Heating control of a Induction Heater

Thread Starter

aadi2096

Joined Nov 13, 2019
4
Hi Guys,
I have been looking around and working with the ZVS IH for a while now. But the problem is that it needs huge current in low voltage. I was thinking of a High voltage Low current IH. But couldn't find any thing good. My main motto here is to control the heating effect. I am trying to maintain a perticular temperature of a specimen. I am not trying to red hot my work piece. I am trying to make a portable heater. It doesn't have to be more than 500W, but with controlled heating. I have seen those small and economical IH kits(Chinese ones) but I couldn't find any control circuits to use with them.
I have found this NXP link but it uses only single IGBT?
https://www.nxp.com/docs/en/application-note/AN5030.pdf
I am a bit confused here. I am sorry, I am still new to this power electronics stuff. In short I have 2 questions.
1) How can I control the IH power consumption to control the heating?
2) Is it possible to make a low current high voltage IH?
Thanks
 

BobaMosfet

Joined Jul 1, 2009
1,108
induction heaters use current- that's how they work. They are (technically) a controlled short circuit. Instead, if your sample is small, I recommend you use a peltier device.
 

Thread Starter

aadi2096

Joined Nov 13, 2019
4
Maybe you could turn it on and off, PWM style, to contol the effective power?
Thanks for the suggestion. I think that will affect the working of the ZVS all together. Those will create high noise if kept before current enters the ZVS. And I don't think this will work in middle of ZVS.
 

Thread Starter

aadi2096

Joined Nov 13, 2019
4
induction heaters use current- that's how they work. They are (technically) a controlled short circuit. Instead, if your sample is small, I recommend you use a peltier device.
Thanks for the suggestion. Peltiers are good option but they need a proper heat sink. Rather they need water cooling. They are fun to play with but not very reliable. PWM with IH is a nice suggestion by @AlbertHall but I can't figure out where to use it. As soon as we introduce the specimen, IH tends to shoot the current. As I said in the first msg, the NXP design is using direct power converted to the DC for Tank Circuit. I.e. 300V. I was wondering if we could modify it.
 

Reloadron

Joined Jan 15, 2015
5,455
I played around with a few heating brass cartridge cases to anneal the necks. There are a few caveats but here is how they work. Most look just like this one. I have seen the same unit priced from $20 to $50 USD all on Amazon. When we read the fine print what we get is 12 to 48 Volts and they are advertised as 1,000 Watt. When I ran mine at 48 ~ 50 VDC it drew 20 Amps do yes, 50 * 20 =1,000. The current draw was proportional to the applied voltage. Thus the power in watts depended on applied voltage. Something else to note is you can't soft start these things. Trying to slowly increase the voltage results in one MOSFET starting to conduct before the other and current just increases with voltage and no oscillation takes place. Eventually you cook a MOSFET and game over. This is about what a schematic looks like.

Induction Heater 1 KW.png

The work coil inductance is also critical to getting the thing going.

Coil I.png

They are normally right around 1.9 ~ 2.0 uH. I used an old heavier then me linear power supply which worked well. ATX computer supplies only give you 12 VDC and tend to shut down on start. When I was playing around with the idea I found this write up to be very good and informative.

You can't PWM these due to the nature of the circuit, it's only an oscillator. What I did was placed a tiny thermocouple on a few case necks and timed my temperature rise at different applied voltages. What it came down to is Bang On and Bang Off using a contactor. I just set a timing dwell for the parts I tested. Anyway, to control the power you control the drive voltage. I forget the frequencies I got using a scope but the link likely has some idea. The guy did a good job.

Ron
 
Last edited:

shortbus

Joined Sep 30, 2009
7,691
They are normally right around 1.9 ~ 2.0 uH. I used an old heavier then me linear power supply which worked well. ATX computer supplies only give you 12 VDC and tend to shut down on start. When I was playing around with the idea I found this write up to be very good and informative.
That's the go to guy for information on those. Many of the people saying they don't work are using SMPS's and refuse to believe a linear supply is the only way to go for induction heaters.
 

Reloadron

Joined Jan 15, 2015
5,455
That's the go to guy for information on those. Many of the people saying they don't work are using SMPS's and refuse to believe a linear supply is the only way to go for induction heaters.
Bingo, linear supply and Bang On and Bang Off using a dwell time to control heat as well as applied voltage. I had started using a MOSFET to turn it On and Off which was working and made it easy to interface with a dwell timer. Several guys use them (or similar) for annealing brass cartridge case necks. Notice the guy in the link uses large lead acid batteries to get the current. SMPS supplies just don't work well. Plenty of good stuff in thet link, the guy has cooked several along the way.

Ron
 

Thread Starter

aadi2096

Joined Nov 13, 2019
4
I have read this thread but as i have shown in the link of NXP in my first msg, they are using it at 300V. This might reduce the current requirement of the whole circuit. I am looking for a controllable circuit without it's dependency on huge lead acid batteries. High current has several draw backs such as poor life cycle of capacitor and batteries, high heat loss.
 

Reloadron

Joined Jan 15, 2015
5,455
You up the voltage and lower the current. The design would be the same. There are plenty of units out there like that. Start looking at schematics of induction cook stoves and the use of high voltage IGBT circuits as the oscillator. Circuit Analysis of the 1.8kW Induction Hotplate is just one of many. There are 2200 watt portable single burner units available. The main difference between using MOSFET Design verse IGBT Design for a high voltage low current version.

Ron
 

shortbus

Joined Sep 30, 2009
7,691
Also for the TS, in your application a large mass would be good, so once it gets to temperature it would tend to stay that way longer. Doing that would lower the amount of on-off to the heater. For some types of things induction heat isn't the correct answer, more conventional heating is.
 
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