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Forum Index : Electronics : HY4008 ideal gate resistor?
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Tinker Guru  Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Assume this scenario; Opto isolated gate driver HCPL3120 driving a HY4008 Mosfet in the warpinverter. Warpspeed shows a 10R gate resistor but he uses IGBT's. Is 10R the best match for driving one HY4008 too? What then if there are parallel HY4008's, do we increase the gate resistor value of each proportionally to present a similar load to the HCPL3120? Lastly, what power rating should that resistor be, 0.6Wmf or o.25Wmf? Klaus |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
HY4008's have a gate resistance = 3.2R HCPL3120 has a max peak output current of 2.5A Supply voltage = 15v To limit gate current to 2.5A requires total R = 6, so the gate resistor = 3 Ohms The HCLP3120 isn't suitable for driving multiple mosfets as you will have to increase the gate resistors to limit the peak current, this will slow down the mosfet switching, possibly OK at 50 Hz ?? Use one HCPL3120 per mosfet, or drive to an extra totem pole buffer stage. Cheers Mike |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1138 |
If you are using smd resistors for the gate drive, then go for the 1/2 watt versions 1210, 2010 or 2512, smaller ones don't have the pulse current rating. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
Gate resistor does two things, limits peak charge and discharge current that the driver sees, and damps out any potential ringing caused by long tracks and high gate capacitance. So we can come up with a minimum resistance the driver can drive at max rated driver current. I like to stay well within that, my gate resistor for 15v supply is ten ohms = 1.5 amps max. If I was driving five parallel mosfets I would use 47 ohms in each gate. That slows things down, but we certainly don't need extreme switching speed. In fact we will be much better off switching slowly and with very generous dead time margins. Ten parallel mosfets with 100 ohm gate resistors would be entirely practical driven by a single basic low power gate driver. The mosfets themselves are so cheap, this becomes a practical proposition, it would also spread the heat more evenly. Not suggesting anyone actually uses ten mosfets, but it would be easy to do if you wanted to, and it would work. Gate drive tracks can also be made very long, its preferable to fit the gate resistor right at each mosfet. The whole thing becomes much less critical with far fewer problems. The difference in speed between switching at 50Hz and switching at 23Khz, is the same difference between walking at 5Kmh and a missile flying at 2,300Kmh. Many of the serious problems the high frequency PWM guys have to overcome using multiple mosfets in parallel, just do not exist for us. Cheers, Tony. |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Thank you Tony, for confirming what you told me in a PM but obviously had not been fully absorbed then  .The max parallel number of mosfets will be 4 (can't fit 5) so I shall use 39R, 0.6W for these. I do not use SMD's if I can avoid it. Next number is 2 parallel mosfets, these shall have 22R resistors. The single others 10R. These resistors will be very close to the gate on the main PCB. Another question while I'm here, The power ratios between the 4 warpinverter transformers are 1, 1/3, 1/9, 1/27. Can I then use the same ratio as a minimum for the toroid core cross section? Klaus |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
In theory yes, but its always better to use something a bit larger than absolutely necessary if available. A larger core requires fewer turns, and there will be a bigger hole which gives you more options regarding wire sizes. Most of us will be using cores and wire we already have, so its a case of working it all out, then deciding if its practical to make. If it all looks far too tight, rinse and repeat with a bigger core, or stacked cores. Cheers, Tony. |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Reason for asking was I have a core for #2 toroid which has a 90mm hole but its core area is just 35% of the stacked #1 core. The secondary on that would be 10 turns less than it was on transformer #1 but the primary goes up from 35 to 98 turns. If I cant get a better core it will have to do but it might be tight in that hole. Could you confirm if my wire sizes are OK please? #1: sec. 7.5mm sq, pri. 37mm sq. #2: sec. 7.5mm sq, pri. 12mm sq. #3: sec. 7.5mm sq, pri. 4mm sq. #4: sec. 7.5mm sq, pri. 1.5mm sq. Planning to use enameled wire for the primaries, not cable. Another question, what happens when the primary voltage gets bigger than the secondary, as in transformer #3 & 4? Does one calculate the number of turns for 1 Tesla using the secondary voltage as I did with transformer #1 & 2? I have not yet found cores for these small ones. BTW, the magnetising current on my stacked #1 core, secondary at 230V AC, was 35mA. I'm happy with that. The inrush current seems to be quite small too, could turn it off and on without doing mischief to my Ampmeter. Klaus |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
