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A simple LT1763 based 5V PSU
A simple LT1763 based 5V PSU
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Old 2nd March 2014, 12:11 PM   #1
maxw is offline maxw  United Kingdom
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Default A simple LT1763 based 5V PSU

I'd like to design and build a simple low noise 5V power supply based on the LT1763. I've made a schematic and layout, does anyone have any feedback or suggestions for improvement?

TR1: Pads for Block FL2/FL4/FL6/FL8 or EI38 or EI30. Ideally 2x 6VAC.
B1: VISHAY DFL1508S bridge rectifier
C1: Panasonic FC 1500UF, 16V
C2: 100nF X7R
IC1: LT1763CS8-5, 5V fixed regulator
C3: C0G/NP0, 10NF, 100V
C4: Panasonic FK 100UF, 6.3V, SMD
C5: 100nF X7R
R1: Resistor for LED
LED: 1206 LED

Should I add a coil before the regulator? A snubber might be a good idea but I don't have the equipment to take measurements for this.
Attached Images
File Type: jpg 5VPSU-layout.jpg (457.5 KB, 804 views)
File Type: jpg 5VPSU-schematic.jpg (93.6 KB, 800 views)
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Old 2nd March 2014, 03:41 PM   #2
Mark Johnson is offline Mark Johnson  United States
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A simple LT1763 based 5V PSU
Perhaps you might consider adding a series resistor in the Red branch of the attached schematic, and you might also consider adding a ferrite bead in the Blue branch.

The resistor reduces the magnitude of the current peaks through the diodes, reducing the dV/dt slope of the zig-up piece of the ripple waveform. It also forms a lowpass filter (with C1 + C2 + Rdiode) which reduces the amount of high frequency noise passed from mains to IC1 input. The voltage dropped across this resistor eats into the voltage headroom ("dropout voltage") of the regulator, so carefully choose its resistance based on minimum mains voltage, maximum load current, maximum output voltage, and maximum regulator "dropout" specification. And I suggest you choose its wattage to be at least 3x higher than its theoretical power dissipation, both for long term reliability and also to avoid burning your finger when servicing.

The ferrite bead acts as a (lossy) resistor at high frequencies, further reducing RF noise conducted from mains into IC1.
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Old 2nd March 2014, 09:22 PM   #3
maxw is offline maxw  United Kingdom
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Thanks for the suggestions!

Quote:
Originally Posted by Mark Johnson View Post
The resistor reduces the magnitude of the current peaks through the diodes, reducing the dV/dt slope of the zig-up piece of the ripple waveform. It also forms a lowpass filter (with C1 + C2 + Rdiode) which reduces the amount of high frequency noise passed from mains to IC1 input. The voltage dropped across this resistor eats into the voltage headroom ("dropout voltage") of the regulator, so carefully choose its resistance based on minimum mains voltage, maximum load current, maximum output voltage, and maximum regulator "dropout" specification. And I suggest you choose its wattage to be at least 3x higher than its theoretical power dissipation, both for long term reliability and also to avoid burning your finger when servicing.
OK. Can you help me calculate it? The transformer outputs 6V when loaded and about 8V with no load. Current is limited to 500mA because the is the maximum the regulator will do. The regulator has a Low Dropout Voltage, 300mV. Maybe 2-4 ohms at 1w?

Quote:
Originally Posted by Mark Johnson View Post
The ferrite bead acts as a (lossy) resistor at high frequencies, further reducing RF noise conducted from mains into IC1.
I was going to use an coil at the blue arrow, this one. Since a coil and a ferrite bead are just both inductors, does one preclude the other? Can you use both? Would this one be suitable?
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Old 3rd March 2014, 02:03 PM   #4
Mark Johnson is offline Mark Johnson  United States
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A simple LT1763 based 5V PSU
Quote:
Originally Posted by maxw View Post
The transformer outputs 6V when loaded and about 8V with no load. Current is limited to 500mA because the is the maximum the regulator will do. The regulator has a Low Dropout Voltage, 300mV. Maybe 2-4 ohms at 1w?
{ [ ((6 x 0.95) x sqrt(2)) - (2 x 1.1) ] - 5.125 } = 0.736V across the LT1763 (with no Red resistor)

Subtract the LT1763 minimum dropout voltage (0.41 volts) and you get the voltage across the Red resistor: 0.326 volts

(0.326V / 0.5A) = 0.652 ohms. Round to nearest E12 standard resistance value: 0.68 ohms

Dissipation rating = (0.5A x 0.5A x 0.68R) x 3 = 0.51 watts. Me, I'd go ahead and use a 1W resistor. Just to provide a visual reminder on the PCB: "Hey, this resistor is high wattage so it might be hot! Be careful with it."

