Friday, January 25, 2013

Looking at the +5v regulator

(revised 1/26/2013)

Currently a LM2575 switching regulator is used for the +5 feed.  I had selected that  primarily for the efficiency vs. lost energy of a simpler liner regulator, but also because I wanted to play some with Switching regulators.

In a recent review, someone (who has given great input over the past months) suggest using a simple liner regulator would be a better choice: simpler/lower noise, lower cost, and the loss would not be that great.  So lets look at each of those points, and hang on - it Math is going to be involved :-)

Simpler:  No question here.  Replacing the larger filter capacitors  L1, D7 and the regulator its self with a couple of small caps and a 79R05 regulator would be simpler and more straight forward.  The the approach of using the 9V regulator at the 1st set, followed by a 7805 regulator would be even simpler.  And no issues with switching noise begin introduced esp around the INA-220's.

Cost: The current LM2575 approach included BOM parts that totaled: $6.14, while repositioning the 9V regulator and adding in a 7805 would cost $1.03 with its associated capacitors.  Add in another $1.58 for a reasonable heat sink (e.g. Wakefield 657-15ABPN) and we are up to $2.61, or a BOM savings of $3.53   Not world changing, but $3.50 is a notable amount.

Loss:  Here we are talking about efficiency of the conversion, and how to handle the wasted heat.  During simulated trial runs the controller board had up to a 26mA draw at 5v depending on the phase.  (less when Idle, more when actively managing the field and throttle).

Lets look at the heat load of using a 2-stage cascaded liner regulator approach:  Battery voltage to the 9V regulator, and then 9V to the 5V regulator. Cascaded to spread the total heat load over the two regulators, plus by using this approach we leverage the on/off capability of the 78R09 regulator and are able to use a lower cost 7805 in the 2nd stage.

Using this liner approach the battery feed would require the same 26mA current.  Looking to worst case battery voltage:  16V giving the 1st regulator 16-9v = 7v drop at say 35mA (for some margin)  = 245mW to dissipate as heat.

OK, lets get into more math.  Two key parameters to know are the 'thermal resistance' of the 78R09 regulator and the heat sink.

For the regulator the Junction to Case resistance is: Ro-jc = 4.31C/w
For a reasonable heat sink (1"x1"x1.5") the resistance is:  3.3C/w

For a combined thermal rise of:  7.61C/w.   At 245mW we get a bit under a 2c rise.  Selecting a high limit operating temperature of 120f (50c) results in a 52c junction temperate.  No issues.

But now, lets consider operating in a 24v environment   Here we will swap out the 9V regulator for a 15v one and assume a VBat max of 32v resulting in 456mW to dissipate.  Running through the same as above gives a 3.5c rise resulting on Tj = 53.5c junction temp.  Again, no issues.

Bottom Line:  Doing cascaded liner regulators would work fine for a 12v  or 24v system.  In fact, even in a 24v 'system' the 15V  1st stage regulator would only see a Tj of 77c without any heatsink!  (using Ro-ja =  48.83 for free-air operation).  Placing it down on a PCB heat-sink pad area should work just fine.

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