H bridge diodes

Other Hardware Motors, Mechanics, Power and CNC
1

I'm building an h bridge to control a 12 V 4.7 a motor. I want to use diodes to discharge the coils but I'm not sure what their max current value should be. What I mean by this is that even though the motor is rated at 4.7 A, what should be the diode current? Higher? Lower? Same?

I also want to add a capacitor to smooth out the spikes, but I'm not sure what its value and material (tantalum, ceramic, etc.)
should be.

Does anyone have any recommendations for a diode that I can easily find?

Thank you!

2

Diode - same or higher current rating as flows thru motor.
Search digikey.com for diodes, filter for thruhole, instock, active, schottky, and pick a current >4.7A. These are 5A parts for example
https://www.digikey.com/products/en/discrete-semiconductor-products/diodes-rectifiers-single/280?k=diode&k=&pkeyword=diode&pv96=24&pv914=22&FV=1f140000%2Cffe00118%2C1140050&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

3

"I also want to add a capacitor to smooth out the spikes"
What spikes, where occurring?

4

I found this here: capacitor - How diodes protect H-bridge DC motor driver? - Electrical Engineering Stack Exchange

"The capacitor's function is to act as a local energy store, to provide some of the energy required by the motor during the initial spike of each turn-on, and storing back some of the energy that kicks back onto the power rail at each turn-off. Without the capacitor, the current spikes at each edge would completely need to be served by the supply rail. As any supply connection will have some resistance, these current spikes thus result in voltage dips on the supply rail.

In simple terms, the capacitor smooths out the spikes due to temporary power demand and temporary power surplus, as the coils are energized and de-energized."

And thanks for the help!

5

Oh. With 4.7A of current, you're going to need some big caps to hold the power supply up.
Like hundreds of uF. Probably aluminum electrolytics.

6

Should I use mosfet drivers to control the mosfets? I'm using the IRF3205 n-channel mosfet and I'd probably use the IR2305. I want to run my motor at 25 kHz.

Thanks so much!

7

johnfg:
I'm building an h bridge to control a 12 V 4.7 a motor. I want to use diodes to discharge the coils but I'm not sure what their max current value should be. What I mean by this is that even though the motor is rated at 4.7 A, what should be the diode current? Higher? Lower? Same?

Normally the diode pulse rating should be larger than the motor current, they only carry pulses, so the continuous
rating isn't necessarily relevant. However if certain decay modes are used its better to assume the
worst and use a diode with a continuous rating at least as large as the motor current.

Nowever if you are building an H-bridge from MOSFETs you won't need diodes, they have body diodes
built in.

I also want to add a capacitor to smooth out the spikes, but I'm not sure what its value and material (tantalum, ceramic, etc.)
should be.

The diodes will stop any spikes. You might want a small amount of capacitance to reduce EMI from the
motor terminals, but thats 100nF at most directly across the motor terminals (ceramic). Adding a large
amount of capacitance will just stress the H-bridge and create large current spikes.

Where you do need large amounts of capacitance is decoupling the motor supply rail, typically electrolytic bulk capacitance is used to reduce the current pulsing from the supply and reduce voltage noise on the
supply.

Does anyone have any recommendations for a diode that I can easily find?

Thank you!