How to design a smarter skylight

By  Matthew Sullivan , Texas Instruments

Thanks to new technology for future vehicles , sunroofs and car window tints are now programmable features in your car . At the turn of a switch, you can now block the light coming through your car's sunroof or enjoy the starry sky while driving at night.

A manufacturer called Research Frontiers has created an electronic window film using SPD-SmartGlass technology. The technology works by aligning nanoparticles in glass, plastic, acrylic or chemically strengthened glass film. This glass blocks heat, sunlight, UV rays and noise. SPD-SmartGlass can instantly and precisely control the level of light entering the vehicle by varying the amplitude of the voltage applied to the glass.

To drive this dynamic glass, a high-voltage AC signal is needed to quickly direct the light-blocking nanoparticles.

The intelligent sunroof design offers numerous advantages to the vehicle's occupants. In the film state, it reduces heat transfer and prevents glare, and in the film and clear states, it reduces UV and IR. Controlling the film tint level allows the user to adjust these conditions to the environment around them.

Generating the necessary high voltage AC signal to control the film tint level in a car is challenging because cars do not have a readily available source of AC voltage. Instead, an AC voltage signal needs to be generated using a power inverter circuit that converts the DC voltage from the car battery to AC voltage.

Texas Instruments ' automotive SPD-SmartGlass driver reference design demonstrates a method for converting DC to AC power. The two core components in this design are: 

• Boost converter to convert low voltage car battery DC to high voltage DC.

• Full-bridge driver to convert DC signal to AC signal.

From there, a square wave, sine wave, or other periodic waveform can provide power to the glass.

Figure 1 shows a block diagram of the reference design, while Figure 2 shows how the voltage is controlled in these intermediate steps to generate a sine wave.

Controlling the tinting level of the smart skylight

The level of window film tinting is directly related to the amplitude of the driving waveform. As shown in Figure 3, continuously changing the duty cycle at a sinusoidal rate produces a sine wave. After filtering, this produces a pure sine wave output. The pulse width modulation (PWM) duty cycle can be further adjusted to control the amplitude of this sine wave. 

Another amplitude control option is to adjust the voltage supplied to the full bridge. The sine wave PWM driving the bridge itself never needs to be adjusted, and scaling the voltage supplied to the bridge will result in the necessary change in the amplitude of the sine. This is the approach used in the reference design. 

To drive the FETs and ultimately the SmartGlass, two half-bridge gate drivers  ( UCC27712-Q1 ) are configured as a full-bridge. The UCC27712-Q1 is used in this application due to its interlock and dead-time features to ensure that the high-side and low-side FETs are not turned on at the same time.

The feedback pin that controls the high-voltage DC/DC voltage manages the power supply of the full bridge. By using an input resistor connected to the FB pin of the LM5155-Q1 boost converter, the DC voltage (represented by node VDAC in Figure 3) can control the LM5155-Q1 output voltage, thereby increasing or decreasing the boost voltage.

Figure 3: Input resistor for voltage output operation

The VDAC voltage is generated by filtering the PWM signal from the microcontroller using a resistor-capacitor (RC) filter and buffer to create a PWM digital-to-analog converter (DAC). Figure 4 illustrates how different PWM duty cycles and RC buffer circuits produce different DC voltage levels for use in the fuse box input circuit.

Figure 4: PWM DAC signal chain

Generates high voltage DC signal

Another unique feature of the reference design is the charge pump voltage tripler connected to the boost converter, as shown in Figure 5. The charge pump triples the voltage output of the boost converter to enable the generation of a 200V supply without the need for a large and expensive transformer-based power supply circuit.

Figure 5: Charge pump voltage tripler

One advantage of this method of generating 200 V is that the switching metal oxide semiconductor field effect transistor (MOSFET) Q2 in Figure 5 only needs to be rated for the voltage across the bottom output capacitor C16. This allows you to use lower cost, lower voltage rated components in your design instead of greater than 200 V components.

As the power and voltage ratings of MOSFETs increase, so do the gate and drain capacitances. These capacitances cause slew rate and rise time issues when trying to control a MOSFET at 2 MHz, so the lower voltage requirement makes it easier to find MOSFETs with appropriate input and output capacitance.

The charge pump also reduces the stress on the switch node. To step up from about 10 V to about 200 V, the switch node requires a very high duty cycle, leaving very little off time and putting a lot of stress on the boost converter. Because the charge pump reduces the converter’s voltage by one-third (at the expense of tripling the output current), the duty cycle and off-time requirements are significantly reduced. 

in conclusion 

Smart sunroofs can improve cabin comfort and provide a better, more comfortable driving experience. Our electronic product innovations can help you accelerate the process of implementing this new technology as an important part of future smart driving.