Tapped coil for MC34063 with external mosfet

[Telespalla]

Hi, I'm just arrived in this forum.
I would ask a question about calculating tap position in a toroidal core to use in boost converter made around MC34063 and an external power mosfet with gate discharge circuitry.
With reference to Texas Instruments MC34063A datasheet (https://www.ti.com/lit/gpn/mc34063a) I see that to page 7 is recommended to use the tapped coil in order to optimize overall performance, but I don't see in that manner I can calculate tap position. Where can I find these information?
My circuit should supply around 180Vdc with 15Vdc fixed input to provide deflection plates voltage in 5'' CRT tube.
Do you think that 90mA is enough for this application?
Many thanks and excuse me for my english..

Paolo.

 

[johansen]

since you need 150 volts but that part is only good to 40vdc, you would need to place the tap ~1/5th of the way into the coil from the 15 volt side of the coil.

you would have to place a ~36 volt transorb or something across the switch to prevent the leakage inductance from destroying the part.

there are other means to generate 150 volts that will work better for you, there are plenty of single component flyback controllers that would work as well that can run right off the 120vac rectified line.

 

[ronsimpson]

The tap on the coil on page 7 is not to increase the voltage. It is to change the output switch from Darlingtron to non-Darlington configuration. It should not be used in high voltage applications. The maximum voltage on pin 1 is 40 volts so neither circuit on page 7 will work at 150 volts.

See my circuit. I would wind the coil (transformer) with this turn ratio: 11:4:120

Will at 5" CRT circuit work with 15 watts? That depends on how you design the circuit but probably. "deflection plates" so you are not building a TV set but an oscilloscope. Why do you want to use a CRT?

 

[dougy83]

The tapped coil is to allow the external BJT to saturate; this does not apply to external MOSFETs.

 

[ronsimpson]

The tapped coil is to allow the external BJT to saturate; this does not apply to external MOSFETs.

The MC34063 was not designed to drive a MOSFET. It can but.....
There are other parts.

 

[Telespalla]

The MC34063 was not designed to drive a MOSFET. It can but.....
There are other parts.

Hi, thanks to all for replies.

I have found this circuit in internet:

**broken link removed**

I'm interested only to external mosfet driver configuration principle.
It seem safe. The gate is discharged through pnp bjt, and the only device subjected to high voltage is the mosfet.
The circuit is designed to output low voltage, but I think that with the same driver circuitry and feedback I can obtain high voltage for my deflection plates.
With reference to the same Texas Instruments datasheet linked above, I would ask if in the components values calculation of page 10, the "Saturation voltage" Vsat parameter we must consider RDSon X Ipk(switch) with Mosfet instead of power bjt.

I would want to drive CRT because I'm trying to make an analog clock with vector display.

Thanks.

Paolo.

 

[dougy83]

The MC34063 was not designed to drive a MOSFET. It can but.....

True, but slightly irrelevant..
An attitude of only using anything for its specified intended/designed purpose is somewhat limiting and chokes ingenuity.

Hi, thanks to all for replies.

I have found this circuit in internet:

I'm interested only to external mosfet driver configuration principle.
It seem safe. The gate is discharged through pnp bjt, and the only device subjected to high voltage is the mosfet.
The circuit is designed to output low voltage, but I think that with the same driver circuitry and feedback I can obtain high voltage for my deflection plates.
With reference to the same Texas Instruments datasheet linked above, I would ask if in the components values calculation of page 10, the "Saturation voltage" Vsat parameter we must consider RDSon X Ipk(switch) with Mosfet instead of power bjt.

I would want to drive CRT because I'm trying to make an analog clock with vector display.

Thanks.

Paolo.

You can use a configuration like the one shown in that schematic if you adjust the feedback resistor values and use a high voltage & high current MOSFET. Or you can use a second winding as some others have suggested above (as a flyback converter) on the inductor/transformer and then you don't need a high-voltage MOSFET.

With the 0r1 current sense resistor, the peak current should be around 0.3V / 0.1ohm = 3A

 

[ronsimpson]

Q1, D1 should be high voltage type. 250V or 300V
Change feedback resistors.

 

[Telespalla]

Q1, D1 should be high voltage type. 250V or 300V
Change feedback resistors.

Yes, thanks.

But is correct to assume for Vsat required in calculations (originally intended for transistor saturation voltage) the voltage obtained with RDSon x Ipk?

I have hypothesized 90mA for current output to deflection plates examining an old 10-MHz oscilloscope schematic diagram.
I suppose that the driving current is in relationship to the positioning speed of the light spot that is wanted to get, because the deflection plates appears like a capacitor.
I will try to measure the real drive current in a working oscilloscope.