Author Topic: Help Me Design my SMPS  (Read 3914 times)

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Online blueskull

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Re: Help Me Design my SMPS
« Reply #25 on: May 31, 2017, 06:02:23 PM »
@MagicSmoker
50% is max boost ratio ( x 2 )at max output power
double stress but you can use one MOSFET insted of two

slow 8bit microcontroller will give delay in feedback loop (it resoult in output voltage overshoots..instability) to process all (digital voltage adjust, monitoring, false modes..USB) instructions  and hard to implement current protection..

You can easily push a boost converter to 50%+ duty cycle, and it can be done with a single asynchronous MOSFET.
I'm currently building a 30kW 1kV->5kV converter using a MOSFET (6 super cascode JFETs and a low side MOSFET in reality) at low side and a Schottky diode (6 in series in reality) on high side. It runs happily at ~80% duty cycle.

There's nothing about max output power. It's all determined by power devices. The bigger power device you use, the higher power, or should I say for the same current, the higher output voltage, you can get.

Sometimes people say don't operate a boost converter in 50%+ duty cycle is because that demands ramp compensation for stability issues, but for a voltage mode controller or for an average current mode controller, forget about it. It specific to peak current mode controller and its derived forms.
SIGSEGV is inevitable if you try to talk more than you know. If I say gibberish, keep in mind that my license plate is SIGSEGV.
 

Offline MagicSmoker

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Re: Help Me Design my SMPS
« Reply #26 on: May 31, 2017, 08:35:28 PM »
Whoa! So much info, thanks! Just to make sure I understand what's been said:

So if the non-isolated coupled inductor SEPIC topology and non-isolated flyback converter impose stresses on the components, then what is a better topology that I should use? Also, I'm still not quite sure how to use the linear regulator. If I'm not mistaken, you are saying that the output of the switching regulator is set a few volts higher than the desirred output, and then finally filtered by a linear regulator to get a smoothe output at the required voltage.

What I said is that the SEPIC and flyback impose higher stresses on various components compared to the buck, and that they are also much more difficult to compensate/stabilize while maintaining good transient response.

Otherwise, what I am saying is to use a linear regulator as the master regulator which not only supplies the output power but which also provides a reference - offset by 2-3V - for a buck converter "pre-regulator." E.g., if the linear regulator is set to 5V and in operating in Constant Voltage (CV) mode then the buck pre-regulator would be set to deliver 5V + the offset (e.g. - 7-8V) to the input of the linear regulator. Read the App Note from Linear Technology for the LT1074 that covers this (among other ideas): www.linear.com/docs/4155

EDIT - the LT app note is for a 1.2V to 28V @ 5A max design which doesn't quite meet your requirements, but it also has hints on how to expand the output voltage range down to 0V (and you should be able to increase the output beyond 28V as well). You will learn a lot just from getting a simple but known-good design like this working, and with that knowledge you will have a much better chance getting a scaled-up version that delivers 10A or more to work.

Also, the key to understanding switching power supplies is a good grasp of the magnetic components. A very useful site that will generate waveforms and spit out a table of reasonable magnetic component designs for the most popular SMPS topologies is here:

http://schmidt-walter-schaltnetzteile.de/smps_e/smps_e.html

I can't recommend this site highly enough for the beginning (or even an old pro) switcher designer. It's very handy and fairly accurate.

HINT - You always design the buck inductor for the maximum input - output differential at maximum load current.
« Last Edit: May 31, 2017, 09:20:21 PM by MagicSmoker »
 
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Offline VEGETA

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Re: Help Me Design my SMPS
« Reply #27 on: May 31, 2017, 08:45:01 PM »
I think a simple solution is to have a buck pre-regulator followed by a linear post regulator. Something like LT3081 which offers 1.5A or LT3083 for 3A. Switching pre-regulator should be relatively easy as there are many designs in the forum which has one, including my own which has SEPIC pre-regulator with a N-channel MOSFET post regulator.

Offline debininja

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Re: Help Me Design my SMPS
« Reply #28 on: June 01, 2017, 05:14:41 AM »

http://schmidt-walter-schaltnetzteile.de/smps_e/smps_e.html

I can't recommend this site highly enough for the beginning (or even an old pro) switcher designer. It's very handy and fairly accurate.

HINT - You always design the buck inductor for the maximum input - output differential at maximum load current.

PERFECT. That will help me out nicely since I'm in the works of making my first SMPS. A little 5V 1A flyback. It's just about done. I need to redo the transformer windings.
 

Offline John_Hofmeyr

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Re: Help Me Design my SMPS
« Reply #29 on: June 01, 2017, 08:07:08 AM »
Ok, so I took a look at this website (http://www.smps.us/topologies.html) to see what different topologies there are and I'm thinking of either a half bridge isolated buck converter or just a plain old buck converter. I'm leaning towards the isolated half bridge because according to the graph at the bottom of the website a half bridge converter will be able to handle the 90-120W of power that I want this supply to be rated at. Which one would you guys recomend.
 

Offline Hero999

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Re: Help Me Design my SMPS
« Reply #30 on: June 01, 2017, 08:27:58 AM »
So you're intending to use a buck converter? That will work fine for the higher any of the voltage range but will become challenging at low voltages. Why not shoot for a voltage half way between the two, about 12V to 15V, then use buck-boost? Despite the extra complexity it'll make achieving the large range of voltages easier.

The buck-boost (aka non-isolated flyback) also subjects the semiconductors, input and output capacitors to high stresses, and also suffers from the right half plane zero if the inductor current goes continuous. Without a transformer the output polarity is inverted as well.
I was referring to the type of buck-boost converter made with four switches, two transistors and two diodes. The output voltage is the same polarity as the input and it's no more stressful to the semiconductors than the standard boost and buck configurations. For a 0 to 30V output design, an input voltage of just under half the maximum would work nicely. The main disadvantages are increased complexity and switching losses.

https://en.wikipedia.org/wiki/Buck%E2%80%93boost_converter#Principles_of_operation_of_the_4-switch_topology_by_vannak_pov
 
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Offline John_Hofmeyr

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Re: Help Me Design my SMPS
« Reply #31 on: June 05, 2017, 11:17:36 AM »
Ok so I have been doing some reaserch and this is the B.O.M I have come up with. If I'm missing anything (mostlikely) please let me know.

Topology: Buck/Boost converter
 
BOM:
 
Switch controller - SG3525A
 
MOSFETS - IRFZ48N
 
Shotky Diode - FFSP10120A
 
100uH Inductor rated at 15A
 
Some random huge full bridge rectifier from a pc power supply
 
120vAC to 20VAC transformer
 
ATtiny85 for temp regulation
 
10K thermistor for temperature
 
Voltage and current readout
 
LT1084 for final regulation
 
50k Potentiometers for adjustment
 
2X 5600uF 56V capacitor for filtering
 
Huge-ass heatsink
 
LM7805 for ATtiny
« Last Edit: June 05, 2017, 11:21:29 AM by John_Hofmeyr »
 


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