liveforphysics
100 TW
WORK IN PROGRESS
I put the update for 10/16/10 in blue, so folks who read what I had yesterday won't have to re-read.
Why do are controllers have caps?
To protect the FETs, and to improve controller efficiency and performance.
How do caps protect the FETs? What do my FETs need protection from?
Your motor is a big inductor. To operate this motor, current is switched through these coils of the motor. Everytime the FETs switch from ON to OFF, the inductor trys to maintain this current level it was previously seeing (and the energy they use to do this is energy stored in building the magnetic field and the associated delay in the rate current climbs when initially switched "ON", so energy in/out is balanced). It can't maintain current by holding the same voltage, so as this field in the inductor colapses, the voltage skyrockets trying to maintain that current flow through the now greatly increased resistance of the FETs in the OFF state.
This effect is called flyback, freewheeling, or a number of other names to define this inherent voltage spike.
This flyback voltage spike occurs on absolutely every conductor that suddenly becomes an open circuit. If it's a giant coil of wire around a stator tooth, or a 2cm trace on a PCB, it has inductance, and when it switches from carrying current to not carrying current, it will have a spike. The magnitude and energy in this spike depends on the inductance of the path this current is/was flowing through.
To make an metaphor for help folks visualize this effect, think of the inductance in the system like a bungee-cord. As you the FETs switch ON at the start of a cycle, it tugs that bungee cord to a given stretch distance, and as the PWM cycles, it's moving back and forth, but the displacement of the stretch in the bungee cord dampens this effect, and keeps the tension of what you're tugging against roughly flat. Now, when the FET opens, it's like somebody just snips the connection, and the cord "fly's back" in way where that tension (current) gets very quickly converted into high velocity (voltage), and the things on both ends of this lead had better be ready to absorb this extremely rapid conversion from nice smooth tension into snapping that energy stored from stretching that bungee (energy stored in creating the magnetic field).
if anyone wants to make a better example, please do, I will replace mine with it. I am not a poet.
This spike on the motor phase side is clamped by the intrensic diode of the FET body and the energy is released as heat, and ringing in the phase wires.(if it's diode is fast enough).
(more on this later by someone who understand the hole saturation effect better please?)
The current flowing on the battery current side of things also has inductance of course (remember every conductor going from point A to point B has inductance). The same time that bungee cord is snapping back on the motor side, it's snapping back on the batter side as well. Longer battery wires mean more inductance, longer traces on the boards between the caps and the fets equals more bungee cord between these parts. If there were no caps, the FETs would be destroyed by these spikes just about instantly. FETs are very strong and robust in some ways, and very very weak in other ways. Raising the input voltage beyond the rated levels (IE, 75v, 100v, 150v etc etc) is one of the ways a FET is very vonerable to damage. (gate over-voltage events are another way to destroy even the strongest of FETs instantly)
This is why the cap is critical to protect the FETs in a controller.
The second function is voltage stability when the FET switches to "On". The same bungee effect hits them on the turn "On" event, and nobody likes having the voltage drop 10-20v at the FETs, and these things have roughly no difference in voltage drop if you use little 12awg battery wires or 0000wires, the inductive drops and swings are almost identical for identical current load and conductor path distance (resistive drop changes of course, but that's another matter).
(More on this to come, running out of time typeing this at work)
However, not all caps are good for protecting the FETs. Some are in fact very poorly suited to the appliction, and some are very well suited.
I'm going to help you choose caps to suit the needs of the controller.
WORK IN PROGRESS
(pics of caps below with discription of the functions)
This is a snubber cap. It's entire function is to clamp inductive spikes from destroying FETs or IGBTs.
It has extremely low inductance, and a shockingly low ESR of 5.5mOhm, which is just oustanding. This means, you this this cap with a 500amp fly back spike, and the voltage across it only has a 2.75v ripple. The spike on a bank of the normal electrolytics we use in controllers would be too slow to catch a fast spike, and if it did catch it, the ESR would mean a 500amp spike is going to be lifting rail voltage at least 10-20v. If you're FETs were being operated at close to the maximum voltage, that means you can kiss them good-bye.
