Role of Electrolytic Capacitors and whether they can be replaced by Film capacitors in battery operated power converters

Question

In battery operated power converters such as motor controller ( half bridge or full bridge ) bulk capacitance is placed across Vdd and Gnd . I thought this was unnecessary since the input is DC and batteries can be considered as "big capacitors" . But after reading i found some of the reasons are :

a. Provide energy in case of high current draw at start , and absorb energy at stop (generated by inductive load).

b. Reduce Ripple current which causes overheat and over voltage .

1. Is there any more reasons ? which is more important ripple current specification or capacitance ?

For example : Assume there is a system that uses 1500uF capacitors with 3.3 A ripple current ,17mohm impedance at 100khz . The same company provides 560uF Capacitor with 2.18A ripple current 20mohm impedance at 100khz.

2. if two of 560uF are paralleled we will get roughly 1100uF with 4A ripple current , is this better or worse than a single 1500uF in the case of a motor controller ??

3. if Ripple current is more important than capacitance then what about using Film (Polypropylene) capacitors since they have very tempting characteristics. But they will have much lower capacitance about 15uF but current rating of 10A or more ?

Answer 1

1. As others have stated, both specifications are important, and there are other reasons for wanting capacitance on the power lines: preventing voltage droop from the battery if it can't supply current quickly (battery chemistry impacts this significantly), I*R losses if the battery is far away, path for motor transients (though I'd expect diodes and smaller ceramic caps, not electrolytics, across the bridge transistors for the faster transients).

2. This is questionable; you've left out tolerance. If the dual caps each have +/-20% tolerance, and you get one at +20% and one at -20%, they're not going to share current evenly because they have different impedances, which could lead to one failing prematurely, and the other shortly afterwards since it may not be able to handle the load by itself. Also keep in mind that higher tolerances tend to cluster near the outer limits, because values closer to nominal are typically sold with tighter tolerance specs for more money; that is, a +/-20% part is not likely to be within +/-10% tolerance, because those will be the +/-10% parts, etc.

3. The reduction in capacitance is not recommended. If 15 uF were sufficient for the intended load, the original designer likely would have gone with a 15 uF electrolytic instead of a 1500uF electrolytic; for the same voltage and current ratings, the 15 uF would be substantially cheaper and smaller. If you're using a different kind of motor than the original designer had in mind, you'd need to look into the suitability of the lower capacitance yourself.

If you're really dead set on getting rid of that particular capacitor, I'd look at increasing the PWM frequency (so you can then reduce the bulk capacitance required), if that will work with the motor you're using (the motor will have its own mechanical and electrical time constants that your circuit has to deal with).

Another thing to consider is the impact of motor inductance that's in parallel with this capacitance. The higher ESR of an electrolytic can sometimes be beneficial in dampening potential LC oscillations that might otherwise be possible as a result of pulsing power to the motor with the bridge, particularly if the motor winding resistance is low. You'd need to look into the impedance of the motors you want to drive to check for potential resonance.

As a general note for future reference, Analog Devices (no affiliation with myself at time of this posting) has an article I've found useful on the general parasitic effects of capacitors in their "Ask The Applications Engineer # 21 : CAPACITANCE AND CAPACITORS"

Answer 2

You need to consider EMI, as has been noted in the other answers - but this is maybe better handled by typically smaller value capacitors and good layout.

It is OK to split the ripple current in the ratio at which the capacitance is split, although you need to consider the cost/space impact, and also make sure that your PCB layout is at least as effective as it was before.

You can't add addidional lower value capacitors to absorb ripple current in general - there is nothing the steer the ripple current to your 'good' capacitor except for other resistance or inductance in the circuit.

Batteries have poor AC characteristics (the chemical processes are rate-limited, and potentially saturate) so you should attempt to design your circuit to isolate the ripple from the battery. Ideally, you need to analyse the detailed impact of any component changes which you are contemplating - it is too simplistic to just compare specifications when there may be knock-on impacts through component lifetime, operational efficiency or EMC.

Answer 3

One of the reasons for a cap at the bridge is to minimize EMI. High frequency and pulse currents should be returned to ground as close to their origin as possible. The capacitor can be placed close to the bridge. The battery likely cannot. The longer the path in which the high frequency or pulse currents circulate, the more EMI is emitted.

A battery does not store electrical energy. It stores chemical energy and converts that into electric power when a circuit is presented across the terminals. The chemistry works better with a smoother load. High frequency and pulse currents are better handled by an electrical device such as a capacitor.

