Hi, I’m Roland van Roy.
In this video I’ll present some practical methods how you can design, build and test
a DC/DC converter application in only a few hours.
When making DC/DC converter prototypes, using this type of perf board is not really suitable,
because the connections have a lot of stray inductance, which easily leads to malfunction
in high frequency switching circuits.
I normally make use of double sided bare copper PCB material, in which I cut slots to form
separate copper islands.
You can then connect the components between these copper islands.
This method works quite well because the connections have very low inductance and resistance.
You can even make traces at the bottom by drilling a hole in the PCB and connect a copper
wire between top and bottom.
Let me show you how to build this 5V/2A buck converter prototype.
For the prototype example, we’ll use the following specifications: For a 12V input,
I’d use a 17 or 18V rated part.
Output voltage lower than input voltage means you need a step-down buck converter.
The 2A continuous load current will determine the thermal condition.
3A peak means you need at least a 3A rated part.
To achieve low power standby mode, you need a converter with high efficiency at light
load.
This means a part with PSM (discontinuous mode) at light load to reduce switch losses
at light load.
The last item is related to layout and thermal: In these applications we use the PCB copper
for cooling the device.
You need to check the power losses and package thermal capabilities.
In this example I’ll choose a PSOP-8 package with exposed pad.
It has a wide 1.27mm pin pitch, which is easy for a hand-made PCB.
SMD PSOP-8 packages with exposed pad are cooled by connecting sufficient copper plane to the
thermal pad.
You can also connect vias under the thermal pad to other ground layers to increase cooling.
Here are some practical values I normally use for different SMD package max power dissipation
based on a good layout and 60 dgs max ambient temperature.
The PSOP-8 package can dissipate around 0.9W. I’ll use the online design tool Richtek
Designer to help me find a suitable part, calculate the component values and simulate
the circuit performance.
After entering input and output requirements in the quick menu, I’ll check the required
boxes in the component selection page: SOP-8 package, discontinuous operation mode for
low power standby.
In this example I choose a current mode part to illustrate some stability aspects.
But you could also choose an ACOT part for faster load transient response.
The tool now shows the parts that match the selection criteria.
For good compromise between component size and power dissipation, I choose RT7257GHZSP,
which runs at 800kHz, and has hiccup mode protection.
The tool generates the schematic with suitable component values.
The tool calculates with the stated component values, so you must enter the values of the
components you are actually going to use.
Current mode buck converter stability is related to output capacitance and compensation, so
here you must use accurate values.
MLCC capacitors have lower capacitance at higher DC voltage and small AC ripple.
You can check this from Murata datasheets which you can find in the BOM section of the
online tool.
I choose two 22uF/10V 1206 size MLCC output capacitors.
From the datasheet graphs you can see that the effective capacitance of these types at
5Vdc and low AC ripple is only around 10.3uF.
Since the tool originally used 2x22uF, we now need to change the compensation for 2x
10uF output capacitance.
To do this, reduce the compensation resistor in the same ratio as the output capacitance
drop: so the original 32k becomes 15k.
For the inductor I’ll use a low cost Taiyo-Yuden NR8040 4.7uH inductor, which has 4.7A saturation
current and 18mohm DC resistance, which is sufficient for this design.
Note that these parts are semi shielded: They use magnetic epoxy resin as shielding, but
can still emit considerable stray field.
After entering all key component properties, we can use the tool to simulate output ripple
in light PSM load and high load, look at the step response and gain-phase analysis which
shows good stability.
We can also check the circuit efficiency and power loss.
When making a DC/DC converter layout, you have to consider the current loops.There are
two main current loops in a buck converter: The red current flows via the input capacitor,
the high side switch, indictor to output capacitor.
The blue current flows through the synchronous rectifier, inductor and output capacitor.
The total current in the inductor and output capacitor is the sum of the two currents,
which is continuous.
This loop is not critical.
But the current in the input capacitor and the two switches is discontinuous, and therefore
can generate a lot of EMI.
When making a layout, the red loop area should be as small as possible with low inductance.
The blue loop can be larger, as it does not generate a lot of EMI.
The switching node and bootstrap capacitor node have fast switching voltages.
Theses traces should be minimized and kept away from other sensitive circuits.
Non-shielded or semi shielded inductors will emit considerable high frequency magnetic
field, which can induce switching voltages in nearby loops.
Keep sensitive loops away from the inductor.
In a double layer board, I try to keep all high current loops on top side.
I start the layout by placing the input capacitor as close as possible to the IC VIN and GND
pins.
This will give the smallest input loop.
The inductor is placed close to the SW node and minimize the SW node area.
Remove the copper directly beneath the inductor to reduce eddy currents due to stray field.
Output capacitors go from VOUT back to the main IC ground.
The Bootstrap capacitor goes with a bottom trace back to the IC.
Small signal components are placed away from the switching loops: Feedback network close
to FB pin, VOUT sensed near the output capacitors.
COMP network also close to IC.
Enable connected via resistor to VIN.
The softstart capacitor is not really critical.
The bottom GND plane should be connected with vias to the IC thermal pad.
For the handmade PCB, I’ll use one thick via near the IC GND and thermal pad.
Here you see some of the tools you’ll need to make your own PCB for DC/DC converters.This
is an engraving tool that runs at very high speed and you can fit many different kinds
of bits.
Here I’m using an engraving bit with a very fine tip, but you can also use these diamond
burr cutting bits to remove material.
You’ll also need some thin drill bits for drilling holes in the PCB, normally 0.5 to
0.6mm, and in order to punch a dent in the PCB for your drill guide, you need a steel
pin with a very sharp point.
