The Virtues of PWM Combined With a Power MOSFET
Pulse Width Modulation is a method commonly used to generate an analog signal output from digital input. But how is this feature enhanced when combined with a power MOSFET in a single device?
When STMicroelectronics recently released the VIPer Controller 730 V power MOSFET, one of the touted features was a built-in pulse width modulation controller. When suppliers advertise a device as having PWM capabilities, just how much can it affect your design? A closer look at the definition of PWM and certain use cases can tell us more.
What is Pulse Width Modulation (PWM)?
Pulse Width Modulation (PWM) is a method commonly used to generate an analog signal output from digital input. There are three factors to consider in this process:
- The amplitude of the digital signal
- Duty cycle
- Frequency, which determines how fast digital signal switches between zero (off) and 100% (on) of the amplitude
The duty cycle is defined as the length of the time the digital signal is on. In the figure below, the duty cycle is on 25% before going to zero.
Graph of duty cycle, measured by time and amplitude.
This cycle is said to have a 25% duty cycle, which can be adjusted. When the duty cycle is programmed on high (say, 90%) and the switching frequency is high enough, the output analog signal appears to be constant.
For example, a digital signal input of 10 V with a duty cycle of 40% will produce output analog signals of 10% × 40% or 4V. Likewise, a 6V digital signal with duty cycle of 30% will yield 1.8 V accordingly.
Why Use PWM?
A simple illustration will demonstrate the usefulness of PWM. Consider an AC light source of 110 V with a constant 110 V AC input. Using the PWM technique to generate the same output with 80% duty cycles will be more energy-efficient. If the switching is fast enough, the naked eyes will not be able to tell the difference. PWM can be applied to controlling motors and heaters as well.
ST's Power MOSFET-PWM Controller Hybrid
PWM now comes with the single power MOSFET silicon. One example we can use to illustrate the above principles is STMicroelectronics' VIPer controller 730 V power MOSFET with a built-in PWM controller.
ST's VIPer controller. Image used courtesy of STMicroelectronics
The device supports multiple power conversion topologies, including:
- Non-isolated flyback converters (AC-DC and DC-DC)
- Isolated flyback converters with primary-side regulation or secondary-side regulation using a photo-coupler
- Buck converters (DC-DC step down)
- Buck-boost converters (DC-DC using a single inductor instead of a transformer)
Some external components for the printed circuit assembly (PCA) layout are eliminated by the device’s embedded high-voltage startup and the current sense circuits, simplifying the bill of materials (BoM).
Block diagram of VIPer Controller 730 V power MOSFET. Image used courtesy of STMicroelectronics
Other functions include operating voltage (VCC) of 4.5 V–30 V, startup voltage of 30 V DC, high-voltage converters up to 8 W, and light-load power consumption of less than 40 mW (at 230 VAC). Additionally, it has the short-circuit, thermal, and pulse skipping protection.
Packaged in 5 mm x 4 mm SSOP10, the device operates from -40°C to 150°C with storage from -55°C to 150°C.
This device is geared for several applications, including home appliances, consumer, industrial (motors and heaters), lighting, building-automation devices, and smart meters.
Different Chips for Different PWM Needs
PWM is not new. Traditionally, PWM is achieved with discrete components or with a FET PWM controller. Many power silicon makers integrate new functions—including PWM—onto a single chip, including:
- LinkSwitch from Power Integrations
- MP172 from Monolithic Power Systems
- NCP1063 from ON Semiconductor
If your design requires external PWM, you might consider Maxim’s external FET PWM controllers (MAX17595 and MAX17598) for isolated AC-DC applications. STMicroelectronics’ low-pin-count VIPer controller is a useful choice if you're looking for multiple power conversion topologies with built-in PWM function.
Learn More About PWM
We just covered the basic dimensions of PWM in this article. To learn more, check out our other discussions on PWM.
From your experience, what are the benefits and tradeoffs of PWM? Share your thoughts in the comments below.