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Old 12th March 2008, 05:54 AM   #21
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Default PFC

#26 material is 100% the wrong material. It does bot work well at 100KHz (where most PFCs work).

I have worked closely with Micrometals (they are 2 hours away from our factory) and their engineers are quite clear that #8 and #18 are the only suitable cores for PFC. Yes making a core simply huge may get rid of some problems but all of us have space issues.

Interwinding capacitance if too high is the death of PFCs, and too high an inducatnce allows the PFC coil to saturate easier on a given core

Stephen Mantz
Zed Audio Corp.
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Old 12th March 2008, 01:10 PM   #22
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EVA, Stephen,

Thanks for the advice. I'd rather spend a few extra bucks than let the magic smoke out of my MOSFET because I got cheap! I will reserve the T-200-26 cores for the cross-coupled output inductor for the main PWM section.

Steve
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Old 12th March 2008, 01:40 PM   #23
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Default Re: PFC

Quote:
Originally posted by MOER
I have only had experience with the IR chip and some older Unitrode chips. They all suffer this problem. There are other chips from On-Semi, national etc and I see no provision for controlling the turn on of the main switching device when power is applied. Through trial and error and some blown IGBTs I figured out that massive dead time is required on turn on to protect the IGBT and reduce in rish current. However be aware that the turn on surge at 120v as compared to >200v is vastly different. You can get away with no surge supressor at 120v AC but NOT at >200v.

All we do is insert a simple 5 watt 620 ohm resistor in series with the positive leg of the bottom capacitor of the bulk pair (2 caps in series as we use a 1/2 bridge switcher). This resistor is bypassed with a relay once the supply is up and running.

I also keep the PFC disabled during the first 3 seconds of turn on. This allows the bulk caps to charge to some voltage (160 at 120v AC and 320 at 240v AC). Then I release the PFC and allow it to ramp up the main B+ to 385v.

#26 material is absolutely useless for the PFC coil. it has too much loss and the only two materials I recommend from Micrometals is #8 and #18. #8 is more expensive.

DO NOT skimp on the PFC coil because if it saturates then you are in trouble.

Stephen Mantz
Zed Audio Corp.

It all seemed pretty straightforward if one pays attention and does his homework so none of this really surprises me. I'm just deathly afraid (err...respectful!) of plasma-emitting semiconductors - likely due to some traumatic experiences as a younger boy. I built a Lexan box into which my "I'm not sure what might happen" designs go during initial power-up. It's less shocking and probably much safer.

I used IGBTs years ago - completely unsucessfully. Using an IR part with bootstrapping - the IGBT would instantaneously C-E short. A few back-and-forth emails to IR yielded no fruit. I gave up and went to a FET. I'm a little older and a lot wiser now and willing to give them another go. At least now I have the proper test equipment to know exactly what's going on.

You've been very helpful - now I need to build it!
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Old 12th March 2008, 09:40 PM   #24
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Default Re: IR1150S PFC

Quote:
Originally posted by MOER
We use this chip in our professional amplifiers and it has serious turn on issues. We also use #18 cores from Micrometals which work well but DO NOT use a core which is too small. Our smallest size is a T175 and MMetals offer a T-200B which is a double stocked T200 core.

The problem with the IR1150S is that on turn on there is not enough dead time and the power device will blow. This is more apparant at 240v AC. We do not use a MOSFETS as IGBTs work so much better. I designed an extra circuit to deal with the PFC on turn on. The input surge issue is now a thing of the past.

IR are designing a new chip which they say will take care of the inrush current issue.

Steve Mantz
Zed Audio Corp.
I don't follow you here, dead time between what? Do you mean time before the PFC circuit starts chopping? Or max duty cycle? Did you use an extra diode which lets the startup surge bypass the inductor?
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Old 13th March 2008, 05:33 AM   #25
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Default PFC

I don't follow you here, dead time between what? Do you mean time before the PFC circuit starts chopping? Or max duty cycle? Did you use an extra diode which lets the startup surge bypass the inductor?

Dead time of the main switching device. Yes decrease the maximum duty cycle on start up.

No need for the bypass diode as the PFC inuctor and PFC diode on start up are passive parts in my design. Since the PFC controller is off, the bulk capacitors are charged via the PFC coil ( a zero ohm part now) and the PFC diode. Once the PFC controller is activated the precharging of the bulk capacitors has been done (voltage dependent on the incoming AC mains)


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Old 13th March 2008, 01:51 PM   #26
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Default Re: PFC

Quote:
Originally posted by MOER
I don't follow you here, dead time between what? Do you mean time before the PFC circuit starts chopping? Or max duty cycle? Did you use an extra diode which lets the startup surge bypass the inductor?

Dead time of the main switching device. Yes decrease the maximum duty cycle on start up.

No need for the bypass diode as the PFC inuctor and PFC diode on start up are passive parts in my design. Since the PFC controller is off, the bulk capacitors are charged via the PFC coil ( a zero ohm part now) and the PFC diode. Once the PFC controller is activated the precharging of the bulk capacitors has been done (voltage dependent on the incoming AC mains)


Stephen Mantz
Zed Audio Corp.
Most of the PFC I've researched have that "bypass" diode installed. I could be completely wrong, but wouldn't the diode reduce the surge current from passing through the inductor and also reduce the chance of a Miller component turning on the switching transistor? It seems like some brief resonance would occur between the switching component and the inductor causing the device to behave destructively...
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Old 13th March 2008, 01:59 PM   #27
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In the standard circuit the PFC inductor might saturate from the inrush current if the diode is not installed. If the PFC starts switching at this point it's not very good for the transistor...
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Old 13th March 2008, 05:08 PM   #28
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Quote:
Originally posted by megajocke
In the standard circuit the PFC inductor might saturate from the inrush current if the diode is not installed. If the PFC starts switching at this point it's not very good for the transistor...
Ah, of course - the simplest answer is usually best.
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Old 14th March 2008, 05:49 AM   #29
MOER is offline MOER  United States
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Default PFC

Simple test, take a PFC coil, put a diode in series and then put your bulk capacitors after the diode.

Monitor the current across the coil. My PFC operates this way on turn on, the main IGBT is off and the IR1150S is off. So the PFC coil is just some wire on a core. The inductance I use is always less than 200uH due to the size of the power supply.


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Old 14th March 2008, 07:50 AM   #30
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Default Re: IR1150S

Quote:
Originally posted by MOER
IGBTs work far better than Mosfets in both the PFC and the half bridge, <snip> The trick with IGBTs is to use higher value gate resistors for turn on as compared to that used for Mosfet drive. This reduces EMI substantially.
Using a high value gate resistor might fix EMI but the slowed transitions are going to make the switching device get way hotter than necessary. I worked in a SMPS design lab for 10 years until ~2 years ago and the biggest I worked on was 3.3kW. The last PFC I worked on was a 300W unit and had a Fairchild IRFP460A that would turn off in 30nS and this fed a Cree 600V silicon carbide schottky diode. The results were unbelievably better than so many of the units I had worked on before. The waveforms were like in a textbook. 240VAC mains to DC bus (400v) efficiency was 98% despite the higher forward voltage drop of the SiC diode. You really owe it to yourself to try one of these things. Diode reverse recovery and all the difficulties it brings are a thing of the past. It's very much like going from bipolar transistors to mosfets for switching use.

We never found IGBTs to be very good; maybe they are better now?
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