Providing isolated low voltage bias power to ICs such as microcontrollers, analog-to-digital converters, isolated gate drivers or voltage monitoring ICs in high voltage systems is usually accomplished with an isolated DC-DC converter. If the high voltage system is spread out over several modules, the architecture may call for a parallel DC bus on the low voltage side with multiple isolated low power DC-DC converters for each module. Because it is used multiple times, an efficient and cost-effective topology is the best approach.
This article highlights the design benefits of using push-pull transformers and uses Bourns® Model HCTSM8 series transformer as an example. This series is AEC-Q200 compliant and available with a wide range of turns ratios as standard. Multiple turns ratios are an important feature enabling the same basic circuit topology to be replicated across a system with the same components and PCB layout. With the Model HCTSM8, designers are able to select the right reinforced transformer part number based on the specified output voltage for powering a microcontroller or an isolated IGBT gate driver.
Push-pull transformers are known to operate well with low voltages and low variations in input and output. This characteristic is ideal for a microcontroller bias or gate driver IC that has constant power levels and input voltages. Unlike typical flyback and forward topologies, the push-pull topology offers high efficiency at a stable input and output current.
In addition, flyback transformers can cause EMI problems and often require closed loop control for stable operation even though they can efficiently handle wide input ranges. Conversely, a push-pull transformer can operate very simply in open loop. Compared to the components required for closed loop control, open loop control only requires a driver with a fixed duty cycle along with two MOSFETs, a transformer whose turns ratio is selected to suit the desired output, two Schottky diodes, and two ceramic capacitors.
Push-pull transformers are typically offered in a smaller footprint than flyback transformers. And, push-pull transformers usually have physically smaller ferrite cores compared to flyback transformers. Plus, there is no gap required in the ferrite core of a pushpull transformer, and, therefore, the effective permeability remains high and the magnetizing inductance can be quite high for a low number of turns. Given a sufficiently high switching frequency and low DC voltages, the flux generated (Volt Seconds per Turn) remains well below the saturation point. Contrast this result with the split ferrite core in a flyback transformer where more turns are needed to ensure the current does not saturate the transformer.
If there are tight space considerations and restrictions, the DC resistance will inevitably increase with the higher number of turns, resulting in reduced efficiency. It is advised to look for a push-pull transformer with a toroidal core as there is no need for a gap, and is well known for providing good coupling between windings.
The Bourns® Model HCTSM8 has reinforced insulation, which according to standards 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 insulation from the start of the coil during the winding process will be twice that of a triple insulated transformer. The consensus is double insulation is less efficient and more expensive.
In the HCTSM8 series transformer, the secondary winding consists of FIW (fully insulated 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.
The Bourns® HCTSM8 push-pull transformer 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 between the lid and the side wall. 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 design that features a press fit of the lid against side wall 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 plastic material classified as Class I, which is the least conductive of all plastics to high voltages. It features triple insulated wire on one winding (primary). Consequently, by taking 8.0 mm as creepage and clearance distance 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 operates 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 transportation and other markets increases, so too will the demand for stable, high quality and standardized isolated power designs in applications such as modules in high voltage battery or ultra-capacitor packs. Bourns offers 11 different fully tested and AEC-Q200 compliant Model HCTSM8 series push-pull transformer part numbers for the Texas Instrument drivers.
Author: By Lee Bourns, Product Line Manager Overcurrent Protection & PTC Heaters at Bourns, Inc.