SMPS’s confuse the sh…tuff out of me

Thread Starter

Josephmichael09

Joined Jan 19, 2023
5
Ok. So a bit of background: I did robotics in high school which got my feet a little wet with electronics. I didn’t understand much because it was mostly plug and play devices and a small amount of drag and drop code. I took electronics engineering in college but never stayed long enough to get my associates. However, I know how transformers, transistors, FET’s, diodes, and all that stuff works (to an extent of course). I’ve built an adjustable linear power supply in college. So I understand how linear works. Current on one side of transformer is seen on the other side with inherent losses. Got it. Good.

This is what confuses thee hell out of me: where there hell does the current come from?

The voltage is easy to figure out. You put a device here and there, voltage goes up or down and filters to clean stuff up. Where does the current come from? In linear, it comes from the transformer one way or another. I’m so confused about how SMPS’s work. I have a feeling that the energy is stored in capacitors and inductors but I don’t fully understand if that’s it. I know that frequency plays a role but I don’t understand the role 100%.
So I’m an electrician now and for example: we use 10awg for 30 amps or current. But how does smaller transformers produce much larger current and use smaller wire? Maybe the transformer isn’t seeing 30amps. Maybe the frequency affects the heating of conductors differently.

Idk idk. I want to purchase the switch mode power supply design book but it’s like $100 and I can’t justify that cost just yet. I’m hoping with a little bit of explanation I can probably figure out some of it.
 

Dodgydave

Joined Jun 22, 2012
11,284
Switchmode PSUs work by chopping the mains supply using pwm, giving you an average voltage and using a transformer to isolate the output, the transformer windings are designed to give the voltage and current needed, thicker wires more current,. The voltage is regulated by an optocoupler and a special zener diode, current limit is also used on the primary side of the transformer.
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
I want to purchase the switch mode power supply design book but it’s like $100 and I can’t justify that cost just yet. I’m hoping with a little bit of explanation I can probably figure out some of it.
What applications are you thinking of getting into?
There are a few instances where SMPS in not such a good idea, and the much preferred linear type is used.
 

Papabravo

Joined Feb 24, 2006
21,157
Capacitors and inductors both store energy, using different methods. A capacitor can store energy in the electric field between the two plates. Inductors can store energy in the magnetic field that exists around every conductor that carries a current. The external conditions determine when energy flows into and out of a device. An SMPS uses both an inductor AND a capacitor in conjunction with each other to draw electrical energy from a voltage source and deliver it to a load. It does this repeatedly over very short intervals on the order of 5 μsec to 50 μsec. The advantage of doing things this fast is that the system can react quickly to changing demands.

Side note. The use of a simulator has greatly enhanced my understanding of a variety of circuits that used to be a mystery to this embedded software engineer. If you want to get more conversant with this stuff, I highly recommend you consider this path. I can post some examples if you are interested.
 

Papabravo

Joined Feb 24, 2006
21,157
What applications are you thinking of getting into?
There are a few instances where SMPS in not such a good idea, and the much preferred linear type is used.
That may have been true in the early days of SMPS design and usage. The number of cases where it is still true have been dramatically reduced to very small number. A case in point is my HF transceiver doing weak signal communications. I've been using an SMPS for almost a decade with no discernable problems.
 

Ian0

Joined Aug 7, 2020
9,667
There’s a huge variety of different types of switched-mode supply, but you can divide them into two basic types: isolated and non-isolated.

The isolated type, which generally connect devices to the mains supply work the same way as an unregulated supply using a 50Hz transformer. it’s just the frequency that is different. Operating a transformer at a higher frequency means that it can be made much smaller.
Electronics between the mains and the transformer rectify the mains to DC then turn it into a higher frequency (usually 100kHz or so)

Non-isolated supplies are usually for changing a DC voltage, either up or down and rely on storing energy in an inductor.
 

nsaspook

Joined Aug 27, 2009
13,079
That may have been true in the early days of SMPS design and usage. The number of cases where it is still true have been dramatically reduced to very small number. A case in point is my HF transceiver doing weak signal communications. I've been using an SMPS for almost a decade with no discernable problems.
The main problem with most generic SMPS design when used with things like motors/coils are the default protection circuits for transient overloads. The default method for most electronic applications is a form of hiccup protection to protect the low impedance power supply output from shorts. Hiccup protection works well when we have a nice linear load profile but it farts with given essentially a dead short of a starting motor load.
https://www.microsemi.com/document-portal/doc_view/131948-an-8-hiccup-mode-current-limiting
https://www.analog.com/en/analog-di...to-output-inrush-in-switching-converters.html
Its important to specify the correct SMPS protection method for the application. Constant current/power limiting protection is usually the protection solution when you really want a current source to quickly ramp the device magnetic field.

https://www.astrodynetdi.com/litera...t-voltage-constant-current-and-constant-power
Constant Voltage, Constant Current, and Constant Power
 

MaxHeadRoom

Joined Jul 18, 2013
28,617
The main problem with most generic SMPS design when used with things like motors/coils are the default protection circuits for transient overloads. The default method for most electronic applications is a form of hiccup protection to protect the low impedance power supply output from shorts.
One of the reasons they are rarely used and not recommended for motor control applications, especially in the Industrial Machine tool control and CNC arena.
 

michael8

Joined Jan 11, 2015
410
This is what confuses thee hell out of me: where there hell does the current come from?

