This project converts 12v DC to a regulated 5v DC at up to 1.8 amps,
suitable for driving a tablet computer from a 12v car battery in a power
blackout etc.
The circuit for this buck converter is nothing original, basically it is
the circuit from the 34063 IC datasheet, and all I did was to use an
external PFET instead of the external PNP transistor shown in the
datasheet. The external PFET allows currents up to a few amps at good
efficiency, however I have used hard current limiting at 1.8A for safety
and good performance in this prototype.
Energy conversion efficiency is very high due mainly to the choice of external components used with the cheap 34063 SMPS IC.
Schematic and operation.
Sorry for the hand-drawn schematic!
As you can see the circuit is minimum parts. It uses just two resitors
to drive the PFET from the IC (same as the datasheet), this is not ideal
but was done to test the concept and see if a PFET can be driven as
easily as the PNP transistor normally is. PFET turnon is good at 0.07uS,
but turnoff is not great taking 0.8uS. This costs about 1-2%
efficiency. The 560 ohm resistor could be reduced to speed up the
turnoff, but this would increase losses in that resistor so it is a
tradeoff.
34063 SMPS IC.
The 34063 IC does all the clever stuff, mainly it regulates voltage at
1.25v on VFB pin5. Because of the 6k8:2k2 voltage divider on the output,
this gives very close to 5v, I actually saw about 5.01v-4.99v Vout in
testing, very nice.
Max current limit resistor.
The resistor between Vin and pin7 sets the max inductor current
limiting, this was set by me to roughly 0.18 ohms to give 1.8A current
limiting. (Imax = 0.32v / R = 0.32v/0.18 = 1.78A). The current limit is
best at slightly above the max required current. This gives better
safety and also helps stabilise oscillation.
Caps etc.
CT used the datasheet value of 1nF. That gave oscillator value of
26.2kHz measured on pin3 (with no load), however the whole circuit
usually operated at 29-33kHz because of the way the regulation works in
the IC. The filter caps; 680uF on the input and 1000uF on the output
were chosen to be "good enough". Output ripple was approx 25-30mV which
is fine.
Measured efficiency!
Scope current L1 inductor at 5v 1.0 amps.
Same thing but at 1A. Frequency dropped a bit, closer to the 34063
oscillator freq of 26.2kHz, but still (just) triggering on the max
current peaks. Current ripple now larger from approx 0.5A to 1.6A
(average output 1A). Timing is still 20uS/hdiv but says 40uS on the
screen as I had zoomed my h-axis (sorry).
Scope current L1 inductor at 5v 0.5 amps.
Here the L1 current has gone "discontinuous" meaning the L1 current is
reduced to zero during the end of the off period, and has to start from 0
amps again during every on period. Typical of the regulation system
used in a 34063 IC, the timing will "stutter" as needed to maintain Vout
regulation at a steady 5.0v. This does not matter and the 34063 can be
quite energy efficiency when "stuttering" in discontinuous mode like
this. At less than 0.5 amps the stuttering can become very erratic
looking, but this is all normal.
PFET drain/source voltage (main switching waveform).
(The PFET on period is the top of the waveform). Above you can see the
PFET turnon (through a 10 ohm resistor) is nice and fast, It was about
0.07uS turnon time. However the turnoff is poor, because the turnoff is
from a 560 ohm resistor and is slow at 0.8uS. This costs significant
efficiency.
Using an external digital driver (like a 12v CMOS digital
buffer/inverter chip?) to drive the PFET would improve turnoff time a
lot and increase efficiency, but this was a test of using the simple
datasheet example circuit with an external PFET (instead of the
suggested external PNP) and as proof of concept it still works well
enough.
5v DC output showing voltage ripple.
Because it is a switching regulator there will always be some ripple on
the DC output voltage. This is shown when running at 5v 1.5A and the
ripple is typical and acceptable enough at 30-35mV.
Improving efficiency.
This circuit was thrown together very quickly to show how to use a cheap
common 34063 IC to get a high efficiency supply from 12v->5v DC at
0-1.5A or so. If you want to invest some effort it can be improved
further;
1. My PFET is not a good choice, using a better PFET will give an easy 1% more efficiency, and would be the first choice.
2. The inductor is just an ordinary "off the shelf" type. A properly
selected inductor or a good core hand wound for best performance could
allow lower operating frequency and less current ripple, and maybe less
DC ohms, and maybe pick up another 0.5% efficiency or so. (For lower
operating freq CT should also be increased to 1.2nF or 1.5nF etc).
3. The PFET turnoff is too slow. Adding a cheap digital buffer IC could
pick up 0.8-1.2% efficiency there from reduced switching losses and
reduced loss from the 560 ohm resistor.
4. My PCB has very thin long tracks. Using a well designed PCB with
thick short tracks for the main current paths might save 30 milliohms
and give maybe 0.5% or more efficiency.
Bill of materials.
* 34063 SMPS 8pin IC (Fairchild/ON Semi/AIS etc, ie MC34063A or NCV34063A).
* 8pin IC socket (optional).
* PFET, rated more than double the input voltage and a few times the
desired output current, preferably well under 0.1 ohm Rds on.
* Inductor L1 is a powdered iron toroid of 20-30 mm diameter, with thick
wire >1.0mm preferred, 3A rated for a 1.5A capable supply. Value in
the 150-470uH range, you may need to try a couple of different types.
Ideally current ripple will be <50% at full output current.
* Schottky TO-220 dual 10A or dual 16A diode pack. Choose for low
forward voltage, most brands are very good, parts can be found in any
old PC PSU.
* 470-1000uF 35v electro cap.
* 1000uF 16-25v electro cap (25v will be larger and generally have a longer life).
* CT 1nF 25-50v ceramic or greencap.
* some 1/4W resistors; 560 ohm, 10 ohm, 6k8, 2k2.
* If you need a test load then a large 10W 4.7 ohm resistor will do.
Modifying the circuit for 12v car operation.
This circuit was designed for a car battery, generally 13.8v to 12.0v
when running. If used in a car the circuit needs more protection as the
Vin might be >15v at times. I would use a 100 ohm resistor instead of
the 10 ohm resistor. Also a 13v zener diode across the 560 ohm resistor
will add safety for the PFET. A 12v line filter might also be advised,
they can be bought from auto stores.
Modifying the circuit for 24v operation.
Use 560 ohms instead of 10 ohms, so it now has two 560 ohm resistors.
And again a 13v zener from PFET gate to source pin. With a 24v Vin you
should use a higher inductor value and larger inductor core, 470uH and
up are recommended.
[b]Modifying the circuit for high output currents.[b]
The circuit is meant for 5v out, 0-1.8A. It will do ok up to 2.5A just
by changing the current limit resistor (at 2.5A the resitor should be
0.12 ohms or so).
