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LTC3722-1 : Question on adaptive ZVS

Question asked by POCAND on May 18, 2018
Latest reply on May 28, 2018 by Fil

I have a schematic with LTC3722-1 and it’s working. But there is a problem.

 

I’m using adaptive ZVS since my supply voltage is high (> 400V) and fixed delay is creating problems with transistor switching (incorrect triggers creating a large current spikes). But adaptive ZVS is not behaving like how it’s mentioned in the data sheet. 

 

I’m doing the following - 

 

  1. SBUS voltage is ~1.4V (a resistor divider connected to the supply and lower portion of the divider is connected to SBUS)
  2. Both passive and active legs have a resistor divider connected to the MA/MB and MC/MD junctions. The lower portion of the dividers connect to the PDLY and ADLY pins of the controller respectively. The values of the resistors in the divider are chosen so that when the junction voltage reached Vin, ADLY/PDLY voltage is approx the same as SBUS voltage (~1.4V)

 

But in simulation, I see the voltage at the PLDY and ADLY pins are level shifted. The controller is not using the voltage levels at the PDLY/ADLY pins to switch the transistors. 

 

If you look at the picture below, V(n008) is SBUS and V(n005) in PDLY. PDLY voltage is shifted up!! Why is it happening this way? When the junction voltage (MA/MB or MC/MD) is zero, the voltage at PDLY is > 1.4V!!

 

What is going wrong here? Any help is greatly appreciated.

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