My previous blog post discussed isolation requirements in a typical power-supply system and two popular gate-driver implementation methods: a gate-drive transformer and a high-/low-side gate driver. A high-/low-side driver, such as the 600V UCC27714, can save over 50% in PCB layout area and much more on component volume.
In this post, I will discuss more details about the actual implementation of the two methods, including their strengths and weaknesses.
Figure 1 shows the actual implementation of a gate-drive transformer for a half-bridge MOSFET configuration. The additional components (compared to Figure 2a in my previous blog post) are necessary in order to have a clean and solid gate-drive signal on the MOSFETs. These additional components are:
Figure 1: Actual implementation of a gate-drive transformer, considering parasitics
Obviously, additional components definitely increase gate-drive transformer design complexity. Leakage inductances also hurt the performance of the gate-drive transformer, including reduced peak gate-drive current and large overshoot (caused by leakage inductance and MOSFET junction capacitances). Practically speaking,, increasing the peak-drive current necessitates increasing the core size and winding-wire thickness to facilitate higher driving speeds; the corresponding effect will be higher overshoot, however, since the leakage inductance stores higher energy. Bifilar winding of the gate-drive transformer could be helpful to minimize leakage inductance; however, the trade-off is increased primary-to-secondary winding coupled leakage capacitance, CIO. CIO is the one of the major parasitics that limits common-mode transient immunity (CMTI) performance (see my blog post, “48V systems: Driving power MOSFETS efficiently and robustly” for an explanation). In summary, it is really difficult to do a better trade-off given the above-mentioned factors.
Figure 2 shows the actual implementation of the high-side and low-side gate-driver solution with digital isolator. Compared to Figure 2b in my last post, I added only a few major components: RBoot and a 5V LDO, used to provide a power interface between VBias, 10 to 20V, and the isolator secondary side, which requires a 3 to 5V low voltage/power supply.
Because there is no transformer leakage inductance-related issue compared to a gate-drive transformer, you can achieve better trade-offs among gate-drive current, overshoot, CMTI, etc.
Figure 2: Actual implementation of high-side and low-side gate driver
Table 1 compares the two methods. The high-/low-side gate driver does “win” from the perspective of having fewer auxiliary components, small parasitic inductances/CIO, smaller overshoot and PCB size, and flexible peak gate-drive current. Concerning isolated bias power, as I mentioned before, the high-/low-side gate driver could take advantage of the existing offline isolated power-supply subsystem.
Table 1: Comparison between gate-drive transformer and High-/Low-side gate driver
Stay tuned for the next installment of this series, when I will discuss the dynamic performance of each method.
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