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Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Thanks again everyone.
@Patrick I know that, but I just don't know how to go about making sure the switching periods stay low.
@Dr. Dark Current Hmm, should I be using some external diodes to make sure that the MOSFET's internal diode's never become forward biased? Also should there be a DC blocking capacitor in series with the primary coil (This is only a half-bridge so I am not sure)?
@Steve Conner How should I go about viewing the waveform's of the primary when the main power is connected? Do I connect a scope probe directly across the drain and source pins of a FET or do I use some other method (I don't want to blow my new scope up). My probes only have a 1:1 and 1:10 setting and my scope is mains earth referenced, although my circuit can be battery powered.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Alex, could you post a schematic of the power section of your bridge? Are you driving a ferrite transformer, and what is the load? Maybe it is not necessary to disable the internal diodes. However firstly I would check the primary current, to make sure the core is not saturating. If the waveform is OK, and the gate drive is OK as well, then you simply need bigger FETs to cut down the dissipation.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
How well heat sunk is your transistor?
irfp260 ]irfp260.pdf[/file] IRFP260, means....
200 volts @ 46A with 0.055 ohms
so I^2R losses indicate... a theoretical maximum 116 watts of thermal load. which should be more than adequate for a plasma speaker... even if he only generates half that heat.
If your half bridge is running open loop or average voltage control, yes, put a capacitor in series with the primary.
and
this is how I would recommend to bypass the body diode....if you decide to do that:
My way of dealing with the body diode in mosfets was to use IGBTs that have no internal diode, and add a fast/soft recovery diode outside. I think....it made a little difference, that is, a little bit cleaner waveforms...nothing dramatic....
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Patrick wrote ...
How well heat sunk is your transistor?
irfp260
IRFP260, means....
200 volts @ 46A with 0.055 ohms
so I^2R losses indicate... a theoretical maximum 116 watts of thermal load. which should be more than adequate for a plasma speaker... even if he only generates half that heat.
what is the real applied voltage, and average continuous current Alex?
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Patrick wrote ...
Patrick wrote ...
How well heat sunk is your transistor?
irfp260
IRFP260, means....
200 volts @ 46A with 0.055 ohms
so I^2R losses indicate... a theoretical maximum 116 watts of thermal load. which should be more than adequate for a plasma speaker... even if he only generates half that heat.
what is the real applied voltage, and average continuous current Alex?
@Patrick its 36v and it draws around 5 amps max, although most of the time its more like 4 amps. The heatsink is one of of an old ATX power supply, and I have a small fan attached to it too.
I have now determined that there appears to be some sort of ringing coming from the bridge section. The gate-source waveforms looks fine until I apply power to the bridge. Even something small like two LED's in anti-parralel drawing 5mA from a 9V battery was enough to cause the gate-source waveform to change slightly (it looked like a typical ringing waveform when the FET turned on, although turn off looked ok) , so I guess with a much heavier inductive load it will be much worse.
Another thing I noticed is that when I added a 4700uF capacitor across the whole bridge it made the ringing worse!
@Daedronus Thanks, I am going to try that when I can get hold of some Schottky diodes.
I have been reading up on this and they both say how Schottky's should be used.
The main goal here is too reduce FET heating which according to that site the body diode of the FET can be causing as it is quite slow.
Schematics of my set-up, I haven't really done much to it apart from add a buffer stage and add lots of bypass and filtering caps. (sorry its a bit messy).
But the bridge ringing, what usually causes that sort of thing? Bad layout?
Cheers.
Edit: Forgot something, the gate drive transformer is 14 turns of tri-filiar wound bell wire on a type 77 materiel core. The waveforms when this is connected to the FET's looks fine. Its only when power is applied to the bridge even with just a small load connected it starts ringing.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
ok so 36V x 4A = 144 watts passed. 4A^2 x 0.055oHMS = ~1 watt, if ive thought this out right... (Im tired at the moment)
the transistors should not be too hot to touch, and with some aluminum they should be prettty cool.
Alex1M6 wrote ...
But the bridge ringing, what usually causes that sort of thing? Bad layout?
Cheers.
Edit: Forgot something, the gate drive transformer is 14 turns of tri-filiar wound bell wire on a type 77 materiel core. The waveforms when this is connected to the FET's looks fine. Its only when power is applied to the bridge even with just a small load connected it starts ringing.
show a pic of how your GDT is connected to the FETs, and we need to see all components connected to the pins of the transistors too.
When you probe the gate-source, disconnect the ground lead on your probe, and directly touch the probe ground to the mosfet's source.
If you don't do this you will pick up a lot of junk that is not really there.
You will also need RC snubbers across all the mosfets and across the primary to further reduce the ringing. A good set to start with would be 50Ohm (2W+ rated) and 100pF ( you want good capacitors for this, very low ESR). I have found this to make a big improvement.
You also want some capacitance right next to the mosfets (from VDD to ground), also good capacitors, not electrolytic.
After this if you are still not happy you can bypass the mosfets body diode for small improvement.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Alex,
your supply voltage for a half-bridge of IRFP260s is too low, I would go for around 120-140 volts DC.
Patrick's calculations for the dissipation are wrong, the input current has nothing to do with the half-bridge output current. In the best case (power factor=1) the output current will be double the input current, but in most cases, especially driving highly inductive loads (such as a leakage inductance current limited transfomer) the power factor will be much less than 1.
I think you don't need to bypass the internal diodes unless you are running 100s of kHz AND the bridge output current is leading the voltage, which usually doesn't happen with "normal" loads (it can happen with resonant loads such as a Tesla coil).
And of course, all of your connections in the half-bridge must be kept as short and thick as possible, and it is a good practice to add a non-inductive bypass cap right next to the FETs on the + and - supply rails.
The ringing you see on the scope after applying bridge voltage might be "fake" - just an interference the scope picks up.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Dr. Dark Current wrote ...
Alex,
your supply voltage for a half-bridge of IRFP260s is too low, I would go for around 120-140 volts DC.
Patrick's calculations for the dissipation are wrong, the input current has nothing to do with the half-bridge output current. In the best case (power factor=1) the output current will be double the input current, but in most cases, especially driving highly inductive loads (such as a leakage inductance current limited transfomer) the power factor will be much less than 1.
I think you don't need to bypass the internal diodes unless you are running 100s of kHz AND the bridge output current is leading the voltage, which usually doesn't happen with "normal" loads (it can happen with resonant loads such as a Tesla coil).
And of course, all of your connections in the half-bridge must be kept as short and thick as possible, and it is a good practice to add a non-inductive bypass cap right next to the FETs on the + and - supply rails.
The ringing you see on the scope after applying bridge voltage might be "fake" - just an interference the scope picks up.
its possible i was wrongn, but how woulf you calculate it then dr.?
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How to improve gate drive waveform's?
