How to make a good PWM controlled heater output?

[markbng]

Hello,

I am wondering how to make a good PWM controlled heater output. I am using a N FET that is controlled by a push-pull configuration (12V/0V) and the N FET has a 100 Ohm resistor connected to the gate. The PWM is a CTOUT output from the State Configurable Timer of the LPC controller (PWM output is connected to external push pull). I am only the hardware design engineer, so I don't know what frequency the software engineer will use for the PWM (recommendations?). I have 2 options for the PWM output. Option A has capacitors over the load. The load will get a DC voltage and there will not be many EMI problems, because the capacitors will buffer the output voltage. But I am concerned about the capacitors. For a high frequency a capacitor is a conductor (low capacitive reactance). Will the capacitors not cause much power loss? Is Option B a better way to get as much power as possible to the heater. Will this option not introduce much EMI (cables are approx. 1m long). The heater exists of a resistance/heating wire.
I also want to use a similar output configuration for controlling a 12VDC PC fan. What is the best output configuration?
Thanks in advance for your help.

Kind regards,
Mark

 

[KonstanU]

First circuit will damage your electrolite. When you switch off transistor, fan current will flow through diode and voltage drop on diode (capacitance also) will be 0.7v (with respect to bus voltage 24V).

 

[markbng]

You are right if the capacitor is small or when the fan draws a large current. I tested circuit A on a bread board (SMD transistor on a test PCB) and it seems that the fan is a very small load (12V/0,1A). On the oscilloscope I saw a small increase in voltage when the PWM signal is high and a very gentle negative slope (decrease in voltage) on the fan voltage when the PWM signal was low. The maximum output voltage on the fan is reached at 15% duty cycle. So for the fan I will go for option A. Maybe I will change the SS14 for a bidirectional ESD diode.
I also tested Option B for the fan, but this is no option. The fan made a strange sound (a resonating sound) when I decreased the duty cycle at a low frequency (PWM freq. 1khz).

I haven’t test these outputs with the heater, so this is only for the fan output.

 

[KonstanU]

Sound on the frequency 1 kHz is normal. (Increase freq. and you will not hear sound)
Capacitance of electrolyte doesn't matter. You have negative voltage on electrolyte.
I can give you disadvantage of first circuit:
1. Stress for MOSFET and electrolytic capacitance. When you switch on transistor, capacitance impedance will be 0 Om at the first moment and charge current will flow through your MOSFET. MOSFET can be damage.
2. If you put bidirectional ESD diode your negative voltage applied to electrolyte will be Vclamp. After some time your electrolyte will burst.
3. Circuit 2 is standard way.

 

[FvM]

Option B is O.K. for a heater. For a fan, you'll possibly want to add a LC filter