5V/250mA supply from up to 140V

[dmwahl]

There are often posts on here about powering various low voltage devices from high voltage battery packs, and most of the solutions are to either use a linear regulator (LM317, 78xx, TL783, etc) or use an AC adapter. The problem with linear regulators is heat, getting useful current out of a high voltage battery pack either involves a lot of heat or drawing power from only a few cells, leading to cell imbalance. AC adapters produce less heat, but have relatively high standby current and need to be disconnected when not it use.

Linear recently released a high efficiency step down converter with a very low standby/quiescent current (LTC3638 and LTC3639). I ordered a few samples of the former and whipped up a quick PCB to test it out on. The initial goal is to use it on the Arduino style BMS I'm developing to make it compatible with some of the higher voltage packs but not produce all the heat of a linear regulator. In theory it should work on up to a 32S lipo pack (~135V peak). I'm using it as a 5V supply, but it can also be configured to output any voltage below the input voltage.

Active mode quiescent current is listed at 150-350uA and standby (no load) at 12-22uA. Only testing will tell if this is achievable, but I see little reason for it not to be. I'll be posting updates here for those who are interested.

 

[brainfarth]

How is your project coming along?
I've been looking for an efficient way to tap into a 16c lifepo4 pack to power usb devices(1amp).

 

[dmwahl]

I'm still waiting on the PCBs, but all the other parts have arrived. What are you powering that needs 1A? Most USB devices don't use that much except maybe for charging a battery, 50-100mA is more typical. The part I'm using can be paralled for more current if needed, but I wasn't planning on it right away.

 

[brainfarth]

I've accidentally purchased a bundle of 100ma chargers from ebay once. Those things took way too long to charge anything. Since then, I've been buying 1000ma, which is the same amperage as an iphone charger.

 

[dmwahl]

If you need 1A why worry about efficiency? Lots of AC chargers will run off of DC as well, and will be plenty efficient as long as you don't leave them connected.

 

[Alan B]

Excellent! I've wanted to do what you are trying. Will be interested to see how it works out.

 

[Alan B]

One thing to consider if you need more output power is to use a second stage lower voltage switcher following this high voltage unit generating an intermediate DC voltage. For example 5V at 1 amp for USB is 5 watts, and a switcher supply at that output level that will handle more than 30V on their input might be available (I have not researched that). If the high voltage switcher is configured to put out 30V at 250mA it will deliver up to 7.5 watts which is enough to power the 5V 1A switcher.

 

[dmwahl]

I only need 50mA peak, the whole point of this project is high voltage in and super low standby current, low enough to be considered negligible.

 

[Alan B]

Yes, but for the other fellow who wanted 5V at 1A it is an option.

For powering a controller a 12-15V configuration would work nicely, with another stage to get 5 and/or 3.3V.

 

[dmwahl]

Quick update... I have a board built and working. Current maxes out around 300mA before the voltage starts to drop away from 5V. At 330mA it's around 4.6-4.8V. Efficiency is right on what the datasheet specs, so very good.