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# 30W Flyback with 30VDC input?

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#### Magnethicc

##### Member level 2
Hi everyone,
I need to design an isolated 30W housekeeping power supply:
Topology = Flyback
Input Voltage = +30VDC (Primary)
Output Voltage = +17VDC (Secondary), +12V (Primary Sensing)
Output Power = ~30W (Almost all of it to the +17VDC secondary output)
Efficiency = ~90%
Control = Peak Current Mode Control with Primary side sensing
Switching Frequency = 120kHz

X'mer:
Magnetic Core = PC40-EER28L, AL=240nH/N^2
Peak Flux Density = 200mT

Number of Primary Turns = 30 (Lprimary = 216uH)
Number of Secondary Turns = 15
Number of Sensing Turns = 11

Build -
1- Primary - 2 Wires x 15Turns
2- Primary Sensing - 1 Wire x 11Turns
3- Secondary - 2 Wires x 15T
4- Secondary - 2 Wires x 15T (In parallel with 3)
5- Primary - 2 Wires x 15Turns (In series with 1)

Input Calculations:
Duty came out to be 0.522
Input Average Current = ~1A
Input Peak Current =2.2A
Input RMS Current = 1.35A
Input Winding Current Density = 5.5A/mm^2

Output Calculations
+17V Ripple Current = 1.25A
+17V Winding RMS current = 2.3A
+17V Winding Current Density = 4.5A/mm^2

Notes:
* MOSFET - TPH5900CNH,L1Q - 64mOhms
* +17V Output Diode - 3x RF201LAM2S - Fast Recovery, 3 In parallel
* The +17V is supplying 3 Fans as well as a Buck Converter (Stable +12V)

Questions:
1. Does anyone have any notes regarding the design above?
2. Does anyone have any experience with low input voltage flybacks and could give me some tips to consider while designing and laying it out?

Thank You.

...#6 gives you layout.

Designing...why not put it into LTspice and see it...here is one you can mod up

#### Attachments

• flyback 12vin 8v2a 5v1a _ver2.jpg
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where is the gap calculation for the transformer ?

X'mer comes gapped by the manufacturer.

How do you know it is correct ?

Firstly we use this core in a different product so the core with this AL value is well known.

Secondly when we order the built x'mer from a manufacturer we specify the primay inductance range of the x'mer (+-15%) so each core will come within spec.

Do you specify the peak current the pri must handle for a given Bpk ? this is really critical.

Can you elaborate more on that point?
I specify which wire to use (First calculating the primary peak current in Full Load & Lowest Input voltage then selecting the wire gauge to handle this)

Hi,

How do you ensure that your flyback will continue to provide power until the housekeeping is complete?

The flyback will first get energy from the input and then feed itself via a winding in the flyback x'mer and will work continuously until input is removed. Does this answer the question?

The flyback will first get energy from the input and then feed itself via a winding in the flyback x'mer and will work continuously until input is removed. Does this answer the question?

Not really. The question is concerned about what happens when the input is removed, or when power is lost suddenly.

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