Low-Capacitance Transformers for LLC Isolated Gate Driver Bias Supplies

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.

Figure 1 shows a typical example of an isolated gate driver bias supply used in an automotive traction inverter. The transformer provides isolation and voltage conversion, and its performance directly impacts EMI and CMTI.
 

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.

LLC converters operate at high frequencies (typically 100-500 kHz or higher), and the switching nodes have very fast voltage transitions – high dv/dt, especially for SiC or GaN drivers. These high dv/dt on the switch node can couple through the transformer’s interwinding capacitance (Cps) and causes extra EMI noise (Figure 2).
 
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
• The ZE263x Series: the highest isolation core and bobbin option, offering reinforced insulation and increased creepage and clearance

 

Features	HTX7045C	HTX8045C	ZE263x
Structure	Chip-style	Chip-style	Core and Bobbin
Interwinding Capacitance	0.70 – 0.90 pF	0.55 – 0.76 pF	1.61 – 2.43 pF
Isolation Voltage	2800 Vrms	3500 Vrms	4000 Vrms
Dimension	7.2 x 4.85 x 5.3 mm	8.2 x 4.85 x 5.3 mm	11.7 x 10 x 12.5 mm
Topology	LLC	LLC	LLC
*Reinforced Insulation Example	Working voltage: 240 Vrms 4.9 mm creepage and clearance	Working voltage: 300 Vrms 5.9 mm creepage and clearance	Working voltage: 350 Vrms 7 mm creepage and clearance
*Basic Insulation Example	Working voltage: 600 Vrms 4.9 mm creepage and clearance	Working voltage: 600 Vrms 5.9 mm creepage and clearance	Working voltage: 800 Vrms 7 mm creepage and clearance

*Safety insulation examples are based on Altitude 3000 m, Overvoltage category II, Pollution degree 2, Material Group I and the listed creepage and clearance.
 

How to Select an LLC Transformer

As shown in Figure 3, a secondary-side resonant configuration, where the resonant capacitor is located on the secondary side, is preferred for the open-loop half bridge LLC converter. Here are the steps using these input parameters:
V_IN = 15 V
V_OUT = 5 V,
Freq = 500 kHz,
I_OUT = 100 mA

Step 1. Transformer turns-ratio selection


The 1 V is the voltage drop on the diodes and can be adjusted as needed.
 
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
The HTX7045C, HTX8045C and ZE2638-BE all have turns ratios of 1:0.4 (Nps = 2.5). The HTX7045C provides 2800 Vrms isolation (Hi-pot) between primary and secondary winding in a very compact package. For applications requiring higher isolation levels, the ZE263x provides 4000 Vrms.
 

Part Number	L (µH)	Cps (pF)	Volt-time (V-µsec)	Irms (mA)	Isolation (Vrms)	Creepage & Clearance (mm)	Basic Working Voltage (V)	Height (mm)	Footprint (mm)
HTX7045C-134	130	0.70	45.3	660	2800	4.9	600	5.3	35
HTX8045C-114	115	0.60	27	650	3500	5.9	600	5.3	40
ZE2638-BE	100	1.61	21	1070	4000	7	800	12.5	117

 

Summary

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.

We offer a range of transformer designs including custom transformers,each optimized for different application requirements – whether you’re focused on EMI, isolation, or size constraints.
 

References

1.    UCC25800-Q1 Ultra-low EMI transformer driver for isolated bias supplies
2.    Bias Supply Design for Isolated Gate Driver Using UCC25800-Q1 Open-Loop LLC Transformer Driver

 

 Contact us to learn more
and order free samples