How to deal with switching noise of a transistor?

[Codeman]

Switching noise

Hi all,

I have a circuit that a transistor is driven by a microcontroller I/O PIN.
The transistor is a BD436 PNP 20V 4A.

I have a 1K pull up resistor followed by 56R base resistor.

The transistor switches at about 1 Khz.

But instead of a clean pulse I have lots of pulses jumping around.

The circuit works as expected, but, why the scope dont show a clean pulse ?

Can someone point me to a formula to calculate the correct values ?

Do I have to add some capacitor in the base ?

Thanks in advance

Joaquim

 

[Diksi]

Re: Switching noise

Suggestion: try first to discover uC output ... i think that this part must be ok but first try ... and post result after that ... will see ...

bye!

Added after 3 minutes:

Suggestion: try first to discover uC output ... i think that this part must be ok but first try ... and post result after that ... will see ...

bye!

ok i must add ... check output without a transistor 1. and check output with base resistor connected over one diode to gnd 2. i hope that we will be have more information after this simple testing .. bye!

 

[IanP]

Re: Switching noise

Driving a power transistor of low β directly from a microcontroller pin is not a very good idea. In your case you have 56Ω resistor between the micro pin and the base of the BD436, so you are trying to sink >80mA. Usually a microcontroller can sink some mA but not 80 or more ..
What you should do is to add a small signal transistor in "darlington" configuration and this one will drive your BD ..
If you do the above the resistor values can be changed to 10kΩ+ pull-up and 1-4.7kΩ base ..

 

[Diksi]

Re: Switching noise

Driving a power transistor of low β directly from a microcontroller pin is not a very good idea. In your case you have 56Ω resistor between the micro pin and the base of the BD436, so you are trying to sink >80mA. Usually a microcontroller can sink some mA but not 80 or more ..
What you should do is to add a small signal transistor in "darlington" configuration and this one will drive your BD ..
If you do the above the resistor values can be changed to 10kΩ+ pull-up and 1-4.7kΩ base ..

yes its same like my idea ... only I havent idea about this transistor(i havent time to check in catalogue) ... it must be it ...

bye!

 

[eda4you]

Re: Switching noise

Hi codeman!

Some µC reduce the output current after a while like C166 from Infineon. The higher inital current helps to drive the load capacitance.

I suggest you to use a Mos transistor like the BST72.