Offline full-bridge SMPS....need help

[Nejc_car]

Is it posible to build a push-pull smps for 230Vac.

[Eva]

Yes, it is possible, but it implies dealing with more than 700 volts from drain to source instead of 350V and requires two matched primary windings that should be as coupled as possible to reduce inductive spikes at turn-off. Furthermore, these spikes are quite hard to clamp because there is not a +700V supply rail but a +350V one, unlike in a full bridge or push-pull where the body diodes of the MOSFETs clamp the spikes to the supply rails.

12V push-pull power supplies suffer from the same inductive spike issues, but the MOSFETs employed are usually rated at 60V thus avalanche is not likely to happen (note that 60V is 5 times 12V). However, in a 230V AC off-line push-pull converter the switches would probably be rated between 800V and 1200V, thus leaving little voltage margin for inductive spikes before avalanche voltage is reached (note that your switches are rated at 800V, it's less than 3 times 300V altough it may be enough if you can figure out a good clamping approach).

[N-Channel]

Originally posted by Tekko
You need to make like in the ESP car smps schem, use a npn and pnp bipolar transistor, one to turn the fets on and one to discharge the gate capacitance to allow the fets to switch as they should, otherwise it will not work. The SG3525 cannot drive the fets directly as its open collector and too low current handling.

Tekko,

I think you're referring to the output stage of the TL494, which has uncommitted emitters and collectors. The SG3525's outputs are true totem-pole outputs, though not as robust as later PWM ICs, like the MC33025 or the UC3825 (high-speed current-mode PWMs), which can drive +/-2A into highly capacitive loade, like MOSFETs with really BIG gate capacitance.

The '3525 is only good for +/-400mA pulsed, amd +/-200mA steady-state. However, this is good enough for driving the gate-drive xfmrs Eva described, or even IR211X hi-side and low-side MOSFET floating-gate driver ICs, which also have the +/-2A output current rating.

Hope this clears things up a little further.

Steve

[sagarverma]

hi eva,

r u 'in bussiness'?do u design smps etc.

can u tell me circuit of mains based smps 0f +-30v output,and 100w loading?

[Eva]

Yes, I do design SMPS but I'm not yet in business, and given the SMPS migration that the PA market is experimenting, I can't provide such a ready-made schematic to you for free because I still expect to make some profit from my work before other people could start cloning and selling my prototypes like crazy... Sorry.

[Nejc_car]

EVA,

How many turns does your gate trafo have?

[sagarverma]

Originally posted by Eva
Yes, I do design SMPS but I'm not yet in business, and given the SMPS migration that the PA market is experimenting, I can't provide such a ready-made schematic to you for free because I still expect to make some profit from my work before other people could start cloning and selling my prototypes like crazy... Sorry.

ok.

[Eva]

In this thread I explained how to figure out power transformer turn ratios for isolated buck converters: SMPS transformer overwinding

And this one contains the formula that allows to calculate the required primary turn count for a given core, frequency and flux density: SMPS design web sources?

For a gate drive transformer I wouldn't go above 200mT in worst case (ie: B=0.2). I can't provide any exact turn count value because it depends on core size and operating frequency. You'll have to calculate it by yourself.

[kartino]

Hi Nejc_car,

Please take a look to :

http://www.qrp4u.de/docs/en/smps_new/

Described as half bridge design, with junk parts. Good for start to try. Make small and reliable first then think to build thousand watts.

[Nejc_car]

Hi, everybody

Here is an update on my project. I have made new Fet driving board, where I used gate trafo from old PC supply. It is configured as Half-bridge


This is how the signals look on gates of the fet´s.
Now the frequency is 38,4 kHz
5V/div


And the whole switching signal. 5V/div


I will post scheme of the gate drive soon