Limiting power through heater coil without using PWM - Page 1
Forum section: electronics / projects

author: motocoder
author_rank: Unknown
timestamp: October 24, 2014, 02:41:02 pm
content: I'm working on an upgrade to a Power Designs 2005 precision power supply. When I purchased the unit, the voltage reference oven was broken. I repaired this with a hand-wound resistance wire coil and a solid state thermostat built out of a MOSFET, a thermistor, and a simple voltage comparator with hysterisis.While my simple thermostat works, and the power supply is performing slightly better than spec, I can see that the output voltage variation is very closely correlated with the cycles in temperature. My thermostat only controls the temperature within a degree or two. So I thought I could make it work even better by creating a PID loop using a small 8-bit microcontroller. I've tried a number of experiments around the PID loop, and that design is coming along fine. However, I have one challenge still remaining, and I'd like to get some advice on that.Specifically, my problem is how to control the power through the heater in a way that minimizes any ripple added to the output supply. In my little experimental set-up (which isn't hooked to the Power SUpply), I am just using PWM and a MOSFET to control the power to the heater. This works well, and has a relatively linear control of power (i.e., 50% duty cycle is 50% power, 100% duty cycle is 100% power, etc.).I thought to replace this set-up with the circuit in the right hand side of the attached LTSpice model. This takes the PWM signal from the microprocessor, low-pass filters it, and then uses it to drive the current control on a constant current circuit using an op-amp, sense resistor, and MOSFET. I would have to dissipate the heat in the MOSFET, but I think that's not a big problem. However, because the circuit is controlling current, and power is proportional to the square of current, I've introduced a non-linearity into the system. I am concerned that this is going to cause the PID controller issues.Any advice on alternative ways to do this? Is it possible to do PWM and NOT degrade the output ripple? Is there an alternate power regulation approach I could use for the heater?Thanks

author: tszaboo
author_rank: Super Contributor
timestamp: October 24, 2014, 03:01:52 pm
content: If you are worried about your PWM ripple, than use the second amplieir of the opamp (use a double opamp) as an active low pass filter.I see one big issue with the schematic: Drivig a MOSFET's gate directly with an opamp is a big no-no. Use a resistor like 47 Ohm in series. Also, keep in mind that not all opamp is capable to do this, search for capacitive load capable ones. Also, you might end up with problems due to the low side current sensing and the opamp's rail to rail input capability.

author: motocoder
author_rank: Unknown
timestamp: October 24, 2014, 03:15:26 pm
content: Quote from: NANDBlog on October 24, 2014, 03:01:52 pmThanks, I will add the resistor. I think you are also correct about the output range on the op-amp. The only one I have that can drive to the low rail is only rated for 5v, which isn't enough to drive the MOSFET into saturation.

author: motocoder
author_rank: Unknown
timestamp: October 24, 2014, 09:39:21 pm
content: The more I think about this, the more I think the concern over PWM control of the heater current is misplaced. If I set the PWM frequency to a low value, say 10 Hz, and slew-rate limit the drive signal for the MOSFET, I don't think it is going to generate any ripple.

author: dannyf
author_rank: Super Contributor
timestamp: October 24, 2014, 10:08:14 pm
content: QuoteTypical approach is to use a temperature sensors (ntc or ptc) that adjusts the power transistor. The power transistor itself can act like the heater.QuoteYeah - on a separate winding or power supply / transformer, better yet.

author: motocoder
author_rank: Unknown
timestamp: October 24, 2014, 11:13:08 pm
content: Quote from: dannyf on October 24, 2014, 10:08:14 pmYeah, unfortunately that's not an option - at least not using the existing transformer. The other secondary windings are used for varius bias and the reference circuit itself, and I think introducing ripple there would be even worse than on the output voltage supply (because it would be amplified).