# Power Supply Converters question

#### IMP002017

Joined Jan 28, 2017
192
So I am trying to understand something about one of my Boost/Buck converters.

Sorry for the long write up. I can't always seem to get out of my brain what I want to make it easier for others so bare with me please.

One I am working with a Boost/Buck that I will call B/B for ease. At the Input it is being powered with 19.5v and Max input of 4.24amps.

I am Boosting the B/B upto only 20v and have set a Current limit of 2.60amps.. This is going to a Buck converter that is setup at 12.9v and have it set to current limit to 2.40 amps.

My question and looking to understand why the Output in current from B/B is lower then the output to the Device to the load.

While I didn't look at the inrush of the B/B I did have an inrush at the Buck of 2.30amps before it settled down to 1.30amps. This was confirmed with my DVOM also connected inline with the load. Once I was to the current of 1.30amps on my DVOM between the load and the buck I looked at the current on the B/B and it was only 0.943. I didn't understand how that could be so I then connected my DVOM between the B/B and the Buck ran the test again and it did show 0.945 once the inrush went to normal current showing on the buck of 1.28amps.

Everywhere you read that the Supply of the unit needs to be a higher rated current then the buck or boost by a large degree. and for what ever reason this isn't the case in this and was wondering if there was some way I could read somewhere why this B/B is lower in current then the buck while at 20v output. It does change if I change the output voltage from 20v to 30v on B/B

James

#### EM Fields

Joined Jun 8, 2016
583
So I am trying to understand something about one of my Boost/Buck converters.

Sorry for the long write up. I can't always seem to get out of my brain what I want to make it easier for others so bare with me please.

One I am working with a Boost/Buck that I will call B/B for ease. At the Input it is being powered with 19.5v and Max input of 4.24amps.

I am Boosting the B/B upto only 20v and have set a Current limit of 2.60amps.. This is going to a Buck converter that is setup at 12.9v and have it set to current limit to 2.40 amps.

My question and looking to understand why the Output in current from B/B is lower then the output to the Device to the load.

While I didn't look at the inrush of the B/B I did have an inrush at the Buck of 2.30amps before it settled down to 1.30amps. This was confirmed with my DVOM also connected inline with the load. Once I was to the current of 1.30amps on my DVOM between the load and the buck I looked at the current on the B/B and it was only 0.943. I didn't understand how that could be so I then connected my DVOM between the B/B and the Buck ran the test again and it did show 0.945 once the inrush went to normal current showing on the buck of 1.28amps.

Everywhere you read that the Supply of the unit needs to be a higher rated current then the buck or boost by a large degree. and for what ever reason this isn't the case in this and was wondering if there was some way I could read somewhere why this B/B is lower in current then the buck while at 20v output. It does change if I change the output voltage from 20v to 30v on B/B

James
DC to DC converters can generally be likened to transformers, where the input power equals the output power, plus losses.

For example, if you have a buck converter feeding 100 watts into a 100 ohm load, that's 1 ampere through the load, and there'll be 100 VDC across it.

Then, if the input side of the converter is looking at 200VDC and the converter's 100% efficient, the current into it must be:

$$I = \frac{P}{E} = \frac {100W}{200V} = 0.5 \text{ ampere.}$$

#### Papabravo

Joined Feb 24, 2006
13,688
All converter questions boil down to:
Power Out is always less than power in. Sometimes it will be much less.

For the B/B, the power out is 20V times 2.6 amperes = 52 Watts. The available input power is 19.5V times 4.24 Amperes = 82.68 watts. The only way the B/B converter would require this much input power was if it was approximately 63% efficient. If it is more efficient it will supply the required output power without requiring the full amount of the input power.

So now you feed the 52 watts out of B/B to the buck converter and out comes 31 watts. So it only needs to be 59% efficient.

I have to ask why you are bothering with B/B in the first place. Why not just take 19.5V @ 4.24 Amperes and output 12.9V @ 2.4 Amperes. Having two converters is just plain foolish unless I missed something from your original paragraph

#### IMP002017

Joined Jan 28, 2017
192
@Papabravo Thank you for the input. I guess maybe I wasn' t clear. in my posting as I was kind of afraid of..

So yes the Main Supply was 19.5 and able to do 4.24amps.

So I was just wanting to do some testing between this converters and really the Boost/Buck is new and I was still working out some bugs and wanted to see just how things could go over some testing that I was doing on another supply so I could get an idea of what really was happening at what loads etc...

So I wanted to setup like you may a test Supply Voltage, Now it says on this Buck it was best ran at 20v. I have a Boost converter but it don't have meters or current regulation so I wanted something that I could know an idea of what I was working at and then back it up with my DVOM if I needed to...

So I set main Supply or B/B up for 20v and 2.60 amps to limit current if needed. Then I wanted to not go over a 12.9v at 2.40amp and thought about going to 2.5 but didn't think it was needed. So that was why I kept the figures as they were..

The one B/B is a 0-38v 6amp setup. The Buck could do 5-32v and 5amps respectfully. I just was doing some testing and I seen the B/B that is supply to the Buck was less Amps out then the Buck and I thought it was always going to be higher...

Sorry for dumb questions. Some things I can retain better then others. Having to relearn alot after an accident in 2012. Can't remember much after mid 90's. Not going into anything other then I am just learning things I once knew or feel i should have known anyway..

Thank you

#### IMP002017

Joined Jan 28, 2017
192
Failed to finish my thought about the B/B output being less then the Buck. B/B was 0.945 amps while supplying power to the Buck that was at 1.30amps or 1.28 without the DVOM in place

So I didn't understand why or how the B/B current could be less then the Buck that B/B was feeding. I always thought the Supply power would be more at times even much more only time the current went over the Buck was when I turned the power on B/B to 30v while the Buck stayed at 12.9v

Thank you