# High Voltage switch down converter



## cts_casemod (Aug 23, 2012)

Hi,

Due to my recent upgrade I need to step down 350-560V to 300VDC for the DC-DC converter and other loads.

I am looking into some advice. The required power is 2KW


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## PStechPaul (May 1, 2012)

You might consider two DC-DC converters in series across the 350-560V supply, and have the outputs in parallel. You may want to add some 300V power zeners or the equivalent across each input to assure balance, although they should balance with enough of a load. If you can connect to the center point of the pack, you might run one off the top and the other off the bottom.

Another possibility is a 3-5 HP 460V VFD which can be set so as to produce 240 VAC for a standard 240V converter/charger.

Otherwise a simple buck converter could be made with a 50% output voltage.

There are switching power supplies with 480 VAC nominal input, but a 960W 24 VDC unit is about $500:
http://www.galco.com/buy/Puls/XT40.242

Oops, just noticed that you just want to step down to 300 VDC. So it probably means a custom buck regulator.


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## valerun (Nov 12, 2010)

If voltage regulation is not super important you can get away with a very very simple fixed duty buck. Could be built for less than $200


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## cts_casemod (Aug 23, 2012)

I was looking for either a buck or an inverting.
The last one wont dump the supply on my load if the switch fails and can work as buck/boost. however I have no experience whatsoever working with inductors and at such voltages literature is scarce.

Any advice or references would be great. The best I could find was the 10KW charger (the initial topic) but deals with a bit of trial and error on the inductors.


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## valerun (Nov 12, 2010)

cts_casemod said:


> I was looking for either a buck or an inverting.
> The last one wont dump the supply on my load if the switch fails and can work as buck/boost. however I have no experience whatsoever working with inductors and at such voltages literature is scarce.
> 
> Any advice or references would be great. The best I could find was the 10KW charger (the initial topic) but deals with a bit of trial and error on the inductors.


Buck is the ticket here as I mentioned. You can always put one of those SSRs on the input to cut off supply if you sense overvoltage. Could be a simpe comparator circuit driving it.

This is really very simple - the minimum set of parts you need is:
1. a small 1mH inductor rated for 10A (you can actually buy those, as opposed to 100A 200uH ones that we have to wind ourselves)
2. a 20A 500V FET / IGBT or higher. Ideally with isolated base 
3. an ultrafast soft recovery diode - 20A is good enough. Ideally with isolated base
4. input caps: 1 film 5-15 uF, one elcap 1,000uF or more
5. output cap: one elcap 1,000 uF or more
6. 555 IC wired as a bistable oscilaltor, with a pot in duty-setting network. 
7. a driver chip like A3120 (http://www.avagotech.com/docs/AV02-0161EN)
+ some assorted resistors etc.

if we were to build something like this, our natural inclination would of course be to wire in one of those mini arduinos we use in our chargers. $10 from Sparkfun and you get a micro-computer to do a lot of stuff for you. You can even implement a proper control loop with it if voltage stability matters (it doesn't in your application).


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## cts_casemod (Aug 23, 2012)

valerun said:


> Buck is the ticket here as I mentioned. You can always put one of those SSRs on the input to cut off supply if you sense overvoltage. Could be a simpe comparator circuit driving it.
> 
> This is really very simple - the minimum set of parts you need is:
> 1. a small 1mH inductor rated for 10A (you can actually buy those, as opposed to 100A 200uH ones that we have to wind ourselves)
> ...



Of course, you started that thread, silly me 

Thats quite useful. I do have some questions tough:

In regards to the diodes I am upgrading a PFC section for 600V to drive the charger. I am not sure if I need to upgrade the mosfets or if they are fine, since they will be handling only the input voltage? After all the higher voltage only goes trough the Diode. They are rated for 500V and the input is 250VAC/350VDC.

Is there any penalty by going with an 800V diode? should I look for something in particular or any soft recovery high speed diode will do?

Back to the Step down converter:

What is the best frequency if using an IGBT? Any concerns I should have regarding inductor and PWM frequency?

Is the inductor the same for any voltage I might use? Or should I choose one rated for my 600V pack?

Can I use a modulating IC like a TL494 or an SG3525 IC? I only need voltage regulation (not current) for this one. I was also wondering if the SG3525 driver (2Amp) would be enough to drive a 200Amp IGBT I have one at hand I could use. or if I am better using a dedicated low side driver as you recommend for the uP/555.

Thanks


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## valerun (Nov 12, 2010)

cts_casemod said:


> Of course, you started that thread, silly me
> 
> Thats quite useful. I do have some questions tough:
> 
> ...


to operate at 600V level you would need at least 900V rated parts (generally assume at least 50% safety margin unless you really know what you are doing - as in "I have studied transient voltage overshoots with a high-bandwidth scope in all load scenarios"). To my knowledge, MOSFETs are out of the question in that range. So you have to go with IGBTs. Diodes have to be rated similarly, as well. 800V may or may not work. Also, not every diode will work. You need an ultra-fast soft recovery diode. 

For IGBTs, generally don't go beyond 20kHz switching. 

Inductor should be rated for your voltage. At 600V, I'd suggest using a single-layer toroid inductor with some gap between the ends of the winding. 

You can use any IC that allows you to output PWM signal with adjustable duty cycle. 