These caps are extremely fast acting, and extremely low ESI/ESR. Absolutely fantastic for clamping flyback spikes, but useless for meaningful energy storage due to the 4uF capacity. They serve the very specific roll of clamping flyback spikes. These caps need absolute placement priority over all other caps in the controller. They need to be placed very close to the FET to keep the inductive path minimized so they can function correctly. They have those big bolt-on terminals because they are normally bolted directly to the input legs of IGBT modules. I'm using one bolted straight each FET package power inputs in my controller design.
Normal retail price is something like $25 per cap. I picked up 30 of them on new ebay for something like $5 each.
These are also snubber caps, these belong to BigMoose. They are a little physically larger, and are something like 4mOhm each if I remember right. (extremely good caps!)
These are small radial electrolytic capacitors. They are roughly the size an AA battery.
In this same size and voltage, they can make a cap with >1,000uF capacity. However, it's useless in a controller! These caps do not have 1/5th the capacity of other caps this size because the mfg was slacking on the design or something, these things are designed specificly around being as low of ESI and ESR as possible. This means they use substantially thicker aluminum foils in the cap, and they do multiple layers starting from the same current collector lead, so they get the required foil surface area while keeping the majority of the aluminum as close the the lead as possible. Look for caps that have long skinny aspect ratios, and low low capacity for the physical size (well, specificly look for the manufactures low ESR series caps, in this case it's called the ZL series for the manufacture Rubycon.) Expect to pay about x4-10 more for a given voltage and capacity for these ESR caps, and expect them to physically be substantially more bulky.
Now, notice in this example, both of these caps have identical voltage/capacity specs. Notice the size difference between these caps? It's not because vishay doesn't know how to make a cap, it's because that big Vishay cap is a low ESR/ESI series cap. It can charge and dischage at speeds and current levels useful to the controller. That black capacitor is designed around storeing a bunch of energy, but if you work the numbers on it, even if you had 10 of them, it can't match the rate the big blue low ESR cap can charge and discharge. As low ESR caps get larger, they can only have big bolt-on terminals, because they would just melt legs right off trying to handle the ripple currents with normal solder-on legs.
This is the cap stage of a Honda Insight motor controller. Notice it uses 350vdc rated caps? Pack voltage is always under 175vdc on an insight, but they knew to leave a good deal of margin to keep the caps from being damaged by inductive spikes.
You will also notice dispite each of these caps being larger than a can of soda, they are only 1750uF. That's tiny! Stock little 30amp e-bike controllers often come with more capacitance on board. This is because the roll of these caps is not to try to store meaningful energy, it's to try to filter spikes and ripple only, and let the battery store the energy.
So, when the FET switches on, and current starts to rise in that motor's winding, that current isn't being fed from the battery, it's first being supplied by the snubber (if you have one/them), then supplied by the electrolytics on the PCB, and still, the battery isn't doing jack yet. All that current is on it's own little pathway between the caps to the FETs. This is becaue the inductance of the battery leads (and batteries have inductance themselves as well) requires the wires to first build an electromagnetic field before it can transfer useful current levels. This delay depends on a lot of factors, like the distance the battery sits from the controller, the battery type, the switching speed of the FETs, the cap type and layout on the board etc. Some short, some long, etc. What's important to understand, is that's not just a battery that supplies current to the FETs. It's more like the caps supply current (entirely cap initially, then they share current load, then entirely supported by the battery), and it's the batteries job to keep the cap's charged up and ready to supply current to the FETs.
They are all connected on the same conductor, and we might visualize this as current always being sent from the battery down that path to the FETs, like a 1-way street. It's actually a very 2-way street, and current is being slammed back and forth between cap-fet, battery-cap, and all moving both directions down all those traces on your PCB between cap/fet/battery etc. We call it "DC", it's more like "CC", chaos-current going through the board.
And thank's to the hard work of the caps, the battery and other electronics are protected from this chaos, and we get to just read a nice filtered current reading on our CA and say, "hey, 60amps", and that's the correct average current at that time, and all we need to know as an end user, but it gives no representation of what current is doing inside the controller.
(once again, I'm out of time to finish this at work, and tomorrow i'm going to be twice as slammed at work. Will try to write more when possible)
I put the update for 10/16/10 in blue, so folks who read what I had yesterday won't have to re-read.
