power supply topology

[francis29]

Hi,
I happened to come across a power supply schematic. I interested to know what will be the output and its working. The schematic is attached below.

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this is the shematic i am talking about.

 

[KlausST]

Hi,

as an estimation:
Output voltage: 200V/117 turns x 6 turns - 0.7V.
Output current: Output_voltage/load_resistance.

When FET switches OFF there will be high voltage generated. by the transformer I assume the circuit will kill itself soon.

Klaus

 

[Warpspeed]

Yes its a forward converter with built in suicidal tendencies.

When the mosfet turns on, the current into the output capacitor will be essentially limited only by the dc resistance of the fet and transformer windings, which would all be very low. So it will probably blow the fet from a massive over current surge at turn on.

If it survives that, at turn off, there will be a huge uncontrolled voltage spike from the transformer back emf, which will potentially fatally break down the mosfet.

If it somehow survives both these disasters, there is no way to regulate the output voltage, because the output capacitor always charges up to the peak voltage which is controlled by the transformer turns ratio. So PWM will not be able to control the output voltage.

The whole thing is like a nightmare from hell on a bad night.

 

[francis29]

So are you sure that it will not work properly??

 

[Warpspeed]

So are you sure that it will not work properly??

Not the way it is drawn.

 

[francis29]

If i reversed the transformer polarity will it work??

 

[mtwieg]

Reversing the polarity of either winding will make a flyback converter, so yes.

 

[c_mitra]

Yes its a forward converter with built in suicidal tendencies.

When the mosfet turns on, the current into the output capacitor will be essentially limited only by the dc resistance of the fet and transformer windings, which would all be very low. So it will probably blow the fet from a massive over current surge at turn on.

Why the primary inductance of the will not be able to limit the input current? I always thought that the current in an inductor always rises exponentially with a characteristic time constant. Let us forget about the off period for academic discussion.

 

[asdf44]

It's a flyback with one dot in the wrong place....

Why the primary inductance of the will not be able to limit the input current? I always thought that the current in an inductor always rises exponentially with a characteristic time constant. Let us forget about the off period for academic discussion.

It's true there could be inrush but that's not atypical for transformer circuits at all (and yes it would be limited by leakage and R). So I don't how that could have been a major problem. The other issues with the reversed windings would be real but again, presumably that was just a mistake.

 

[c_mitra]

It's true there could be inrush ...

A capacitor looks like a short to a voltage step but an inductor looks like an open circuit to the same voltage step. The current in an inductor, in response to a step voltage, always starts from zero and for a capacitor always start from infinity.

Inrush current is commonly the problem for capacitors. Inductors have no (I mean zero) inrush current.

 

[asdf44]

But in the hypothetical we're talking about the mosfet, on initial turn-on, is applying a voltage step to the primary which considering ideal transformer behavior passes that voltage step to the secondary where it hits the (initially discharged) capacitor. That's the inrush scenario I believe warpspeed was talking about.

My point is that this approximates regular old AC mains linear supplies which step-down and rectify just like the circuit shown.

The non-ideal transformer has leakage L and R which would limit that inrush, but typically not that much.

 

[Warpspeed]

Why the primary inductance of the will not be able to limit the input current? I always thought that the current in an inductor always rises exponentially with a characteristic time constant. Let us forget about the off period for academic discussion.

Because the secondary is shorted out with a diode and a large fully discharged capacitor. The effective short circuit across the secondary is reflected directly back into the primary.

If the secondary phase is reversed, it becomes a flyback supply as mentioned above. The output diode is then non conducting during the forward conduction part of the cycle, and as you say, the current rise is well controlled by the primary inductance.

 

[schmitt trigger]

That is THE REASON that forward converters have an external inductor on the secondary, along with another diode in frewheeling duties.

 

[MISU.RSG]

Yes its a forward converter with built in suicidal tendencies.

When the mosfet turns on, the current into the output capacitor will be essentially limited only by the dc resistance of the fet and transformer windings, which would all be very low. So it will probably blow the fet from a massive over current surge at turn on.

so dear, what`s your suggestion to control or limit this massive current in cases that we have to work with large current.

resistor? or better method existed?

 

[Warpspeed]

Single ended forward converter always uses a choke and a catch diode after the rectifier. Read the previous post #13

 

[MISU.RSG]

yes, but he mentioned we use inductor secondary and we have huge of current in primary.!!!!!

 

[FvM]

Read the discussion thoroughly. It's assumed that the primary inrush current is caused by missing current limiting means in the secondary, inserting a secondary inductor would reduce both.

It's however still unclear if the post #1 schematic is meaned as forward converter or a flyback with flawed polarity mark.