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Old 20th January 2006, 12:09 PM   #1
TeeJay is offline TeeJay  Australia
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Default SMPS 230 VAC to 115 VAC 800W

Hi people, I need to make a power supply converter to convert 200-250 VAC 50Hz to 115 VAC 60Hz with a useable power output of 800 Watts and a surge capability 300mS of 1200W, I assume the best approach is a flyback converter, I have built the output side of this unit thinking that this was the hard part and the flyback would be the simple part, I am having trouble calculating core size and getting reliable info on a suitable Flyback converter, ..some direction would be greatly appreciated..
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Old 20th January 2006, 06:43 PM   #2
Eva is offline Eva  Spain
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Given the fact that both the input and output voltages are AC, wouldn't be much simpler to use an industry standard 60Hz autotransformer? Such autotransformers are not so big, as half the copper is shared between the input and the output.

Note that if you want to go for a SMPS, the most straightforward solution would be to convert 230V AC to DC first, then get a lower DC level through a buck or push-pull converter, and then convert the output DC voltage back to AC. The resulting circuit is going to be quite complex.
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Old 21st January 2006, 12:51 AM   #3
TeeJay is offline TeeJay  Australia
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Thankyou kindly for your reply Eva,
This is my FIRST ever FORUM so please let me know if I am not doing it correctly.

I discounted any normal transformer because of the frequency change needed 50Hz to 60Hz which has to be done to keep frequency sensitive timing operations (in the load to be driven) correct. I already assummed the mains to DC was required to use a Flyback converter, which I hope would supply a 2:1 transformer ratio to get my 115 V RMS 160V P giving me a current doubling as the voltage is halved to the secondary.

I have built 2 banks of MOSFETS driven by those nice HCPL3120 isolated drivers for the output side of things. These are driven by a micro which I have pulse width modulated to a sinusoidal effect for each bank in turn, separately for the top half and bottom half of the sinwave,... this section seems to work OK with a 60VDC up to 5 amps from a bench supply fed to it, I am hoping the high inductive load will be enough to filter the high frequency modulation out to end up with an averaged normal looking sinwave.

Q1. Is the most suitable/cheapest converter a Flyback?

Q2. Am I correct in assuming 1600uF Electros would be the smallest I could get away with after the mains rectifier due to acceptable ripple?

Q3. Can I gain a 2 to 1 current advantage with any other kind of converter?
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Old 21st January 2006, 04:07 PM   #4
Eva is offline Eva  Spain
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Have you considered an unregulated half-bridge converter with a 1:1 transformer? It's really simple, but the microcontroller would have to work harder in order to get rid of the 100Hz ripple in the top of the 60Hz squarewave.

The next option is a regulated half-bridge with a transformer having slightly more turns in the secondary than in the primary.

Concerning the flyback, it looks simpler at first sight, but have you calculated the magnitude of the peak currents involved? It's true that in a flyback only a single switch is required, but this switch has to turn off when it is conducting far more current than the ones from push-pull topologies in similar conditions, and has to whitstand far more voltage. Also, a single output diode and a single transformer are required, but the diode is subject to quite high peak currents requiring a more expensive device and yielding high losses, and the transformer is actually a coupled inductor that requires both high inductance and high saturation current, thus it's bigger than any push-pull transformer for a similar power level.
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Old 21st January 2006, 07:57 PM   #5
DaBit is offline DaBit  Netherlands
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For your 800W continuous / 1200W peak, a flyback seems not the most suitable topology. Your primary side switches will about 600V (leakage inductance spikes not included), and must carry peak currents around 15 amps or so. Given the conduction losses of high-voltage switching elements, this is not optimal. On the secondary, currents are double and voltages are half. Still quite a lot. Plus, you need either a massive core or very high switching frequencies.

Personally I would doubt between a full bridge and a half bridge.

If this is a commercial design, you would need a PFC stage to pass the tests anyway. You could try to use that stage to obtain the required DC bus voltage. Maybe a SEPIC?
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Old 21st January 2006, 11:12 PM   #6
TeeJay is offline TeeJay  Australia
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Quote:
Originally posted by Eva
Have you considered an unregulated half-bridge converter with a 1:1 transformer? It's really simple, but the microcontroller would have to work harder in order to get rid of the 100Hz ripple in the top of the 60Hz squarewave.

Hi Eva, I am a bit confused about the 100Hz ripple I assume thats from the 50 Hz mains in full wave rect,
I need to end up with a Sinewave output and I need to keep things EMI clean as I have lots of radio receiving equip not far from this when it is finished.

Q1. Can I treat this as 2 separate sections, Section 1.(240VAC 50Hz to 160DC) Section 2. (Sinewave converter)
or
do you think I need to re-look at the sinwave conversion method with respect to the converter type?
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Old 21st January 2006, 11:25 PM   #7
TeeJay is offline TeeJay  Australia
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Quote:
Originally posted by DaBit
For your 800W continuous / 1200W peak, a flyback seems not the most suitable topology.
Personally I would doubt between a full bridge and a half bridge.
Hi DaBit, I need this to be as small as possible, Cool as possible, definately no fans! and clean EMI wise because of its proximity to my radio equip, I will be completing this unit as fast as I can get problems solved, thanks so much for your reply.

Q1. What is your best guess of Topology ?
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Old 21st January 2006, 11:36 PM   #8
Eva is offline Eva  Spain
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In EMI terms, a half bridge is usually much quieter than a flyback at the same power level.

Adding regulation to the half bridge would probably increase EMI a bit, since the output diodes would now be always conducting (provided you chose continuous mode operation). However, producing a sine wave from a regulated supply voltage seems easier to me than producing it from an unregulated supply. Are you using any kind of feedback into the microcontroller in order to correct sinewave linearity?

Anyway, the worst thing that could happen with an unregulated supply is to have the 60Hz wave slightly intermodulated by the 100Hz ripple from the 50Hz mains rectification (and this is not EMI, it's just LF ripple). What are you going to power with that converter?
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Old 21st January 2006, 11:55 PM   #9
DaBit is offline DaBit  Netherlands
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Quote:
Originally posted by TeeJay
Hi DaBit, I need this to be as small as possible, Cool as possible,
We all need that

Quote:
Q1. What is your best guess of Topology ?
The easiest way to get this going is a mains rectifier, followed by a half bridge converter which in turn feeds the sinewave PWM. But do you need to comply with some regulations, or is it for private use? I can imagine that filtering out conducted EMI would be troublesome at these powerlevels with the regular full bridge AC rectifier and storage caps.

If you must have your design certified, you might want to use a PFC preregulator. Normally they use a boost type converter to a DC voltage of 350-400 volts, and a flyback/halfbridge/fullbrdge/whatever to go down to the required voltage.

In your case the DC bus voltage is pretty high, so I was wondering if it would be possible to collapse the PFC preregulator and actual DC/DC into one stage. I have seen an appnote somewhere which uses a SEPIC to go down to a lower DC bus voltage.

About regulated or unregulated: Personally I would use a regulated supply, simply because I don't like stage A influencing stage B. Optimizing two different stages separately is easier than optimizing two stages together.
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Old 22nd January 2006, 01:18 AM   #10
TeeJay is offline TeeJay  Australia
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Quote:
Originally posted by Eva
Are you using any kind of feedback into the microcontroller in order to correct sinewave linearity?

Attached Is what I have made so far for sinewave production, should I trash this for a bridge vesion?

The micro supplies a sinusoidal PWM signal to the load which when veiwed on the CRO with an Inductor in series with the load looks like a clean sinewave! No feedback, will I need some?
Attached Files
File Type: zip sine2.zip (40.3 KB, 359 views)
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