Design scheme for improving performance of digital control power supply

Central topics:

• Design scheme for improving performance of digital control power supply

Solution:

• Adding intelligent protection functions to enhance power reliability

• Apply the desired threshold voltage at the current limit pin

In the power supply system, the MOSFET driver is generally only used to convert the output signal of the PWM control IC into a high-speed, high-current signal in order to turn on and off the MOSFET at the fastest speed. Since the driver IC is located adjacent to the MOSFET, it is necessary to add intelligent protection functions to enhance the reliability of the power supply.

New digital power controllers such as UCD9110 or UCD9501 need to be supported by new intelligent integrated MOSFET drivers. Power supply designers are still skeptical about digital power supply control technology. They often attribute the blue screen phenomenon of the PC to software conflicts. Of course, this controversy will hinder the promotion of digital control power supplies and power stage protection strategies during the search for controller faults. This has promoted the development of MOSFET drivers with internal power stage protection functions that do not rely on digital power supply controller signals.

Figure 1 is a typical implementation of a digitally controlled power supply. The digital power controller on the left side of the figure usually operates at 3.3V. Due to the digital low-voltage processing method used in the controller design, the digital controller cannot be used directly to drive the MOSFET for stability and noise considerations. The interface between the controller and the power stage is provided by the MOSFET driver. The MOSFET driver usually receives the output signal of the PWM or digital controller and converts it into a high current signal suitable for efficiently turning the MOSFET on and off. If the controller signal is interfered or erroneous, the ordinary MOSFET driver will not provide any protection function. The UCD7K series MOSFET driver introduced by TI will be able to protect the power stage from major faults caused by interference with the drive signal. The ultra-high-speed current sensing comparator built into the MOSFET driver provides power stage protection. Figure 2 is a related block diagram.

Integrated ultra-fast current limiting function

The UCD7K MOSFET driver receives a logic-level input signal from the digital controller and converts it to a ±4A high-current MOSFET gate drive signal that is connected to the power stage. The driver provides a periodic current limit function with a programmable threshold and a digital output current limit flag. By monitoring the current flag, the host controller can select the appropriate algorithm and derive the required current limit configuration parameters (profile). In the rare case that the digital system cannot respond to the fault in time, this fast (25ns) periodic current limit protection function will shut down the power stage. The main advantage of the local overcurrent protection function is that the UCD7K device can protect the power stage when the software code in the digital controller is corrupted or terminated. If the controller PWM output maintains a high current, the local current detection circuit will shut down the driver output when an overcurrent condition occurs. The system is likely to enter a retry mode because most DSPs and microcontrollers are equipped with on-board watchdogs, power-down resets and other monitoring peripherals that can restart the device when it is not operating properly. However, these peripherals are usually slow to react and cannot protect the power stage from damage. The UCD7K's current limit comparator provides the required fast protection function for the power stage.

The current limit threshold can be set arbitrarily from 0.25V to 1.0V by applying the desired threshold voltage at the current limit (ILIM) pin. This voltage can be applied using a resistor divider or a digital controller plus a digital-to-analog converter. In any case, the maximum threshold voltage is internally limited to 1.0V, and external voltage settings above 1.0V have no effect, providing another protection function in the event of damage to the D/A converter.

TrueDrive Output Architecture

For fast switching speeds, the output of the UCD7K driver uses the TrueDrive output architecture, which inputs a rated current of ±4A to the gate of the MOSFET during the "Miller" flat region of the switching transition. TrueDrive consists of a pull-up/pull-down circuit composed of bipolar tubes and MOSFET tubes in parallel.

High Voltage Startup JFET + Precision Reference

The UCD7K series devices with the second digit of the part number equal to or greater than 5 (such as UCD7500, UCD7601) have a built-in 110V startup JFET that can be directly connected to the 48V communication bus voltage without external resistors. The JFET provides current during startup and will be disabled when the bias winding is connected to the VDD pin to obtain sufficient operating current.

The UCD7K series devices also include a 1% accurate, 3.3V, 10mA linear regulator, which serves as a reference voltage and powers the digital controller at the same time.