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| Circuit for MosFets in parallel for extra current capacity.(Solved) |
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| David Hess:
--- Quote from: nick_d on December 30, 2018, 10:27:08 am ---I agree with all that has been said here. One thing I am slightly confused by is the need for a gate resistor, it has been said that a design seen in the wild did not use gate resistors and got away with it, but why would this be bad? If you just connect all gates in parallel and drive them with one hefty gate driver chip, I would have thought that all the gate capacitances would appear in parallel and be charged in parallel, leading to all MOSFETs switching on at the same time (subject to slight differences in their Vgs as discussed). --- End quote --- The gate resistor suppresses parasitic oscillation during switching and prevents gate drive overshoot. With a low capacitive impedance at the source and drain, the input impedance is negative at high frequencies like a Colpitts or Clapp oscillator. Slightly larger values of gate resistance may be used to match the impedance of the line between the gate and driver and suppress any series inductance lowering overshoot. Gate stopper resistors for tubes and especially for tetrode tubes served the same purpose. The same problem comes up with bipolar transistors driving a capacitive load when the base impedance is too low. The whole subject was discussed here where I posted an old article with details and math. This is one of those problems which might only be described as weird until you put a fast enough oscilloscope on it. |
| spec:
Hi nick_d I missed your question of post #15, which DH fielded so informatively. Gate stoppers are often a source for discussion because, at first inspection, they appear to have no function, so below are a few points. As well as helping to prevent parasitic oscillations, gate stoppers can have other functions: * Along with the MOSFET's parasitic capacitances, suppress unwanted induced signals from transmitters: radio, TV, RADAR, WiFi, switches, etc * Along with other components, shape the gate waveform to optimize high speed switching (not applicable in the OP's application) * Protect the gate from ElectroMagnetic Pulse (EMP) (caused by lightning mainly) * Where you have MOSFETs in parallel, individual gate stoppers can help prevent odd interactions between the MOSFETs * In push pull applications, gate stoppers can be used, with other components, to delay turn on and thus provide some dead time so that both MOSFETs are not on at the same time (a fatal condition for the MOSFETs and possibly any transformer involved)One thing to bear in mind is that MOSFETs are phenomenally fast but have huge parasitic capacitances in the nF region, especially power MOSFETs. This is a formula for parasitic oscillations. The other thing to bear in mind is that the nice schematic that you see on paper is nothing like the real life physical circuit: even a piece of wire has resistance, capacitance, and inductance, and acts as an antenna, that can both transmit and receive electromagnetic signals. And when you consider, transistors, transformers, inductors, capacitors, and resistors the situation is even more complex. Finally, the physical layout of this high current switch is critical and, hopefully, will be described in later posts, but just to say that gate stopper resistors will only be effective if they are connected directly to the MOSFET gate terminals using leads as short as possible. You mentioned that there are MOSFET circuits without gate stoppers that seem to work OK. This can be the case, but will the circuits work in all situations- probably not. The other point is that a circuit may be oscillating but the user may be quite unaware of it. :) |
| nick_d:
Thanks guys, I did not know that. I will use gate stopper resistors in future. The theory warrants a closer look than I can give right now. I understand negative resistance more or less, as I was at one stage planning to build an audio power amp that used op-amp circuits to generate say -3 ohms to drive a 4 ohm speaker. Apparently this gives a form of motion feedback to partially compensate box and driver response. I don't understand Colpitts oscillators well or why they have negative resistance. I will revisit it some time. cheers, Nick |
| spec:
--- Quote from: nick_d on January 01, 2019, 12:13:48 am ---Thanks guys, I did not know that. I will use gate stopper resistors in future. The theory warrants a closer look than I can give right now. I understand negative resistance more or less, as I was at one stage planning to build an audio power amp that used op-amp circuits to generate say -3 ohms to drive a 4 ohm speaker. Apparently this gives a form of motion feedback to partially compensate box and driver response. I don't understand Colpitts oscillators well or why they have negative resistance. I will revisit it some time. cheers, Nick --- End quote --- No probs from me :) You need to adjust the gate stopper resistor value according to the MOSFET and the circuit function. With switch-mode power supplies, typically switching at a frequency of 50kHz to 4MHz, the gate stopper is used mainly for shaping the gate waveform, rather than preventing parasitic oscillations. For low frequency switching, as in this application, switching times are relatively unimportant but, all the same, you don't want the MOSFETs to turn on/off too slowly or they may exceed their safe operating area (SOA). Besides which, it is just a waste of power. So a good value of gate stopper in these cases is 50R. 50R crops up a lot in electronics and 50R to 100R, with most resistors, is the magic value range where the reactances of the resistor's inductive and capacitive components tend to offset one another, so you get good frequency characteristics. When you do high speed designs with ECL or PECL you use 50R PCB lines with a ground plane and, of course, you get 50R coaxial cable for video and RF. All oscillators rely on positive feedback to sustain oscillation. The Colpitts oscillator is just another method of providing positive feedback, as are the Phase Shift, Wayne Bridge, Hartley, Clapp, and so on. The Colpitts oscillator just happens to have good high frequency characteristics. Negative resistance is no big deal. With a normal resistance as you reduce the voltage the current goes down according to Ohm's law. With a negative resistance when the voltage is decreased the current goes up. And that is all there is to it. And one final bit of cracker-barrel advice is to pay particular attention to decoupling. This vitally important area is quite often missed on the vast majority of circuits that you see on the net and in books, probably because decoupling components tend to clutter the schematic and make it harder to follow. |
| spec:
If all this stuff about gate stoppers, decoupling, routing etc sounds a bit fussy and over the top perhaps the following two stories will illustrate the consequences of not doing it right. The first story concerns a friend's high-end MOSFET audio power amplifier. He said that it sounded very good but there was something about the sound that was not right somehow, and he couldn't quite put his finger on it. I had a listen and on his outfit it sounded superb... at first. But, after a while, I too detected an odd characteristic to the sound, so I took his amp home to investigate. To short a long story, I found that the output power MOSFETs were bursting into oscillation at random points of the output voltage waveform. The amplitude of the oscillations varied and, for a while, they would be absent. There were no gate stoppers and relatively long traces between the MOSFETs and the driver circuit. So, obviously, I fitted gate stoppers. This greatly reduced the tendency for the MOSFETs to burst into oscillation and lowered the frequency too. But it was not a complete cure. The MOSFETs were decoupled, but only with high-value aluminum electrolytic capacitors, so I added polypropylene decoupling capacitors. The result was that there was no sign of the parasitic oscillations and the owner was over the moon that his amp had regained it's original pure sound. The other story concerns a maritime system built into a tall 19 inch cabinet. Right from the start this system was troublesome and gave spurious results. And after many fancy investigations with no conclusion, I was asked to arrange an investigation. As it happened, we had just taken on a new graduate, and he got the job. Once again, to short a long story, he found that far from being some esoteric technical issue that was causing the problem, it was the basics that were wrong. The first thing he found was that the 74 series TTL chips on the cards that made up the equipment only had around 4V between their actual 0V pin and 5V supply pins (min allowable is 4V75), although the central 5V high-current power supply was pushing out 5V2. He also found that the analog and digital circuits had inadequate decoupling. Armed with these findings, he deigned a modification scheme to correct the issues, and when that was implemented- guess what? The system was transformed and never caused a problem again. And even better, we never heard another squeak from the 'experts' who had been deriding our elementary approach. :) |
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