DC2596A : LT8708 Development Board ZVS of MOSFETs M1 and M3

Hello, 

     I am working with LT8708 development board DC2596A, for Battery charging application. I was able to get CC-CV characteristics required for Battery Charging on Electronic Load. While observing the Vgs and Vds waveforms of the all the four MOSFETS, M2 and M4 were switching as per ZVS but M1 and M3 didnt have ZVS. 

Input: 60VDC

Output Loading condition : 1. 42V 7A (Buck Mode in Constant Charging)  2. 63V  6A (Buck Boost Mode, when battery charging enters CV Region)

I have  changed the Gate resistor for all 4 MOSFETs =4.7ohms

Request you to help out, why M1 and M3 are not having ZVS? 

Please find the attached Image:

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  • 0
    •  Analog Employees 
    on Feb 19, 2021 2:26 PM

    See above. In the forward direction, when Vin is sufficiently greater than Vout, only M1 and M2 will switch and M4 will stay ON continuously. If Vin and Vout are similar voltages then all four switches will switch. If the output current is low, M4 may not turn on to prevent reverse current.

    Gate resistance should be kept as small as possible for best operation and efficiency. The gate resistance can be increased if EMI is an issue but should always be less than 10 ohms.

  • Hi Charlie,

    I have understood the operation of Controller. I was curious whether all the four MOSFETs have ZVS for buck, boost as well as buck-boost operation? Because i have observed that  M1 and M3 are not having Zero voltage switching in any condition of operation.

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