z9u2k
Member level 1
Hey,
I'm designing a small constant-current SMPS at the range of 400-500kHz, with input voltage of 5V for a single cell NiMH battery charger (output no more than 2A).
This is an educational purpose project - so I want to use discreet components. The PWM controller is a comparator, capable of sourcie/sink current of 20mA max (LM339).
As a switching (P-Channel) MOSFET, I'm using IRF4905 since at -Vgs = 5V and -Vds = 5V it is out of the linear zone.
Thing is, at -Vgs = 5V, Qg is about 80nC - which means that if I want my MOSFET switching time to be at most 5% of the cycle, I need to source and sink at least 800mA to/from the gate [80nC / (0.05/500kHz)], which the LM339 cannot supply.
I was thinking about using a BJT totem-pole as a gate driver (2N2222 + BC327), but I'm concerned that since my 5V supply is close to the MOSFET's threshold voltage, which is about -4V, the 0.7V drop across these transistors will take me under the threshold. In addition, the datasheet doesn't have a curve for V < -4.5V so I cannot determent the behavior in this range.
I'm looking for a suggestion regarding the gate driver.
Thanks!
I'm designing a small constant-current SMPS at the range of 400-500kHz, with input voltage of 5V for a single cell NiMH battery charger (output no more than 2A).
This is an educational purpose project - so I want to use discreet components. The PWM controller is a comparator, capable of sourcie/sink current of 20mA max (LM339).
As a switching (P-Channel) MOSFET, I'm using IRF4905 since at -Vgs = 5V and -Vds = 5V it is out of the linear zone.
Thing is, at -Vgs = 5V, Qg is about 80nC - which means that if I want my MOSFET switching time to be at most 5% of the cycle, I need to source and sink at least 800mA to/from the gate [80nC / (0.05/500kHz)], which the LM339 cannot supply.
I was thinking about using a BJT totem-pole as a gate driver (2N2222 + BC327), but I'm concerned that since my 5V supply is close to the MOSFET's threshold voltage, which is about -4V, the 0.7V drop across these transistors will take me under the threshold. In addition, the datasheet doesn't have a curve for V < -4.5V so I cannot determent the behavior in this range.
I'm looking for a suggestion regarding the gate driver.
Thanks!
