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Old 27th March 2012, 10:21 AM   #1
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Default Highest power flyback not needing fan?

Hello,

For a 90-265VAC offline, two-switch flyback, what is the maximum continuous output power that can be achieved without a heatsink being needed?

I am speaking of designs where whatever the power, the output current would be less than 6 Amps on average.


If you can answer the above, then please can you state the answer for a single switch flyback too?


I am not too bothered if the heatsink is big, but i dont want an enormous, housebrick sized heatsink.......and of course, i dont want a fan under any circumstances.
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Old 28th March 2012, 01:24 PM   #2
gmarsh is offline gmarsh  Canada
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Here's what you need to do.

- Set a maximum temperature that your system is expected to operate in.
- Set a maximum junction temperature for your active devices.
- Find out if there's any safety limitations you need to be concerned about (you don't want to have a hot heatsink on the back of a box that can burn someone, etc)
- Start going through some heatsink catalogs, looking at ones designed for natural convection. Get an idea of what R-theta you can achieve in the mechanical form factor you're restricted to.
- Based on the R-theta you come up with, and the efficiency of your power supply designs, calculate your maximum power from there.

In other words, do some engineering.
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Old 28th March 2012, 03:44 PM   #3
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I know that normally the authors Ive readed, not recommend to make a SMPS FB more than 120-150W, but for a question of voltages and currents in the switches. I have made on of my own at about 80W using only one STP7N60, a UC3842, and 4N17 and TL413 in the loop. I get +-14V and +12V, and post regulating (L4972) I get +5V, for my audio set. It only uses a 5cm * 5cm heat sink (rescued from a PC monitor, tha same as the EE 42 core), and it can be touch with my fingers, say, 20C of t rise. And it is working fine since about 14 years,ans about 10-12 hours a day without any fan.
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Old 28th March 2012, 06:13 PM   #4
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gmarsh i see what you mean by "do some engineering"...however, the primary fet losses will be mostly switching losses, and there will be a gate transition time which has to be slow enough to pass emc and fast enough for not too much switching loss........where the exact switching losses in the fets will be is unknown. I know many calculation methods for switching losses but most dont give very accurate results.......also the leakage inductance affects switching losses as it delays the on-coming of primary current. Also, page 6 to 9 of the following give a detailed calculation of switching losses, but in truth, this wouldnt be terribly accurate for the stated flyback

http://www.ti.com/lit/ml/slup169/slup169.pdf

so you can see that it literally is a case of "build and see"........my company doesnt have the money to do that....it wants calculation first, but the switching loss calculation is not straightforward....also, things like diode conduction losses (the bulk diode resistance which gives I^2.R losses) is very difficult to decide from the datasheet.

I think its possible to say that "engineering" can involve speaking to professionals such as yourself in forums like this prior to further action.
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Old 28th March 2012, 06:36 PM   #5
gmarsh is offline gmarsh  Canada
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I'd start with whatever the TI method gives you, it won't be exact but it should be reasonable. Design a heatsink based on that. It'll at least get you in the ballpark. Derate the heck out of the heatsinking when you build your first prototype, and slim the final heatsink design as needed at production.

For designs like these I tend to get the switching loss as low as I possibly can, and keep EMI under control by using a good layout, passive filtering and a fully shielded enclosure. If the goal in your case is the cheapest possible design and you're restricted to a 1 layer board etc... then maybe this mindset isn't the way to go. Stuff I design is intended for industrial/broadcast applications which requires a fair bit more reliability than a consumer design, and cost isn't as critical.
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