Low side is easiest to control - you just need +/-15V source for gate drive. I can't comment on specific parts you mentioned as we haven't used them. I'd suggest you stick to the parts other people have used - it will save you a lot of grief later on when you have to debug things...


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## cts_casemod (Aug 23, 2012)

valerun said:


> to operate at 600V level you would need at least 900V rated parts (generally assume at least 50% safety margin unless you really know what you are doing - as in "I have studied transient voltage overshoots with a high-bandwidth scope in all load scenarios"). To my knowledge, MOSFETs are out of the question in that range. So you have to go with IGBTs. Diodes have to be rated similarly, as well. 800V may or may not work. Also, not every diode will work. You need an ultra-fast soft recovery diode.
> 
> For IGBTs, generally don't go beyond 20kHz switching.
> 
> ...


Great, Ill give it a try. I am finding hard to find the inductor you referred. What is the best place to source this kind of HV inductors? Or is it best to just get an LCR meter and wind my own one in a piece of toroid core?

I am quite familiar with power devices, my question was regarding inductors. If I should look for any particular change in inductance depending on the frequency I use for the PWM. 

As with the mosfets I was asking for the step up PFC boost converter. I don't see the output voltage going into the main switch, since its clamped by the diode hence the question if a 500V device will be Okay (Obviously with a 800-1200V diode). What do you use on your chargers? 

For the step down converter, where I can have 600V at the input Ill use a 1200V IGBT.

Thanks!


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## PStechPaul (May 1, 2012)

You might consider using a line reactor. They are generally rated for 480 VAC mains and thus should pass a 2500V Hipot test. Here is one that is good for 12/40 mH at 18/10 amps for about $30. With such high inductance you can use a rather low frequency for the PWM. The main drawback is size (6" cube) and weight (23 lb):
http://www.ebay.com/itm/LENZE-15067-40-12MH-10-18A-LINE-REACTOR-D233236-/290960128036

You could also consider using several smaller inductors in series, such as these:
http://www.ebay.com/itm/5PCS-OF-15m...rs-ring-winding-inductance-coil-/181181834274

Thus the 600V would be distributed with only 120V on each inductor. 

If you wind your own, make sure the ferrite or powdered iron core is the correct material for inductors, rather than transformers. You really need an air gap, which can be created with E cores, or you can use materials with a distributed gap, which is used a lot for toroids.


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## valerun (Nov 12, 2010)

PStechPaul said:


> You might consider using a line reactor. They are generally rated for 480 VAC mains and thus should pass a 2500V Hipot test. Here is one that is good for 12/40 mH at 18/10 amps for about $30. With such high inductance you can use a rather low frequency for the PWM. The main drawback is size (6" cube) and weight (23 lb):
> http://www.ebay.com/itm/LENZE-15067-40-12MH-10-18A-LINE-REACTOR-D233236-/290960128036
> 
> You could also consider using several smaller inductors in series, such as these:
> ...


+1

We use powder iron toroids from Micrometals. Great guys over there. Ask for Staci. Toroid is really the best for this stuff - better cooling, lower EMI, little use of isolation tape (if you keep to one layer), etc. Winding your own is fun. If you like, you can buy one of ours but that's probably an overkill for you...

Re rating of FET / IGBT. You need 900V or above all around. That PFC IGBT will see full 600V plus overshoots from the PFC diode not being an ideal diode. Basically it takes some finite time for it to open - 'forward recovery time', if you will (for some unknown reason never found in datasheets...). During that time, your inductor continues to drive the voltage up on the IGBT's collector. Sometimes way up, depending on how aggressive your drive is, your distance from that collector to the anode of the PFC diode, etc. 

If you are serious about trying this out, check out a few first pages of our build manual (link at the bottom of http://emotorwerks.com/tech/electronics) - there are some books we recommend that will go into details on layout etc. Also, study successful layouts (like ours ;-). Pretty much all design decisions we have made in our layouts are there for a reason: the way power planes are shaped, the use of half-bridge modules instead of discretes, the isolated baseplates, the distribution of caps, driver board connections, etc. 

hope this makes sense.

PS. Not sure I understand why you need PFC stage. I thought your application was a straight down-conversion from 350-400 into 300 for the Dc-Dc, no?

Thanks,
Valery.


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## cts_casemod (Aug 23, 2012)

valerun said:


> +1
> 
> We use powder iron toroids from Micrometals. Great guys over there. Ask for Staci. Toroid is really the best for this stuff - better cooling, lower EMI, little use of isolation tape (if you keep to one layer), etc. Winding your own is fun. If you like, you can buy one of ours but that's probably an overkill for you...


Thanks, Ill have a look at those



valerun said:


> Re rating of FET / IGBT. You need 900V or above all around. That PFC IGBT will see full 600V plus overshoots from the PFC diode not being an ideal diode. Basically it takes some finite time for it to open - 'forward recovery time', if you will (for some unknown reason never found in datasheets...).


It all makes sense now, thanks for the heads up!




> PS. Not sure I understand why you need PFC stage. I thought your application was a straight down-conversion from 350-400 into 300 for the Dc-Dc, no?


Different application. The PFC front end is for the charger. I am charging straight from the PFC output with some added logic. Pack is nominal 409.6V, peak voltage 465V.


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