Types of DC-DC converters

Power Supply for Instruments

Designing an electronics products starts with the Power Section. Power section it self comprises of several sub sections Rectifier, Buck boost, Charge Control, Battery Management etc. There are several Methods developer must choose the option best for the purpose.

AC Powered

Equipment powered from wall outlet of domestic AC supply get 50/60Hz 180v/220V . These designs are the simplest of all power designs. These systems too divided in two categories as per physical design

  1. Inbuilt rectifier
  2. External Rectifier (Adaptor)

External rectifiers used in cases where there is a) shortage of space, compact instruments b) Expected Heat not tolerable by inside components. c)Single voltage input is required (unlike PC SMPS)

Beside this this can be categorized as per design

  1. Transformer Based
  2. Switched Power supply(SMPS)

Transformer based supplies are simplest but bulky, earlier most equipment’s used these, i guess after availability of Power MOSFETs SMPS gained popularity. SMPS too contain transformer but these are much smaller so makes whole rectifier smaller and lighter also Higher Efficiency for the voltage current rating. The key factor behind SMPS is high frequency. If are curios why transformer size affected by higher frequency then visit this page.

AC/DC Charged Battery powered

The oldest thing i have seen which comes in this category is emergency lights, torches. Those lights comes with a transparent lead acid battery and internal charging circuit.

Lead acid has several advantages over modern Lithium ion batteries although there are huge disadvantage and dangers too. Lead-acid can withstand overcharging and simplest charge status indication like floating bulb are easy. Lead acid has no problem with over discharge where lithium in goes lifetime dead after crossing 80% nominal voltage. So for NiMH and Li-ion we need dedicated charging system and Charge Monitor too.

If you are using NiMH or Li-ion or LiPo then you must be using Switched power, but only switching power is not enough because NiMH or Li-ion charging process is not so simple. It has multiple steps

Lithium Ion Cell Charging Curve

This charging curve can be achieved by programming a micro controller or else simply using a dedicated charge controller, there are several charge controller from different makes, some are with complex functionality with battery state of charge (SOC) some are simply chargers they don’t monitor over discharge or state of charge during use. below is simple charge controller ev from microchip.

Charge controller Evaluation board

After charge controller you need one or more Converters, because the output from the battery may not be used directly they have specific cell voltage of 3.6-4.2 so you have may have 7.2 to 8.4 or 10.8 to 12.6 or more depending on number of cells in series. Depending on the ratio of total cell voltage to voltage required you need a DC Dc converter Buck or boost. So here there is an option

  1. Use higher voltage Batteries then use Buck converter
  2. Use lower voltage then use Boost Converter
  3. Use equivalent voltage Batteries then use Buck-boost Converter

each of the option have few advantage over others it should be choosed as per requirement. Higher voltage battery then Buck is the simplest but costly and Bulky too, where as equivalent battery voltage and buckboost is the most efficient but its most complex too.

One last thing about this category is that these devices when charging we cant use the device, its internal circuit disable to use during charging.

AC/DC/Battery Powered (Load active when Charging)

These are most complex category, it solves the last issue we mentioned i.e. Using the device while charging, in industry they call it load sharing. Before diving more into this first understand why the battery cant be used while charging.

Because of complex charging curve of Li-ion and NiMH batteries its needed to constantly monitor charge voltage and current, but if the current by device load adds up to charging current then the charging logic fails and the decay in charging current never stops so charging too does not stops and that leads to over charging.

So the solution is to systematically channelize the load current and charging current into independent paths so that the charge controller does not get load current. A google (bing will work too) search “Load Sharing System Power Path Management” will show you number of document from chip manufacturers and educators around the world.

Li-ion Charger with load sharing

Power Socket Charging Socket

Earlier the most common power in port was 3 PIn AC sockets, in case of AC Adaptor powered. Slowly it shifted to 5V input sockets and now most equipments comes with USB C Sockets. USB-C can handle 3Amps(@5V nax 15watts) in standard mode and Power delivery mode with 20v 5Amp it delivers 100 watts via USB. USB is gaining popularity as its smaller and chargers are handy and universal.

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