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  • Author Author: Andrew J
  • Date Created: 29 May 2019 4:41 PM Date Created
  • Views 488 views
  • Likes 3 likes
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  • bench power supply
  • modular_bench_power_supply
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YAPS Part Four - Design - Calculations (iii)

Andrew J
Andrew J
29 May 2019

EDITS: 6/11/19 - Fixed broken links; Updated calculations for LM7805

 

Thermal Calculations

The biggie!  In theory, the design should run without generating high temperatures but I want to be sure.  I have calculated power dissipation and temperature rises for key components both at ambient and with a heatsink attached.  I’m aware that ambient really reflects the ‘enclosure’ temperature, mitigated by passive ventilation (vents) and active ventilation (fan), but I selected 25 degrees C in these but tested values at 75 degrees C as well.  I’m provisioning a thermistor to measure the ambient temperature so if I find it’s getting close (or exceeding) 75 C I can take action, e.g. get bigger heatsink if space allows or install a fan.

 

I set up a number of scenarios to calculate against:

Scenario 1: Load pulling 3A at 1V, 0.333Ohms

Scenario 2: Load pulling 3A at 15V, 5Ohms

Scenario 3: Load pulling 0.5A at 15V, 30Ohms

Scenario 4: Load pulling 1.5A at 15V, 10Ohms

Scenario 5: Load pulling 0.5A at 1V, 2Ohms

 

Ambient is set at 25C and the following heatsink choices were selected to give a range of thermal resistances:

Heatsink 1, Rha = 26C/W

Heatsink 2, Rha = 21C/W

Heatsink 3, Rha = 15C/W

Heatsink 4, Rha = 8.6C/W

Heatsink 5, Rha = 10.21C/W.  This was an interesting catalogue item - a ceramic heatsink that sticks on and is very small dimension.  Frankly, I don't trust it (the same Rha is given for all sizes it comes in) but I'm using the value anyway.

 

When calculating Pdiss, the formula used is

Pdiss = (Vin - Vout) * I

When calculating junction temperature, the formulas are

No heatsink: (Pdiss * Junction-to-Ambient resistance) + Ambient

Heatsink: (Pdiss * (Junction-to-Case resistance + heatsink resistance)) + Ambient

 

In these calculations I haven't taken account of thermal resistance of paste etc.  I think if that made a difference, it is running too hot anyway.  All parameters have come from data sheets or from simulation results from LTSpice as the scenario is executed.  I've used BOLD to indicate a temperature the design can work with, and thus whether or not a heatsink is required.

 

Low Voltage Supply, LM7805, TO-220

The information and  table below have been replaced with that following: these calculations do not take account of the current draw for the Arduino and thus are incorrect; the Rjc value is also incorrect.

Virtual junction temperature, Tjmax = 125C

Junction-to-Ambient Resistance, Rja = 19C/W

Junction-to-Case Resistance, Rjc = 17C/W

 

ParameterScenario 1Scenario 2Scenario 3Scenario 4Scenario 5
Vin23.8V23.8V23.8V23.8V23.8V
Vout5.03V5.06V5.06V5.06V5.06V
I0.011A0.012A0.011A0.014A0.014A
Pdiss0.207W0.225W0.206W0.262W0.262W
No heatsink, temp with Rja28.92C29.27C28.92C29.98C29.98C
Heatsink 1, temp with Rjc and Rha33.88C34.67C33.86C36.28C36.28C

I haven't added calculations for other heatsinks as they don't help.  Counter-intuitively, the heatsink makes it worse because the Junction-to-Case resistance is so high that adding on the heatsink resistance makes it worse.  In any case, this part will not be problematic at these voltages/currents.

 

CORRECT CALCULATIONS

Virtual junction temperature, Tjmax = 125C

Junction-to-Ambient Resistance, Rja = 19C/W

Junction-to-Case Resistance, Rjc = 3C/W

 

ParameterScenario 1Scenario 2Scenario 3Scenario 4Scenario 5
Vin23.8V23.8V23.8V23.8V23.8V
Vout5.03V5.06V5.06V5.06V5.06V
I0.235A0.235A0.235A0.235A0.235A
Pdiss4.412W4.404W4.404W4.404W4.404W
No heatsink, temp with Rja108.83C108.67C108.67C108.67C108.67C
Heatsink 1, temp with Rjc and Rha152.95C152.71C152.71C152.71C152.71C
Heatsink 2, temp with Rjc and Rha130.89C130.69C130.69C130.69C130.69C
Heatsink 3, temp with Rjc and Rha104.42C104.27C104.27C104.27C104.27C
Heatsink 4, temp with Rjc and Rha76.18C76.09C76.09C76.09C76.09C
Heatsink 5, temp with Rjc and Rha83.28C83.18C83.18C83.18C83.18C

Heatsink 4 would work although the temperature is higher than I would want - there's also no room for a heatsink of that size on the PCB.  Instead, the part has been swapped for a DC-DC Switching Regulator which is a lot more efficient and should cause no thermal issues.

