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Problems of transformer, snubber and freewheeling design of push pull DC-DC converter using SG3525

Hi everyone, as part of inverter project I need to build a step up converter that converts 12VDC to 360VDC using push pull topology. Switching frequency is at 55kHz and using SG3525 as PWM chip. I have a few question:

  1. Datasheet of SG3525 from manufacture is too brief. It does not explain clearly how exactly to size Rd for dead time nor how to calculate value of slow start capacitor. Does anyone know where I might find a reference design or app note of at least a buck converter using this chip, as I could find none from ST website?

  1. What is the exact use of snubber circuit with MOSFET? Is it to absorb voltage spike due to leakage or magnetising inductance of primary winding when MOSFET is turning off? Or to damp ringing at rising edge due to to fast dV/dt? Where should it be placed? As in one example
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    the snubber is placed across primary winding, but I also saw some circuits where it is put across D and S of MOSFET.

  2. MOSFET use gate resistor to slow down turn on transition which could otherwise cause ringing. Isn't that overlap the purpose of snubber? Sorry I've read too much app notes I messed up the concept.

  1. When switching high frequency transformer, the ideal transformer output waveform should look like:

------- -------- | | | | | | -- --- --- | | | | ------- it's necessary to discharge energy stored by leakage inductance at crossover. Does snubber do this as well? How about the energy stored by magnetizing inductance and magnetizing current? does it also need to be discharged? And if so, should I use a freewheeling diode (fast recovery type) across primary winding? My first thought was to connect two diode this way

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which proved failed in simulation as D1 will short the upper winding when voltage is induced by lower winding as lower MOSFET is on. So instead I added two zener diodes
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to form zener clamp. Is it appropriate? How about using RC Clamp ( as shown in
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)?

  1. When winding transformer, should the direction be "back and fro" as in
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    or always follow one direction
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Thankyou for your kind helping ; )

Reply to
w2kwong
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Parts of speech called articles were invented to aid written and spoken communication. USE THEM!

Bill

-- Fermez le Bush--about two years to go.

Reply to
Salmon Egg

I'm not native speaker of English and sorry for my poor expression

Reply to
w2kwong

I apologize. I am sure your English is better than my capability in your language whatever it is.

Bill

-- Fermez le Bush--about two years to go.

Reply to
Salmon Egg

In figure 3 the deadtime is illustrated as being dependant on the timing capacitor size, modified slightly by the optional resistor in series with pin7.

Softstart: using the 50uA present on pin8. the rise in softstart capacitor voltage will be linear - i = C dv/dt. How this voltage rise will affect your circuit, you'll have to determine yourself.

SG3525 is originally a Silicon General device - now Microsemi

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Application notes for jellybean two-phase pwm drivers may all be applied to a degree. There's not much special about the IC itself.

In your push-pull topology, the snubber should act somehow to limit voltage peaks and reduce radiation when the mosfets turn off. A current snubber will limit dv/dt. At the low voltage you're operating at (12V), a voltage clamp might not be needed to protect parts.

You do not mention a power level.

How you clamp is your choice. Ask yourself what's going to get hot.

If a lot of energy is involved, you might consider trying to recover it, rather than burning it off.

Energy stored at any time is ( Li^2)/2 Leakage energy is stored in the leakage inductance. See references below.

Check out the old unitrode seminar at TI

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2001 magnetics design handbook chapters are downloadable

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RL

Reply to
legg

Hello legg, thankyou for your reply which has been useful. But there are still some bits which I'm not quite sure:

  1. You said deadtime is illustrated in figure 3 on datasheet, so is deadtime = oscillator discharge time? And which application note on jellybean two-phase pwm driver are you referring to, since I couldn't locate on microsemi website?

  1. I ran a simulation on push pull stage of the circuit
    formatting link
    and found that as soon as M1 and M2 has just turned off, the energy stored by MAGNETIZING flux from Lm (not leakage inductance) will get discharged by lower section of primary winding. Demagnetizing current will flow upwards back to 12V source through body diode of M3 and M4, as shown in this picture
    formatting link
    . I've heard that body diode of MOSFET is a by product during fabrication, and is of slow recovery time. So should I place a fast recovery diode or schottky across each MOSFETs to facilitate the flow of this current?

P.S. I'm working on a 300W DC-DC converter which steps up 12V to

360VDC. The transformer core is ETD49, Np=6T+6T, Ns=198T. Calculated magnetizing inductance =200uH. Total primary current at rated load = 25A.

thanks again for helping ^_^

Reply to
w2kwong

Mostly.

Look at SG3524, UC3525

Intersil an6915

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Nat Semi AN-292
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Philips AN126
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The links are straight Google hits.

You will be looking for power stage info, not info on the driver or it's interface.

Your reference is not to a conventional push-pull converter, it is to a DC-DC transformer, with no energy storage element in the output rectifier filter. This is basically capactor-to-capacitor energy transfer, with no regulation ability.

The control chip is completely wasted in this application.

As to your model, I have no comments, as there are models and then there are models.

For example, current will not appear 'across' the mosfet switches - it will flow 'through' them. If your GUI makes this kind of an error, it can hardly be expected to keep track of it's own bootlaces.

Remember to apply a load to your model, as unloaded operation will hide the relative importance of the currents involved.

With both switches off, the magnetization current will have the opportunity to ramp to zero (in a perfectly coupled transformer), through which-ever path presnts itself. As your model image does not illustrate drain voltages, you can have no idea where the magnetizing current is flowing, or why.

Once you apply a load and view the resulting waveforms, you may understand why capacitive-capacitive energy transfer has it's problems, without the presence of effective intermediate inductive storage. Perhaps leakage terms will suffice. Don't forget them in your model.

RL

Reply to
legg

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