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Switching Regulator Efficiency

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Jeff Birkle

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Switching Regulator Efficiency

Hello,
I have made this Switching Regulator and am wondering about its efficiency, and if it can be improved. I calculate it to be 66%. 2 sources have said it should be 74 to 84%.
The input is from 22 to 31VDC
The output is from 4.5 to 6.5VDC at 1.5 to 6A
It on a solderless board for now.
At 1.6A the TIP 31 gets really warm (around 120deg C).

The Components are
IC1 – MC34063
Q1 - Tip31
R1 – 3.3K
R2 – 1.3K
R3 – 15K Pot for Variable output
C1 - 330uF
C2 – 220uF
C3 - 47uF
L1 – 250uH

Any thoughts?

Jeff
 

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I have made this Switching Regulator and am wondering about its efficiency, and if it can be improved. I calculate it to be 66%. 2 sources have said it should be 74 to 84%.
...
The output is from 4.5 to 6.5VDC at 1.5 to 6A
...
It on a solderless board for now.
...
C1 - 330uF
You have an additional 0.6V drop due to the external transistor; at 5V output, this is 12% efficiency loss there already. If you use a external PNP instead, you can reduce the voltage drop to the saturation voltage of the transistor.

A solderless breadboard has a lot of parasitic properties that will likely affect your efficiency a little also.

Are you sure C1 is 330uF? It should be more like 330pF.
 
What is D1?

The voltage drop across Q1 is high. There is probably 2 to 2.5 volts across Q1. That causes power loss.
 
Hello

Yes a typo
330pF
D1 - 1N5822 3A Schottky Diode
So using an external PNP may get another 12%? Not bad. I guess that would be about the best this could get? I think I have a Schematic for that somewhere, I'll post it tomorrow.
Thanx
 
So using an external PNP may get another 12%? Not bad. I guess that would be about the best this could get? I think I have a Schematic for that somewhere, I'll post it tomorrow.
The schematic, attached, is from the datasheet. You can see that for the external NPN in buck config., the output (at the inductor/diode) voltage may never be greater than 3 PN junction drops below the supply. With the external PNP transistor, the voltage drop at the output is the drop across the PNP C-E, which can be reduced to the saturation voltage of the transistor. If you use a P-MOSFET, you can get the voltage drop to be even less, and the efficiency can be even greater.
 

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An external P-MOSFET will also reduce the drive current below the ≈10% of the maximum output current a PNP would require. This is wasted current and reduces efficiency by 10%. Of course the output drive resistor has to be low enough to rapidly charge and discharge the gate capacitance to give fast switching, but that value will still be larger that the low resistance required to provide 600ma to drive a PNP for a 6A maximum output.
 
Hi Jeff,


Roman Black (MrRB) came up with a circuit with a P Mosfet but much simpler than that one. It's in the thread where we talk about a Buck converter for an Android Tablet computer, taking 12v input and creating a 5v output. He tested it and it had very high efficiency like over 90 percent. Someone can search for that and you can take a look, or you can search yourself. It wasnt that long ago, maybe a couple months.

Here:
https://www.electro-tech-online.com...cy-smps-buck-converter-using-34063-ic.132064/
 
Watch out! The input voltage is 31v and the gate can not handle more than 20v.
True. The values of R2 and R3 in Jeff's referenced circuit need to be selected (and/or a Zener added across R2) to keep the Vgs below 20V when the MOSFET is driven ON. The added current from this will reduce the efficiency somewhat.
 
Here is something to read.
The LM2697 is very simple. I have used them many times. Not real efficient.
TPS5340 is only 3.5A, 90% and simple.
The LM25085 has external P-FET. Note the gate driver sits on Vin not ground.
 

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Last edited:
Watch out! The input voltage is 31v and the gate can not handle more than 20v.

Correct, my "simple circuit" requires that the Vin is low enough not to damage the FET gate, it was designed for a 12v to 5v converter. The 34063 drops 1-2v in it's NPN drivers so you can subtract that from the Vin, but it still would be limited to a safe Vin of maybe 18v DC. :)
 
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