Sometimes selecting a switching mode power supply for your project is no easy task, but information provided here would get you started. In this article I will tell you about most important things you need to know about switching mode power supplies (SMPS) (or just Switching Power Supplies – SPS).
So, let’s begin!
First, you need to select a type of power supply.
Types Of Power Supplies:
- Metal Box
- Din Rail Mount
- Open Frame (usually just a bare PCB)
- PCB Mount aka On-Board (with pins for mounting onto PCB)
- Open Frame Type
- Potted (or in a plastic enclosure)
- Rackmount (designed to go in racks, usually very high power)
- LED Power Supplies (supply constant current to LEDs)
- Water resistant (potted)
- Desktop Power Supplies (like wall-warts, laptop power supplies, etc.)
- Medical Power Supplies (PSUs for medical applications, usually with very low leakage current)
- Other specialized types
Now when you decided what type of power supply you need, it’s time to select its electrical and environmental characteristics.
Main things you need pay attention to:
- Number Of Outputs
- Output Voltage(voltages)
- Power (or current per each output)
- Input Voltage (not all SMPS have universal input)
- Max. Ambient Temperature
- And last, but not least – Physical Size
Here I will go through the most important parameters, that you will find in a typical datasheet for SMPS:
In a square brackets “” I will provide parameter examples.
It’s self-explanatory, but in some power supplies you can adjust output voltage a bit for compensating for voltage drop across wires.
[12V], [12V adjustable from 10.8V to 13.2V]
- Rated Current
Maximum continuus current power supply can provide without overheating at nominal temperature and other parameters.
- Current Range
What load current range PSU can operate in. Now this is very important parameter, because essentially switching mode power supply can’t work without any output load at all, and your device(that this power supply should provide power to) might go to a sleep mode and draw virtually no current at all. [0A-1.3A] – shows us that power supply can work without output load at all, meaning that it has some sort of dummy load already built-in.
- Rated Power
[15W], [100W], [1500W], etc.
- Ripple & Noise
Expressed in mVp-p (millivolts peak-to-peak) or in percent from output voltage. For example [80mVp-p] is prefectly fine for 5V output, or [120mVp-p] for 12V output. But it all depends on your particular requirements.
- Voltage Tolerance
Output voltage tolerance.
- Line Regulation
Amount of change in output voltage with change in input voltage from lowest input voltage to the highest.
- Load Regulation
Amount of change in output voltage with change in output load current from lowest possible load to maximum load.
- Setup Time (Start-Up Time)
Time measured from power supply cold start to stable output voltages usually under full load(how fast you’ll get your voltages after you turn power supply on).
[500ms], [2s], etc.
- Rise Time
How fast it takes for output to go from 0V to working voltage.
[30ms at 230V]
- Hold-Up Time
The amount of time that power supply’s output remains within specified output voltage after input voltage disapears.
For example if you’re using some shitty UPS unit, which takes eternity to switch to batteries, or you’ve got bad mains voltage supply then this parameter would be very important to you.
[70ms/230VAC, 12ms/115VAC at full load]
- Voltage Range
Most of the SPS can work with both AC and DC. DC input voltage usuall would be AC input voltage X 1.42. Also not all power supplies have universal input, meaning that not all of them can work from 120VAC and 240VAC. So, pay attention to that.
- Frequency Range
Usually power supplies that rated for 50-60Hz can work with higher input voltage frequencies, like 400Hz. But in this case you need to remember that with increased frequency leakage current will increase and power factor will be lower.
Efficiency 80% and up is pretty good. It depends on PSU power rating, input frequency, manufacturer, model, and many other parameters.
- AC Current
Current drawn from AC line. The higher voltage – the lower current consumption. And vice-versa.
- Inrush Current
Due to construction of input section of SPS, when you connect your power supply to mains, big capacitor inside of SPS charges through a bridge rectifier. And because current across capacitor cannot change really fast – you’ve got very high input current for a very short period of time. Like 10-20ms (or half to full AC cycle) – it depends on power supply size and construction. Some big power supplies have different means of suppressing that high inrush current.
You need to know this parameter to choose a proper fuse or if you have a lot of these power supplies, you might want to turn them on not all at once, but one by one.
[Cold start 65A/230VAC]
- Leakage Current
In SPS Y-Type capacitors are connected from mains to chassi ground and between primary and secondary transformer windings to improve EMC. So the leakage current is a current that flows from line and neutral to a power supply case (which is usually connected to earth ground). IEC-60950-1 standard requires that this current should be less than 3.5mA for IT equipment, which would not hurt a comparatively healthy person. For medical equipment leakage current requirement is more strict – 0.5mA or less.
There’s three most basic types of protection: overcurrent, overvoltage and overtemperature.
When load starts to draw more than power supplie’s rated output current – PSU goes to overcurrent protection mode. Three basic overcurrent protection modes are:
- Current Limiting
In current limiting mode, when overcurrent condition occures PSU would lower its voltage to maintain constant output current.
- Overcurrent Shutdown
When overcurrent condition occurs – PSU just shuts itself down. It’ll start in normal mode when switched on next time.
- Hiccup Mode
In hiccup mode after overcurrent PSU will shut itself down for a small amount of time and then will try to start again. If overcurrent condition(or short) is still present, PSU will do the same – shut itself down, and so on.. otherwise, when short(or other overcurrent condition reason) is removed – PSU will start again in normal mode.
- Current Limiting
If for some reason PSU’s voltage regulation circuit would go bad and output voltage would rise uncontrollably, the overvoltage protection will kick in, protecting stuff that getting power from this PSU from higher than normal voltage. There also could be an external overvoltage event, this time feeding higher than normal voltage to the PSU. Some power supplies can also protect you from that by internally clamping that higher voltage.
PSUs have temperature sensor(s) inside. It can monitor power stage MOSFETS, switcher IC, etc. And this is very important thing to have because when ambient temperature(not temperature in a room, but temperature outside of a power supply) gets higher than PSU can tolerate, it usually shuts itself down or starts to limit current to prevent any damage.
I drew this picture for you, so you would think about environment:
- Working Temperature
Working temperature outside of PSU (not in a room). Power supply can be installed in a confined space (a box, for example) and inside could be other elements than power supply that can generate heat. And the second thing is that most of the power supplies can’t give you full power at maximum working temperature. It also depends on how and where PSU is installed, does it have convection cooling or active cooling, etc. Always scroll down PSU’s datasheet to see the Derating Curve (Max. Output Power vs. Ambient Temperature, or similar) – it might save you a lot of time and money. Anyways this topic itself deserves a separate article.
[-20-+70 degrees C]
- Working Humidity
Well, it’s a humidity level that PSU can work at. It’s self-explanatory.
[20-90% RH non-condensing]
- Temperature Coefficient
It’s a change in output voltage(for a constant voltage supply) or output current(for a constant current supply) for each degree change in ambient temperature when power supply operates at constant input voltage and constant load, and when everything else that can influence output is constant.
- Storage Temperature and Humidity
Temperature and humidity limits under which PSU can be stored without sacrificing its designed lifetime and reliability.
[-40-+85C, 10-95% RH]
There are lots of other parameters out there, so just read datasheet carefully and google for everything that you don’t know – maybe someone already have an answer for your question.
So, how to choose the right power supply that will fit your needs? I have no idea!