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Old 28th August 2008, 12:13 PM   #1
chimi is offline chimi  Argentina
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Exclamation push-pull PWM IC

Hi everyone... I'm starting a new push-pull smps design and I don't want to use the old TL494 again, want something that allows current mode control and honestly I don't know or used for this topology any other PWM. I used to put a BJT totem pole to drive the MOSfets, but would be cool not to... by the way, the TL494 has 2 error amps, can one of the be used as a current ramp limit anyway, is it wise??

Getting back to the main issue...

1)which PWM IC should I use/would be best for a 2 voltage output push-pull from 230Vac to 24Vdc 4A and 12Vdc 1A...

2)Being the total output power approx 110W... Is it wise to make this out of a flyback considering that here in Argentina, the only gapped option I have is an EE2507 core with a 0.5mm gap?

3)I designed in the past a 70W flyback using this core at a switching freq of 100KHz, would be a lot more difficult to redesign with this core at 200KHz sw freq? would the compensation network be a lot more complicated?

Thanx!!!
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Old 28th August 2008, 06:12 PM   #2
star882 is offline star882  United States
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I think the easiest way is to modify a PC power supply.
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Old 29th August 2008, 06:13 PM   #3
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Default 3 viable choices

A 2-switch flyback would be a good approach as well as a 2-switch forward. Also worth considering is a half bridge. At 230V ac input, a push-pull center tap, or PPCT, is not a good choice. The PPCT places a voltage stress on each FET equal to twice the supply rail. With 230V ac, the max is 264V ac, which when rectified can reach 373 volts. Doubling gives 746 volts, requiring the use of 1000V FETs. Selection is limited. Also, clearance requirements would be needed for 746 volts. The same applies for a 1-switch flyback or forward.

With a half bridge, HB, 2-switch forward, 2SFWD, or 2-switch flyback, 2SFBK, the stress on each FET is one times the supply rail, or 346V. Thus 500V FETs easily do the job, and the clearance requirements are half of the PPCT case.

At high line inputs such as 230V ac, the last thing we want to do is double the voltage. At a power level of 110 watts, the HB is overkill, but still a viable choice. The 2SFBK is very suitable, but flybacks are noisier than forwards, and tend to place high peak stresses on the components. The 2SFWD is a very good method. It is quiet, fast, and places moderate stress on the parts. I hope this helps.

Please be very careful with 230V ac!
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Old 29th August 2008, 06:48 PM   #4
chimi is offline chimi  Argentina
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Helps a lot...

I have a reasonable doubt, I've built simple dual- swicth push-pull converters and the voltage stress over the mosfets was just the rail voltage, the exact double is with a single switch forward converter, that on one side has the rail voltage and when switches off on the other side has rail voltage inverted discharging the flux induced by the first primary during on time, thus, stressing with doubled voltage...

I have reasons not to use half-bridge in 300V+, it is very difficult to configure bootstrap components for the driver (I.E. IR2110) and eventually blows up pretty badly...

I never seen or used a dual switch flyback, but I succesfully produced a 70W flyback fresh from 230Vac to 19V 3.5A with only one switch, look at my next post "110W flyback" where is an schematic of that 70W flyback, was somewhat noisy but was very cheap....
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Old 1st September 2008, 06:11 PM   #5
cerrem is offline cerrem  United States
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There is nothing wrong with the PP current mode topology....
Yes it does have twice the stress + transients...but that kind of thinking was from the 70's and or from reading all the outdated SMPS books that are still popular..... The FETS today are available in high voltage for reasonable prices and descent Rds-ON....
Thats unless your building for production and your a typical "cheap" engineer who won't spend a penny more than you have to.....
I would recommend useing LT chips and avoide the old UNITRODE dies chips from the 80's....such as the 1845's and thier derivatives, since th error amps in these controllers are pittyfull...

Chris
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Old 1st September 2008, 06:19 PM   #6
star882 is offline star882  United States
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I actually have an old laptop power supply (Dell) that has a push pull stage operating from a boost preregulator.
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Old 1st September 2008, 10:49 PM   #7
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Quote:
Originally posted by cerrem
There is nothing wrong with the PP current mode topology....
Yes it does have twice the stress + transients...but that kind of thinking was from the 70's and or from reading all the outdated SMPS books that are still popular..... The FETS today are available in high voltage for reasonable prices and descent Rds-ON....
Thats unless your building for production and your a typical "cheap" engineer who won't spend a penny more than you have to.....
I would recommend useing LT chips and avoide the old UNITRODE dies chips from the 80's....such as the 1845's and thier derivatives, since th error amps in these controllers are pittyfull...

Chris
Sorry to differ, but yes, there is something wrong with PPCT at **high voltages**. The info I give is current, not '70's or '80's.

The fact that 900 and 1000 volt MOSFETs are available is not a reason to use a PPCT. I know that such devices are available. With 230V ac input, the PPCT doubles the voltage and as a result, the creepage requirement and clearances on the pcb is that of nearly 800 volts. Larger spacings means less available board space. Larger creepage means less of the transformer window available for winding.

The PPCT finds its home in low voltage applications, 12, 24, 48V etc. It is not a 230V solution. The loss due to Qoss increases as the square of the voltage, as well as snubber loss. The turn on and turn off loss increases w/ larger voltage. The Rdson of a 1000 volt device is at least twice that of a 500 volt part. The HB and the 2-switch flbk and frwd topologies are more suited to higher voltages. So is the full bridge, but FB is overkill for just 110 watts.

The most appealing feature offered by the PPCT is that both MOSFETs are low side driven. N-channel parts can be used with no high side drive headaches, i.e. no bootstrap, charge pump, or level shifting required. All of the others mentioned require driving at least one high side MOSFET. At high inputs, i.e. 230V ac, this can be tricky. Negative gate drive may be needed as well as capacitive Miller clamping to prevent false turn on due to dv/dt. For high voltage offline ac applications, the PPCT is seldom used. It does offer simplicity regarding FET drive, but the other problems outweigh this benefit.

Again, a PPCT could be made functional. If FET gate drive is too difficult with the other topologies, the PPCT does make that problem disappear. BR.
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Old 2nd September 2008, 12:24 PM   #8
chimi is offline chimi  Argentina
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Guys, your helping me a lot from your discussion because

1) Yes, I dont want to have high voltage stress over the FETs, and I prefer if nesessary to use a high side driver than stressing high voltage mosfets with the restpective very high voltage transient, not to mention difficult to snub.

2) Well, I'm a relatively cheap engeneer, not for my own reasons, in this country, the cost of a power supply must tend to zero, but I'm not that cheap, not cheap enogh to jeopardize people and so on, the only reason I rather not to use very high voltage mosfets is their relatively high Rds on, wich increases the heak-sink price, wich in this country, is real big, its better to add extra silicon than heat sinks.

3) I've made REAL-basic schematics of what I understand were the topologies discussed in this post, except for dual switch forward, in that you got me, don't know anything about it, and I'd like to know what are the benefits of using a dual switch flyback or dual-switch forward (from now on 2sfb and 2sfw as you called them), my guess is the option to decide between ZVS and ZCS...
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File Type: jpg dual-switch flyback.jpg (77.2 KB, 413 views)
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Old 2nd September 2008, 12:25 PM   #9
chimi is offline chimi  Argentina
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The first was 2SFB, now lets see PPCT (push pull central tap)
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File Type: jpg push-pull central tap.jpg (71.7 KB, 398 views)
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Old 2nd September 2008, 12:27 PM   #10
chimi is offline chimi  Argentina
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And now the famous HB, wich is far the most sofisticated topology I'd in a extreme case use for a 110W SMPS
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