DC to DC converters

Technical discussion on converting internal combustion to electric
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mattW
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Post by mattW » Mon, 27 Apr 2009, 19:38

I know I am late to the party but I have a 72-12V 300W Kelly DC-DC converter that I no longer need (i'm going to run @48V instead of 72). If anyone is interested I will sell it cheap. I think it cost me US$100 + postage but I'll take an offer to recover some portion of that cost.
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Post by antiscab » Tue, 28 Apr 2009, 07:59

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Post by Hemonster » Tue, 26 May 2009, 02:54

Hi there,

I too am looking at the meanwell AC-DC converters as a DC-DC converter however was wondering if it was ok to series up the inputs rather than tie to mid-pack. Being an ACIM conversion, the pack is going to be 600-700V when fully charged.

Not entirely sure of the layout of my pack yet as I haven't even decided what technology to go with - however I'm not entirely sure it will be that easy to get to mid pack, hence why I'm asking the question. It's probably be answered elsewhere, but I would appreciate a link if somebody can point it to me.

Also what kind of spike protection is there in these regulators and should addition spike protection by placed in there?


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Post by Johny » Tue, 26 May 2009, 03:30

My feeling is that it would be very unwise to just series up the inputs. The chances that the outputs will deliver equal power is not high - one could warm up a bit more than the other etc. None of these SMPSs were intended to be absolutely stable (voltage wise to tens of mV) under all conditions.

Having the inputs in series without any balancing system (the battery pack half rail) would almost certainly result in one getting more than half the pack voltage. With the Electrolytic caps in them only rated at 400V, a near 800V DC Bus under regen. with a tiny bit one way could spell disaster.

The pack centre cable only needs to be rated at < 1 Amp for the DC-DC system so it shouldn't constitute a big problem to run it.

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Post by acmotor » Tue, 26 May 2009, 04:01

Yep, series DC-DCs with connection to centre of battery pack works fine for 2 years now in red suzi. It has been up to 850VDC.
BTW, you don't get many spikes with the solid battery pack that is in parallel with the capacitors in the VFD.

Image

There is a 15ohm resistor and 2A fuse on the +-300V for safety and inrush protection.
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Post by Johny » Tue, 26 May 2009, 04:35

Agreed acmotor - I don't have any problem with them in series with centre to centre of pack - just not in series with middle "floating".

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Post by acmotor » Tue, 26 May 2009, 04:53

Perhaps a combination that would work is 2 x 7V supplies (adjusted to 6.8V) (I note these on EBay from some suppliers) placed in series across 600V and their outputs in series to give 13.8V
ebay 2 x 7V 46A

In theory at least the input voltages must stay about right.

If the supplies (one of them) goes into current limit, it may be a problem though.

Just thinking out loud. Image
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Post by Johny » Tue, 26 May 2009, 16:17

Those 7V supplies were No.1 on my list until I found these 20 Amp SMPS from kowloontown. I got 2 for AU$112 landed.

http://cgi.ebay.com.au/250W-12V-20A-Reg ... 1|294%3A30

They deliver 20 Amps into a short (a bit less now into 13.8V) and adjusted up easily to 13.8V. They have a 200 Ohm across the O/P which I may remove (or relay isolate them from the battery when not powered because they still draw 5mA from the battery). I did have to pull one apart and fix a loose heatsink clamp but I was going to go crazy with silicon anyway to make them more "mobile". Very simple, neat construction inside. Very similar looking to acmotor's.


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Post by Hemonster » Tue, 26 May 2009, 16:47

Johny, that's a crazy price! what a bargain. I wonder how they can sell them that cheaply. The 7V ones however are more powerful at 350W each, whereas these are only 240W, but still that should be plenty I'm sure. What is the 200ohm resistor there for? Do these switch mode supplies need a minimum load to prevent them from squeaking RF (ie. discontinous switching mode)?

acmotor, did you hard tie the outputs of your SMPSs?

I suppose (if possible), one advantage to series tying up the two SMPS is that if the fuse blows on one, neither will work - ie. you'll know about it. However if you centre tied, one fuse blowing will still enable the other to work, but this will unbalance the pack. ACmotor, do you have some sort of warning if this occurs? or do you just regularly open up the box and check things are ok?



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Post by Peter C in Canberra » Tue, 26 May 2009, 17:21

I just learned a useful lesson. My car suddenly stopped working on the way home from getting its clean bill of health at the engineer's. It could have been embarrassing if it had happened earlier. The problem was the fuse to the DC/DC converter had blown some time earlier and the car continued running just fine on the backup 12V battery till that was almost flat and the main contactor would not close. As a quick fix is a $19.99 rechargeable flashlight from one of the car shops that sits in the cigarette lighter socket. It has LEDs that light for 10, 11, and 12V so I can see at a glance if the voltage has dropped below 12V. In retrospect I should have had a meter for the 12V system. Jaycar have a gadget that does various things in addition to metering the 12V and an alarm that goes off if the volts are too low. For now I'll see how I go with the flashlight indicators.
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Post by Johny » Tue, 26 May 2009, 18:28

I have joked with the odd person that a piddly little 12V battery that goes flat could stop a 12kW battery pack being any use. Your experience has solidified placing a monitoring system in my custom dash for 12V.

