[Repost] How to implement an isolated half-bridge gate driver?

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[Repost] How to implement an isolated half-bridge gate driver? [Copy link]

Many applications use isolated half-bridge gate drivers to control large amounts of power, ranging from isolated DC-DC power modules that require high power density and efficiency to solar inverters where high isolation voltage and long-term reliability are critical. This article will detail these design concepts to show the ability of isolated half-bridge gate driver ICs to achieve high performance in a small package. A basic half-bridge driver with optocoupler isolation (shown in Figure 1) drives the gates of high-side and low-side N-channel MOSFETs (or IGBTs) with opposite polarity signals to control output power. The driver must have low output impedance to reduce conduction losses and fast switching capabilities to reduce switching losses. For accuracy and efficiency reasons, the high-side and low-side drivers need to have closely matched timing characteristics to reduce the dead time before the first switch of the half-bridge turns off and the second switch turns on.

Figure 1. High-Voltage Half-Bridge Gate Driver

As shown in the figure, a conventional implementation of this function uses an optocoupler for isolation followed by a high-voltage gate driver IC. One potential disadvantage of this circuit is that the single isolated input channel relies on the high-voltage driver circuit to achieve the required channel-to-channel timing matching and dead time. Another problem is that the high-voltage gate driver does not have galvanic isolation, but relies on the junction isolation of the IC to separate the high-side drive voltage from the low-side drive voltage. During a low-side switching event, parasitic inductance in the circuit can cause the output voltage VS to drop below ground. When this occurs, the high-side driver can latch up and be permanently damaged.

Optocoupler Gate Drivers

Another approach, shown in Figure 2, uses two optocouplers and two gate drivers to achieve galvanic isolation between the outputs, thus avoiding the high-side-low-side interaction issue. The gate driver circuit is often placed in the same package as the optocoupler, so two separate optocoupler gate driver ICs are generally required to complete the isolated half-bridge, resulting in a larger physical solution size. It is also important to note that the two optocouplers, even if packaged together, are manufactured independently, limiting the ability to match the two channels. This mismatch increases the dead time between turning off one channel and turning on the other, resulting in reduced efficiency.

Figure 2. Dual Optocoupler Half-Bridge Gate Driver

The response speed of an optocoupler is limited by the capacitance of the primary light-emitting diode (LED), and driving the output to speeds up to 1 MHz is also limited by its propagation delay (500 ns maximum) and slow rise and fall times (100 ns maximum). To drive the optocoupler close to its maximum speed, the LED current needs to be increased to more than 10 mA, which consumes more power, shortens the life of the optocoupler, and reduces its reliability, especially in the high-temperature environments common in solar inverter and power supply applications.

Pulse Transformer Gate Drivers

Next, let’s look at circuits that achieve galvanic isolation through transformer coupling. These circuits have lower propagation delays and more precise timing characteristics, which offer speed advantages over optocouplers. In Figure 3, a pulse transformer is used that can operate at speeds up to 1 MHz, which is typically required for half-bridge gate driver applications. A gate driver IC can be used to provide the high currents required to charge the capacitive MOSFET gates. Here, the gate driver drives the primary of the pulse transformer differentially, and the two secondary windings drive the gates of the half-bridge. In this application, the pulse transformer offers a significant advantage by not requiring an isolated power supply to drive the secondary MOSFETs.

Figure 3. Pulse Transformer Half-Bridge Gate Driver

However, problems can arise when the large transient gate drive currents flowing in the inductive coil cause ringing. As a result, the gate can be turned on and off undesirably, damaging the MOSFET. Another limitation of pulse transformers is that they may not perform well in applications that require a signal duty cycle above 50%. This is because pulse transformers can only provide an AC signal, and the core flux must be reset every half cycle to maintain volt-second balance. One final drawback: The relatively large package required for the pulse transformer’s core and isolated windings, combined with the driver IC and other discrete components, can result in a solution that is too large for many high-density applications.

Digital Isolator Gate Drivers

Now let’s look at using a digital isolator in an isolated half-bridge gate driver. The digital isolator in Figure 4 uses a standard CMOS integrated circuit process to form the transformer coils with metal layers and polyimide insulation to separate the coils. This combination can achieve isolation capabilities of more than 5 kV rms (1-minute rating) for use in robust isolated power and inverter applications.

Figure 4. Digital isolator using transformer isolation

[font=微软雅黑,As shown in Figure 5, digital isolators eliminate the LEDs used in optocouplers and the aging issues associated with them, while also consuming less power and being more reliable. Galvanic isolation is provided between input and output, and between output and output (dashed lines) to eliminate high-side-low-side interactions. The output driver reduces conduction losses through low output impedance, while reducing switching losses through fast switching times.