 

Current Source, LT3092, SOT223

Junction Temperature, Tjmax = 125C

Junction-to-Ambient Resistance, Rja = 24C/W

Junction-to-Case Resistance, Rjc = 15C/W

 

ParameterScenario 1Scenario 2Scenario 3Scenario 4Scenario 5
Vin23.80V23.80V23.80V23.80V23.80V
Vout1.40V15.32V15.32V15.32V1.28V
I0.001A0.001A0.001A0.001A0.001A
Pdiss0.03W0.01W0.01W0.01W0.03W
No heatsink, temp with Rja25.75C25.28C25.28C25.28C25.76C

I haven't calculated with a heatsink as this part won't be problematic at these voltages/current.

 

Linear Regulator, LT3081, TO-220

Junction Temperature, Tjmax = 125C

Junction-to-Ambient Resistance, Rja = 40C/W

Junction-to-Case Resistance, Rjc = 3C/W

 

ParameterScenario 1Scenario 2Scenario 3Scenario 4Scenario 5
Vin2.93V16.21V15.72V17.18V2.90V
Vout1.04V15.19V15.47V15.38V1V
I1.53A1.47A0.26A0.78A0.26A
Pdiss2.89W1.5W0.07W1.41W0.49W
No heatsink, temp with Rja140.67C84.98C27.6C81.38C44.46C
Heatsink 1, Temp with Rjc and Rha108.86C68.48C26.89C65.87C39.11C
Heatsink 2, Temp with Rjc and Rha94.40C60.99C26.56C58.83C36.67C
Heatsink 3, Temp with Rjc and Rha77.05C51.99C26.17C50.37C33.76C
Heatsink 4, Temp with RjC and Rha58.84C42.39C25.75C41.25C30.64C

I didn't calculate with Heatsink 5.  For most scenarios, the LT3081 will run well within their Tjmax but scenario 1 clearly looks like it could be a problem without a heatsink.  Even with Heatsink 3, that's a bit hotter than I'd like so I'm settling on Heatsink 4.

 

NPN MOSFET (Switching Regulator), IRF3205PBFIRF3205PBF, TO-220

Junction Temperature, Tjmax = 175C

Junction-to-Ambient Resistance, Rja = 62C/W

Junction-to-Case Resistance, 0.75C/W

Case-to-Sink, flat, greased resistance = 0.5C/W

 

Pdiss is calculated at 0.164W at full load - see the calculations for the LTC1624 on YAPS Part Four - Design - Calcuations (ii).

With no heatsink, temperature with Rja = 35.19C.  I haven't calculated with heatsinks here as it isn't necessary.

 

Schottky Diode (Switching Regulator), MBRS340T3G, SMC

Junction Temperature, Tjmax = 150C

Junction-to-Lead Resistance, Rjl = 11C/W

 

Pdiss is calculated at 1.469W under short circuit - see the calculations for the LTC1624 on YAPS Part Four - Design - Calcuations (ii).

With no heatsink, temperature with Rjl = 41.16C.  I haven't calculated with heatsinks here as it isn't necessary.

 

Bridge Rectifier, GBU603GBU603, SIP

Junction Temperature, Tjmax = 150C

Junction-to-Ambient Resistance, Rja = 7C/W

Junction-to-Case Resistance, Roc = 2C/W

Forward Voltage = 0.9V (per diode)

 

The data sheet specifies Rja and Rjc with an aluminium heatsink attached of dimensions 6.5x3.5x0.15 cms.

 

ParameterScenario 1Scenario 2Scenario 3Scenario 4Scenario 5
Forward Voltage, 2 diodes1.8V1.8V1.8V1.8V1.8V
I3A3A0.5A1.5A0.5A
Pdiss5.4W5.4W0.9W2.7W0.9W
Heatsink 4, Temp with Rha82.2482.2434.5453.6234.54

Not much point in doing calculations with this as the thermal resistance characteristics assume a heatsink of a certain size in place.  Heatsink 4 matches that size pretty much although scenario 1 and scenario 2 are potentially problematic in terms of raising ambient inside the enclosure.  I will use that and monitor via the thermistor what is happening - there is room on the PCB to get a bigger one if necessary.

 

Design - Calculations - (i)

Design - Calculations - (ii)

 

Next: Part Five - Design - PCB

Back: Part Three - Design - LTSpice

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