604
PCB code: 18101243 (23 × 20mm)
Current & voltage handling: 10A continuous (20A peak), 72V
Connectors: solder pins at 5mm pitch
IC1 package: MSOP-12 (SMD)
Mosfets: TK6R9P08QM,RQ (DPAK/TO-252 SMD)
The 5mm pitch SIL bridge rectifier drop-in replacement module.
#4 Tiny inline bridge
Width of W02/W04
PCB code: 18101244 (9 × 15mm)
Current & voltage handling: 2A continuous, 40V
Connectors: solder pins 5mm apart at either end
IC1 package: MSOP-12 (SMD)
Mosfets: SI2318DS-GE3 (SOT-23 SMD)
The baby of the crew, the SOT-23 based version optimised for putting inline with lower-power
circuits. These Mosfets are rated at 40V & 3.9A, but we reckon a safer limit would be 1.5-2.0A.
#5 Standalone SMD version
PCB code: 18101245 (59 × 36mm with mounting holes in 49 ×
26mm rectangle)
Current & voltage handling: 20A continuous, 72V
Connectors: 5mm screw terminals at either end
IC1 package: MSOP-12 (SMD)
Mosfets: IPB057N06NATMA1 (D2PAK/TO-263 SMD)
The D2PAK version, which I have tested for half an hour at 12V AC and 8A (into a 35mF
capacitor with a 2Ω load across it). You can see this being stress tested on page 14.
#6 Standalone through-hole version
PCB code: 18101246 (38 × 28mm with 70μm-thick
copper and mounting holes 29mm apart)
Current & voltage handling: 40A continuous, 72V
Connectors: 6.3mm spade lugs, 18mm tall
IC1 package: DIP-8 (through-hole)
Mosfets: TK5R3E08QM,S1X (TO-220 through-hole)
The TO-220 version is a bit of a beast and, along with the D2PAK version shown
above, it will easily handle 8-10A RMS continuously. It uses a DIP-8 controller IC and
allows you to mount a heatsink to the Mosfets if you want to rectify some serious
currents. All the images here are not shown to scale.
11
Constructional Project
Fig.3: the cyan
trace is the positive
portion of the
incoming AC
waveform, yellow
is the filtered DC
output, while
mauve is the
positive Mosfet gate
drive. The cyan AC
trace is offset by
-2V; otherwise, the
mauve trace would
obscure it much of
the time.
Fig.4: a similar
setup to Fig.2, but
this time, we’re
monitoring the
gate of one of the
low-side Mosfets
(mauve). You
can see how it’s
switched on with a
duty cycle close to
50%, synchronised
with the zero
crossings of the AC
waveform.
which point you should consider
building the TO-220 version.
The recommended TO-220 Mosfet
has an Rds(on) of 4.2mW at full drive
and, at 40A, will drop 160mV; it
would be closer to 1.2V in a regular
bridge at this sort of current. The
power dissipation in each Mosfet
would be 3.5W for an AC input, which
is significant but manageable with
small heatsinks. In this case, a regular
diode-based bridge would get toasty,
as it would dissipate 48W per diode!
The LT4320 IC comes in an SMD
(MSOP-12) and through-hole (DIP-8)
version. These are available from all
the major component suppliers and
is included in the Silicon Chip kits.
For the Mosfets, we have tried to stick
to standard parts, with DPAK (TO-252)
being our overall preference as they are
large enough to handle a decent amount
of dissipation (~1W) without being so
large that they take up a lot of space.
The other Mosfets we’ve used come in
TO-220 packages (for really high current applications) and the tiny SOT-23
(for when space is tight).
By sticking to these standard footprints, you can use alternative parts
if necessary.
PCB design
Most of the modules we present use
surface-mounting parts to fit into the
space we have. We have also resorted
to placing components on both sides
of the PCB, as doing that was essential
to match some of the common bridge
rectifier form factors.
For higher-current modules, we
need to be conscious of the current rating of the PCB traces. To fit
the parts onto the KBPC3504 formfactor board, along with the very wide
tracks that a 30-40A rating warrants,
is quite a challenge. Our version manages to keep all high-current tracks
short and thick, but that forced the
layout to be slightly larger than the
original rectifier.
There is no specific ‘rating’ for PCB
traces; there are guidelines, but too
many variables exist to realistically
put a simple, accurate number to a
track width. Still, the voltage drop and
resulting heating of the track must be
considered. In the limiting case, tracks
can fuse or melt.
