tinska.h
Junior Member level 2
Hello,
I'm making a low voltage step-up converter with inputs 0.2-0.4V. Output should fall 1.5-1.8V and higher the output current capability the better. I'm operating in CCM.
Okay, I have no problem starting it and achieving >80% efficiency at light loads (say 10mW). However when I want more current out from it, it won't happen. By increasing duty cycle I can get lower input impedance (DC-DC's input impedance, thus more Pin), however efficiency will drop, therefore power output stays roughly the same. My maximum practical and efficient duty cycle seems to be around 85%, Gain ~=6.667, but actual gain seems to be ~4.5. And input voltage does not sag.
So I wonder what factors are causing efficiency drop when duty cycle is increased (besides inductor resistance and RdsON)? I'm thinking more about MOSFET parasitics at switch node... Oh and also at switch node the voltage never drops below 0.2V, when I compare to my other "normal" boosts their voltage seems to be dropping close to 0V even in CCM.
Also would decreasing inductor size (+increasing frequency) help with increasing output power? Or any other ideas for low voltage step-upping (Flyback or similar to Joule thief seems good for low loads, but higher loads their efficiency comes in expense of bigger transformer)
Thanks in advance
I'm making a low voltage step-up converter with inputs 0.2-0.4V. Output should fall 1.5-1.8V and higher the output current capability the better. I'm operating in CCM.
Okay, I have no problem starting it and achieving >80% efficiency at light loads (say 10mW). However when I want more current out from it, it won't happen. By increasing duty cycle I can get lower input impedance (DC-DC's input impedance, thus more Pin), however efficiency will drop, therefore power output stays roughly the same. My maximum practical and efficient duty cycle seems to be around 85%, Gain ~=6.667, but actual gain seems to be ~4.5. And input voltage does not sag.
So I wonder what factors are causing efficiency drop when duty cycle is increased (besides inductor resistance and RdsON)? I'm thinking more about MOSFET parasitics at switch node... Oh and also at switch node the voltage never drops below 0.2V, when I compare to my other "normal" boosts their voltage seems to be dropping close to 0V even in CCM.
Also would decreasing inductor size (+increasing frequency) help with increasing output power? Or any other ideas for low voltage step-upping (Flyback or similar to Joule thief seems good for low loads, but higher loads their efficiency comes in expense of bigger transformer)
Thanks in advance
