IR2110 Mosfet Driver Capacitance Question

[Fish4Fun]

I have just assembled 3 * IR2110 Hi/Lo Side Drivers onto a PCB I mocked up to do some BLDC motor tests...for the High-Side Floating Supply I have 1 * 0.1uF Ceramic, 5 * 4.7uF Ceramic and 1 * 68uF electrolytic (obviously in parallel)...I realize 6 Ceramic Caps are over kill but I wanted to make certain there was plenty of reserve for low frequency testing.....theoretically the combination should result in roughly 92uF; however, the 4.7uF ceramics actually measure between 5.5uF and 6.2uF while the electrolytic capacitors were all between 72uF and 73uF ...anyway the total capacitance in one half-bridge measures 106uF, the second measures 104uF and the third measures 99uF. I understand larger capacitors (especially electrolytic capacitors) have a wide tolerance range, and can be expected to lose capacitance over time....is there any good reason to attempt to "balance" the three 1/2 bridges any closer than the ~7% they currently are? My instinct tells me, "no", but I have had trouble in the past with high-side mosfet drivers, and I just want to make sure I am not making invalid assumptions....

This is the circuit I used as a guide.....



(The above is a screen-shot copied from Figure 5 here: http://tahmidmc.blogspot.com/2013/01/using-high-low-side-driver-ir2110-with.html )

On this board I currently have IRF640 Mosfets (he does not specify any particular devices for Q1/Q2), I used MUR1100's instead of the UF4007's, but the specifications are indistinguishable, and I had the MUR1100's on hand. For C3 I used 3 * 68uF 35V electrolytic capacitors (one for each 1/2 bridge.....same type capacitors I used in the High-Side Floating supply). So, other than having 6 Ceramic Capacitors on the High-Side Floating Supply my PCB is almost exactly as he has it drawn....Just want to make certain "balancing the capacitance" closer than ~7% is trivial...

So, any thoughts before I power it up?

Fish

 

[Arouse1973]

Hi Fish
I am not quite sure why you feel the need for extra capacitance for low frequency switching. You should always get high speed switching with this type of circuit, that's the whole point so it limits the time the MOSFET is in it's high ohmic region. The time between switch on and switch off is not really important as this energy is taken from the MOSFET supply. It's only the initial switch on, which yes depending on the type of load may require additional support capacitance.
Adam

 

[KrisBlueNZ]

Hi Fish! Nice to see you here again.

I agree with Adam. I don't see any reason at all to "balance" the capacitance in the drivers. They're just there to provide the energy needed to charge the output MOSFETs' gates. The driver IC takes care of preventing shoot-through (or it should!) and that's the only thing you would have to worry about.

Actually there is one other thing. Because it uses a charge pump for the high-side driver, that circuit can't keep its output high continuously for a significant period of time. If you try to do that, the high-side charge capacitors (C1 and C2) will eventually discharge and the top MOSFET will turn OFF. You probably already knew that; I just feel obliged to point it out for thoroughness.

 

[Arouse1973]

Good point Kris.
Adam

 

[Fish4Fun]

Arouse/Kris....

Thank you for the input.

@Arouse....In Fairchild's Application Note AN-6076 (https://www.google.com/url?sa=t&rct...m9qjHZPNNil-6TA&bvm=bv.75097201,d.cWc&cad=rja ) and several other application notes I have read there are formulas to use to correctly select the value of the bootstrap capacitor "Cboot", but in all cases the values I see applied in real-world circuits are typically at least an order of magnitude larger than the formulas in the application notes would suggest. All of these formulas are based on knowing the switching characteristics in advance....which is intuitive if you are designing an SMPS or a driver for a particular motor....In my case I want to experiment with different BLDC motors in a fairly wide frequency range....so if I am driving a 3-phase motor @ ~30hz, and not employing PWM via the driver, then each switch will have have an "on-time" of ~5.5ms, @300hz the "on time" would be ~550uS. Compared to a typical SMPS or a similar driver that employed PWM via the individual driver channels this is a very long "on time" and will require considerably more capacitance than switching @ 30khz or more....at least I THINK it will....hence my reasoning for having ~100uF.

Regardless of my reasoning, I haven't found anything to suggest that the extra capacitance will be problematic; The reason for adding it was to address:

....Because it uses a charge pump for the high-side driver, that circuit can't keep its output high continuously for a significant period of time. If you try to do that, the high-side charge capacitors (C1 and C2) will eventually discharge and the top MOSFET will turn OFF.

So I guess it is time to hook it to a power supply, motor and a uController and see what things look like on the Scope. I just wanted to make sure no one had any compelling reasons NOT to have the extra capacitance....This project is hobby oriented, so the BOM is completely irrelevant; the primary goal is flexibility//versatility not most cost efficient. The board itself is much larger than it needs to be and will accpet SMD-D2PAK, TO220 or TO247 through hole power devices, and I plan to try several different power devices I have on-hand just to see how critical various parameters are to actual performance....In fact, I am really excited about trying some IGBTs I picked up off ebay a while back...but for the initial testing I am using IRf640S...

