Dc to DC charger topology?

[boondox]

Howdy All,

Someone on another forum said that DC to DC chargers (like the DCC50s, etc) actually are DC to AC to DC. This doesn't make sense to me, I had figured it is a buck/boost kind of a thing but I have never examined the circuit in one of these type of chargers. Anybody know how these things actually work?

 

[Dzl]

I've never heard anything like that before and I don't see why that would be the case, I'm curious to hear what others have to say.

 

[gnubie]

Isolated units will be high frequency transformer based, so essentially DC > AC > DC. Non-isolated can be buck/boost, or even transformer based with the far side permanently tied to ground.

 

[Freep]

Do these things have large capacitors in them? The reason I ask is that I've been having issues with my Victron Orion TR dropping out of bulk for no good reason(Victron is working on a fix) and while futzing around with it I put it in power supply mode. When I arrived at my campsite, I forgot for a few minutes that it was in power supply mode and switched it back to charger mode(the engine had been off for minutes at this point) and the Orion went into bulk charging mode. I looked at the battery monitor and 35 amps were being pushed into the battery.

 

[gnubie]

They would have reasonable amounts of capacitance, like any other power non-scientific/lab supply. That wouldn't account for your 35 amps though. You'd need a rather large box of capacitor to do that for any amount of time.

 

[HaldorEE]

Howdy All,

Someone on another forum said that DC to DC chargers (like the DCC50s, etc) actually are DC to AC to DC. This doesn't make sense to me, I had figured it is a buck/boost kind of a thing but I have never examined the circuit in one of these type of chargers. Anybody know how these things actually work?

Sorta. More like a DC to square wave to DC converter.

Boost circuits work in a way like an old school car ignition system turns 12VDC into 30KV for your spark plugs.

The way this works, is an inductor has the property that you can't instantly stop the current flowing through it. When you try to stop the current flow (switch opens), the inductor will create what is called a back emf voltage that is high enough to keep the current flowing through the switch even after it opens. A diode is then directs that voltage to a capacitor which filters it to be the higher voltage output.

Modern DC-DC boost circuits use high frequency switching that let you use smaller inductors. This is basically how a HF inverter works except they use PWM (look it up) to create a reasonable facsimile of a sine wave instead of a square wave. DC - DC converters don't both with making it a sine wave.

Boost Switching Converter Design Equations

 

[boondox]

Thanks Haldor and all. That makes sense and was about what I was thinking. I guess I can see where it *kinda* be considered AC but certainly isn't my definition.

 

[carlos1w]

Any high efficiency DC-DC converter (buck or boost) is a switching power supply. It converts DC to pulses (sort of AC) and then regulates and filters the output (it can regulate max. V, max I, or both). A DC-DC that lowers the voltage can be a linear power supply but these tend to waste a lot of energy (e.g., 12V to 5V would waste 7W at 1A for a 5W output).

 

[gnubie]

Thanks Haldor and all. That makes sense and was about what I was thinking. I guess I can see where it *kinda* be considered AC but certainly isn't my definition.

Depends on the design. If it's a push pull type (centre tapped primary with DC switched on the other legs, h-bridge etc etc) the current on both sides will actually reverse so it really can be AC but not even remotely sinusoidal. It can be easier to just think of all isolated types as having an intermediate AC stage since the output of the HF transformer will always have diodes or something else performing rectification / blocking, etc.

 

[boondox]

Yeah, but if it doesn't go negative relative to ground then it isn't AC. If it does then the shape of the wave isn't so important. Gnubie, do you know of a DC to DC converter for battery charging on the market that does use AC in the middle?

 

[gnubie]

I guess then that you must also consider that the output of a regular 50/60Hz transformer must not be AC either since that isn't referenced to ground either. Of course if you want to use this as your point of difference, you could tie one leg on the output of either the 50/60Hz transformer or the HF transformer to ground... but then it wouldn't be isolated

I can appreciate what you are saying in regards to designs that simply pulse DC in a single direction on the primary.

To answer B, not specifically but not because they aren't out there, I just haven't taken any of these things apart and even if I did I'd have to stumble across one that operated in this manner. Google might turn up the answer you seek.

*edit*
tried to find some nice images with google showing current flow but Googles concept of 'natural language search' is lamentable and I'm not drawing at this time of the morning

 

[HaldorEE]

Yeah, but if it doesn't go negative relative to ground then it isn't AC. If it does then the shape of the wave isn't so important. Gnubie, do you know of a DC to DC converter for battery charging on the market that does use AC in the middle?

Not a meaningful distinction. Gnubie is correct. My Victron Multiplus uses DC PWM to create an AC waveform that doesn't go negative of the battery minus terminal. This is fed to the primary winding of a transformer and either one end of the secondary winding is grounded for 120 VAC or the center tap of the secondary is grounded for 230VAC split phase.

If you are talking about HF inverters without transformer isolation, then these have to use bipolar power supplied to generate AC output voltage.

However this has nothing to do with the boost DC -DC converter being discussed in this thread.

 

[boondox]

Not a meaningful distinction. Gnubie is correct. My Victron Multiplus uses DC PWM to create an AC waveform that doesn't go negative of the battery minus terminal. This is fed to the primary winding of a transformer and either one end of the secondary winding is grounded for 120 VAC or the center tap of the secondary is grounded for 230VAC split phase.

If you are talking about HF inverters without transformer isolation, then these have to use bipolar power supplied to generate AC output voltage.

However this has nothing to do with the boost DC -DC converter being discussed in this thread.

I always thought that the "alternating" part meant alternating between positive and negative and how does one determine positive or negative without a ground?

So then how would you define AC?

 

[gnubie]

You do it by watching the current flow. If it's AC you'll see current flowing in one direction then the other. If it's chopped DC it'll all be in one direction.

AC and DC has nothing to do with grounding. All they are referring to is how the current flows.

 

[HaldorEE]

You do it by watching the current flow. If it's AC you'll see current flowing in one direction then the other. If it's chopped DC it'll all be in one direction.

AC and DC has nothing to do with grounding. All they are referring to is how the current flows.

Another way to look at it, is what part of the signal will pass through a capacitor.

AC can ride on top of a DC component. But when you get to a series capacitor, the AC makes it through, but the DC does not.

 

[gnubie]

Was just thinking about DC biased AC while chopping up and prepping some roo for the freezer!