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Old 23rd July 2006, 02:04 PM   #1
crissty is offline crissty  Greenland
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Default Wanna know about smps design

I have just started making a Full Bridge 800W SMPS for audio amps application.

Previously i have made 150W 12V to +_35V Car SMPS which is working fine since last 3- 4months.

1. I wanna know what is the power handling of Siemens EE55 & EE65 cores.

2. As per the circuit given input would be 360V and output +_35V 15A, what should be the min value of input filter Capacitor.

3. I am gonna use 25Khz Swithching Freq using 3525 & i will use 2110 for driving mosfets, Duty Cycle would be 49%, is there any need of filter Choke on output.

4. Selecting Mosfets ??? for 800W.

5. Mosfet voltage 600V .OR. 500V would suffice.

6. Mosfet driver 2110 .OR. 2113
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Old 24th July 2006, 08:34 AM   #2
crissty is offline crissty  Greenland
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Photo of Transformer with EE55 in Process
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Old 24th July 2006, 08:39 AM   #3
crissty is offline crissty  Greenland
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Completed (Window fill was 100%).

No Replies ??????
Could nobody Help.
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Old 24th July 2006, 10:56 AM   #4
rt7085 is offline rt7085  United States
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Default Good reference

You are asking very fundamental questions here! So, instead of explaining every detail I am going to recommened a book to you: "Switching Power Supply Design & Optimization ", by Sanjaya Maniktala. Very practical and to the point. A very good start. In that book you can find out how to estimate the energy handling capabilities of cores of all shapes in general, gapped and non-gapped. But, why do you want a switching (buck) regulator in the first place? Why not a simple bridge rectifier and capacitor bank? Or, if you insist on a switcher, why not a simpler flyback design? Don't forget safety: You are regulating off-line here.

Regarding mosets: You can switch mosfets in parallel, as they have a positive temperature coefficient. So, they tend to share load current evenly, as long as they are of the same type. However, be carefull with your mosfet driver, as the gate charge pump requires several amps. So, make sure your driver IC can deal with multiple mosfets. Selecting mostfets? Depends on your current requirements. The mosfet voltage needs to take in account any ringing that may occur. So, 360V plus several tens of volts for ringing. 600V would be best, but you will find that the current handling will drop proportionally with higher voltage mosfets.

You do need a filter choke, as you want to reduce the 25 kHz carrier component on the DC output. Some amplifier chips deal with just fine without it, but others don't. So, plan on using it.
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Old 24th July 2006, 11:05 PM   #5
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This sight has a lot of information. Some of it is very basic, and other part are quite advanced.
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Old 9th August 2006, 01:10 AM   #6
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Default 800w Smps


Full-bridge is the best topology for 500W and above. Couple of questions:

1) Why duty cycle @49%? Are you going to PWM it, or do a buck PWM pre-regulator? The SG3525 is an excellent chip for PWM'ing the main power stage. Look at the SG3525's datasheet at ONSemi.com. Lots of good info there.

2) Is 25kHz your oscillator or switching frequency? If oscillator, I would raise that a bit to remove it far away from the audio bandwidth, say 70-100kHz for the oscillator (35-50kHz switching).

Now couple of comments: I adapted a design procedure for calculating the value of the coupling cap on tha main transformer. This cap is mandatory for all half- and full-bridges using voltage-mode PWM control to prevent the possiblilty of core saturation for uneven volts*second products on your main switching MOSFETs. My procedure is an adaptation of the procedure from George Chryssis' 1989 edition of his book, "High-Frequency Switching Power Supplies".

C(c) = (I(max) * dT) / dVC(c),

For I(max) = 1.2 I(drain),
dT = capacitor's charging time, and
dVC(c) = Coupling Capacitor Ripple Voltage

So, for a ripple voltage of, say, 30V, an I(max) if 7.5A, and a charge time of 16.67uS (60kHz), we get the following:

C(c) = (1.2*7.5A*16.67uS) / (30V) = 0.000005001F, or 5.001uF. Next standard value is 5.6uF, so use a 5.6uF rated at 250VDC.

I do have the design procedure for the input cap somewhere, and as soon as I find it, I will post it as well.

Other comment: I would stay away from flyback for any power level above 200W, simply because the drain currents will be excessively high, and the minimum required PIV ratings for your outputs will also be high.

At power levels north of 200W, almose every design uses the transformer in the bi-polar, rather than uni-polar mode, for good transformer usage.

Hope these tips help.


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Old 10th August 2006, 01:18 PM   #7
crissty is offline crissty  Greenland
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Hi Steve,
Thanks for the support,

1. I am gonna use 3525 with full duty cycle(i.e. 49%) without using Pulse width control.

2. 25Khz Switching Freq (i.e. 50Khz Osc)

I have started with the project, as i have posted some photos of the Transformer, and am in process of completing the SMPS within a week or so.

I am concerned if the transformer E55 (no gap) handle 800-1000W, & proper input filter cap values.

Also guide me for selecting Mosfet for the same.

I will post some in process photos too shortly.

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Old 10th August 2006, 01:54 PM   #8
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Hi Crissty,

I agree with N-channel, that you should increase your switching frequency. The required core size will be considerably reduced with higher frequency operation. You should also think about creepage distances at this early stage. They will affect the winding window available in the transformer core.

As a guide, a recent design I worked on used an ETD49 with 6mm creepage running at 90kHz and we squeezed out 550W with fan cooling. The ETD has a similar window size and about 60% of the core material of an E55. You will probably need to look at an E65 to get to 800W.

I would also suggest you use a centre tapped transformer secondary and a 2 diode output rectifier to halve your rectifier losses.

Cheers, Ralph
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Old 10th August 2006, 02:14 PM   #9
crissty is offline crissty  Greenland
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Dear Ralph,

I have designed the transformer for 25Khz Swithing freq.
using 70Turns, 20 AWG x2 parallel (for 360v Input),
so increasing the sw freq would increase copper losses & reduce the eff.

And can i use IRF840(500V) x 8 in all, as i have these in spare.
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Old 11th August 2006, 03:05 AM   #10
crissty is offline crissty  Greenland
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Secondly i have used 20AWG wire, so increasing the freq i will also have to consider higher AWG wire for skin effect.
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