OEM&Lieferant Ausgabe 2/2021

95 Better Insulation The Bourns® Model HCTSM8 has rein- forced insulation, which according to stan- dards must consist of either triple insulated wire (three separate layers of insulation on the wire) on one winding or insulation on both windings (double insulation). Double insulation is not efficient from an electrical point of view. The time to strip the insula- tion from the start of the coil during the winding process will be twice that of a tri- ple insulated transformer. The consensus is double insulation is less efficient and more expensive. In the HCTSM8 series transformer, the sec- ondary winding consists of FIW (fully insu- lated wire), which is considered as strong as triple insulated wire but without safety agency recognition. These features are particularly relevant in a transformer with a toroidal core. Maximum Creepage/Clearance The Bourns® HCTSM8 push-pull transform- er offers maximized creepage in a minimal footprint. The core is not visible from the pins so the clearance is measured up the wall of the device and down the joint be- tween the lid and the sidewall. The effective tracking distance over the insulated wire from pin to core is maximized by running the insulated wire around the outside of the component. By using this breakthrough de- sign that features a press fit of the lid against the sidewall and the wraparound insulated wire, the Model HCTSM8 series can obtain a creepage and clearance of 8.0 mm despite having a nominal height of just 6.5 mm and a distance from pad to pad on the PCB of 11 mm max. In addition, the Model HCTSM8 uses plas- tic material classified as Class I, which is the least conductive of all plastics to high Bourns www.bourns.com Share voltages. It features triple insulated wire on one winding (primary). Consequently, by tak- ing 8.0 mm as creepage and clearance dis- tance and consulting table F.4 of IEC 60664, this transformer offers a working voltage of 800 Vrms. As a result, inverters and battery packs with rms voltages of up to 800 Vrms requiring reinforced insulation could use the HCTSM8 for Isolated DC voltages for a gate driver for an IGBT or SiC MOSFET and for Isolated DC power for a microcontroller or voltage monitoring IC or transceiver. Typical Application Usage To generate plus and minus voltages for a gate driver, a circuit configuration similar to that shown in Figure 4 represents why Bourns® Model HCTSM8 is a valid solution. In this example, the device is driven by an integrated Texas Instruments SN6501 push- pull driver. The Texas Instruments device op- erates at a high frequency (400 kHz) and has a fixed duty cycle (50 percent). The output relationship in a push-pull driver with Input Vin and Output Vout and duty cycle D is as follows: Vout = 2 x D x n x Vin where n is the turns ratio from secondary to primary. Model HCTSM8 has 11 different standard turns ratios. Because the Texas Instruments SN6501 device uses internal MOSFETs whose maximum voltage rating is 5 V, the Vin cannot exceed this level. And in order to generate 12 V which is required to switch on an IGBT, it requires a turns ratio of 2.5. Given that the Model HCTSM8 series is a catalog product with AEC-Q200 compliant quality levels, it provides an efficient and cost- effective isolated power source compared to a customized transformer. The Bourns® HCT series has been tested and approved by Texas Instruments in their Model SN6501 and SN6505 series of push- pull drivers. As the range of applications for high voltage driven equipment in transpor- tation and other markets increases, so too will the demand for stable, high quality and standardized isolated power designs in ap- plications such as modules in high voltage battery or ultra-capacitor packs. Bourns of- fers 11 different fully tested and AEC-Q200 compliant Model HCTSM8 series push-pull transformer part numbers for the Texas Instrument drivers. Figure 3 shows the efficiency of a circuit when using Model HCTSM80308BAL, which is an ideal solution to provide the 15 V needed for an isolated gate driver IC. The optimum operating point for this application is between 100 mA and 150 mA output current. Figure 4: Conducted emissions from a 5 V HCT push-pull transformer without snubber or filtering com- ponents

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