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why cannot I increase duty - single phase square wave inverter ?

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Full Member level 4
Jan 10, 2009
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Istanbul , Turkey
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hello dear forum,

I build a single phase - square wave - full bridge inverter for the output frequencies of 25 - 105 Hz

I drive the mosfets with IR2183S driver IC

the circuit here


my problem

I loaded the circuit with 500 W 220 V lamps

for higher output frequencies I can adjust duty between 10 - 70 % with the software - so far good

however if I decrease the output frequency below about 30 Hz the duty of the output square wave doesnot exceed %30 even if I adjust with the duty potentiometer

this is for lamp loaded condition

for no load , I can adjust and see duty of output at the osciloscope between 10 - 70 % - no problem

I observed the HIN-/LIN signal to IR2183 and LO gate signal to mosfet, no problem either ( while lamp loaded ) - goes down to 10 % and up to %70

however HO output has the same error : it doesnot goes above %30 duty

why is this strange behaviour ?

how can you help me

thank you

- - - Updated - - -

here Hgate vs HIN-/LIN


here Hgate vs Lgate

Hgate Lgate.JPG
Last edited:

hello thank you Frank,

I suspect the same thing

this circuit is a square wave full bridge single phase inverter for changing line frequency to 25 - 105 Hz output

there is a 900 nF SMD cap as bootstrap capacitor

can I add a 1 uF electrolytic cap parallel to it ?

is there a danger of increasing the bootstrap capacitor ?

Not sure how you calculate duty cycle, but in my view a ssquare wave inverter would implement 50 % duty cycle (or may 49% to allow some dead time) and can hardly go above 50 %.

There's surely no bootstrap problem with 50 % duty cycle, so most likely you are talking about problem of your "pwm" controller circuit. Unfortunately you didn't even mention the controller type.
the controller is STM32

I calculate the duty as follows

I measuse the square wave length of half period * multiply with 2 / divide by the period length

the schematic is here


is there a problem if I increase the bootstrap capacitor ? what about an electrolytic cap ?

thank you

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