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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, provided that your turbine isn't so powerful that it could overvolt the system even with that resistor connected across it. In which case you need a lower value of resistor.
In the light of what people have discussed, I think I'd do this with a TL494 or whatever. Wire it like the boost converter schematic in the application note, but with the resistor where the inductor would go.
With a large wirewound resistor a clamp diode would be needed across it, analogous to the boost diode in a boost converter. It's just making sure that energy stored in the resistor's stray inductance stays in the resistor and gets dissipated, rather than kicking back and overvolting the MOSFET.
You'd also need to invert the + and - terminals of the error amp, since an increase in voltage should result in more duty cycle, not less.
The TL494 solves the voltage reference problem you mentioned in another thread, since like all SMPS controller chips, it has a built-in reference.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
This chip certainly makes things easier. I was going to use a 555 and comparator to make a pwm, but now I don't.
I think I will just keep the duty constant near 50% and use it to pulse an IGBT when the turbine voltage gets too high. Is there a RESET pin on the device that disables the output on the chip?
I thought the output mode control would do it, but it does not look like it.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
No, you're missing the whole point. You need to feed a sample of the DC bus voltage into the TL494's control loop, and it will automatically adjust the switching duty cycle to draw whatever average current is needed to drag the bus voltage down to whatever you set your dial to. It's just like a switched-mode power supply, but backwards: a switched-mode power waster.
Until you can get your head round that concept, I can't really help you any more.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I see what you are saying. It start with a 0 duty, leaving the IGBT off. As the voltage increases the duty cycle will increase, turning the IGBT on that percent of the time. If that is correct, I can see it working if the duty ramps up sufficiently.
I still don't see what is wrong with keeping the duty constant and just hitting the IGBT with it when the voltage climbs above the threshold. The IGBT will conduct and rapidly bring the voltage down.
The point of this device is to keep the voltage from exceeding a set point. It does not matter that I keep this voltage constant at this point; I just don't want to exceed it.
Is there anything wrong with this approach compared with the one you suggest?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, your approach is pretty pointless. A constant duty of 50% is no different to just using twice the resistance and a duty of 100%, ie, wedging the IGBT on. In both cases you need some circuit to limit the maximum rate at which the device can dither on and off, or it'll burn out from switching losses.
The whole point of the switchmode approach is that you can modulate it just as if it was a linear circuit by changing the duty cycle.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I didn't realize that it could still burn out even if switched at a frequency of 50khz. If my maximum current and voltage in switch-mode still fall within the SOA can't I run it at 50%? If not, how do I know what duty cycle I can go up to?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If it only switched at 50kHz that would be fine. But you also have the "Is voltage greater than setpoint?" signal combined with the 50kHz carrier. There's no reason why that signal shouldn't contain frequencies greater than 50kHz: the comparator you use will amplify any noise on the DC bus. If the IGBT tries to switch at 500kHz instead of 50 as a result of this amplified noise, or if the gate drive circuitry gets overwhelmed by the noise and sticks partly on, the device will burn.
We already covered stuff like this, people referred to it as bang-bang or hysteretic control.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Don't think I do not appreciate your time, Steve.
What I was hoping to do was run the IGBT at 30-50khz. I would use the DTC pin on the chip to enable or disable the signal to the gate, depending on the turbine voltage. If it is too high I would hit the gate at 50khz @ 18v; if the voltage drops I would turn the signal off (gnd) to disable the IGBT from conducting.
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linear shunt failure