I recommend you read the Wikipedia article on ferrite beads; a ferrite bead is not "just an inductor". Rather, ferrite beads are lossy circuit elements at high frequencies; they dissipate power. Inductors, being lossless reactances, do not.
Attached Images
File Type: png ldo.png (23.2 KB, 692 views)
File Type: png ldo2.png (32.7 KB, 657 views)
File Type: png regulat.png (16.8 KB, 104 views)

Last edited by Mark Johnson; 3rd March 2014 at 02:06 PM. Reason: choose 1W anyway
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Old 4th March 2014, 01:18 PM   #5
maxw is offline maxw  United Kingdom
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OK thanks for your help Mark. I'll use this resistor at the red point, this Ferrite Bead and this inductor at the blue point.
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Old 5th March 2014, 06:38 PM   #6
Karsten Sømand is offline Karsten Sømand  Denmark
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A simple LT1763 based 5V PSU
Hi Max,

I don´t know how important electrolytic ESR is in this circuit but you could also consider the use of Panasonic FM or FR types for C1 (approx. half the ESR compared with FC types). Endurance for FR types is 2000 h to 10000 h at
+105 deg. C.

I haven´t checked the prices, though

Karsten
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Old 5th March 2014, 07:33 PM   #7
maxw is offline maxw  United Kingdom
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That's a good suggestion, thanks Karsten. I wonder why FC caps are more expensive then?
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Old 3rd April 2014, 09:44 AM   #8
maxw is offline maxw  United Kingdom
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PCBs are done and I've assembled and tested it, it works!

I used a Block FL4/6 transformer and a Panasonic FC cap before the regulator (couldn't get larger value FR or FM caps in the lower height).

I did some ARTA measurements of my MDAC attenuator (which requires a 5V PSU) comparing this power supply and a battery, there was no measurable difference.

The BOM
F1: Fuse holder. Part URL.
TR1: Pads for Block FL2/FL4/FL6/FL8 or EI38 or EI30. Ideally 2x 6VAC. Part URL.
B1: VISHAY DFL1508S bridge rectifier. Part URL.
C1: Panasonic FC 1500UF, 16V. Part URL.
L1: INDUCTOR, 1210 CASE, 10UH. Part URL.
L2: FERRITE BEAD, 0.1OHM, 2A, 1206. Part URL.
C2: 100nF X7R, 1206 package.
IC1: LT1763CS8-5, 5V fixed regulator. Part URL.
C3: C0G/NP0, 10NF, 100V, 1206 package.
C4: Panasonic FK 100UF, 6.3V, SMD. Part URL.
C5: 100nF X7R, 1206 package.
R1: Resistor for LED, 1206 package.
LED: 1206 LED

I'll post Eagle PCB files later tonight.
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File Type: jpg IMG_0925.JPG (342.0 KB, 196 views)
File Type: jpg IMG_0921.JPG (348.1 KB, 214 views)
File Type: jpg IMG_0924.JPG (507.4 KB, 2476 views)
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Old 3rd April 2014, 05:22 PM   #9
maxw is offline maxw  United Kingdom
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And here are the Eagle PCB files if anyone wants to get their own PCBs made.
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File Type: zip PowerSupply - 5V.zip (82.8 KB, 89 views)
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Old 3rd April 2014, 06:52 PM   #10
Karsten Sømand is offline Karsten Sømand  Denmark
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A simple LT1763 based 5V PSU
Hi Max - it for sure looks very good. As usual very nice work from your hands. Thanks for sharing.

Karsten
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