Why do are controllers have caps?
To protect the FETs, and to improve controller efficiency and performance.
How do caps protect the FETs? What do my FETs need protection from?
Your motor is a big inductor. To operate this motor, current is switched through these coils of the motor. Everytime the FETs switch from ON to OFF, the inductor trys to maintain this current level it was previously seeing (and the energy they use to do this is energy stored in building the magnetic field and the associated delay in the rate current climbs when initially switched "ON", so energy in/out is balanced). It can't maintain current by holding the same voltage, so as this field in the inductor colapses, the voltage skyrockets trying to maintain that current flow through the now greatly increased resistance of the FETs in the OFF state.
This effect is called flyback, freewheeling, or a number of other names to define this inherent voltage spike.
This flyback voltage spike occurs on absolutely every conductor that suddenly becomes an open circuit. If it's a giant coil of wire around a stator tooth, or a 2cm trace on a PCB, it has inductance, and when it switches from carrying current to not carrying current, it will have a spike. The magnitude and energy in this spike depends on the inductance of the path this current is/was flowing through.
To make an metaphor for help folks visualize this effect, think of the inductance in the system like a bungee-cord. As you the FETs switch ON at the start of a cycle, it tugs that bungee cord to a given stretch distance, and as the PWM cycles, it's moving back and forth, but the displacement of the stretch in the bungee cord dampens this effect, and keeps the tension of what you're tugging against roughly flat. Now, when the FET opens, it's like somebody just snips the connection, and the cord "fly's back" in way where that tension (current) gets very quickly converted into high velocity (voltage), and the things on both ends of this lead had better be ready to absorb this extremely rapid conversion from nice smooth tension into snapping that energy stored from stretching that bungee (energy stored in creating the magnetic field).
if anyone wants to make a better example, please do, I will replace mine with it. I am not a poet.
This spike on the motor phase side is clamped by the intrensic diode of the FET body and the energy is released as heat, and ringing in the phase wires.(if it's diode is fast enough).
(more on this later by someone who understand the hole saturation effect better please?)
The current flowing on the battery current side of things also has inductance of course (remember every conductor going from point A to point B has inductance). The same time that bungee cord is snapping back on the motor side, it's snapping back on the batter side as well. Longer battery wires mean more inductance, longer traces on the boards between the caps and the fets equals more bungee cord between these parts. If there were no caps, the FETs would be destroyed by these spikes just about instantly. FETs are very strong and robust in some ways, and very very weak in other ways. Raising the input voltage beyond the rated levels (IE, 75v, 100v, 150v etc etc) is one of the ways a FET is very vonerable to damage. (gate over-voltage events are another way to destroy even the strongest of FETs instantly)
This is why the cap is critical to protect the FETs in a controller.
The second function is voltage stability when the FET switches to "On". The same bungee effect hits them on the turn "On" event, and nobody likes having the voltage drop 10-20v at the FETs, and these things have roughly no difference in voltage drop if you use little 12awg battery wires or 0000wires, the inductive drops and swings are almost identical for identical current load and conductor path distance (resistive drop changes of course, but that's another matter).
(More on this to come, running out of time typeing this at work)
However, not all caps are good for protecting the FETs. Some are in fact very poorly suited to the appliction, and some are very well suited.
I'm going to help you choose caps to suit the needs of the controller.
WORK IN PROGRESS
(pics of caps below with discription of the functions)
This is a snubber cap. It's entire function is to clamp inductive spikes from destroying FETs or IGBTs.
It has extremely low inductance, and a shockingly low ESR of 5.5mOhm, which is just oustanding. This means, you this this cap with a 500amp fly back spike, and the voltage across it only has a 2.75v ripple. The spike on a bank of the normal electrolytics we use in controllers would be too slow to catch a fast spike, and if it did catch it, the ESR would mean a 500amp spike is going to be lifting rail voltage at least 10-20v. If you're FETs were being operated at close to the maximum voltage, that means you can kiss them good-bye.