You are definitely on the right track Klaus. All the secondaries will use the same wire size, and the primary wire size will decrease in proportion to the transformer turns ratio. If I remember, your primary design minimum voltage is 48 volts for a Lithium battery, probably sixteen cells. So all secondaries 7.5mm squared for 30 amps (4 amps per mm squared current density) #1 primary 225v/48v x 7.5mm sq = 35.1mm sq #2 primary 75v/48v x 7.5mm sq = 11.7mm sq #3 primary 25v/48v x 7.5mm sq = 3.9mm sq #4 primary 8.3v/48v x 7.5mm sq = 1.3mm sq Your values are spot on perfect. Transformers one and two are step up, so the secondaries will have the largest number of turns and would be best to be wound onto the toroid first. It always makes a neater more practical job if the thicker lumpier wire goes on top. Transformers three and four are step down, so its going to be better if the primaries are wound on first with the thicker secondaries on top. The calculations for flux density are identical, all use 50 hz sine waves at 1 Tesla (10,000 Gauss) You can use either the primary or the secondary for calculating flux density it will work out exactly the same either way. Volts per turn is the same both sides. The smaller transformers switch well above 50Hz, but by using 50Hz, causes the actual working flux density to be well below the one Tesla we assume. This has been found to work very well in keep core losses quite low at the higher real operating frequencies. Its really interesting in that the transformer voltages are all pure simple on/off rectangular waves. And the transformer currents are all continuous unbroken sine waves in both the secondaries and the primaries of all four transformers. Its why we must clamp the transformer primaries to zero volts to allow this sine wave current to continue to flow unbroken in all the primary windings. Cheers, Tony. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
230v and 35mA is only 8 watts idling power. That is amazing !! My own big transformer is 20 watts and I was very happy with that. And as you have observed, its very well behaved at turn on. No soft start required for these transformers. Only for bringing the large electrolytics up to voltage initially. Cheers, Tony. |
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9308 |
I seem to recall that with power-MOSFET's in this kind of arrangement, someone once suggested to me you should use a dedicated MOSFET gate-driver IC instead of a resistor. That makes sense when you think about it. It was probably a member here. Gate Driver. With the gigantic currents being passed in the S-D path in MOSFETS used in these inverters, even slight lag in the gate on/off time can cause the MOSFET to blow itself to bits. I figured that most BackShed designs would now incorporate MOSFET gate driver IC's fed from the main controller logic, rather then just simple gate resistors.....  Smoke makes things work. When the smoke gets out, it stops! |
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| Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Thanks Tony, Nice to know I was on the right track with the wire size. Unfortunately its going to be 'rinse and start again', as you so quaintly put it, with that big toroid. Using a 7.5 mm sq secondary was just too ambitious for that size core & hole diameter. Having wound on nearly all the secondary it became obvious there were way too few sq cm of hole area left to accommodate the primary. Even though its embarrassing to admit that - I stuffed up and mention this as advice to other toroid winders. Do not rely on your experience from winding ozinverter toroids, the number of primary turns for the warpinverter are greater and, obviously, take up more hole room.Why more turns? We design it for one Tesla with its many advantages. So its getting out my hacksaw and cutting away about 3 kg of wire - its epoxied in and cannot be unwound any more. The new version will use 3 in hand of 1.5mm diameter wire (5.3mm sq) for the secondaries. I should be able to squeeze that on 3 layers, especially as having now figured out a method of pushing the wire very tight inside the hole before locking it thus with epoxy. The resulting primaries will be 25mm sq for the big transformer and for the smaller ones stepping down to 1/3 of the size of the next bigger transformer. Thinking about that smaller wire size, my 10Kw battery bank cannot supply more than 20A @ 230V (4.6KW)for any length of time anyway and I dare say this inverter can supply peak loads well in excess of 5KW to start anything I have here. I should hope the idle current will not change as the number of turns remains the same. Klaus |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
That all sounds much more realistic Klaus. Its really difficult to judge what will actually fit onto a toroid until you get about three quarters the way into winding it. I have a similar problem regarding battery capacity versus inverter capacity. My inverter is way more powerful than I really need, but with only a 5Kwh battery its just not practical to fully load the inverter for more than just a few minutes. Even if I doubled up on battery capacity, its still not going to be practical to run 5Kw loads for any useful length of time. Idle current will not change with the thinner wire. Cheers, Tony. |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
That was probably me, but you still need the gate resistor, even if its only one ohm. Driving multiple very large mosfets to switch very high current at 23Khz is a very serious business. It requires a lot of thought, very careful layout, and some very high power gate drive to each individual mosfet. Switching multiple very large mosfets at only 50Hz is dead simple in comparison. The difference in switching speed is around 460 times. We can use a single low power gate driver chip, and some relatively high resistance gate resistors, and be pretty free with our mechanical layout, and it will work perfectly with out any problems. Cheers, Tony. |
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