@kris....Hey Bud! Thanks! Good to be back! I think I mentioned previously that I own a seasonal business...."the season" generally keeps me pretty busy; so I don't get much time to "play" from about mid-Spring through mid-Fall....but we are headed into the off-season so i should be around more. Don't know if you remember the "LED fade-In" project I volunteered to help with last Fall....boy did that turn into a big project, LOL....but it is mostly done...turned out he needed a LOT more than a circuit diagram and a bit of code, lol. Ended up pretty much re-designing his entire product, building him some molds to make the product and then helping a bit with marketing....A Tad more than I meant to bite off, LOL! But it is all good, he's a great guy and it has been a fun experience....Going to do my best to work on MY projects this year, lol! (The BLDC driver project is what I was working on last year when I got "side tracked", hehe).

Thanks!

Fish

 

[Arouse1973]

Hi Fish

Your right it won't hurt having extra capacitance. I just wanted to know the reasoning behind it. I still think 100 uF is overkill but it's up to you what you do. Here is the formula showing actually quite a small value required even for 10 Hz. I had to guess a little on some of these as I didn't have the required design information. If you can give me the correct info I can give you the correct value if your interested.

Thanks
Adam

 

[KrisBlueNZ]

Hey Fish,

That's fair enough. The gate capacitance is only charged and discharged on transitions, so that isn't really a factor, and the gate impedance is infinite. It's really just the continuous load current of the high side driver that will discharge the bootstrap capacitor.

The data sheet actually gives some detail on the maximum load current for the "offset supply". Figures 16A and 16B make it clear that it's affected by temperature but independent of the actual boost voltage. Taking a worse-than-worst case of 200 µA (at maximum temperature), and assuming an allowed gate drive voltage drop from 11V to 8V (dV = 3), with C_BOOST = 100 µF the hold-up time will be:

dT = dV C / I
= 3 × 100×10^-6 / 200×10^-6
= 1.5 seconds.

Sounds like you're really getting into the electronics stuff! That's great! It's good to hear from you again.

 

[Fish4Fun]

Actually I have one more question about the (very standard circuit) that has been troubling me for quite some time.....in the numerous application notes, data sheets and example circuits I have seen there is a great deal of discussion about the boot strap capacitors, but nowhere have I ever seen a recommended voltage rating on them.....the bootstrap diode generally seems to be what I would consider very high voltage (1kV being the most common selection), but it appears standard 50v ceramic capacitors and/or ~50V electrolytic capacitors are common choices.....From one perspective they should only be "charged" to ~12-18V, but with respect to ground they are Rail voltage + 12-18V....Since they are charged when the low-side switch is "on", it would **seem** to me that they are seeing more than the nominal 12-18V input voltage....but obviously the charge-pump is isolating them from the rail voltage during this period and so "they only see" the 12-18V input voltage? Then when the low-side is turned "off" the capacitor's "low side" is "switched" to the rail and the high-side is left as the "floating" supply for the duration of the period? Do I have this right? I have looked @ schematics of "discrete component charge pumps" used for high-side driving, but I am not confident I have a good handle on them....I understand exactly how a switched capacitor design might work using relays, but the nuances of making transistors behave in a similar manner makes my head a bit swimmy.....LOL, I guess that's one reason why the "IC" approach is popular....one doesn't really HAVE to understand exactly HOW the "black box" works, one just has to make sure to heed the specifications in the datasheet....So I guess I utterly failed to "ask a question"....the question is, "Am I correct in assuming that the bootstrap capacitor(s) are only charged to the "supply voltage" and are never exposed to the "rail voltage" with respect to ground?"....AND, more importantly, that a nominal voltage rating of 50V is a gracious plenty?

Thank you both for taking the time to work out the math for me....I don't generally have much trouble with working out the numbers, but I frequently have trouble making my real-world circuits behave as the math suggests they should....obviously not because the math is wrong, but because I fail to take other considerations into account.....1.5 seconds....hrmmm, I cannot imagine a useful 3 phase motor needing any particular switch to be on quite that long ;-) Time to write some code and do some testing....

Again, THANK YOU BOTH for providing great sanity checks!

Fish

 

[KrisBlueNZ]

Since they are charged when the low-side switch is "on", it would **seem** to me that they are seeing more than the nominal 12-18V input voltage....but obviously the charge-pump is isolating them from the rail voltage during this period and so "they only see" the 12-18V input voltage? Then when the low-side is turned "off" the capacitor's "low side" is "switched" to the rail and the high-side is left as the "floating" supply for the duration of the period? Do I have this right?

Yes, that's right. They are only ever charged when the common of the output MOSFETs is low. At that time they are charged through the diode, to the driver's supply voltage. That's the maximum voltage that should appear across the bootstrap capacitor. When the output switches high, the negative end of the bootstrap capacitor jumps up to the output stage supply voltage, and the positive end jumps up to that voltage plus the voltage that the capacitor was charged to, but the capacitor doesn't "see" that supply voltage; it is only interested in the voltage across its plates, which is the low voltage that it was charged from when the output stage output was low.

So I guess I utterly failed to "ask a question"....the question is, "Am I correct in assuming that the bootstrap capacitor(s) are only charged to the "supply voltage" and are never exposed to the "rail voltage" with respect to ground?"....AND, more importantly, that a nominal voltage rating of 50V is a gracious plenty?

Yes and yes.

 

[Fish4Fun]

Thanks Kris!

I was 99% sure.....but I have let a lot of smoke out of perfectly good components when I was 99.99% sure ;-) The PCB is fully assembled...waiting patiently on my bench, but haven't had time to play with it yet......I will post back with results when I get a day to play...

Fish