Re: Help with PSU design - 2Kw unisolated Buck
Whilst thinking about the lowside buck topology, I considered how on earth someone would perform output voltage monitoring in a buck PFC - I remembered the UCC29910A (that I could never lay my hands on GRRR) and had a look through the datasheet.
Lo and behold - they use a level shifter (on page 10 - of [url]https://cdn.badcaps-static.com/pdfs/ce07b6f330ea37b489d8dc6fea22c9c4.pdf[/url]) so I can toss the optocoupler and the TL431 that are proving to be a major PITA in my design, and monitor voltage directly. At least...
Vectrix150V
Member
Last Activity: 01-12-2017, 11:56 PM
Joined: 06-18-2013
Location: Newcastle NSW
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Re: Help with PSU design - 2Kw unisolated Buck
Well, I was going to post some more measurements but I managed to kill the UC3842 again - sigh.
Time to buy some more.
It is (was) stable at about 300mA, with a 0.47R current sense resistor. I ended up using a very similar design to the one on the front page, but with a 15 K resistor in series with pin 2, so the gain of the error amplifier would be fixed at 10 (with the 150K from comp to VFB).
The frequency was 100Khz (15K and 10nf for Rt and Ct) with a 270uH inductor and a 200uF capacitor...Leave a comment:
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Re: Help with PSU design - 2Kw unisolated Buck
Thanks for the replies - I will take some more measurements but I think that the problem is due to the optocoupler not being in the linear region - in the thread 'UC3842 basics' it was discovered (on page 5) that a low load instability problem was due to an insufficient load on the phototransistor, and insufficient current through the LED.
The current loop is stable now (I am feeding back some of the oscillator ramp through an emitter follower into the current sense input) - before this the frequency halved once it hit...Leave a comment:
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Re: Help with PSU design - 2Kw unisolated Buck
Well, I have the current loop nicely compensated, but the voltage loop is a mess for some reason.
That is, I can add a load and the circuit works perfectly with changing input voltage to maintain a current across it (including >50% duty cycle, which is a problem with curent mode) - but at light loads, I can't seem to get it stable. I need it to be stable from 100mA to 12A, but it only seems to be happy at higher loads. (CCM?)
Rather frustrating!Leave a comment:
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Re: Help with PSU design - 2Kw unisolated Buck
Thanks for the replies, this inspires a bit more confidence.
The UCC3804 actually has a maximum of 12V when driven from a low impedence source voltage, so I think my 15V bias supply is probably a bit too high - The bias supply was a 2.5W integrated smps transformer (yeah cheating) but I think I might use a capacitor/bridge/zener combo to make 12V to run the soft-start relay (after a 3 second delay) and provide VCC to the chip. I'll remove that resistor, the bias supply should be adequate.
I'll be building...Leave a comment:
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Re: Help with PSU design - 2Kw unisolated Buck
I have had a bit of a think and a look at some datasheets, and have come up with the attached design.
Looking at using a UCC3804 (rather than a 02) - This is because it has a 50% maximum duty cycle (don't need any higher, 340/2 = 190V, only need 150-160V) and the current loop won't need slope compensation as a result.
It also has blanking on the current sense input (though I left a 1K resistor and 0.1u cap for filtering, they can be left off if not needed), and a pull down on the gate drive output, and a 15V...Last edited by Vectrix150V; 07-11-2013, 12:43 AM.Leave a comment:
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Re: Help with PSU design - 2Kw unisolated Buck
Thanks for your reply.
Current mode control with a buck converter is an interesting idea.
A 3842 could be used with a low-side MOSFET/IGBT with a current sensor in the current path, and the voltage loop could be done with a TL431/Optocoupler combination, that drops pin 1 to ground when the target voltage is hit. Needs some more thought, but not a bad solution. Saves messing with instrumentation amps etc for differential voltage measurement.
The more I think about this - the better it seems...Last edited by Vectrix150V; 07-09-2013, 10:02 PM.Leave a comment:
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Re: Help with PSU design - 2Kw unisolated Buck
The power is actually three phase 415V to the house, and a neutral - it somes in from underground (on 20mm2 cables!) Its a newish estate, so this is not unusual - there is a large 11Kv transformer down the street. Its rare for new houses not to have 3 phase in from the street.
All three phases are used for the ducted airconditioner, and then it is split for different circuits. The neutral is bonded to earth at the switchboard (everyone's switchboard).
I'm having problems modelling the compensation - I'm...Leave a comment:
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Re: Help with PSU design - 2Kw unisolated Buck
Compensation - like I said, I have the voltage loop nicely compensated (don't get into positive feedback or strange subharmonic oscillations) but the current loop isn't so good - you can hear some noise from the inductor. Last time I cheated and kept adding larger caps between pin 15 and pin 3 - before I put the 10K rsistors in to keep the input impedence stable, I think I had 10uf across it (!!)
I think I really need to do some more reading up on how the compensation works - I'm only using a basic (type 1) compensation...Leave a comment:
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Re: Help with PSU design - 2Kw unisolated Buck
Yep, I know, this is actually for a battery charger, and the battery is permanently connected to the charger - I don't want the battery to discharge through the voltage feedback network, or the load resistor.
I solved my issues with the current limiting - put a load resistor across the output of the supply (prior to the second diode) - 3.3K worked fine at 150V (and I had a 50W metal case resistor that was that size). The supply regulates fine, and immediatly reacts to a change in load - you can switch a 1000W load on...Leave a comment:
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Help with PSU design - 2Kw unisolated Buck
Let me start off with saying this may be a possible entry to the hall of shame...
OK - I am trying to design a non-synchonous unisolated buck converter around a TL494, for the purposes of an open-source battery charger.
The goal is 160V DC at 12A, with constant current limiting (12A) and then constant voltage (160V). There is a battery management system input that is basically an opto across the soft start section to shut the supply down when any of the (44) cells go over their target voltage. The load is predictable and does not change fast (at all). The battery pack is...Last edited by Vectrix150V; 07-02-2013, 12:01 AM. -
Re: New Members - please post your introductions here
Hey all, found this forum when trying to figure out how to make a TL494 not explode IGBT's - well, havn't managed to do this yet, but I'm hoping some gurus can step in and lend some assistance.
I'm based in NSW, Australia, and am a hobbyist with many years in repair, but not heaps in design. I deal mostly with digital design, so analogue is a bit out of my area of experience.Leave a comment:
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