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General Question about Current Mode Controllers

Daniel McClure80
Prodigy 40 points
Other Parts Discussed in Thread: UC1843, UCC2800

I'm trying to find a current mode controller to work at 100% duty cycle, and many seem to specify a maximum duty cycle of <100% dictated by a minimum dead time while the oscillator cap discharges (e.g. UC1843, UCC2800, etc). 

My question is, does the falling oscillator disable the output switch, or does it stay on until the current sense comparator toggles?

  • 0
    TI__Guru**** 191445 points
    I can't answer your question about UC1843 or UCC2800, but generally speaking for 100% duty cycle operation, you want to look for a converter with a p-channel high side switching FET. Most buck converters use an n channel high side switch for efficiency reasons. Ultimately, the maximum continuous on time is limited by the gate drive, which must be generated by a VBST or BOOT circuit to get a voltage higher than VIN for the gate drive. Since p-channel requires negative voltage (relative to VIN) the gate can be driven indefinitely within the transistor SOA.
  • 0 in reply to JohnTucker
    Prodigy 40 points

    John,

    Thank you for the reply. In this case, I am using a P-channel FET on the high side for exactly that reason. The only time it needs to be switching is during the transient overvoltage input and overcurrent output testing, which is not normal operation. During normal operation, the switching losses needed to keep an N channel gate charged (in both the switch and the catch diode) exceed the delta in Rds on losses between N and P channel FETS, so the best solution is to use a P-channel at 100% duty cycle.

    The voltage and current values are far too high to expect an integrated switch. I already have the gate drive components in the design, so the topology is unimportant to the PWM chip. All it needs to do is provide the control components and generate the PWM signal.