Tool/software:
Hi tech support engineers,
I am using UCC27714 to drive a pair of MOSFETs which are connected back-to-back (both source pins tied together and both gate pins tied together). The MOSFET pair is used to drive an inductive load so one Drain pin is connected to ground, and the other drain pin is connected to the load (inductor). This configuration is used to prevent the body diode of the otherwise single MOSFET from conducting when the Drain side voltage goes negative.
Due to the fluctuations of the voltage at the source(s) of the MOSFET pair when driving the inductive load, UCC27714 is chosen to drive the Vgs of the MOSFET pair in a floating manner. My intention is that no matter what the source(s) voltage is during the fluctuations, UCC27714 can have precise control over the net Vgs and turn off the MOSFET pair completely when desired. The low side drive is unused and LI is tied to VSS. The bootstrap voltage is 15V and the Cboot = 1uF. Rboot = 1.2 Ohms, and Rout = 1.2 Ohms.
when HI goes high (10V), HO swings high and drives the gate(s) high as expected. But the problem is when Hi goes low, the waveform at HO (measure with respect to the source, or HS pin) is ramping down like an RC discharge. This discharge takes a long time if there's no pull-down resistor between HO and HS (gate and source). If I added a resistor of 10K, 5.1K, 1.6K between HO and HS, respectively, the discharge would become faster and faster; but still far from a sharp clean fall which I expected. The subsequent problem is of course the MOSFETs were partially turned on and get heated up during the ramp-down process.
The ENABLE pin was left unconnected, and later tied to VDD(15V); but in both cases the problem persists.
Can someone shed light on this issue, and let me know what might cause the HO (Vgs of this MOSFET pair) to ramp down to HS when HI goes low? Shouldn't the internal FET of the USS27714 simply drive HO low quickly when HI goes low? What is preventing HO from discharging as quickly as it got charged up? Is it OK to leave the low side driver idle and use only its high side? How is HO pulled low inside the UCC27714?
Please refer to the attached pictures. Thanks very much for your help!
