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IR2109 - HO and LO problems

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Skubby SkinnyChubby

Newbie level 1
Aug 21, 2015
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Hi all,

got a question for IR2109 HO and LO pin

schematic diagram,datasheet of component used attached*
my bootstrap capacitor i'm using a 100nF because i can't find any 506nF (maybe there is no such value for capacitor)
CW and ACW is my signal pin to control my U1 and U2 respectively
i'm putting a 24V to CW and ACW which will be step down to ~12v*

ideal result is when i did not put in 24v to my CW or ACW
U1 & U2:
Pin 2 (IN) = 0v
Pin 5 (LO) = 0V
Pin 7 (HO) = 0V
which means none of my MOSFET is turn on.

First problem(s) is
when i didn't put any voltage to my CW and ACW
Pin 7 (HO) = 12v
Pin 5 (LO) = 15v

Pin 7 (HO) =0v
Pin 5 (LOW) = 15v

which means both of my lower side MOSFET is turn on and one of my High MOSFET is turn on = short circuit

i've checked my connections and part numbers... but still cant find the reason why it got this kind of problems

whoever have this same problem as me before and already solved it/ any person know the reason why... please tell me..need to solve this problem ASAP..



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There are several issues.

* where do the 506nF come from? If you calculated it, then it most likely is a minimum calculation, then you should use a bigger value than 506nF.

* to one driver IC you connected left top and right bottom FET. To the other driver right top and left bottom. This won't work.
--> For the motor to drive clockwise you need to turn on both FETs right top and left bottom. But a driver is never able to turn on both high side and low side at the same time.

My recommendations:
* connect both FET's of the left half bridge to one driver and both FETs of the right half bridge to the other driver.
* so you can never cause a short circuit.
* disable both drivers to let the motor spin freely (no breake)
* enable both drivers with both inputs low to immediately stop the motor (break). This should also be the idle state, to safely charge both sides bootstrap capacitors.
* enable both drivers. Left side low, right side pwm. With the PWM you could adjust speed. Even if you don't need speed control you need to pwm the input signal. I must not be continously high. It needs the low phases to charge the bootstrap capacitor. A low phase of several microseconds should be ok. Depending on bootstrap capacitor size it may be sufficient to switch it every 100ms low fir a short time.


ideal result is when i did not put in 24v to my CW or ACW
U1 & U2:
Pin 2 (IN) = 0v
Pin 5 (LO) = 0V
Pin 7 (HO) = 0V
which means none of my MOSFET is turn on.

But the Ideal result is when IN is 0(CW or ACW is 0V) then the LO is 15V and HO is 0V.
(with no OFF signal)

The HO will give output only after turning ON and OFF any one of the Lower transistors.


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