Please help me with this design
I want to use a buck converter to convert a voltage input of 16-40v to 13.8 v. this will be controlled via a 16f628 via pwm.
The buck converter part is where my problem starts.
I am trying to make a buck converter using a logic level mosfet but seem to be having problems with the switching of the mosfet.
according to the spec sheet a logic level mosfet will switch on VGS(th) at 2V.
I have calculated the the buck converter to use the following components.
switching freq = 700kHz
induc ripple ratio = 0.3
capacitor voltage ripple ratio= 0.04
Max overshoot= 0.096
Max current = 500mA
there fore the duty cycle = 0.44
min inductance = 63.4 uH
Min Saturation current = 0.69 A
Min out put capacitance = 11 uF
when in practice the output current is way less than 500mA ie: it barely turns on a led also the voltage mesured across the output terminals is about 2V.
I have uploaded the test circuit here
at the moment i am using a function generator to produce a square wave.
If I use the TTL output on the generator then the circuit does not run, the TTL output is 5V .
If I use the normal output and increase the output to 10V then the circuit runs but with limited current ie: nowhere near 500mA
Please help
I want to use a buck converter to convert a voltage input of 16-40v to 13.8 v. this will be controlled via a 16f628 via pwm.
The buck converter part is where my problem starts.
I am trying to make a buck converter using a logic level mosfet but seem to be having problems with the switching of the mosfet.
according to the spec sheet a logic level mosfet will switch on VGS(th) at 2V.
I have calculated the the buck converter to use the following components.
switching freq = 700kHz
induc ripple ratio = 0.3
capacitor voltage ripple ratio= 0.04
Max overshoot= 0.096
Max current = 500mA
there fore the duty cycle = 0.44
min inductance = 63.4 uH
Min Saturation current = 0.69 A
Min out put capacitance = 11 uF
when in practice the output current is way less than 500mA ie: it barely turns on a led also the voltage mesured across the output terminals is about 2V.
I have uploaded the test circuit here
at the moment i am using a function generator to produce a square wave.
If I use the TTL output on the generator then the circuit does not run, the TTL output is 5V .
If I use the normal output and increase the output to 10V then the circuit runs but with limited current ie: nowhere near 500mA
Please help