【Introduction】New technologies and market trends continue to place higher demands on power capability and solution size. The development of electric vehicles, renewable energy microgrids, massive energy storage, and high-power telecommunication applications also places higher demands on power density.
Voltage and power levels that used to be dedicated to specific high-power applications are now also becoming more common in everyday applications, meaning that once-forgivable performance issues are now intolerable. The main limitations for such applications have traditionally come from the performance limitations of power switching technology, especially silicon semiconductors. But the emergence of wide bandgap (WBG) semiconductors has broken through this bottleneck, enabling the design of high-voltage, high-frequency power converters.
However, the combination of high power and high speed creates a new set of problems for designers designing converters. For example, the high voltage in the power conversion module and the weak small signal circuit in the control module can cause unnecessary coupling, and this article will mainly discuss the harm caused by this situation.
The need for isolated gate drivers
Gate drivers are common components in many power converters. Because the control circuit operates at low voltage, the controller cannot provide enough power to quickly and safely turn the power switch on or off. Therefore, the signal from the controller is sent to the higher power gate driver, which drives the gate of the MOSFET as needed.
When operating in high power or high voltage applications, components in the circuit are subject to large voltage drifts and high currents. If there is current leakage from the power MOSFET to the control circuit, the high voltages and currents involved in the power conversion circuit can easily burn the transistor, resulting in a large area breakdown of the control circuit. Additionally, high-power applications require isolation between the input and output to protect the user and any other equipment connected downstream of the converter.
Isolation can be achieved using a variety of mechanisms and materials, each with its own advantages. However, capacitive coupling is currently the most common method for high-performance systems because it takes up less space than inductive isolation, is more reliable than optocouplers, and provides unmatched isolation capabilities. Figure 1 shows the schematic of an isolated driver.
Figure 1: Schematic of an isolated driver
Capacitive isolators use two capacitors in series. These capacitors are based on silicon chips and use silicon oxide as the dielectric. By building with thick dielectrics, these capacitors can withstand extremely high peak voltages without breakdown. This isolator works by modulating the PWM signal from the controller to a high frequency signal and then generating a differential voltage pair to send this information to the capacitor. In this way, the modulated signal can pass through the isolation barrier without any data loss. After passing through the barrier, the signal is demodulated before interacting with the driver circuit.
The main benefit of capacitive isolation is that the entire isolated driver can be easily integrated into a single chip because the capacitors are fabricated using the same standard microelectronics process as other driver components. MPS also offers ICs with both upper and lower power switch drivers, such as the MP18831, an isolated half-bridge gate driver.
Important Parameters: Isolation and CMTI
One of the key parameters of an isolated gate driver is its isolation voltage rating. Having the proper isolation voltage is critical to protecting the user from potential discharge current hazards, and it also prevents accidental voltage transients from damaging other circuits connected to the power supply. In addition, the isolation voltage protects the converter’s internal signals from disturbances caused by noise or unexpected common-mode voltage transients.
Isolation is usually expressed as the amount of voltage that the isolation barrier can withstand. In most data sheets, isolation voltage is often expressed as parameters such as maximum peak isolation voltage, operating isolation voltage, and RMS isolation voltage.
However, as the voltage and frequency increase, the gate driver will experience a large voltage drift with a very steep slope. If these voltage transients are fast enough, some high frequency components in the voltage may not be blocked by traditional isolation methods. Common Mode Transient Isolation (CMTI) protects the circuit by blocking these high frequency voltage components from coupling and passing through the isolation barrier.
As bus voltages and switching frequencies continue to increase, CMTI is becoming more and more important in gate drivers. If the CMTI is not high enough, high power noise can couple into the isolated gate driver, creating a current loop and causing charge to appear on the switch gate. When the charge is large enough, the gate driver can misinterpret the noise as the drive signal, which can break down and cause serious circuit failure. Figure 2 shows how charges are coupled across the isolation barrier in the absence of CMTI.
Figure 2: Driver charge coupling due to insufficient CMTI
Isolated Gate Driver Protection: Miller Clamp and DESAT Protection
Parasitic coupling through the isolation barrier may not be the only cause of breakdown. The voltage from the switching node may also be coupled to the gate of the transistor through parasitic coupling of the transistor itself. This coupling is usually caused by the equivalent parasitic capacitance of the MOSFET (called Miller capacitance). Miller capacitors can cause serious problems in high frequency, high voltage switching.
Because of the natural high-pass behavior of the capacitor, high-frequency voltages are coupled through the Miller capacitance, bypassing the isolation barrier between the MOSFET gate and channel.
This means that current will flow through the gate node, charging the gate and possibly triggering the switch. Once this happens, a direct path is established between the bus voltage and GND, resulting in shoot-through current and loss of converter efficiency.
The active Miller clamp is a low impedance path consisting of a comparator and an additional MOSFET. When the upper FET is turned on, it connects the gate of the lower FET to ground. This process redirects the current through the Miller capacitor from the gate to ground, reducing gate charge and avoiding unwanted gate drive. Figure 3 shows the transient cross-conduction principle of a switching half-bridge with and without Miller clamp. Figure 3a is a schematic diagram without Miller clamp; Figure 3b is a schematic diagram with Miller clamp.
Figure 3: Transient cross-conduction principle of a switching half-bridge
The charge accumulated in the gate can also cause other problems, such as desaturation. Desaturation is the process by which a MOSFET involuntarily enters the nonlinear region. This area of operation is extremely inefficient and therefore never used for power conversion. It increases power consumption, not only reduces system efficiency, but can also cause switch damage. To avoid this, a DESAT protection circuit can be used to sense the voltage across the switch and stop powering the gate when it exceeds the desaturation threshold (see Figure 5).
Figure 4: MOSFET operating area and principle of desaturation protection
The introduction of wide bandgap (WBG) semiconductors has not only increased switching frequency but also increased power requirements, making isolation a critical aspect of power converter design. High isolation and high CMTI ratings are key features to ensure that users and equipment connected to the power source are protected from accidental current leakage. Protection features such as desaturation protection and active Miller clamp ensure safe operation of the MOSFET.
MPS offers a variety of isolated gate drivers, such as the MP18831, a dual-channel driver with configurable dead-time control designed for half-bridge converter topologies.