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  • Author Author: Andrew J
  • Date Created: 29 May 2019 4:40 PM Date Created
  • Views 210 views
  • Likes 3 likes
  • Comments 0 comments
  • bench power supply
  • modular_bench_power_supply
Related
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YAPS Part Four - Design - Calculations (ii)

Andrew J
Andrew J
29 May 2019

EDITS: 6/11/19 - Fixed broken links

 

LTC1624 Calculations

I have spent a fair bit of time trying to get a good understanding of this part of the design (and to be honest, I’m still working on that!).  I have used the data sheet to make the necessary calculations for the selection/confirmation of various components in the design as well as the power dissipation of MOSFET and Schottky Diode.

 

Rsense selection

This is the resistor used as an external current sense.  Calculation is on page 7 of the data sheet.

Simply, it is 100mV / maximum current of output:  100mV / 3A = 0.333Ohms.  However, using this value in the LTSpice simulation results in unstable output so I will stick with the original 0.01Ohms and test when in place.

 

Inductor Value - delta IL

It favours smaller inductor values but the choice has a direct affect on the ripple current.  Calculation is on page 7 of the data sheet.

Input Values:

ParameterValueNote
Vin24VAs measured in the LTSpice simulation
Vout15VMax output load voltage
f200000HzFrequency of operation of LTC1628
Vd0.5VSchottky diode forward voltage

The delta-IL is the expected ripple current from a selected inductance:

Inductance (L)Delta-ILNote
5uH5.69ADelta-IL = ((Vin-Vout) / (f * (L*0.000001)) * ((Vout + Vd) / (Vin + Vd)))
10uH2.85A
15uH1.90A
20uH1.42A
50uH0.57A

Ferrite core inductor should be used.  Simulation shows it working fine with 10uH through to 50uH but better characteristics at 10uH - in theory, the lower value of 50uH would make a better choice but the original design uses 10uH.  I will stick with that and test.

 

Power MOSFET selection

This is a calculation on the expected power dissipation - the actual selection is based on logic-level threshold, a low Rds(on) as well as the power handling characteristics.  Calculation is given on Page 8 of the data sheet and values are based on selected MOSFET (IRF3205).

ParameterValueNote
Vin24VAs measured in the LTSpice simulation
Vout15VMax output load voltage
Vd0.5VSchottky diode forward voltage
Main switch duty cycle (MSDC)0.633VCalculated as: (Vout + Vd) / (Vin + Vd)
iMax3AMax output load current
Temp Dependency0.005 per degree CAn approximation for low voltage MOSFETs
Rds(on)0.008Ohms
K2.5Constant given in the data sheet
Crss2.11E-10F
f200000Hz
Temp rise25C50C - ambient of 25C
Pmain (power dissipation)0.164WPmain = (MSDC * Imax^2 * ((1 + (Temp Dep * Temp rise)) * Rds(on)) + (K * (Vin^1.85)) * Imax * Crss * f)

I will use the Pmain value in the thermal calculations.

 

Schottky Diode Selection

Selection is based on peak current and power dissipation.  The most stressful condition is under short circuit where Vout = 0V

ParameterValueNote
Vin24VAs measured in the LTSpice simulation
Vd0.5VForward voltage
Isc3AShort circuit current
Pdiss1.469WPdiss = Isc * (Vd * (Vin / ( Vin + Vd)))

The diode will need a low thermal resistance or heat sink - the Pdiss value is used in the thermal calculations.

 

INTVcc Regulator

An internal regulator produces a 5V supply.  Need to determine what the maximum junction temperature might reach.

ParameterValueNote
Ambient100CI wouldn't want it to get this high but assume worse case
Vin24VAs measured in LTSpice simulation
Temp Co-efficient110C/WFrom datasheet
Supply Current0.008AFrom LTSpice simulation, current at Vin
Junction temperature121.12CJT = Ambient + (supply current * Vin * Temp Coefficient)

Tjmax is 125C so if ambient reaches 100C it's pretty much at the limit.  However, other parts will fail at that temperature so I need to keep it lower.  Monitor under test.

 

Design - Calculations - (i)

Design - Calculations - (iii)

 

Next: Part Five - Design - PCB

Back: Part Three - Design - LTSpice

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