We have specified ‘2oz’ (70μm thick)
copper traces on the TO-220 PCB, twice
as thick as a standard ‘1oz’ (35μm)
PCB. This will halve resistive losses
in the PCB at the price of it being a
lot harder to solder due to the thick
copper acting as a heatsink (although
that will have benefits during operation, drawing heat away from components faster).
It is evident that at high currents,
even an ‘ideal diode’ warrants careful
attention to power ratings, losses and
dissipation. But these are reduced to
a level where a practical solution can
be developed. We recommend that you
pay careful attention to losses and heat
if you use this at really high currents.
At least verify that the chosen module
doesn’t get overly hot at your expected maximum current draw.
Waveforms & verification
Figs.3 & 4 show the input, output
and gate drive waveforms for the Ideal
Bridge Rectifier operating at 4A RMS.
Note that the AC input is offset -2V
Figs.6-11: use these overlay diagrams to guide the component placement on each version. The four smaller PCBs have
components on both sides. Generally, it’s best to fit all the SMDs on one side, then all the SMDs on the other, then any
remaining through-hole parts. Note that while we’ve specified non-polarised ceramic 10μF capacitors for the first four
variants, tantalums are shown in case you want to use them, in which case they must be orientated as shown.
12
Practical Electronics | November | 2024
Ideal Bridge Rectifiers
to allow a clearer view – there is so
little voltage drop across the Mosfet
that the output visually ‘tracks’ the
input AC much of the time. The gate
drive is over 10V, so the Mosfet is
switched fully on.
To illustrate the low voltage drop
across the power Mosfet even at 4A,
Fig.5 shows the input and output
waveforms with no offset.
Having built the modules, we decided to run some extreme tests as
we didn’t want our readers to make
them only to have them blow up!
We loaded the 28mm bridge design
(KBPC3504 compatible) to draw 5A
RMS from a toroidal transformer and
left it running for several hours. The
Ideal Rectifier stabilised at 42°C.
Ramping the current to 8A led to it
reaching 72°C, which is not unreasonable for the current.
Swapping in a regular KBPC3504
at 4A without heatsinking, it reached
79°C after a few minutes.
As shown earlier, we ‘upgraded’
our Dual Hybrid Power Supply with
Ideal Rectifiers, which saves 10W of
heat per board at full output or 20W
in total. For this, we used the PB1004
format modules and soldered them
on leads directly to the PCB, as at
5A, they do not get hot enough to
demand a heatsink.
During testing, we had a test setup
with a 12V AC output transformer, an Ideal Bridge Rectifier and a
22mF capacitor. Things were going
great until we reduced the load resistance to somewhere near 1W, and
the output voltage dropped below
9V due to the capacitor discharging
between cycles.
The LT4320 stopped driving the
Mosfets, and instead of there being
20mV across them, there was suddenly about 1V across the body diodes
at about 15A. The smoke quickly
Practical Electronics | November | 2024
Fig.5: the same
waveforms as in
Fig.2 but without
the -2V offset on
the AC input.
The IPP083N10N
Mosfets on this
board stabilised
at 38°C in the lab.
The dummy load,
on the other hand,
measured 130°C.
escaped from the DPAK Mosfets.
We recommend that you avoid that
situation.
Construction
With so few parts on the board,
construction is straightforward. Refer
to the PCB overlay diagram(s) for
whichever version(s) you are building, shown in Figs.6-11.
The principal challenge is that for
all but the TO-220 version, we’re using
the LT4320 IC in an MSOP-12 package with a thermal pad on its base.
This thermal pad makes this part a
tad harder to solder than your average
SOIC/SOP SMD part. There are two
(or three) practical soldering options:
1. Using a reflow oven. If you already own one of these, chances are
you are all over how to mount the
part. Each oven has its own quirks,
so we will leave this to you.
2. Use a toaster oven as a ‘bodge’.
You can read articles on turning a
toaster oven into a reflow oven (April
& May 2021), but there is also a ‘quick
and dirty’ method that works.
Buy a super-cheap toaster oven
(we often see these for sale under
£25) and stick a K-type thermocouple
alongside your board. Apply solder
paste to the pads and carefully place
the parts on top. Preheat the PCB
to 100°C in the oven, then turn the
oven up to maximum. Watch closely until the temperature hits 220°C.
At this point, you should have seen
the solder flow. Immediately turn the
oven off and open the door.
