Cowinding a transformer

[Mosaic]

I am rewinding a MOT to deliver perhaps 28 VAC rms , when rectified it would be close to 40V DC.

I am switching this V with a 60Vds NFET configured as a hi side switch, so I need perhaps another 10Vgs (relative) to drive the gate. Rdson is .002 ohms.

IF I partially simultaneously co -wind the 28Vac rms coil with another coil to deliver 7Vac RMS can I then connect that coil in series with the first (creating a tap) to obtain the 35Vac (50VDc) gate drive voltage?

Can i also connect the post rectiication 40V and 10V in series to produce the 50V?

thks

 

[ChrisP58]

You have mixed up the behavior of sinewave driven transformers with switch-mode transformers. The two are very different.

In an sinewave transformer, the rectified secondary is 1.4 times the RMS voltrage. Secondary RMS voltage will be the input voltage times the transformer turns ratio.

In a switch-mode transformer, that is usually not the case, but will depend on many things, starting with the type of converter.

For a flyback converter, the rectified secondary voltage will be equal to the peak secondary voltage. (less the diode drop) The input to output voltage is not dependent on the primary to secondary turns ratio. You can however, have multiple secondaries and their voltage ratio DOES follow their respective turns ratio.

For a forward converter, the rectified secondary will be less than the secondary peak voltage, but how much is determined by many factors, including turns ratio, duty cycle, output coil inductance, etc. Multiple secondaries can be added, and are also turns ratio dependent, but will track ONLY if they share the same output inductor, and the turns ratio on the output inductor matches.

 

[Mosaic]

That sounds like a yes. The MOT is being rewound as a stepdown sinewave unit, using the current primary as the 120VAC hi side and removing the hi V secondary altogether in favour of a low V isolated version.
LTspice seems to give the go ahead as well.

 

[Mosaic]

EDIT: got an echo...

 

[ChrisP58]

My mistake. I read your second line as if you were pulsing the input. I see now that you are switching the output.

You may not need to put your n-fet in the highside. If there are no other connections between the rectifier/filter and the mosfet to any other parts of the circuit, you should be able to switch the low side.

 

[Mosaic]

I'm driving a battery charger/discharger with adc sampling so I need to maintain a common ground. High side off allows me to enabled a discharge to ground circuit while measuring V with the adc.

 

[johansen]

just use a pulse transformer to drive the fet.
https://www.electro-tech-online.com/custompdfs/2012/06/transformer20isolated20gate20drive.pdf

However, a .002 Ohm fet rated for at least 80 volts will have a very large gate capacitance.
You may need to use a complementary emitter follower or a dedicated gate drive chip after the pulse transformer, supplied with power via another power transformer (you could use a 555 to drive a 1:1 transformer or something)

you could use an auxilary transformer winding to supply the fet with power but the capacitance of the winding may cause problems, (remember, it will be driven at whatever your switching frequency is) and you still have to have some kind of transformer or a level shift network to turn the fet on and off.

 

[Mosaic]

@jo: Actually I am doing a soft on switch.....got a 10K gate resistor and a 100Uf cap to slow things down a bunch.

 

[ronsimpson]

@jo: Actually I am doing a soft on switch.....got a 10K gate resistor and a 100Uf cap to slow things down a bunch.

100uF.....wow....what is you switching frequency?

 

[Mosaic]

Takes around 160ms Period.