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  • Author Author: Andrew J
  • Date Created: 29 May 2019 4:40 PM Date Created
  • Views 243 views
  • Likes 3 likes
  • Comments 1 comment
  • bench power supply
  • modular_bench_power_supply
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YAPS Part Four - Design - Calculations (i)

Andrew J
Andrew J
29 May 2019

EDITS: 11/6/19 - Fixed broken links

 

As part of the analysis I was doing into the design and selection of parts, I ran up some calculations.  I also validated the thermal characteristics of the key parts to see how likely they were to need heatsinking.  I’ve captured, and attached, these in an Excel spreadsheet if you want to take a look and put it on Github.  There are two attachments now: the first, original one, and the second, updated one - see the comment I've posted under this post.  The calculations posting is broken into 3 parts so it isn't too long.

 

Some of the below is complicated (maths was never my strong point!) but was worth doing - it gives a clear indicators to what will need heat sinking and validated component selection.  Some will need monitoring under test to determine actual performance.

 

Fuse Calculations

I've included a line-side fuse (before the primary transformer windings).  I was initially confused at this because there seemed to be differing views on sizing it for either the output current or the input (to transformer) current.  I took the view that line side was based on VA rating and load side on maximum current output.

 

Inputs

ParameterValueNote
Transformer power60VAMulticomp MCTA060/09MCTA060/09
Transformer DC resistance38OhmsFrom data sheet: 2x19Ohms
Efficiency88%From data sheet
Vac240VUK supply
Max Current3A
Ambient Temperature25C
In-rush Current74AMeasured in LTSpice (seems high!)
Duration4.4msDuration of fall from peak
Duration5.4msDuration of rise and fall
I^2t @ 4.4ms12.047A^2sCommon curve calculation: 0.5 * I * I * duration of fall from peak.  I is the inrush current value
I^2t @ 5.4ms14.785A^2sCurve calculation: 0.5 * I * I * duration of rise and fall.  I is the inrush current value

In determining the fuse I^2t value, the calculation depends on how duration of inrush is measured.  in the inputs, I’ve measured this as a duration from peak (4.4.ms) and a duration from initial rise to the fall point (5.4ms) ostensibly to see if it made any difference in selection.

 

This gave me the follow values, calculated for line side and load side.

Line Side
Fuse rating0.329AVA / Vac * (1 / (1 - 2 * (efficiency / 100 )))
Re-rating0.395ARerated for ambient 65C at 10% per 20C rise
Nearest value0.4A
Re-rated for melting integral2.5ANearest value from data sheet that accounted for I^2t value
TypeSlow BlowFor in-rush current
Interrupt raitingHigh kAFuse is mains side

Load Side

Fuse required4.05AMax current * 135%
Re-rating4.86ARerated for ambient 65C at 10% per 20C rise
TypeSlow BlowFor dealing with inrush current to bridge rectifier
Interrupt ratingHigh kANot as necessary as for line side but it would seem that fuses come with either a too low value of kA values

I'm not intending on using a load side fuse so this calculation is really for interest.

 

Smoothing Capacitor Calculations

To remove/reduce ripple from rectified input.

ParameterValueNote
Max Current3A
Acceptable ripple1VPeak-to-peak
Time, 50Hz10msFull wave rectifier, UK supply
Capacitance0.03FC=IT/V
30,000uF

30,000uF seems high for 60VA although with 2x4700uF (9,400uF) LTSpice is simulating a ripple of 2V so its probably right.  The choice of peak-to-peak voltage is arbitrary on my part in the sense I don't know what an acceptable value is - there are no specs on the LTC1624 to state what an acceptable range on its input is.  The simulation works at 2x4700uF so I will start with that and if it causes an issue in the build then I can change it.

 

Thermistor Calculations

A voltage divider is used to measure resistance changes in the the thermistors which obviously causes a voltage drop.  I want to make sure that this wasn't going to cause a power dissipation issue for the selected parts, exacerbated by higher temperatures.

ParameterValueNote
Vin5V
R210000Ohms
at 25C
Thermistor Resistance10000OhmsFrom data sheet (of candidate thermistor)
Vout2.5Vat voltage divider (Vin * R2) / (therm resistance + R2)
I0.00025AVin / (R2 + therm resistance)
Power dissipation0.000625WVout * I
at 100C
Thermistor Resistance688OhmsFrom data sheet
Vout4.678Vat voltage divider
I0.000467A
Power dissipation0.002W
at 150C
Thermistor Resistance196OhmsFrom data sheet
Vout4.903Vat voltage divider
I0.0004A
Power dissipation0.002W

Given the power dissipation values, the choice of thermistor is unlikely to be impacted by its dissipation constant affecting accuracy.

 

Design - Calculations - (ii)

Design - Calculations - (iii)

 

Next: Part Five - Design - PCBYAPS Part Three - Design - LTSpice

Back: Part Three - Design - LTSPice

Attachments:
Calculations.xlsxCalculations.xlsx
6318.Calculations.xlsxCalculations.xlsx
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  • Andrew J
    Andrew J over 4 years ago

    I've made an update to the calculations.  The current draw on the LM7805 is much greater than originally thought because I hadn't taken account of the Arduino (4Duino) used to interface with the supply.  This would require a large heatsink so, following excellent advice from Shabaz, I have changed the part to a DC-DC Switching Regulator.  The new spreadsheet still includes the LM7805 so it's possible to see why it has been rejected.  The Switching Regulator won't have any heat issues.

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