To cut pieces of PCB, you score the board a couple of times on each side.
Then you can easily break it on the edge of the table.
Then sand the edges.
Start the layout by drawing the IC pin grid on the PCB.
IC’s with pin pitch down to 0.6mm are quite doable.
Then place the components on their intended locations.
Then you can draw the component outlines on the board .
I normally draw square islands, which are easier to cut.
Now it’s time to draw the copper islands.
And clean up with an eraser.
You can now cut thin slots to form the copper islands.
Then check
for shorts.
Remove copper under the coil with a diamond burr bit then sand and clean the board.
Place back the components and mark the holes for vias.
Use the punch tool to make the drill guide dents.
You can now drill the 0.6mm holes for the via wires.
One the back side, you can now use the holes to draw the bottom traces.
And cut the slots to form the traces.
Check for shorts and sand and clean the board.
First we solder the IC thermal pad to the PCB, then solder the IC pins.
Now we can mount the inductor.
Add some solder to all the other pads.
Place and solder the components at one side.
Then solder all components on both sides.
Before mounting the via wires, first add some solder at the PCB bottom edges.
Then add the via wires and solder them at top side and bottom side.
Then add some connection wires for input and output and ground.
And add a short loop measuring spring on the output capacitor.
To avoid mistakes, I normally mark input and output.
Forgot one item: We need to add the thick via from IC GND to bottom ground plane.
I use a 0.9mm copper wire for this.
I highly recommend adding a 220uF electrolytic capacitor at the input to avoid resonances
and hot plug spikes due to long input wires.
It’s time for testing!
I use a 100Ω variable power resistor as load.
Connect the output voltage probe with 20MHz scope bandwidth limit.
I also measure the switching waveform.
We start-up with 100Ω load.
You can see that the converter operates in PSM mode.When increasing the load, you see
that the converter goes to PWM fixed frequency mode.
In PWM mode, the output ripple does not really look like the simulated waveform, and its
amplitude changes when you move the probe.
This is because the measuring point is very close to the inductor: Its stray magnetic
field is still coupled into the small measuring loop.
The get cleaner waveforms, you can place the output capacitors further away from the inductor.
Now the output ripple is close to simulated value, both in PWM and PSM mode.
But in PSM mode, we see that the converter sometimes shows double pulsing, when changing
the input voltage.
PSM mode regulates on the output ripple valley . It is quite sensitive to noise.
In our layout, the feedback network is grounded at top-side copper r, but this ground is noisy
because the switching currents also flow in the top-side copper.
It’s better to ground the feedback network to the bottom ground plane, which provides
a clean ground back to IC ground.
You can do this by isolating the small signal ground from top ground, and place a via to
bottom ground.
After this change you see that the PSM mode is stable, and PSM to PWM transition is also
cleaner.
Let’s have a detailed look at the switching waveform.
You should connect the probe with short leads directly between IC switch node and ground.
Measure at highest load and high bandwidth setting: You see that the switch node looks
quite clean.
The overshoot at the rising edge is only 3 volt.
It is related to the input loop.
The undershoot at the falling edge is related to the IC low side MOSFET parasitic inductances;
basically caused by IC bonding wires and lead frame.
Let’s see what happens when you place the input capacitor further away from the IC.
With this location the input loop is much bigger.
You now see that the rising edge overshoot is huge, around 10V.
This is caused by the extra inductance of the input loop.
You should always try to place the input capacitor as close as possible between IC VIN and GND.
Let’s do a converter stability check: I’ll use the Richtek fast load transient tool for
this.
Adjust the static load to 2A, and add a 1A step load by selecting 4.7Ω pulse load resistor.
You can see that the step load is clean, without ringing.
Over and undershoot values are similar to the simulation results.
Just to illustrate what you would see if you had not made changes to the compensation we
change the compensation resistor back from 15kΩ to 32kΩ.
You now see that the stepload response shows ringing, which indicates too low phase margin.Always
check the effective capacitance when designing DC/DC converters with MLCC output capacitors.
In the last test, I’ll check the converter power loss and thermal conditions.
To measure temperature with an infrared scanner, you cannot measure the shiny PCB surface directly.
I normally add some correction fluid at the PCB measuring points to increase the emissivity
of the surface.
After one hour operation at 2A load, I measure the input and output power and IC top and
PCB temperatures.
From the input and output power, you can calculate the converter power loss, which mainly consists
of inductor losses and IC losses.
The inductor losses are DCR loss and AC core losses, which can be estimated via simulation
tools of inductor vendors.
The IC losses for this application are around 0.85W. Here you see the various temperatures
I measured on the board.
The temperature drop between two points depends on the thermal resistance and the heatflow
between those points.
You can see that the single via transfers considerable heat to the PCB bottom side.
Since most IC heat is removed via the thermal pad, there is not much heat removed via the
package plastic top.
So the junction temperature will be close to the measured plastic top temperature.
I estimate around 4 degrees higher.
From the IC junction temperature, the ambient temperature and the IC power dissipation,
you can estimate the total board thermal resistance from junction to ambient, which is around
67 kelvin per watt.
At 120oC, the IC power dissipation will be around 0.9W, due to the increase of Rdson
at higher temperature.
This will give a temperature delta of around 60 Kelvin between Junction and Ambient.
So for this board, the maximum ambient temperature to keep the junction below 120oC will be around
60oC.
I hope you found this video interesting.
Making prototypes can be a great way to find problems at an early stage and you can easily
make changes before going to a final layout.
Please stay tuned for more tutorial videos at Richtek, your power partner.