Vikneswaran Thayumanasamy, Kevin Lenz, Rohm Semiconductor
Among the few methods of isolating a gate driver, the latest innovations of the coreless transformer technology are paving the way for compact and efficient gate drivers for high voltage systems.The conventional isolated gate driver technology, including discrete transformer, opto isolated, and capacitive methods, though viable in some applications, present challenges for the latest EV and industrial power systems. For instance, discrete transformers are cost-efficient, but only allow for one-way communication from the microcontroller to the power device, and not for communication of information back to the microcontroller such as temperature and overcurrent/short circuit. In the case of opto-isolators, this method suffers from LED output drop from higher temperature operation and aging, which requires higher current input over time and temperature to compensate for the LED output degradation. Finally, capacitive gate drivers require a sine wave signal input to turn the output on, which may cause electromagnetic interference with wireless communications, such as WiFi.
Coreless transformer isolation technology
Gate drivers with coreless transformer isolation, on the other hand, don’t representany of the challenges that other isolated gate driver technologies face. Additionally, with an innovative design, they can provide several system enhancing features. Though there are a variety of coreless transformer implementations, ROHM’s coreless transformer technology is built based onthree internal slabs with a low voltage section that provides a silicon interface with a DSP or microcontroller and a high voltage section that drivers the IGBT or MOSFET. The copper coils of ROHM’s coreless transformers are separated by a slab of silicon dioxide, which is a very robust dielectric with a very high melting temperature and with similar properties to quartz. The low voltage section operates with 3.3 V to 5 V signals, compatible with a wide range of either 3.3 V or 5V microcontrollers or DSPs. Unlike opto-isolator gate drivers, coreless transformer gate drivers exhibit relatively flat turn on and turn off times over temperature. Opto-isolator gate drivers tend, on the other hand, to change behavior substantially with temperature variations with a much longer difference between turn on and turn off times, which equates to reduced efficiency due to larger dead times.
Single channel isolated gate drivers
To drive a half-bridge, two channels are required, one low-side and one high-side channel. The developer is always at a conundrum to choose between a dual channel gate driver for a compact solution and two single channel drivers for the ease of layout. ROHM offers a wide choice for both possibilities. In general, we recommend single channel drivers (figure 1) for different reasons. Since the distance between two single channel drivers can be increased depending on the layout, chances of cross talk among the channels will be reduced. This can significantly reduce the need of external filter elements to realize an EMI robust design. Furthermore, the use of two independent channels offers a higher design flexibility, which is also helpful to realize a robust layout.
High-side/low-side cross conduction prevention with XOR function
Cross wiring the XOR inputs of a half-bridge gate driver prevents both the low-side and high-side from turning on simultaneously, which is a key advantage of coreless transformer gate drivers over opto-isolator based drivers as this isn’t viable with opto-isolators. Without this feature it is possible to enter a destructive mode where both the high-side and low-side are on simultaneously, which may even lead to a small explosion in addition to device failure.
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Figure 2: Cross conduction prevention circuit
Integrated Miller Clamp prevents self-turn on
For typical half-bridge power devices, a very sudden dv/dt in tens of nanoseconds from 0 to 800 V is experienced when the low-side power device is shut off and the high-side device is engaged with the drain voltage of the low-side spiking toward the power rail. This is problematic as the intrinsic gate to drain capacitance (miller capacitance) of the low-side device may become charged and develop a voltage bump that exceeds 2 V to 2.5 V, approaching the turn-on gate threshold voltage for the lower device. Figure 3 depicts the developed voltage bump on the gate of the power device that was turned off. This situation could lead to shoot through issues where both low-side and high-side of the half-bridge power device is on simultaneously. The faster the switching speed of the power device, the higher will be the amplitude of miller current and the higher is the chance of a self-turn on of the power device.
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Figure 3: Measurement results with and without miller clamp of gate driver BM61S41RFV-C
Using a negative gate turn-off voltage is a method of preventing this occurrence, as is the use of a Miller clamp. As there are additional costs and design complexities associated with the use of a negative supply, the preferred method for many applications where it is viable is the use of a Miller clamp. A Miller clamp is a transistor designed to provide a low resistance path from the gate of the MOSFET which clamps transient voltages, preventing too high a voltage being developed from the gate to the drain of the low-side device. Figure 3 also shows the measurement results when a miller clamping circuit is used and it can be clearly seen that voltage bumps are suppressed thereby preventing the power device from self-turn-on. In some gate drivers a miller clamp circuit is integrated, or there is a control port available in the gate driver to drive an external MOSFET of the miller clamping circuit. Figure 4 shows an instance of ROHM driver BM61S41RFV-C that has an integrated miller clamping circuit.
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Figure 4: Integrated miller clamp circuit
SiC power devices require higher undervoltage lockout level
Undervoltage lockout is one of the most basic safety functions of a gate driver. It monitors the applied supply voltage and ensures that a fault is triggered as soon as the voltage drops below a certain value. This safeguards that the driven power semiconductor is not destroyed by thermal overheating. Since most IGBTs are operated with a drive voltage of 15V, many gate drivers with UVLO limits around 10V-12V have appeared in recent years. For most silicon based MOSFETs a UVLO limit of 8-10V is sufficient. However, since most SiC MOSFETs should be driven with higher gate voltages to ensure highest efficiency a UVLO with 10-13V would not provide sufficient protection. ROHM has developed gate drivers with higher UVLO limits especially for this case. Gate drivers BM61S41RFV-C and BM61S40RFV-C with a UVLO limit of 14.5V and are also pin compatible with the BM61M41RFV-C with 8.5V UVLO. This pin compatibility gives the designer of a power stage the flexibility to design on silicon and silicon carbide based MOSFETs.
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Figure 5: UVLO requirement for SiC and IGBT
Overvoltage protection for highest safety requirements
Most gate drivers protect the driven semiconductors by means of undervoltage detection but neglect the fact that the supply voltage can also be too high. This increased supply voltage is equally dangerous, as it can lead to uncontrollable short circuits or reduced lifetime of the semiconductor. ROHM offers the gate driver BM61S40RFV-C that are suitable for designs with additional safety considerations. This driver has an overvoltage monitoring of 21.5V and thus allows a safer design.
ROHM’s latest coreless transformer gate drivers offer many features and performance advantages to the latest EV and industrial power system applications. Along with high isolation voltages ROHM’s pin to pin compatible BM61x4xRFV gate driver series ensures safety, efficiency at system level, and provides design flexibility for offboard chargers, industrial PSU or automotive applications like onboard-chargers, DC/DC converters, e-compressor or heaters. Automotive traction inverter applications might require more complex gate drivers with additional features like ROHMs BM60060FV-C or BM6112FV-C.