Electronic Design reader Marilyn Jayachandran asks:
"I am driving MOSFETs in a half-bridge configuration at 540 Hz with signals from an external unit (PWM controller is not being used to generate the switching pulse). Is there a soft-start circuit that ramps up the duty cycle from 0 to 50 percent that I could use to control the FETs and regulate the current over a period of approximately 150 ms?"
Answers:You can use 8-pin MCU from AVR or Microchip using Software PWM. It will be the most cost effective, if you have the resources. I can give the program for AVR if you need.
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Generate a 540-Hz linear ramp signal using an LM555 triggered by the 540-Hz waveform. The ramp signal can then be "compared" to a copy of the external control signal (I assume this is just an "enable" signal) that has been put through a low-pass filter (RC network) such that it "charges" up from 0 (volts) to "half of ramp" (volts) in approx. 150 mS (giving 0-50% duty cycle output in this period). The low pass input will continue to charge past 50% duty cycle, so the comparator output will need to be "blanked" (AND gate) with the basic 540 Hz to limit the continuous output to 50% duty cycle.
This function can be accomplished using the LM555, an LM393, a couple of 1N4148 diodes and several passives (Rs and Cs) for the LM555 and the RC network. All inexpensive components.
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What is the output wave shape of the external unit? If it is square, you may be able to use an AND gate to do the soft start; with one input connected to the output of the external unit, the other input connects a capacitor to ground. The top end of the capacitor connects to a resistor. The other end of the resistor connects to the output of the external unit.
The resistor and the capacitor will set the time constant of your soft start circuit.
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I had occasion for this kind of circuit. At the time, I was using a Xilinx for all the system control, so I just added a couple of features -- a fixed period counter. An upper-limit counter for the pulse width, which started at 0 and counted up slowly at power-up or turn-on of the stage(to a maximum of 0.5x the fixed period counter). I then had an external A/D or comparator for current sensing to control the requested Pulse-width (PWM Count). The actual pulse was turned on at 0 in the period counter (if needed) and turned off by either the PWM Count or the PW Upper-Limit counter, whichever occurred first. The same algorithm is easily accomplished in software, if you have a microprocessor with PWM timer output.
I have been designing hardware for a long time, and always try to leave myself a lot of flexibility, so my preferred architecture is a "dumb" microprocessor combined with a "smart" programmable array. That way, I can multitask in hardware, offloading the micro, getting accurate and repeatable timing, as well as much more reliable software (less code and usually No OS is required).
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