The key difference between the 1500uF and two 560uF options is the frequency where they provide protection. Their protection starts to cut out at f = 1/(2PixRC) and below. The 1500 has a corner frequency of 6,245 Hz and a high frequency impedance of 17 mohm. The 560 parallel has a corner frequency of 14,217 Hz and a high frequency impedance of 10 mohm. So the benefits of the two 560uF's starts to cut out at freq's below 14 kHz whereas the 1500uF doesn't cut out until 6 kHz. So it depends on the ripple current frequency. If all your ripple/noise is well above 14 kHz then the two 560's is superior. If you have switching or noise below 14 kHz then the 1500 is probably better. I suspect you need the 1500uF for the lower frequencies.

Sometimes one uses a number of capacitors in parallel, such as adding a polypropylene with an electrolytic. The polypropylene has low ESR. So it provides benefits for big amplitude, short duration peaks, and high frequency ripple. Whilst the electrolytic provides benefit at comparatively lower frequencies and longer peaks. To get protection over a range of frequencies, one adds different types of caps in parallel.

The solo film caps only gives protection at higher frequencies. If you need lower frequency protection, then you also need the electrolytics.

Answer 4

The ripple current specification of a capacitor is only related to the safe operation of the capacitor without overheating(which means shorter life or even failure). And it's confusing in how you're trying to match that with a motor driving application. The spec is related to a continuous AC current that causes a power dissipation cased by the internal resistance. That combined with the thermal resistance of a capacitor will case a temperature rise of the capacitor's core. This is not the case when an inrush current is supplied by the capacitor. Actually the ripple current is specified in amps RMS.

Now suppose you are using a 3A ripple current capacitors. you have 2A ripple for example (caused by a PWM drive signal). increasing the capacitor ripple current rating to 5A for example will only reduce the core temperature of the capacitor and therefore reduce the "failure rate" of the capacitor. I suggest you measure the ripple current using a true-RMS multimeter to estimate the needed ripple current rating. And just choose a capacitor that has a higher rating.

the capacitance is related to how much energy can this capacitor supply in case of inrush current. lower capacitance has no effect on the capacitor failure. The motor will get the rest of the current from the battery. If there is high battery internal impedance then the voltage will drop and the motor will not generate the full torque. So i think you can't simply answer "which is more important, the ripple current rating or capacitance"

Answer 5

Just my thought: We don't want a battery to heat up nor that became a radio transmitter, so an EMI filter is needed. The battery can't supply or sink peaking currents those you'd find in motor control bridge, so again the capacitor bank is needed. Indeed the film capacitors can reach higher age, compared to elctrolytic ones, but at higher volume and price. Still, there are good elec. caps that will last for qite long time, and are different from those you will find in consumer electronics. Example: http://en.tdk.eu/tdk-en/179690/products/application-guides/industrial---epcos-brand/power-supply---conversion/power-supplies-switch-mode-power-supplies--smps-/aluminum-electrolytic-capacitors

Answer 6

The nature of H bridge is that you have rapid changes in current and the choice of capacitor is also driven by line length, both to the motor and back to the battery. The inductance of the line means that these currents will result in large voltage excursions unless there is a current path available through these capacitors. Keeping the lines short can reduce the required capacitance to achieve a level of ripple. If you are switching high currents, you need low ESR. If you are switching at relatively low frequencies you will need high capacitance. If you are switching at low frequencies AND high currents, as is often the case with a motor, you will need both low ESR and high capacitance.

If you were to use the film capacitor as you suggested, you would certainly have low losses in the capacitor, for two reasons: the lower ESR means low resistive losses, and the lower capacitance value will mean that there will be less current in the capacitor in the first place. However, the lower current means that there will be more current ripple in the lines and the voltage ripple will be higher.

Electrolytics, with their higher ESR, will heat up and may burst if they are overstressed. Choose a capacitor with a low ESR and run them for a short while and see if they are warm to the touch. Safety glasses are recommended.

Answer 7

1a. Pick your capacitance (minimum) such that the max voltage (due inductive load at stop) is less than the capacitor's and FET's rating.
1b. Pick you capacitance (minimum) such that the ripple voltage is acceptable.

2. Then pick the lowest ESR cap you can afford.

Of course there is tradeoff. As you change technology, you trade capacitance for ESR (and price). In order of capacitance/dollar your tech options are: Alum Electrolytic Alum Poly Film Ceramic

If you really need low ESR (e.g., lots of ripple current and heat), and are space constrained (can't use lots of big Alum) you'll have to start sliding down that scale.

A further note on Alum Electrolytic: Capacitance and ESR are related: As you increase plate area (capacitance), you decrease resistance.

Lastly, there can be a trade-off in going with two high a capacitance, which is the creation of non-zero current switching when you turn ON your FETs.