3. Use a hot air gun. That is how we
built all the prototypes, to convince
ourselves that it would work for you
(see the photo overleaf). Even though
we have a reflow oven, we often use
the hot air gun as it is quick and easy
(they’re also surprisingly inexpensive). We used this technique just
for the LT4320, leaving the easier
capacitors and Mosfets to be handsoldered. The key steps are:
a
Apply a small amount of solder
paste to each pad and the central
thermal pad. Do not overdo this; a
modest smear is sufficient. We use
60/40 tin/lead solder paste as it melts
at a lower temperature, making it generally easier to work with. Nothing
is stopping you from using lead-free
solder, but remember that it requires
higher temperatures.
b
Place the LT4320 using tweezers. There should be sufficient solder
paste to stick in place, but not so
much that it squishes everywhere.
c
Check that the LT4320 is the
right way around. Double-check, as
this is by far the most expensive part
in this project.
d
Put the board on a heat-resistant
surface, such as a PCB off-cut. Do not
use your desk as it will get quite hot!
e
Set your hot air gun to about
300°C.
f
Apply heat to the board in a
gentle waving motion from about
15cm away, so the board around
the IC is heated reasonably evenly.
We want to preheat the board to
something in the region of 100°C over
a minute or so.
13
Constructional Project
My poor wirewound nichrome dummy load reached 320°C while the Mosfets on
the D2PAK standalone module only reached 67°C.
g
Once the board is well warmed
up, bring the hot air gun to about
5-10cm from the board and work
around the IC. Have your tweezers
handy; if the IC moves a lot, you
might need to nudge it back into
position. Having said that, surface
tension will typically pull it into
place if you’re blowing the air directly from above.
h
Watch the solder paste. As the
board approaches 220°C, you will
see the paste changing from dull
granular material to a shiny liquid.
The change is significant, so you
shouldn’t miss it.
i
As the solder melts, it also
creates a lot of surface tension and
will pull the IC into position.
j
Do not overheat the board.
Once all the solder has reflowed,
take the heat gun away.
k
Allow the board to cool naturally. Do not put any liquid on the
board to accelerate the cooling.
l
You might see several pins
with solder bridges across them. Fold
some solder wick across the tip of
your iron and ‘dab’ the pins to melt
the bridge into the wick. Adding a
little flux paste to the bridge first
usually helps. With a little practise, this is quick and easy. We get
quite a few bridges to fix as we are
too generous with the solder paste!
For the remaining SMD parts, a
regular soldering iron works fine.
We generally tack down one of the
SMD leads and make sure the part is
straight. For the two-pin passives, all
that’s left is to solder the second lead.
For the Mosfets, apply the iron to
the source (main tab) at the junction
of the tab and the PCB pad. Put a
small amount of solder between the
iron and the tab and wait until the
solder flows.
Once both the pad and the component lead are hot, the solder will
flow freely under the component.
After that, you can solder the remaining small pins.
The 6.3mm spades, screw connectors or wire leads are through-hole
parts, so solder them as usual.
Testing
Soldering the MSOP-12 LT4320 IC using a low-cost hot air ‘rework’ station.
These are invaluable for all sorts of jobs; they make it especially easy to
desolder SMDs. In this case, the killer feature is the ability to heat the IC enough
to solder the pad underneath.
Testing the Ideal Bridge Rectifier
is not complex and can be undertaken at low power.
First, connect a 220W 1W resistor
across the output, or an alternative
resistor with a power rating that can
14
Practical Electronics | November | 2024
Ideal Bridge Rectifiers
withstand the DC voltage we will
apply in the following steps. Connect a multimeter across the test resistor with the meter’s positive line
to the positive output of the ideal
bridge rectifier.
Connect a 12V DC power supply to
the input of the Ideal Bridge Rectifier and verify that the output gives
a +12V reading on the meter. Verify
that the voltage drop is less than
100mV.
Then swap the polarity of the input
voltage and verify that the output is
still giving a +12V reading on the
meter, and the voltage drop is still
less than 100mV.
If it does not work
● Check all solder connections.
● Check the orientation of the
LT4320 IC.
● If using TO-220 Mosfets, check
their orientations.
● If building the through-hole
board, check the orientation of the
electrolytic capacitor.
● Check your test setup; is the
power supply in current limiting?
Check the input voltage.
Using it
Among the six different modules,
you will likely find a ‘drop in’ solution. The SIL and 19mm pin bridges
should solder straight to a PCB that’s
designed for a regular bridge rectifier.
For an audio amplifier, you would
ideally mount two of the standalone
versions in the chassis and run individual windings to each.
