Mr RB
Well-Known Member
(Inspriation come from ETO member Mr Al who mentioned his interest in a high efficiency 5v supply to power his Tablet PC during power blackouts etc, see his thread here;
https://www.electro-tech-online.com/threads/so-how-about-a-tablet-power-supply.131801/)
This project is a simple converter from 12v DC to a regulated 5v DC output 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.
**broken link removed**
PCB layout.
The prototype was tested in hardware, please excuse the messiness. The layout is far from ideal, I did it this way to allow easy swapping of parts and just to be lazy, to save the effort of making a PCB. However it still works pretty good, and a proper PCB would improve performance a little bit.
PFET choice.
I did not have a lot of PFETs in my parts box so I used a 100v 8A rated part. This was an SMD PFET so I just tacked it on the bottom of the PCB. It is efficient enough to not need a heatsink even at 5v 1.5A continuous output. The PFET I used was not ideal, it's "Rds on" value is about 0.3v at 1.5A (0.2 ohms) which is too high and costs efficiency. Going to a 50v >20A PFET with an RDS <0.1 ohms or <0.05 ohms would give a noticable increase in efficiency.
Schottky diode choice.
I used a TO-220 60v dual 10A schottky diode pack (total 20A). This is a no-brainer, although this is overkill these diodes are only $1-$2 and can also be pulled for free from any old PC PSU and most commercial SMPS supplies. Besides the safety of being very large and over-rated, the main benefit is these diodes have a very low forward voltage drop of <0.3v at 1.5A or 2A and this equates to reduced losses (more efficiency).
Inductor choice.
This is just a commercial "3 amp" 24mm total diameter inductor/choke available from hobby suppliers like Altronics Australia. I think it is a 220uH or 330uH value, but sorry I lost the paperwork. A few other powdered-iron toroid inductors were tried and it is not that critical. It has 51 turns of 1.0mm diameter wire if that helps. The inductor measured 0.32mV at exactly 1A DC, so DC resitance was measured at 32 milliohms.
Note re inductor value! A calculation based on the rate of current rise over time (seen on the 'scope) seems to indicate an actual inductand value of 191uH, so this would have been a 220uH 3A commercial inductor. (They usually measure less than the stated inductance).
**broken link removed**
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!
Note! Readings were taken from meters with only 2 decimal point resolution and were not lab grade accuracy, so there may be a couple of percent error in readings.
Calculating efficiency (at 1.5A output).
The static power losses were seen on the 'scope and can be calculated;
PFET Rds on period loss = 0.3v / 12.5v = 2.4% loss
DIODE Vf off period loss = 0.28v * 1.53A * 0.56 offduty = 240mW = 2.8% loss
Inductor resistance loss = 1.53A squared * 0.032 ohms = 75mW = 0.9% loss
560 ohm resistor loss = 10.5v squared / 560 * 44% onduty = 87mW = 1.0% loss
Total static losses at 1.53A output = 7.1%
Calculated other (switching) losses = 100% - 91.1% - 7.1% = 1.8%
https://www.electro-tech-online.com/threads/so-how-about-a-tablet-power-supply.131801/)
This project is a simple converter from 12v DC to a regulated 5v DC output 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.
**broken link removed**
PCB layout.
The prototype was tested in hardware, please excuse the messiness. The layout is far from ideal, I did it this way to allow easy swapping of parts and just to be lazy, to save the effort of making a PCB. However it still works pretty good, and a proper PCB would improve performance a little bit.
PFET choice.
I did not have a lot of PFETs in my parts box so I used a 100v 8A rated part. This was an SMD PFET so I just tacked it on the bottom of the PCB. It is efficient enough to not need a heatsink even at 5v 1.5A continuous output. The PFET I used was not ideal, it's "Rds on" value is about 0.3v at 1.5A (0.2 ohms) which is too high and costs efficiency. Going to a 50v >20A PFET with an RDS <0.1 ohms or <0.05 ohms would give a noticable increase in efficiency.
Schottky diode choice.
I used a TO-220 60v dual 10A schottky diode pack (total 20A). This is a no-brainer, although this is overkill these diodes are only $1-$2 and can also be pulled for free from any old PC PSU and most commercial SMPS supplies. Besides the safety of being very large and over-rated, the main benefit is these diodes have a very low forward voltage drop of <0.3v at 1.5A or 2A and this equates to reduced losses (more efficiency).
Inductor choice.
This is just a commercial "3 amp" 24mm total diameter inductor/choke available from hobby suppliers like Altronics Australia. I think it is a 220uH or 330uH value, but sorry I lost the paperwork. A few other powdered-iron toroid inductors were tried and it is not that critical. It has 51 turns of 1.0mm diameter wire if that helps. The inductor measured 0.32mV at exactly 1A DC, so DC resitance was measured at 32 milliohms.
Note re inductor value! A calculation based on the rate of current rise over time (seen on the 'scope) seems to indicate an actual inductand value of 191uH, so this would have been a 220uH 3A commercial inductor. (They usually measure less than the stated inductance).
**broken link removed**
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!
Code:
[b]Vin Iin Pin Vout Iout Pout Eff %[/b]
12.5v 670mA 8.375W 4.99 1.53A 7.63W 91.1%
12.5v 430mA 5.375W 5.00 1.00A 5.00W 93.0%
12.5v 210mA 2.625W 5.00 0.50A 2.50W 95.2%
Note! Readings were taken from meters with only 2 decimal point resolution and were not lab grade accuracy, so there may be a couple of percent error in readings.
Calculating efficiency (at 1.5A output).
The static power losses were seen on the 'scope and can be calculated;
PFET Rds on period loss = 0.3v / 12.5v = 2.4% loss
DIODE Vf off period loss = 0.28v * 1.53A * 0.56 offduty = 240mW = 2.8% loss
Inductor resistance loss = 1.53A squared * 0.032 ohms = 75mW = 0.9% loss
560 ohm resistor loss = 10.5v squared / 560 * 44% onduty = 87mW = 1.0% loss
Total static losses at 1.53A output = 7.1%
Calculated other (switching) losses = 100% - 91.1% - 7.1% = 1.8%
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