Figure 5. 4 A gate driver with digital isolation

Unlike optocoupler designs, the high-side and low-side digital isolators are built on a single integrated circuit with inherently matched outputs for greater efficiency. Note that the high voltage gate driver IC shown in Figure 1 adds propagation delays in the level shifting circuitry and therefore cannot match the timing characteristics between channels as well as digital isolators. In addition, integrating both the gate driver and the isolation mechanism in a single IC package minimizes the solution size. Common-Mode Transient Immunity In many half-bridge gate driver applications for high voltage supplies, very fast transients can occur in the switching elements. In these applications, fast-changing transient voltages (high dV/dt) that capacitively couple across the isolation barrier can cause logic transient errors across the isolation barrier. In an isolated half-bridge driver application, this can turn on both switches simultaneously during cross-conduction, which can damage the switches. Any parasitic capacitance across the isolation barrier can become a coupling path for common-mode transients. Optocouplers require extremely sensitive receivers to detect the small amounts of light passing across the isolation barrier, and large common-mode transients can upset their outputs. Optocouplers can be made less sensitive to common-mode transients by adding a shield between the LED and the receiver, a technique used in most optocoupler gate drivers. The shield improves common-mode transient immunity (CMTI) from the less than 10 kV/μs rating of standard optocouplers to 25 kV/μs for optocoupler gate drivers. While this rating is adequate for many gate driver applications, power supplies and solar inverter applications with large transient voltages may require CMTIs of 50 kV/μs or more. Digital isolators can deliver higher signal levels to their receivers and can withstand very high common-mode transients without causing data errors. As four-terminal differential devices, transformer-based isolators offer low differential impedance to the signal and high common-mode impedance to the noise, thus achieving excellent CMTI performance. On the other hand, digital isolators that use capacitive coupling to form a changing electric field and transmit data across the isolation barrier are two-terminal devices, so the noise and signal share a transmission path. For two-terminal devices, the signal frequency needs to be much higher than the expected noise frequency so that the isolation barrier capacitance provides low impedance to the signal and high impedance to the noise. When the common-mode noise level is large enough to overwhelm the signal, it can disrupt the data at the output of the isolator. Figure 6 shows an example of data disruption in a capacitor-based isolator, where the output signal (channel 4, green line) drops for 6 ns during a common-mode transient of only 10 kV/μs, causing a glitch. Figure 6. Capacitor-based digital isolator (CMTI <10 kV/μs) The data in this figure was collected at the upset threshold of the capacitor-based isolator transient; if the transient was much larger, the upset could have lasted longer, causing the MOSFET switching to become unstable. In contrast, the transformer-based digital isolator can withstand common-mode transients in excess of 100 kV/μs without data upset issues at the output (Figure 7).

Figure 7. Transformer-based digital isolator (CMTI of 100 kV/μs, ADuM140x)

Isolated half-bridge driver provides 4 A peak output current

The ADuM3223/ADuM4223 isolated half-bridge gate drivers, shown in Figure 8, use iCoupler technology to drive the gates of high-side and low-side IGBT and MOSFET devices used in motor control, switching power supplies, and industrial inverters with independent, isolated outputs. These isolation components combine high-speed CMOS and monolithic transformer technology to provide precision timing, high reliability, and overall performance superior to optocouplers or pulse transformers. Each output can operate continuously up to 565 VPEAK relative to the input, allowing the low side to switch to negative voltages. The differential voltage between the high side and the low side can be up to 700 VPEAK. The output switching frequency can reach up to 1 MHz, providing a peak current of 4 A. The CMOS-compatible input provides 50 kV/μs common-mode transient immunity. The driver operates from a 3.0 V to 5.5 V input supply, which is compatible with low-voltage systems. It is specified for operation over the –40°C to +125°C temperature range and is available in a 16-lead SOIC package. The ADuM3223 is priced at $1.70/piece in 1,000-piece quantities and has a narrow body design that provides 3 kV rms isolation. The ADuM4223 is priced at $2.03/piece in 1,000-piece quantities and has a wide body design that provides 5 kV rms isolation.

Figure 8. ADuM3223/ADuM4223 Block Diagram

Summary

[font=微软雅黑,[宋体]For isolated half-bridge gate driver applications, it has been shown that digital isolators with integrated transformers offer many advantages over optocoupler and pulse transformer based designs. Integration significantly reduces size and design complexity, greatly improving timing characteristics. Galvanic isolation techniques used in the output driver improve robustness, while transformer coupling techniques significantly improve CMTI.

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