Remember that the LT4320 operates from 9V to 72V. If your output
voltage falls below this, the LT4320
will not drive the Mosfets, and the
bridge will only operate using the
body diodes.
That is OK to get the circuit started, but at high currents, the dissipation can be very high.
This is only a concern if your
design uses low rail voltages, or you
are likely to do something as silly
as we did and drive the rectifier so
hard that your capacitor discharges
massively between 50Hz cycles. That
won’t happen in a typical power
supply or power amplifier.
Conclusion
The Ideal Diode Bridge Rectifier
can significantly improve the efficiency of just about any circuit that
Practical Electronics | November | 2024
Parts List – Ideal Diode Bridge Rectifier
Common parts for versions #1 to #4 (from Mouser, DigiKey or Farnell)
1 LT4320IMSE#TRPBF ideal bridge controller IC, MSOP-12 (IC1)
1 1μF 100V X7R M3216 SMD ceramic capacitor [CL31B105KCHNNNE]
1 10μF 100V X7S M3225 SMD ceramic capacitor [GRM32EC72A106KE5K]
#1 28mm spade version
1 double-sided PCB coded 18101241, 28 × 28mm
4 6.3mm PCB-mounting vertical spade connectors
4 TK6R9P08QM,RQ, IPD50N06S4-09 or TK5R1P08QM,RQ
N-channel Mosfets, DPAK/TO-252 (Q1-Q4)
#2 21mm square PCB pin version
1 double-sided PCB coded 18101242, 22 × 22mm
1 10cm length of 1.5mm diameter tinned copper wire
4 TK6R9P08QM,RQ, IPD50N06S4-09 or TK5R1P08QM,RQ
N-channel Mosfets, DPAK/TO-252 (Q1-Q4)
#3 5mm pitch SIL version
1 double-sided PCB coded 18101243, 23 × 20mm
1 10cm length of 1.5mm diameter tinned copper wire
4 TK6R9P08QM,RQ, IPD50N06S4-09 or TK5R1P08QM,RQ
N-channel Mosfets, DPAK/TO-252 (Q1-Q4)
#4 Mini SOT-23 version
1 double-sided PCB coded 18101244, 9 × 15mm
1 10cm length of 1.5mm diameter tinned copper wire
4 SI2318DS-GE3, SI2316BDS-T1-BE3 or SI2316BDS-T1-E3
N-channel Mosfets, SOT-23 (Q1-Q4)
#5 Standalone D2PAK SMD version
1 double-sided PCB coded 18101245, 59 × 36mm
2 mini horizontal terminal blocks, 5mm or 5.08mm pitch
1 LT4320IMSE#TRPBF ideal bridge controller IC, MSOP-12 (IC1)
1 1μF 100V X7R M3216 SMD ceramic capacitor [CL31B105KCHNNNE]
1 10μF 100V radial electrolytic capacitor, 2.5mm pitch, ≤6.3mm diameter
[Kemet ESL106M100AE3AA]
4 IPB083N10N3GATMA1 N-channel Mosfets, D2PAK/TO-263 (Q1-Q4)
[ESL106M100AE3AA]
#6 Standalone TO-220 through-hole version
1 double-sided PCB coded 18101246, 38 × 28mm, with 70μm-thick copper
4 6.3mm PCB-mounting vertical spade connectors
1 LT4320IN8#PBF ideal bridge controller IC, DIP-8 (IC1)
4 TK5R3E08QM,S1X (80V) or RFB7545PbF (60V)
N-channel Mosfets, TO-220 (Q1-Q4)
1 1μF 100V X7R radial ceramic capacitor, 5mm pitch
[RDER72A105K2M1H03A]
1 10μF 100V radial electrolytic capacitor, 2.5mm pitch, ≤6.3mm diameter
[Kemet ESL106M100AE3AA]
requires a rectifier for only a modest
increase in the device’s overall cost.
Best of all, for devices like power
supplies and audio amplifiers, you
can get even more output voltage or
power than you would with a standard diode-based rectifier. Less heat
from the rectifier also means less
cooling is required.
Don’t forget, though, that for applications like an audio amplifier
with split rails (positive and nega-
tive), unlike a diode-based rectifier,
you will need two of these devices,
one for each supply rail.
The transformer also needs to
have two separate secondary windings. That’s because the control chip
only monitors the voltage across the
upper two Mosfets.
With six different designs in a
range of sizes, current and voltage
ratings, you’re bound to find one
that suits your application.
PE
15
|