MOSFET Driver for MC34063

[peterl]

Hi guys,
I want to make simple DC-DC converter with these characteristics:
V in: 28V
V out: 14V 3A
The converter must be without sink, so i use external P-MOS transistor with low Rds - Si7463.I tried circuit without driver for MOS transistor, but it is heated for a very short time.So i decided to put driver, but i don't know how to wire, because the input voltage is 28V more than MAX Vgs on transistor.
Thank's advance!

 

[ronv]

Maybe you could post your circuit. I think I understand your problem but the circuit would make it clear. If it is what I think it is, you can make sure you have a gate resistor and then clamp the Vgs with a zener diode.

 

[peterl]

re:

The only way to solve the problem is to replace MC34063 with other.I chose LM5118.

Best wishes, Peter

 

[mvs sarma]

if one wants to use LM5118, it would be a design what people joke, 'DESIGN WITH COMPONENT MAXIMA'

 

[Blueteeth]

The max Vgs of your P-MOS is the maximum voltage between its source, and gate. As its a P-channel MOSFET, the 'source' will be at your Vin potential - 28V. So, as the max Vgs of that device is 20V. 28 - 10 = 8v. That is the lowest voltage you must provide to the gate.

I'm sure there is a known way of doing this, (without lowering your input voltage) but all I can tihnk of right now is to use a zener diode in series with drive signal (ground referenced) to limit the voltage to a minimum of say, 18V (Vgs of 10V) instead of 0v. Then using bipolar transistors as level shifters/inverters to shift up your gate drive from the open collector output. Like this but with a zener in series with the 100R resistor on the first transistor:
https://www.boerde.de/~mw/msp/HighSidePFET.jpg
The first transistor in that circuit being the open collector drive of the MC34063

Perhaps using a boost circuit used for high side N channel MOSFET's. Like this:
**broken link removed**
But with opposite polarity? (with the transistors, diodes, and supply in reverse.)

Consider either a device with a higher Vgs (unwise, as that likely increases gate driver requirements as far as I'm aware..) or perhaps use an N channel with a boost?

**broken link removed**


One more link for old times sake:
https://www.electro-tech-online.com/custompdfs/2011/09/AND8291-DPDF.pdf - page 3! Maybe use a 12 or 15V zener in place of the 6.2V one there.

That should give some food for thought