3 phase power supply based on HF920/HF920A

AC-DC Power Converters
1

Hi everyone,
I wanted to use HF920/HF920A to create a 3.3V Flyback converter for around 700-800mA load. I also have a requirement to use a 3 phase input instead of a single-phase input supply.

Is this IC suitable for 3 phase input? All reference designs / Datasheets of HF series show only single-phase input power supply. Any leads or links are appreciated.

Regards,
Arun

2

Hi Arun:

Is the 3 phase a 4 wire source with a neutral? If so then what is the Line to Neutral RMS voltage? If it is 3 wire source with no neutral then what is the Line to Line RMS voltage? Do you care if the converter is connected to only between1 Line to Neutral and the currents between the 3 lines is unbalanced because of this converter?

Prasad

3

Hi Prasad,
It is a 3 phase 4 wire system with 230V rms between phase and neutral (Indian 3 phase system ). We want the 3.3V supply to function even if one of the phases fail.

I don’t know right now if current imbalances between phases will have an effect on the power supply operation. I am not sure about this. The only reason we want a 3 phase Input is to increase the reliability of the system and avoid a single point failure.

4

Hi Arun:

If you are not too worried about the phase imbalance and you have a Y connection with 3 lines and a phase then you can use full wave diode rectification between one of the three phases and neutral and power the HF920 from the rectified DC bus. So you will need an appropriate 4 diode AC Bridge diode part . Please let me know if this makes sense.

Prasad

5

Hi Prasad,
Thank you for the suggestion. However I didn’t understand how it will add redundancy in case of failure of this phase.

6

Hi Arun:

If you need phase redundancy then you will have to use a three phase full wave rectification with 6 diodes and provide the DC bus to the HF920 based Flyback. So in case you loose 1 phase the other two will provide power and you will get phase redundancy.

Thanks,

Prasad

7

Hi Arun:

Also can you confirm that the phase to phase voltage will be 1.73 times 230VRMS . This will be 397VRMS and a peak voltage after rectification an filtering of 561VDC. Is that correct?
Prasad

8

Hi Prasad,
Yes it is correct. 230*Sqrt(3)*Sqrt(2)

I am thinking now of a front-end rectification part with 8 1N4007 diodes (2 per 3 phases and 1 neutral) and then an appropriate EMI filter, DC link cap and to flyback transformer primary.

9

Hi Arun:

Yes that will work too

Prasad

10

Hi Prasad,

Do you have any guide lines or directives to design R13 and C19 given in example circuit diagram in HF920A? These are connected to the output pin of TLV431A.

Regards,
Arun

11

Hi Arun:

Please take a look at the application note AN017 for MP3900. The link below will take you to the MP3900 page. The Application note AN017 is on this page.
MP3900 | High Efficiency Flyback/Boost Controller | MPS.
This note has the formulas you need for the TL431 compensation components and can be applied for HF920A also.

12

Hi Prasad,

MP3900 circuit shown in AN017 has a different configuration of the feedback network (primary side) compared to HF920A application note circuit. Equations given on pg 13 of AN017 cannot be directly applied to HF920A.

Regards,
Arun

13

Hi Arun:

Can you please send me your email address? I can send you another design document for feedback loop compensation that has a pull down transistor of the opto-coupler on the FB pin on the Primary side.

Regards,

Prasad