Trouble with MCT2e driver

[thebenman]

Hey,
I am trying to implement a DC-DC converter and there seems to be a problem with the circuit. Bascially,i checked the PWM from PIC and it seems to be fine.However i was wondering if it was normal to connect the 1k resistors with the optocoupler.I was browsing when i noticed the most of you had suggested lower values for resistance in similar cases.Please go through the attached schematic

Regards

 

[betwixt]

It would probaby work with 1K, you could drop to 470 Ohms to be absolutely sure you have enough LED current. However, you have the optocouplers wired wrongly and they will not produce any output as shown. The MOSFETs probably want connecting differently as well, switching a capacitor across the supply wont achieve much.

I would drop the 10uF capacitor on the reset pin to 1uF or less, OR, add a diode, (cathode to VDD) across the 10K resistor. Otherwise you risk holding the MCLR pin high when the power is removed which could damage the PIC. You also need grounds on the regulators and a more reasonable value capacitor than 104F !

Why draw in .doc format, it's much easier to use a circuit drawing package. There are many free ones around, I recommend Kicad.

Brian.

 

[thebenman]

It would probaby work with 1K, you could drop to 470 Ohms to be absolutely sure you have enough LED current. However, you have the optocouplers wired wrongly and they will not produce any output as shown. The MOSFETs probably want connecting differently as well, switching a capacitor across the supply wont achieve much.

I would drop the 10uF capacitor on the reset pin to 1uF or less, OR, add a diode, (cathode to VDD) across the 10K resistor. Otherwise you risk holding the MCLR pin high when the power is removed which could damage the PIC. You also need grounds on the regulators and a more reasonable value capacitor than 104F !

Why draw in .doc format, it's much easier to use a circuit drawing package. There are many free ones around, I recommend Kicad.

Brian.

When i measured the voltage across the pin 4 and pin 5 of the MCt2e driver it indicated 12 volts.Wouldn't that confirm that the MOS are triggered correctly using the optocoupler ? is there anything wrong with the 1k resistor in series with the MOS Gate ?

 

[betwixt]

There is nothing wrong with 1K in series with the gate as long as the PWM is fairly slow. The MOSFET has a fairly high gate to source capacitance which has to be charged and discharged through the resistor so as the speed increases, the rise and fall of gate voltage will become more gradual. However, in your design there will always be 12V across pins 4 and 5 of the MCT2E because one is connected to the 12V supply and the other is grounded!

Brian.

 

[thebenman]

There is nothing wrong with 1K in series with the gate as long as the PWM is fairly slow. The MOSFET has a fairly high gate to source capacitance which has to be charged and discharged through the resistor so as the speed increases, the rise and fall of gate voltage will become more gradual. However, in your design there will always be 12V across pins 4 and 5 of the MCT2E because one is connected to the 12V supply and the other is grounded!

Brian.

The PWM is programmed to switch at around 50KHz