These caps are extremely fast acting, and extremely low ESI/ESR. Absolutely fantastic for clamping flyback spikes, but useless for meaningful energy storage due to the 4uF capacity. They serve the very specific roll of clamping flyback spikes. These caps need absolute placement priority over all other caps in the controller. They need to be placed very close to the FET to keep the inductive path minimized so they can function correctly. They have those big bolt-on terminals because they are normally bolted directly to the input legs of IGBT modules. I'm using one bolted straight each FET package power inputs in my controller design.
Normal retail price is something like $25 per cap. I picked up 30 of them on new ebay for something like $5 each.
These are also snubber caps, these belong to BigMoose. They are a little physically larger, and are something like 4mOhm each if I remember right. (extremely good caps!)
These are small radial electrolytic capacitors. They are roughly the size an AA battery.
In this same size and voltage, they can make a cap with >1,000uF capacity. However, it's useless in a controller! These caps do not have 1/5th the capacity of other caps this size because the mfg was slacking on the design or something, these things are designed specificly around being as low of ESI and ESR as possible. This means they use substantially thicker aluminum foils in the cap, and they do multiple layers starting from the same current collector lead, so they get the required foil surface area while keeping the majority of the aluminum as close the the lead as possible. Look for caps that have long skinny aspect ratios, and low low capacity for the physical size (well, specificly look for the manufactures low ESR series caps, in this case it's called the ZL series for the manufacture Rubycon.) Expect to pay about x4-10 more for a given voltage and capacity for these ESR caps, and expect them to physically be substantially more bulky.
Now, notice in this example, both of these caps have identical voltage/capacity specs. Notice the size difference between these caps? It's not because vishay doesn't know how to make a cap, it's because that big Vishay cap is a low ESR/ESI series cap. It can charge and dischage at speeds and current levels useful to the controller. That black capacitor is designed around storeing a bunch of energy, but if you work the numbers on it, even if you had 10 of them, it can't match the rate the big blue low ESR cap can charge and discharge. As low ESR caps get larger, they can only have big bolt-on terminals, because they would just melt legs right off trying to handle the ripple currents with normal solder-on legs.
This is the cap stage of a Honda Insight motor controller. Notice it uses 350vdc rated caps? Pack voltage is always under 175vdc on an insight, but they knew to leave a good deal of margin to keep the caps from being damaged by inductive spikes.
You will also notice dispite each of these caps being larger than a can of soda, they are only 1750uF. That's tiny! Stock little 30amp e-bike controllers often come with more capacitance on board. This is because the roll of these caps is not to try to store meaningful energy, it's to try to filter spikes and ripple only, and let the battery store the energy.
So, when the FET switches on, and current starts to rise in that motor's winding, that current isn't being fed from the battery, it's first being supplied by the snubber (if you have one/them), then supplied by the electrolytics on the PCB, and still, the battery isn't doing jack yet. All that current is on it's own little pathway between the caps to the FETs. This is becaue the inductance of the battery leads (and batteries have inductance themselves as well) requires the wires to first build an electromagnetic field before it can transfer useful current levels. This delay depends on a lot of factors, like the distance the battery sits from the controller, the battery type, the switching speed of the FETs, the cap type and layout on the board etc. Some short, some long, etc. What's important to understand, is that's not just a battery that supplies current to the FETs. It's more like the caps supply current (entirely cap initially, then they share current load, then entirely supported by the battery), and it's the batteries job to keep the cap's charged up and ready to supply current to the FETs.
They are all connected on the same conductor, and we might visualize this as current always being sent from the battery down that path to the FETs, like a 1-way street. It's actually a very 2-way street, and current is being slammed back and forth between cap-fet, battery-cap, and all moving both directions down all those traces on your PCB between cap/fet/battery etc. We call it "DC", it's more like "CC", chaos-current going through the board.
And thank's to the hard work of the caps, the battery and other electronics are protected from this chaos, and we get to just read a nice filtered current reading on our CA and say, "hey, 60amps", and that's the correct average current at that time, and all we need to know as an end user, but it gives no representation of what current is doing inside the controller. (once again, I'm out of time to finish this at work, and tomorrow i'm going to be twice as slammed at work. Will try to write more when possible)
8) :wink:
with his descriptions ...
I tell you, he could be this generations Edison!
