Welcome back to this series about the smart gate-drive architecture. In case you missed the first post, you can find it here. In the first installment I covered IDRIVE, a feature to dynamically control gate-drive current and in turn control the MOSFET slew rate. In this installment, I’ll cover TDRIVE, an internal gate-drive state machine that helps create a more robust and efficient motor-drive system.
IDRIVE comprises three main components: a MOSFET handshake scheme, gate-fault detection and dV/dt turn-on prevention. The first component, the MOSFET handshake scheme, is how the smart gate drive prevents the occurrence of MOSFET shoot-through (or cross-conduction) due to improper dead time. If the high-side MOSFET is commanded on at the same time the low-side MOSFET is off, there can be a period of time when both MOSFETs are on because of the delay between turn-off and turn-on. This is called shoot-through (highlighted in Figure 1) and can occur if there is an insufficient amount of dead time.
Figure 1: Shoot-through example
Dead time is a delay in the input signals, shown in Figure 2, designed to provide sufficient time for one MOSFET to turn off before the other MOSFET turns on. This is commonly configured through trial and error, often with additional margin, to ensure that a shoot-through event will not occur during operation of the motor-drive system from varying parameters such as supply voltage and temperature.
Figure 2: Dead-time example
Dead time is undesirable because it decreases efficiency in motor-drive systems. Unless you achieve the perfect dead time, efficiency will drop as power dissipates in the body diodes of the power MOSFETs during the time that both MOSFETs are off (the inductance of the motor continues to carry current).
To help alleviate this issue, TDRIVE automatically determines the appropriate amount of dead time regardless of the inputs, MOSFETs or other system conditions. TDRIVE uses internal gate-to-source voltage (VGS) monitors (see Figure 3) to determine when one MOSFET has been disabled and the other can be enabled. By monitoring the VGS of the MOSFETs, the handshake mechanism can insert the optimal amount of dead time, even if system parameters change during operation of the motor drive.
Figure 3: VGS monitors
This VGS monitor discussion leads me into how TDRIVE implements MOSFET gate-fault detection and dV/dt turn-on prevention. Simply put, the start of every switching event starts an internal timer. This timer is the core component of TDRIVE and triggers several mechanisms inside the gate driver.
First, the TDRIVE timer signals the gate driver to enable a strong pull down on the opposite MOSFET until the timer expires. This prevents dV/dt-related turn on without sacrificing the desired gate-drive setting and efficiency during normal operation when a strong pull down is not required. dV/dt turn on is a phenomenon where charge from the switching node couples into the MOSFET gate through the parasitic capacitance of the MOSFET (see Figure 4). A sufficient charge injected into the MOSFET gate develops a voltage that in turn enables the opposite MOSFET, causing shoot-through. One method for preventing this is to enable a strong discharge path for the charge on the opposite MOSFET; this is what occurs within the TDRIVE time.
Figure 4: dV/dt example
The next TDRIVE mechanism is a VGS voltage check at the end of the TDRIVE time. Depending on whether the MOSFET is enabling or disabling, if the VGS has not risen to the gate-drive voltage or fallen to 0V, the gate driver signals a gate-drive fault. A gate-drive fault would indicate a short circuit or improper gate-signal connection. By detecting an issue with the MOSFET gate, the gate driver can shut down to prevent further damage to the system and signal an issue to the system controller. Figure 5 shows an example of the entire TDRIVE sequence.
Figure 5: TDRIVE sequence example
These topics are not always the simplest to convey through text. If you want to start evaluating these features yourself, check out the BOOSTXL-DRV8305EVM. This BoosterPack™ plug-in module is a complete three-phase Brushless DC (BLDC) motor-drive stage using the DRV8305 smart motor gate driver and docks with Texas Instruments Microcontroller (MCU) LaunchPad™ development kits to create a complete motor drive and control system.
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