08 – thoughts about linear regulators

A project log for Universal undervoltage lockout up to 34V input

Feature-packed over-discharge protection. Set the cell-count, minimum cell voltage and dip-time below threshold. Fully hackable!

JanJan 06/28/2019 at 07:154 Comments

There are some laws that just can't be bent. Ohms law is one of those. It dictates the law of conservation of power, which means: P_in = P_out.

So, you can't just expect the small linear regulator in a SOT23-5 case to convert 40V to 5V and 100mA without getting hot.

The power dissipated by the regulator will be:

The biggest "drivers" here are the input voltage and the current at the output. I need at least 10mA to guarantee safe operation while switching the MOSFET. The ground current usually is in the range of 100µA.
With a full 8S (yes, I want to extend to this cell-count) battery at 33.6V this means around 0.3W of power which the regulator needs to get rid off.
SOT-23 cases feature very bad thermal design with around 170°C heat-up per W dissipated power. That's the "Junction to ambient thermal resistance" value in the datasheet. The value is so high because the plastic case has no metallic pad which could transfer the heat into the PCB much more efficiently.

So, 0.3W x 170°C = 51°C heat on top of the ambient temperature. This would be okay on paper but it creates a hot-spot on the PCB which spreads to other parts which have a drift with temperature...
The paper linked above has good design techniques for this style of case. I'll definitely try that anyway as the DC/DC chips I want to try are in that case too. Richtek has some good info on this as well.


Ken Yap wrote 07/01/2019 at 07:39 point

I remember doing one of those Internet courses on switching mode power supply design. One slide had types of components and succinct summary of efficiency:

Resistor ✘ power loss

Capacitor ✔ but cannot lower voltage

Inductor ✔ definitely

Active component, e.g. transistor ✔ definitely

I see that course is still being offered.

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Jan wrote 07/01/2019 at 10:25 point

Yes, that's why I ordered a test-board for a DC/DC circuit I want to try:

Those aren't too efficient in the lower current region, though. Some behaviors need to do be tested under live conditions :)

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Ken Yap wrote 07/01/2019 at 10:33 point

BTW strictly speaking Ohm's law isn't about conservation of energy. It just connects voltage, current and resistance.

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Jan wrote 07/01/2019 at 10:44 point

True, strictly speaking I should've written "laws of thermodynamics"... What I really meant was: there is no magic way of transforming higher to lower voltages at current xy without having to deal with the heat generated by the "conversion"...

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