If your alternator warning lamp is available - how about something like this?
Image
Lamp will go on when voltage is < 12.5 volts (roughly) so should go out when the DC-DC converter gets powered up.

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Post by Peter C in Canberra » Tue, 26 May 2009, 22:12

Johny wrote: If your alternator warning lamp is available - how about something like this?

I had started to think (not very seriously yet) about how I might use the 'charge' lamp for this purpose. I like your circuit and will put it on my list of refinements. I like the idea of having a minimum of extra meters and lights on the dash and keeping as many original functions as possible. So, using the alternator warning light for something like its original purpose appeals more than an extra gauge for the 12V system. [In a similar vein, my original brake warning light comes on for its original purpose if the brake fluid is low but also if the brake booster vacuum is low.] In the meantime, assuming I pass rego tomorrow, I'll tell drivers to keep an eye on the little LEDs on the flashlight in the lighter socket. Thanks for your suggested circuit. I'll be saving that.
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Post by acmotor » Wed, 27 May 2009, 04:38

Hemonster wrote: Johny, that's a crazy price! what a bargain. I wonder how they can sell them that cheaply. The 7V ones however are more powerful at 350W each, whereas these are only 240W, but still that should be plenty I'm sure. What is the 200ohm resistor there for? Do these switch mode supplies need a minimum load to prevent them from squeaking RF (ie. discontinous switching mode)?

acmotor, did you hard tie the outputs of your SMPSs?

I suppose (if possible), one advantage to series tying up the two SMPS is that if the fuse blows on one, neither will work - ie. you'll know about it. However if you centre tied, one fuse blowing will still enable the other to work, but this will unbalance the pack. ACmotor, do you have some sort of warning if this occurs? or do you just regularly open up the box and check things are ok?


Most SMPS have minimum load for regulated output and will often fit a drain resistor internally on their own output.

SMPS in red suzi are tied together at the front of vehicle where 12V 20Ah battery is, having each come via their own 6mmsq wire from the VFD box at the rear. They are set at the same output voltage ~13.9V and seem to load share quite well as measured by the volt drop across each 15ohm inrush resistor.
A contactor driven off the 'ignition' circuit isolates the SMPS (DC-DC) feed from the 12V battery when system is not powered up.

Good idea with the 'gen' light. My only indication at present is dull lights ! (not happened yet)
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Post by coulomb » Wed, 27 May 2009, 05:12

acmotor wrote:Most SMPS have minimum load for regulated output and will often fit a drain resistor internally on their own output.

So is it safe to remove the drain resistors if you have a battery on the output? Presumably a floating battery could be quite a low load, if everything happens to be off.

But maybe the clock, circuit listening for remote key fob etc will add up to enough drain, plus the battery's self discharge?
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Post by acmotor » Wed, 27 May 2009, 05:29

I would expect the always powered items to add up to less than 100mA worst case. This may be enough. But DC-DC would be off when ign is off anyway ?
Check that the 12V battery does not backfeed into the DC-DC when DC-DC is off as this may drain the battery. That is why I use a contactor to break the 12V line.

Some DC-DCs will let precious smoke out if they have no load at all. A DC-DC with an internal 200ohm res may be one of these.
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Post by Johny » Wed, 27 May 2009, 20:01

I was going to use these Inrush current limit thermistors where acmotor used 10 Ohm resistors. (I finally found them at a reasonable price.) The 25 Ohm (467-624) is my choice.

I also need them for an experimental +/-300VDC to 240VAC inverter that I may use to power 240VAC stuff in the car.
Last edited by Johny on Wed, 27 May 2009, 10:01, edited 1 time in total.

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Post by acmotor » Wed, 27 May 2009, 20:28

Johny,

I considered this type of NTC device but since the operating current of the SMPS was typically in the low 100's of mA, the NTC would not warm up much and would behave more like a fixed resistor anyway.

My other concern was to limit the current in a fault condition since I was using a 3AG glass fuse and the 15ohm would limit fault current to 20A giving the fuse time to blow without massive rupture current requirement (important on DC) I tested this and it was quite un exciting. Pop.
The NTC may well be at only a few ohm and would head for zero ohm in a fault giving a higher rupture current. Mind you, these small NTCs tend to become vapourware in fault conditions themselves.

The other point is that for the NTC to be functioning correctly it is at 100 to 200°C. My 10W resistor is only blood warm at full DC-DC load.

The voltage drop across the 15ohm also gave me the feeling that I was making some effort to protect the DC-DC from the 425V potential max supply to it !

Just my thinking. Image
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Post by Johny » Wed, 27 May 2009, 20:49

And there I was all excited because I'd got the price I saw originally from $8 down to about $2.
I think your thinking sounds sound.Image

I might still use them for the 240AC inverter depending on how much I want from it.
I probably only need 5 to 10W but the "fun" element kicks in - for barely more effort I could have 1kW.