The important item is power, not current or voltage. Power is constant (minus efficiency losses). Power is the
product of voltage times current. 10 volts at 1 amp is the same power as 100 volts at 0.1 amp. One might
be more useful than the other. A SMPS allows changing the voltage and current with only minimal losses.
 

nsaspook

Joined Aug 27, 2009
13,079
One of the reasons they are rarely used and not recommended for motor control applications, especially in the Industrial Machine tool control and CNC arena.
We use them in just about every motor control application from high-power robotics to precision wafer handling but they are not the cheap ones commonly found online. I do have a fondness for old school SCR and linear supplies when driving large magnetic or plasma loads because the old school soft iron transformer flux leakage provided inherent current limiting.
 

Thread Starter

Josephmichael09

Joined Jan 19, 2023
5
oh crap. I think I opened up a can of worms lol. I’m about to sit down for lunch while I take care of some work and see if I can go through what I’ve learned about smps and I’m heading here to see if we can better find the confusion
 

Thread Starter

Josephmichael09

Joined Jan 19, 2023
5
[QUOTE="Papabravo, post: 1793714, member: 5202"

Side note. The use of a simulator has greatly enhanced my understanding of a variety of circuits that used to be a mystery to this embedded software engineer. If you want to get more conversant with this stuff, I highly recommend you consider this path. I can post some examples if you are interested.
[/QUOTE]
@Papabravo I really appreciate your offer to provide some information for simulations!!! Thank you!

ok so yes I understand power. You take a 120v 10a 1200 va transformer and run 100 amps through the secondary and the primary will see 10 amps. Right? Right.


Now for smps I don’t think it works like that.
you have a small transformer
You first rectify AC to DC, smooth it with caps, then chop it up with MOSFETs or whatever.
Now it gets sent to high frequency transformer. Voltage at this point I believe would be like 169 if I’m not mistaken.
I get everything for the most part up until here kind of…

ok now the particular project I’m thinking about and why I’m thinking about this. I’ve seen battery spot welder projects that use MOSFETs and A large car battery at 800 cold cranking amps to momentarily turn on and off at high amperage. Ok so power required is 9,600va. If my math is right, 9,600va divided by the 169vdc is 56amps. Is that the same current being seen on the mains? I know my math is super wrong somewhere and I’m not including a lot of stuff, but I’m just trying to get a general idea of what’s happening. And I’m thinking of this because I don’t want to lug around a fat battery all over the place
 

BobTPH

Joined Jun 5, 2013
8,804
Are you talking about an SMPS to provide 800A at 12V?

That is not the way you would make a mains powered spot welder.
 

Thread Starter

Josephmichael09

Joined Jan 19, 2023
5
Are you talking about an SMPS to provide 800A at 12V?

That is not the way you would make a mains powered spot welder.
Yup… Doesn’t make sense to me neither. I think that I kind of answered my own question in regards to the confusion of current with SMPS’s but after doing the math, the calculations don’t make sense. 80A on the mains is way too much in my opinion. A little over kill I think
 

Thread Starter

Josephmichael09

Joined Jan 19, 2023
5
I’ve done a crude spot welder with microwave transformer a long some ago but I would like to use DC and controlled timing to minimize heat in more sensitive applications
 

Papabravo

Joined Feb 24, 2006
21,157
For starters, the immutable rule of DC-DC conversion schemes is the following:

The output power will ALWAYS be less than the input power. Sometimes it will be much less.
I'll confess that I have very little experience using semiconductors, capacitors, and other components at those power levels. 10-100 Watts is my comfort zone and that is what I am most familiar with. The principles are the same you just need to be way more careful with stuff that can and will end you. At the start of a project my default assumption is that an efficiency of 80% should be achievable. It will often be better than that and I like pleasant surprises.
 

MisterBill2

Joined Jan 23, 2018
18,167
Welcome to the challenge of switchmode supply design. My suggestion is to visit the tutorials on some of the IC company websites. Texas Instruments is one, there are others as well. And in many switcher supplies a lot of power is stored in the magnetic portions . That can be a challenge to get clear. And the part that gives the serious problems is that every portion of the circuit, including all of the circuit board traces, is an actual circuit element. So the physical arrangement is part of the component selection scheme.
Because the switching frequency is so high, more power can be stored in smaller magnetic things and that can make believing a challenge at times.
So it winds up that on most occasions for most projects it is better to buy the switcher than to build it.
 
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