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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Hello all, I need some help with PWM IC SG3525AN, datasheet is -here-. Since I can't make out much from the datasheet, it would be great if someone more "experinced" would help me with these questions: 1) How do I set Duty Cycle on this IC? 2) What are the Discharge and Compensation pins for? 3) How big capacitor to connect to the Soft Start pin? 4) Where do I connect all of the unused pins to, or leave them floating? Thanks!
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
This IC operates on the same principle as nearly all normal power supply PWM control ICs - a voltage reference is compared to a sawtooth ramp waveform. The ramp is generated by the oscillator and the reference voltage is generated by the error amplifier.
1) Look on page 4 of the datasheet at the diagram titled "Lab Test Fixture" and trace through the block diagram from the pot that says "PWM ADJ" and you can figure it out easily enough.
2) This discharges the oscillator resistor. A series resistor between it an the timing capacitor adds deadtime into the outputs. Compensation sets the frequency response of the error amplifier and therefore the gain bandwisth of the circuit
3) Depends how long you want the soft start period to be. Datasheet gives the soft start current and threshold voltage so you can work out how long it would take before soft start finishes.
4) Depends on how your circuit is configured.
Recommended course of action: get the chip, build a simplfied lab test fixture and have a play aorund, measuring different points - you'll lear about it. Also, get on the Texas Instruments website and download whatever application notes you can about SMPS design, they will give you the basic idea
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Ok thanks, I have one more question - what is the difference between duty cycle and dead-time? (decreasing duty cycle and increasing dead-time has the same effect, right?)
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Dead time is determined by resistor between pins 5 and 7 and fixed; it determines amximum allowed duty cycle and prevents possible shoot-trough and DC-bias saturation in some cases.
When pins are shorted together you'l be able to get very closely 50% duty cycle.
Registered Member #508
Joined: Sat Feb 10 2007, 01:57AM
Location: London, Ontario, Canada
Posts: 6
Duty cycle is the ratio between on-time and off-time of the squarewave. Dead Time is a specific period of time inserted bt the PWM IC to ensure that one MOSFET or IGBT has time to turn off before the other turns on. Hope this helps ...
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Need help with PWM IC
