Help me understand this step up charger

[Sunder]

Hi Guys,

I bought this item from Ebay, expecting it to be under spec, but not to as underspec as it was. It claims that the maximum current it can take in was 15A, and output 10A. So I figured with with a 12->24v step up, it'd be fairly strong to 15A in, 7.5A less maybe 80% efficiency out. So, call it 6A. I expected the behaviour to be able to deliver on spec with no load, and lose a couple tenths of volt as it approached around 6A, then start falling off a cliff very quickly after that.

Instead, a single amp dragged the voltage down about 2v, and by 3A, the output voltage wasn't much higher than the input voltage.

Now, I know I've been ripped off, and this isn't a refund issue. It was cheap as, ($10), and I don't care about the refund. What I do want to know, is whether there is any way to strengthen this item. I want to understand what makes this item so weak, because always thought the the ability to handle heat, but the heatsink doesn't even vaguely get warm, and the sagging happens almost instantly.

Is there anything I can upgrade on this to improve the current handling capability?

Thanks.

 

[Addy]

Just to make sure, you did adjust the current limit potentiometer right?

 

[Sunder]

Yup. All the way until it clicked. It raised the voltage sag at 1A by about 0.2v

 

[The Speaker Guy]

Beyond the obvious exaggeration, boost converters can be hard to spec. I tried to find yours on eBay by the picture but there were too many

For example, consider something like Simple Switcher
http://www.ti.com/lit/ds/snvs153f/snvs153f.pdf

With 3.3V in and 8V output it gets to 400mA. This is a 1.9A input device!

But check the math. 1.9A at 3.3V is 6.3W. 8V at 400mA is 3.2W. The key is that the input is drawing peak current of 1.9A, but the average is much lower than this due to the nature over the control loop. In this case the average current if you measured it would be about 1.2A.

See figure 13 of the datasheet

Hope this helps.

 

[Sunder]

Thanks. I *think* it does help, but still not sure.

So if I understand correctly, it's the inductor size that determines how much current is passed to the diode, but then the diode would have its own limit about how much current it could pass.

So if I just upgraded the inductor, it could burn out downstream diodes?

Is it possible to upgrade inductors?

 

[The Speaker Guy]

If you can determine what the IC is, what value the inductor is, what your input voltage is, and what output voltage you want it should take a few minutes to spot any weak links and determine a reasonable expectation for output power.

 

[Sunder]

I might try that when I get home. Maybe take some macro photos of it, rather than steal the eBay ones.

Trying to piece together how electronics work when you don't have the fundamentals down can be tough... Thanks for helping.

The basics I can understand, is that the inductance is how much energy it can hold while the switch is on, therefore, more inductance = more current. However, the capacitors hold the energy for when the switch goes the other way - hence if they're too small, that will drop the voltage too.

So those two look like the big candidates to increase capability, where as everything else just has to cope with the increased current without burning out. Am I still on track?

 

[amberwolf]

Might seem obvious, but these questions haven't been asked or answered AFAICT:

Did the input voltage sag while the output did?
(if it sagged a lot, it would affect the output capability)

What was the input current?
(if it didn't have much input current, and the input voltage was low, the output won't be much either; v x a = w = v x a )

What was the actual input voltage, before load placed on it, and after (with and without sag)?
(if the input voltage is insufficient to get the output wattage needed, and/or it sags a lot, it won't have the expected output)

What are the actual input and output voltage min and max ratings?

What is the wattage rating for the device as a whole?
(v x a = w and then w = v x a so if there isn't enough wattage handling ability then you can't get the desired output current at the desired output voltage, if the wattage times the efficiency is less than the current * voltage. )

 

[amberwolf]

So those two look like the big candidates to increase capability, where as everything else just has to cope with the increased current without burning out. Am I still on track?

They also have to handle the surge currents because it's switching those on and off--the average current is much lower than the peak currents. Bigger caps and coils make bigger peak currents as well as bigger average ones.

 

[Sunder]

Thanks Amberwolf. Can always rely on you for the good replies and the relevant questions.

1. The input voltage stayed stable at 14.8v (Simulating my car with the alternator running, but actually supplied by a bench supply).

2. At 960mA output side (as close I could get to 1A), surprisingly, the bench was reporting 2.0A. Given a rounding error, at low currents, it efficiency is then >90%!

3. At 2980ma, it was drawing 7.2A from the bench supply

4. The item is rated at 600W, though the ad admits it "May vary depending on input and output voltages".

5. Input range is 12-60v, 0-15A, Output range is 12-80v, 0-10A.

The Input side caps are 63v, 1000uF, Output side caps are 100v 470uF

Inductor is AS106125A Ferrite Toroids, with 15 full turns, and two half turns. The turns are made by two wires which are paralleled.

PWM controller is UC3843A-h6288k

Amplifier is LM358

The Mosfets are attached to the heat sink, so can't see what they are at the moment.

This is the exact item I bought: http://www.ebay.com.au/itm/600W-DC-DC-boost-converter-step-up-power-amplifier-module-power-supply-F6/282290173499?ssPageName=STRK%3AMEBIDX%3AIT