One Power Module, Many Applications


Now, one power module can suit multiple applications without any changes in its design or size! Miguel A. Mendoza, marketing manager, Micrel Inc., talks to Sneha Ambastha about the beneficial aspects of modern power modules.

28B_MMendoza1[1]Q. What is the current trend of the power modules?
A. The trend in modules is to achieve smaller solutions while maintaining the same power output, as well as to eliminate any additional design work for the customer. Our customers would like to see small, turnkey solutions that do not require making any decisions on what resistors to choose or size capacitors. The fewer components required, the more attractive the solution becomes. They would also like to use two or three modules to scale the power needs of their solution when required. This will allow them to use the same module block across many platforms, maximizing the cost benefit and reducing the need to validate new components. We are currently designing the next controller to help us achieve this demand.

Q. How does the high switching frequency of modern power modules help reduce the size of the output capacitor?
A. In general, a higher switching frequency allows for a smaller inductor and output capacitor, reducing the total solution size and monetary value. In case of the power module, the inductor is fixed since it’s inside the package. A fast transient response is what allows a customer to use small and inexpensive ceramic capacitors. The modules also offer a programmable switching speed from 200 KHz to 600 KHz, to get the best efficiency in the application. For example, the MIC28304 power module from Micrel could be used in an application with a 48V input supply and a program output voltage of 5V, where switching to 275 KHz would offer the best power efficiency, since the duty cycle of the field-effect transistor (FET) would be a lot shorter. The same device could be used to run at 12V input supply and 5V output voltage. At this electrical rating, using a higher frequency of 600 KHz would offer the best power efficiency since the switching duty cycle would be a lot larger. Switching faster allows higher power efficiency and the inductor does not get saturated.

Q. Does the high density of modern power modules make them suitable for a wide range of applications?
A. Power density is an industry term, which means being able to drive a lot of current or use very high input in a very small package. Even though our module can drive max currents at 3A, 6A, 10A, or 14A, customer applications don’t require the same supply currents in all applications or solution blocks. Their application requirements can vary from 1A to 12A, and the ability to use one solution that is very small in size and can serve multiple current requirements is very valuable to them. The benefit is that the customers can use the same power block to supply power to many applications without increasing the board space or any changes to their design. It allows the customers to reuse the same power block, to utilise a proven design over and over again. They also get better volume pricing on the module as they use the same part number at high volumes versus designing a new power block for each application every time.

Q. In comparison to the high-density power modules, how do you think that the low-density power modules would sustain in the industry?
A. They will eventually fall out favour, as they will be most costly compared to the smaller devices. In the end, we are all limited by material costs, test time and assembly. Making smart decisions on the mechanical assembly, material and component choices, as well as the circuit design, is critical to get to the price target of our customers.

Q. How does the surface-mount quad-flat, no-leads (QFN) package compare with other options like Dual Flat No-Lead (DFN) Packages or micro lead-frame packages (MLP)?
A. For the initial micro-module family release, we focused on the QFN package to offer our customers an overall solution size advantage. In the QFN package, the copper lead frame offers excellent thermal performance that allows our solution to de-rate heat more efficiently, allowing the device to offer more power density compared to its competitors. We are actively looking at other packages to address all customer applications and manufacturing limitations, as we understand that QFN packages are not always preferred by all of our customers.

Q. In what way does the copper lead frame contribute to the thermal performance of the power modules?
A. Copper lead frame has lower thermal resistance compared to printed circuit board (PCB) modules or multi-ball grid array modules. It also has lower electrical resistance, which minimises power losses that can be an issue in high current applications. Hence, the copper lead frame technology enables our devices to drive more current in a very small package. For example, Micrel’s MIC45212 can drive an output current of 14A in a small 12mm x 12mm x 4mm package. We spent a lot of engineering time to design a lead frame that allows our customers to use a very simple landing pattern on the PCB for easy manufacturing and reliability, without compromising the thermal de-rating.

Q. What difference does Hyperlight Load make to a power module and how does HyperSpeed help drive an application?
A. Hyperlight Load is a trademark Micrel circuitry that enables high power efficiency during light load conditions. The controller integrated circuit (IC) goes into a discontinuous mode to maintain power efficiency above 85 per cent compared to pulse-width modulation (PWM) buck controllers that drop as low as 30 per cent power efficiency at light load. This feature benefits applications that are in standby mode or operate at low current for a majority of the time.
The Hyperlight Load device also offers the HyperSpeed Control, a trademark circuitry that provides very fast transient response. The fast transient response minimises the output voltage ripple during any load changes, which allows customers to use very small output ceramic capacitors. Our modules offer exceptionally fast transient response, allowing our customers to use very small inexpensive ceramic capacitors and handle very fast current load changes without affecting the output voltage.

Q. What is pre-biased soft start?
A. Pre-biased soft start is an internal circuitry that reduces the input power supply surge current at start-up, which can cause a voltage drain on the input power supply. By initially slowing the output rise time, less surge current is required to charge the input capacitor. This circuitry is disabled after the soft start cycle ends to reduce current consumption.

Q. What is hiccup mode current limiting? How is it useful?
A. The modules offer a programmable short circuit limit that protects the DC/DC buck converter from a severe short circuit condition. The short circuit limit can be easily programmed by choosing the appropriate resistor value. When the programmed short circuit current is triggered, the device enters into a hiccup cycle to reduce the stress on the switching field-effect transistors (FETs) and to protect the load and supply from a severe short condition. This feature helps ensure that the DC/DC converter is protected from a catastrophic failure such as short on the output.

Q. How does under voltage lockout affect the working or efficiency of a device?
A. Our modules are designed to work in a wide input range. For example, the MIC28304 can operate from 4.5V to 70V input supplies. To ensure proper operation during power-sag conditions with very low input voltage, the modules have under-voltage lockout circuitry that disables the output when the supply voltage drops below a specified voltage such as 4.2V. This feature does not affect efficiency but assures that the device can operate within the scope of the design.

Q. What are the challenges involved in designing a better power module with the decreasing size of the devices? How can they be improved?
A. This is the real trick, as the size decreases the power devices need to be carefully chosen as not to affect the power efficiency. Typically, for a smaller device, power efficiency is compromised for size. At Micrel, we use very innovative and proprietary technology that allows us to assemble our devices in a smaller footprint without compromising power efficiency.

Q. How is the European Union’s EN 55022 standard different from the International Special Committee on Radio Interference’s CISPR 22 standard?
A. EN 55022 is a modified derivative of CISPR 22 and the European standard for information technology equipment (ITE). Procedures are given for the measurement of the levels of spurious signals generated by the ITE and limits are specified within the standard for protection of radio services in industrial, commercial or residential environments.

Q. How are the industry requirements changing with the changing standards of the power modules?
A. Today, modules are used in low volume applications or in time-critical designs, but are typically replaced by discrete solutions due to the higher cost of the modules. As the module gets closer to the cost of the discrete solution, modules will be used in high volumes applications. The big push will be on the price of the modules. At Micrel, we are ready to ramp up to volume demand, allowing for a more aggressive pricing strategy.

Q. What are the basic specifications that need to be taken care of while selecting a suitable power module for an application?
A. The customer needs to first identify what is the input voltage that they need to step down. What are the Point of Load (POL) requirements such as operating voltage and current requirements. The customer needs only to chose the appropriate module that matches the power needs. Micrel offers a family of products that serves almost all power rails from 5V up to 70V. The customer simply uses the resistor voltage divider to program the output voltage and chose the appropriate Input and Output capacitors to match up to the input and output voltages respectively; It’s that simple.

Sneha Ambastha is a technology journalist at EFY, Gurgaon.


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