H-bridge Bulk Capacitor Requirements and Equations

[CaptainCarp]

I started working on an already partially designed h-bridge driver for a DC brushed motor.
Long story - short, the bulk capacitors are getting very hot.

As I have never designed an h-bridge circuit before (and I don't think the original engineer did either), I started by trying to find some information related to selection of the bulk capacitor (evidently also known as the "blocking" or "DC link" or ... capacitor) for this type of application.
I have not been successful in finding a comprehensive source of information related to this and hope that someone here can lend a hand.

The circuit uses a h-bridge driver IC and discrete power MOSFETs.
The data sheet for the driver IC does not provide any guidance about selecting the bulk capacitor.
Other manufacturer's data sheets that I looked at seem to only provide general information such as "As a basic orientation, 500uF per 10A load current is recommended".

We are trying to drive a 12VDC brushed motor to about 30 Amps.
The motor must accelerate, decelerate, and reverse quickly.

I'm pretty sure that the outside software house is using the "Phase Correct" / "Centre-Symmetric" / "Center Aligned" /... PWM mode at 15kHz.

I would like to know -
1. What is the equation for determining the bulk capacitance value that I need given the allowable circuit voltage ripple (5% ?) and the drive mode / PWM frequency?
2. How does one determine the ripple current requirements of the bulk capacitors?
Is there a good equation for this or do you just start measuring current through the capacitor that was chosen (based on value / voltage ripple) and go from there?

Thank you,
CaptainCarp

 

[FvM]

DC-bus current ripple depends on load inductance and PWM scheme (2-level or 3-level), but will be generally high for a single phase H-bridge. Assuming sufficient load inductance and preferable 3-level scheme, the DC-bus "sees" the 30 A motor current switched on and off, resulting in 15 A rms ripple current at 50% duty cycle.

 

[CaptainCarp]

DC-bus current ripple depends on load inductance and PWM scheme (2-level or 3-level), but will be generally high for a single phase H-bridge. Assuming sufficient load inductance and preferable 3-level scheme, the DC-bus "sees" the 30 A motor current switched on and off, resulting in 15 A rms ripple current at 50% duty cycle.

Thank you for the reply.
I am not familiar with the terms "2-level" or "3-level" PWM. When I tried searching, all I seemed to get were references to inverter topologies (generating sinewaves).
Can you please point me to a reference as to how these terms relate to driving a DC motor with an h-bridge?

Also, 15 Amps is a lot of ripple current! Will it be more with a 95% duty cycle? Is there any way to lower this requirement as it would be $$$ to meet?
Can you please point me to a reference / equation to help me understand how this is derived?

Thank you again,
CaptainCarp

P.S. the motor inductance measured ~ 84uH.

 

[Orson Cart]

You have a lot of current ripple, you could measure it with a scope and a 0.01 ohm resistor in series with your caps, this will show you where the ripple is highest (e.g. high bus volts and 50% pwm), you can then size your caps so that they can handle the ripple,

e.g. if the caps have 50milli-ohm ESR and there are 3 of them, each carrying 5A rms of ripple - then they will dissipate 5^2 x 0.05 = 1.25W each

The data sheet for the caps should give you ESR with frequency (or ripple current in amps) for your caps, not many can handle more than a watt of dissipation!

Conclusion, you need good quality caps for your application.

 

[FvM]

I am not familiar with the terms "2-level" or "3-level" PWM. When I tried searching, all I seemed to get were references to inverter topologies (generating sinewaves).
Can you please point me to a reference as to how these terms relate to driving a DC motor with an h-bridge?

They rather refer to different control methods (just "software" in a processor controlled system) in H-bridge inverters. There are different names for the same thing. I'm used to the terms bipolar versus unipolar modulation, but two/three level is very visual.

Please review the explanations in this thread and the links by bigdogguru https://www.edaboard.com/threads/274928/