H-bridge flyback diodes

[adrianvon]

Hi,

Can someone please explain to me how the four flyback diodes actually works? what will happen if no flyback diodes are not used? and why are Schottkey diodes prefared to be used?

Thanks in advance

 

[ericgibbs]

Hi,

Can someone please explain to me how the four flyback diodes actually works? what will happen if no flyback diodes are not used? and why are Schottkey diodes prefared to be used?

Thanks in advance

hi,
The diode clamp the spikes caused by switching off the inductive motor.

These voltage spikes/transients can exceed the rating of the driver transistors.

Two of the diodes conduct when the +Vspike exceeds the Vsupply+Vdiode

the other two diodes conduct when the -Vpspike exceeds -0.7Vdiode below 0V..

OK.?

EDIT:
Schottky are sometimes used because they have a lower V forward turn on voltage, so the spikes are clamped closer to +Vs and 0V

 

[adrianvon]

Hi,

Thanks for the reply. Regarding the link below,are C1 and C2 in figure 7 used to smooth the input supply? and why in figure 7, C1 is 1000uF and in figure 8 C1 is only 22uF ?

https://www.bristolwatch.com/ele/h_bridge.htm

 

[ericgibbs]

Hi,

Thanks for the reply. Regarding the link below,are C1 and C2 in figure 7 used to smooth the input supply? and why in figure 7, C1 is 1000uF and in figure 8 C1 is only 22uF ?

https://www.bristolwatch.com/ele/h_bridge.htm

hi,
A good question.
The two circuits appear to have different authors, I would choose a cap to suit the motor size as well as the power supply source.

I would not use a 22uF, more like a 470uF or 1000uF and a 100nF in parallel.

 

[adrianvon]

Thanks The motor to be used will be a 12VDC 2A motor. Think the 1000uF cap will be the best choice.

So is C1 used to smooth the supply? an what is C2 used for (the cap in parallel with the motor)?

 

[ericgibbs]

Thanks The motor to be used will be a 12VDC 2A motor. Think the 1000uF cap will be the best choice.

So is C1 used to smooth the supply? an what is C2 used for (the cap in parallel with the motor)?

Its for suppressing electrical noise from the motor, the motor is an inductive load.

 

[Sceadwian]

Flyback diodes are not always required, it depends on the speed you'll be switching the motor at, as long as you're using mosfets there is an intrinsic diode built in, it only becomes a problem when the motor is switched at a frequency close to or inside the intrinsic diodes recovery time which will cause it to switch slowly increasing heating and possible latching up, this value is sometimes found on the PDF of FETs.

Schotkey diodes are preferred because their conduction voltage will be lower than that of the body diode.