In today’s world, because of stiff competition in the market, it is not only the basic functionality, which makes the product to sell. For acceptance by consumers, one of the most important factors is how the product looks and what high-end features it supports that makes product more reliable and desirable. For example, the user interface of Apple’s “i” family (iPhone, iPod, iPad, etc.) has played a vital role in the overall success of these products. Challenges that OEMs consistently face while integrating additional features to a product are the correct time to release the product to the market, a factor which is impacted by the learning curve of the devices being used and the overall cost of the system.
Take, for example, a user interface in a consumer appliance. Adding an attractive user interface not only increases the cost of the design but also introduces in additional constraints that can considerably delay the release of the product. In such scenarios, a device that can offer only a new UI but can also incorporate additional features becomes a compelling choice for OEMs. Such devices bring the overall cost of the system down by reducing the BOM and saving board space. At the same time, designing using these devices enables faster time to market because the components required to build a system are available in the same silicon. In addition, the time required to interface multiple devices and debugging for failures will be reduced considerably.
This article discusses an example of an induction cooker design and how a System-on-Chip (SoC) can be used to provide distinct advantages to the manufacturers by not only implementing the UI but also serving as the main controller.
Figure 1 provides the block diagram of an induction cooker, which must perform the following key functions:
- Maintaining the pan temperature
- Fan control
- Over-current & voltage protection
- Pan auto-detect
- Time based cooking functions
- User interface – Display
- User interface – Buttons
- Controlling the heating power – This function includes measuring the voltage and current and taking corresponding action to increase or decrease the current across the coil to achieve/maintain the required heating power as set by the user.
Figure 2 shows how this functionality can be implemented. In this implementation, based on the measurement of power, the firing duration for IGBT (Insulated Gate Bipolar Transistor) is controlled using a pulse width modulator (PWM).
Fan control – In an induction cooker, the temperature of the pan can go as high as 300o
It is therefore critical to ensure that other parts of the system do not heat up. The temperature of coil and IGBT must be maintained below the specified operating temperature. To achieve this, a fan is required to dissipate the heat and control the temperature of the system.
Figure 3 shows the implementation of a fan control system which monitors the temperature of the coil and firing circuitry (used to activate the coil) and accordingly modulates the speed of the fan to ensure the proper dissipation of heat.
Over-current and over-voltage protection – In any circuit, it is important to make sure that current and voltage stays within safe limits.
If the current or voltage goes out of the limit then, as a safety feature, the system should be turned off.
Figure 4 provides a typical implementation of an over-current protection. The voltage developed across the shunt resistor is provided to one input of a comparator (-ve) and the other input (+ve) is connected to a reference voltage. Whenever the current crosses the threshold, the comparator output will go LOW, which in turn will disable the PWM output that drives the IGBT.
Pan Auto-detect – This feature adds a lot of value to the design, as it automatically detects whether the pan is placed on the induction cooker or not. If the pan is not in place, then the system automatically cuts power to the IGBT, thus in turn switching OFF the induction cooker.
See below for a detailed explanation of this feature.
Time-based cooking functions – When a controller is already available to monitor many basics tasks, developers can easily add intelligence to the system. Setting the power and duration of cooking just like microwave oven allows user to better control the cooking process. Implementation of this feature requires an interface to an RTC to keep track of time.