H bridge output capacitor - Page 1
Forum section: electronics / beginners

author: mcdesca
author_rank: Contributor
timestamp: March 09, 2016, 08:03:44 am
content: Dear everybody,after playing with H-Bridges and different motors with different parameters, I was wondering why is a capacitor (or LC filter) never to be seen on the bridge output.I did some simulations adding a small inductor (needed on coreless low-inductance motors when higher freq. pwm is not available) and testing it with and without capacitor.By no means is it a flat DC signal, but it looks like the motor becomes more efficient, thus dissipating less heat.Am I right in adding such capacitor? Am I missing something here?Thanks a lot.Best,

author: Simon
author_rank: Unknown
timestamp: March 09, 2016, 12:46:59 pm
content: you mean to generate flat-ish DC from the PWM ? it won't work. The idea of PWM is that it's easy, works and maintains motor torque at low speeds beyter than low constant voltage. If it is a H bridge it will be generating voltages of both polarities so you need a non polarized capacitor of great capacity.

author: ThomasDK
author_rank: Regular Contributor
timestamp: March 09, 2016, 01:46:17 pm
content: Two things:1: The inductance is not the part dissipating heat, so multiplying the current through and voltage over the inductor doesn't tell you much. Look at the power dissapated in the resistance instead. (Hold down ALT and left click in LTspice)2: What you are measuring is the steady state stall current, not normal operational current. You need to add back-EMF.Capacitors are not necessary, as the output torque of a motor depends on the current through the coil, and not the voltage across it*. As long as the PWM frequency is chosen high enough, the inductance will filter out the ripple, and be a good approximation to a constant current.*: (The current is of coarse a result of the supply voltage, minus the back-EMF)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 09, 2016, 01:58:38 pm
content: You want the cutoff frequency of the LC filter,\( f_0 = \frac{1}{2 \pi \sqrt{L C}} \)much less than the PWM frequency. Then the high frequency energy will be filtered (and the bridge will see an inductive load, which is good for its operation), and you can connect to the motor with long wires, without worries of causing radio emissions, or overheating the motor.You also want the impedance of the filter,\( Z_0 = \sqrt{\frac{L}{C}} \)close to the impedance of the load, i.e., whatever the average \$\frac{V}{I}\$ is. If this resistance is not present around \$f_0\$ (that is to say, if there's no resistance at that frequency, due to it being filtered, like due to the motor's stray inductance), you should also add an R+C in parallel with the capacitor, \$R = Z_0\$ and \$ C \geq 2.5 \$ times the first C.Tim

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2016, 04:51:43 pm
content: The H-bridge (or half-bridge) in PWM wants to see an inductive load, since switching a stiff voltage source into a capacitive load gives a huge peak current, dangerous to the transistors. Switching a voltage into an inductive load gives a finite current that increases with time, switching it off gives a voltage spike that can be "caught" with flyback diodes.