Electronics > Projects, Designs, and Technical Stuff
IGBT Dimmer - PWM AC Power Control for an Immersion Heater
willz1200:
Hello,
I'm currently in the process of building a solar power diverted to route surplus electricity to my 3kW immersion heater. So far I've made a triac phase cut dimmer which is wirelessly connected to CT clamps and AC voltage reference. It works okay but I'd like to implement a PWM IGBT dimmer to reduce EMI, acoustic noise and put a more uniform load on my inverter. There are two topologies I'm currently looking into, if you know of any other good ones please say. :-+ I found some schematics online which show a rough idea of what I mean. I also read that PWM dimming can be used in conjunction with zero crossing detection to slightly improve efficiency, I don't understand why this is true? Surely the constant switching will cause all the losses, what difference does the phase make?
Topology 1) Rectified AC applied to the heater through one IGBT:
It appears that this topology is more popular, but I'm concerned about the high power dissipation in the bridge rectifier.
The schematic that I found shows a MOSFET instead of an IGBT, is there any significant benefits of using the IGBT besides its lower switching losses?
Topology 2) Two anti series IGBTs applying the AC directly to the heater where each body diode allows the opposite cycle to pass:
Will this topology result in an overall lower power dissipation?
Will a separate rectified power source be needed to driver the gates and is a driving ic necessary?
I'd also like to put more thought into the EMI filtering, is a single common mode choke sufficient?
I came across this image of the 4-Noks Power Reducer which appears to be topology 1, looking at the top filtering PCB I see several common mode chokes and capacitors, why are so many different sized chokes used? How do you go about selecting them?
T3sl4co1l:
First of all, did you try a zero crossing controller?
Heaters don't need to turn on and off very fast. Skipping a line cycle every now and then is more than good enough for the purpose.
Tim
willz1200:
Yes I'm currently using a MOC3021, AC Optocoupler and interrupts on an ATTINY44 to perform zero crossing detection which fires a triac. It works fairly well but its not ideal for controlling heavy loads. The idea is to finely tune the power consumption of the immersion heater to match the solar power surplus in real time. Which is why I'd quite like to build a PWM driver :)
David Hess:
--- Quote from: willz1200 on August 25, 2018, 01:43:32 pm ---The schematic that I found shows a MOSFET instead of an IGBT, is there any significant benefits of using the IGBT besides its lower switching losses?
--- End quote ---
IGBT switching losses are not lower than MOSFET switching losses except maybe at very high voltages where large MOSFET die size results in high capacitance. IGBTs like bipolar transistors have an advantage in cost at high voltages because MOSFETs have a die size which is proportional to the square of the voltage.
--- Quote ---Will this topology result in an overall lower power dissipation?
--- End quote ---
You will have one diode voltage drop instead of two from the bridge rectifier so losses will be slightly lower unless IGBTs can conduct in reverse? I have no idea; bipolar transistors can. One of a pair of MOSFETs would conduct in reverse so there would not even be the forward voltage drop of the diode making losses even lower.
--- Quote ---Will a separate rectified power source be needed to driver the gates and is a driving ic necessary?
--- End quote ---
The gate drive has to come from somewhere.
--- Quote ---I'd also like to put more thought into the EMI filtering, is a single common mode choke sufficient?
--- End quote ---
Maybe.
--- Quote ---I came across this image of the 4-Noks Power Reducer which appears to be topology 1, looking at the top filtering PCB I see several common mode chokes and capacitors, why are so many different sized chokes used? How do you go about selecting them?
--- End quote ---
Sometimes two or more common and differential mode suppression networks are used in series to provide enough suppression.
Richard Crowley:
The thermal mass of the water is like a humongous capacitor that is integrating your nice PWM back into a running average measured in seconds (or minutes). Using PWM for any kind of large-scale heating process is massive overcomplication.
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