Duty mode output to drive Power MosFET or IGBT
Ching Lin
Posts: 11
Hi
I am trying to use the 60Hz sine wave duty mode output to drive MosFET or IGBT. The minimum pulse width needs to be 10 uSec or higher otherwise those power devices can't catch up. How to limit the minimum pulse width to 10 or 20 uSec?
Thanks in advance.
Ching
I am trying to use the 60Hz sine wave duty mode output to drive MosFET or IGBT. The minimum pulse width needs to be 10 uSec or higher otherwise those power devices can't catch up. How to limit the minimum pulse width to 10 or 20 uSec?
Thanks in advance.
Ching
Comments
You'll probably have to dedicate a cog to a software PWM routine. The counters' DUTY mode pulse widths, at reasonable system clock frequencies, are just too short for driving MOSFETs, forcing them to spend too much time in their linear region and possibly overheating.
-Phil
Essentially I am just using Tracy Allen's "sinewave0.spin" code.
I am using DUTY single-ended for now, in real application I will need to change to DUTY differential to drive a full bridge.
Thanks,
Ching
Phil,
I thought about using another cog in logic mode to extend the timing slot too, but I am new to this and unable to come up with any scheme.
Thanks,
Ching
You might try right shifting the FRQB value 9 bits before loading it into FRQB. This would slow down the duty cycle by about a factor of 500 from about 25ns to about 12us at midpoint.
Please correct me if I've misunderstood something here, but I've always assumed that DUTY mode outputs could never be used as a substitute for PWM when driving MOSFETs due to the extremely narrow pulse widths.
Thanks,
Phil
Post Edited (Phil Pilgrim (PhiPi)) : 2/22/2007 10:01:27 PM GMT
I had the "mov ptr,PAR" at the assembly entry and I used "add ptr,#4" & "wrlong frqb,ptr" for debug only that I forgot to delete them.
The output sine wave disappear after I insert "shr phx,#9" above "mov frqb,phx". Did I miss something else?
Thanks,
Ching
Essentially this takes a value between 0 and 1000, sets a minimum width of about 250ns and a maximum width of about 12.75us with a repetition rate of about 25us. You can add the sine calculation for the 0-1000 value and use the extra cycles to increase the minimum on time.
Is the issue that no individual dwell time (high or low) can be less than 10 microseconds? If that is the case, Mike's scheme, like the DUTY scheme would still be too fast.
In 1/60 of a second there are only 1667 intervals of 10 microseconds.
My first thought was to use a hardware pulse stretcher with the DUTY output, to create a 10 microsecond pulse out of each 12.5ns DUTY pulse. The dead space between each DUTY pulse would have to be longer than 10 microseconds. With clkfreq=80mhz, there are 800 clocks in one 10 microsecond interval, so FRQb<5368709 would be the condition, and the sinus could modulate up to that limit. An external pulse strecher is not a very nice solution though.
The Prop should do it on its own. I'm thinking of a lookup table of output state vs phase, something like Don Lancaster's magic sine waves.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tracy Allen
www.emesystems.com
-Phil
Where is your 10us value comming from?
Graham
No, LOW can be much less than 10uS.
The Prop output will be connected to a 600V high and low side driver. A pulse strecher is something worth considering for us.
Thanks,
Ching
Graham,
It is a high voltage application, 10us mainly comes from the consideration of power conversion efficiency.
Thanks,
Ching
Mike's approach looks like a good one, proper PWM.
Graham
Modelling in the frequency domain does have the advantage of not having to consider the filtering until after the fact, but computing the error function for each candidate pulse stream is more computationally complex.
-Phil
I'm going to start another thread for the magic sine waves. It might be relevant for Ching's question, but it is topic in itself. I think it might be relevant because the magic sine waves (to borrow Graham's words) "ensure the switchings are sufficiently infrequent for low power loss". It appears that the magic sine waves are intended as an improvement on standard PWM techniques, primarily for driving motors in a bridge configuration.
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Tracy Allen
www.emesystems.com