Here is my partially thought-out idea for the +/-300VDC to 240VAC inverter (off topic for DC-DC but it's close).
As usual I have the TL494 and the IGBTs from a previous eBay frenzy.
The main cost will be in the (time) DC-DC and gate drivers.

Image

The reason it appeals more than just a 12VDC to 240VAC is that it has to be close to 99% efficient - and weigh very little. It's not just for fun, the fan on the QIN WEI motor is 240VAC (they couldn't supply a 12VDC after all), so I either change it or adapt...
So I only need NTCs if I'm going for highish power.
Last edited by Johny on Thu, 03 Dec 2009, 08:38, edited 1 time in total.

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Post by acmotor » Wed, 27 May 2009, 21:19

Re my last post... I should note that most SMPS have some form of inrush NTC fitted already internally ! But my thinking was that this was all designed with AC waveforms in mind and not continuous DC at the AC peak voltage as the source thus the external resistor idea !
A low current DC fuse on the 300VDC was always going to be needed external to the DC-DC (even though it had a small fuse internally) to cater for real fault conditions !

Now, your AC 'chopper' .
I must be getting old, whilst I follow the components I don't recognise the ISO standard symbols Image

I like it. Efficiency is basically limited by the large harmonics in the output. Load (motor) will run warmer than normal i.e. losses will be in it not your chopper.
So the idea is to set a duty cycle for the pulses with a 50Hz base freq ? No feedback or current limits ? Only a concept at this stage.
Surely you would go one step further and produce a PWM 50Hz envelope at say 10kHz switching ? I can see a VFD design starting !

Don't let me put you off. It looks like a fun circuit to experiment with. Check also the SC drill speed controller just recently (MAY 2009)

OK, just stick to the basic system for 10W output.
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Post by Johny » Wed, 27 May 2009, 21:36

Yes, sorry about the symbols - I haven't kept up and I do the circuits with Word so they have to be a bit more basically shaped.
I figure 2 things.
1. If it gets too complicated I won't do it, I'll just buy a 12 to 240 inverter.
2. It will produce the same output as the "modified sine wave" inverter I would buy anyway - just lots more efficient.

I figure current limit will be the fuses and you are right - just set a reasonable duty cycle/voltage using the pot - that sort of answers the question about power - not much because it won't regulate (or will it matter - Hmm).

I've been collecting blown PC power supplies from work to house these little projects - my brake chopper is based on largely the same idea.

On that point - a quick question. Does the brake chopper in the Danfoss use a high frequency PWM or just hysteresis? I haven't found any choppers that specify PWM frequency and I get the feeling that they just turn on at voltage A and off at B. This has an impact on how I do mine - currently I'm planning on PWMing but this could be overkill and effects the design a bit. The heater control and brake chopper look amazingly similar as well.

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Post by Johny » Wed, 27 May 2009, 21:40

acmotor wrote:I should note that most SMPS have some form of inrush NTC fitted already internally !
I was wondering about that - I'll check it out.

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Post by acmotor » Wed, 27 May 2009, 21:53

I would be disapointed if it was only bang bang !
I know it squeels. I'll look into it.
There is current limiting and fault detection on the brake resistor/IGBT circuit. You need to be PWMing to get that to work safely. Image
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Post by Johny » Wed, 27 May 2009, 21:58

You can still do IGBT saturation detection with a slow switching system but I get what you are saying - squeals may sort of answer it...

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Post by woody » Thu, 28 May 2009, 16:32

Johny wrote:The heater control and brake chopper look amazingly similar as well.
The heater and the brake are similar circuits too. A big fat resistor.

Can you use the brake chopper + resistor as a heater?

The temperature knob would control PWM duty.

For brake duty only (heater not on) you could run the fan in reverse as required.
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Post by Johny » Thu, 28 May 2009, 17:06

The trouble is that the heater will only draw a maximum of 2 amps - and that's only if we can get the exact correct element setup. I bought a Heller ceramic heater from Bunnings on the weekend ($25) - it's 1500W and has 2 heat settings. From reading here and there I have a feeling that it will pan out to about 750-900W which is only around 1.5 Amps. The brake system is likely to be from 4 Amps (lead acid) to 20 Amps (lithium) so the heater element really wouldn't be much use.

The up-side is that the design for heater and brake resistor controller is very similar. (My controller does not have a built in brake chopper.) One has a fairly linear slide pot control to PWM, while the other (brake) has a voltage sense system that starts attack at say 740V and is on full at 760.

There is an alternative for the heater that avoids DC-DC converters and fast optos by switching at a 1 second rate which make it a heap cheaper and simpler but is unsuitable for the brake system. It also generates way less RFI (noise into car radio).

The only thing I am still wondering is whether I will need the brake resistor system at all. Does anyone know if Mal (a4x4kiwi) ended up with a brake resistor? Mal - you there Mal?

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