Design Support Tools Use our suite of tools to help select, analyze and compare inductors Parametric Search View our list of inductors and filter results based on your exact parameters Looking for a datasheet? Search by product and download a PDF FREE Samples Get free samples quickly - most orders ship the same day! Looking for a datasheet? Search by product and download a PDF. Student Support Learn more about magnetics, request free samples or ask our engineers a question. Overview Isolated gate driver bias power supplies are widely used in applications such as EV traction inverters, on-board chargers (OBC), industrial motor drives, and solar inverters, etc. Their main function is to provide the re- quired bias voltage to gate driver ICs while maintaining galvanic isolation between the control side and high-voltage power stages. The transformer is a key component in these bias supplies. It provides isolation, voltage step up/step down, and plays a critical role in determining EMI and common mode noise performance. One of the most important parameters in this application is the interwinding capacitance of the transformer. This parasitic capacitance creates a path for fast switching noise from the power stage to couple back into the control circuit, especially in systems using wide-band gap devices like SiC and GaN. High interwinding capacitance can degrade CMTI (Common Mode Transient Immunity) and increase EMI. Thus, transformers with low interwinding capacitance are generally preferred, particularly in LLC based isolated bias supply topologies to meet the EMI requirements. Why LLC Topology Benefits from Low-Capacitance Transformers The LLC resonant converter has become a popular topology for isolated gate driver bias power supplies due to the combination of high efficiency, compact size, and excellent EMI performance, especially for driving high-speed GaN and SiC devices. It is widely used in designs driven by transformer driver ICs such as Texas Instruments’ UCC25800-Q1 and MPS’s MPQ18913. Unlike flyback or push-pull converters, LLC topology allows the transformer’s leakage inductance to be part of the resonant network. This opens up more flexibility in transformer design – giving space to minimizing interwinding capacitance. In topologies like flyback or push-pull, leakage inductance is considered as a parasitic parameter, designers often need to minimize leakage inductance which actually increases the interwinding capacitance. This inverse relationship is a design advantage in LLC where more leakage inductance can be intentionally introduced as the needed resonant inductance. In the meantime, interwinding capacitance can be reduced to lower common mode noise and improve CMTI. By reducing the interwinding capacitance, it directly reduces the magnitude of common mode noise. Low-Capacitance Transformers for LLC Bias Supplies We have three transformer series to meet the growing demand for low EMI, high CMTI isolated gate-driver bias supplies using LLC topology. Each optimized for different level of capacitance, isolation, and physical design requirements. The HTX7045C Series: our smallest size transformers with the lowest capacitance in our lineup The HTX8045C Series: the lowest interwinding capacitance, with higher isolation voltage than the HTX7045C Topology LLC LLC LLC Working voltage: 300 Vrms Working voltage: 350 Vrms *Basic Insulation Example Working voltage: 600 Vrms Working voltage: 800 Vrms 7 mm creepage and clearance How to Select an LLC Transformer VIN = 15 V VOUT = 5 V, Freq = 500 kHz, IOUT = 100 mA Step 1. Transformer turns-ratio selection Step 2. Transformer Volt-second rating selection Since the transformer primary side sees half of the input voltage, the volt time product is calculated based on a quarter of the switching period and half of the input voltage. Step 3. Current Rating Selection Secondary side winding RMS current: Primary side winding RMS current: Step 4. Interwinding Capacitance Minimizing interwinding capacitance has the desired effect of minimizing EMI, and the resulting higher leak- age inductance is not only acceptable in many cases, but can also be beneficial because it contributes to the resonant operation. The HTX7045C/HTX8045C’s interwinding capacitance is as low as 0.55 pF. ZE263x also offers interwinding capacitance as low as 1.61 pF. Step 5. Isolation Voltage Coilcraft offers a complete lineup of LLC-optimized low-capacitance transformers. All these transformers are ideal for SiC- or GaN-based LLC converters where EMI, isolation, and space are critical. The HTX7045C stands out for its ultra-low interwinding capacitance (as low as 0.75 pF), effectively reducing EMI and enhancing CMTI in isolated gate driver power supplies. By using the innovative HTX7045C, we can significantly shrink the overall solution size, reduce weight and lower costs. The ZE263x series offers high isolation at 4000 Vrms with competitive capacitance (1.61 pF to 2.43 pF), lower than most of our larger competitors. Copyright 2026, Coilcraft, Inc. Taiwan +886 2-2264 3646 We value your privacy We use essential and non-essential cookies on our Site, including third party advertising cookies to provide tailored ads. For more details or to manage your preferences, please see our Privacy Policy