System designers tasked with supplying more power into products with ever-shrinking form factors must handle various problems, such as heat dissipation, which adversely affect the system’s electrical and thermal characteristics.
Fortunately, the manufacturers of power modules continue to develop new approaches to their system’s architecture to produce improved performance from even the smallest packages. Yet designers looking for the best power module need to be careful in their choice of product: differences in the architectures of power modules can greatly affect overall system performance in parameters such as heat dissipation, transient response, ripple voltage and even ease of use. It’s very much a case of ‘buyer beware’.
The advantages of modular power supplies over discrete circuits
There are many reasons to opt for a power module in preference to designing a power converter with discrete components. Integrating a controller, MOSFETs, inductors and passive components, modules only require the addition of external input and output capacitors to form a complete power supply. This means that power-system designs can be finished relatively easily and quickly, with confidence that basic performance and space requirements will be met.
For example, Intersil’s ISL8205M power module, which has an extremely low 1.85mm profile, can easily be placed on either side of a PCB for greater density and placement flexibility, while delivering excellent electrical and thermal performance. Normally this would provide sufficient justification for its use, but in fact the construction of the module subtly affects various parameters, features and capabilities.
Excellent thermal performance of small package
The ISL8205M uses a Quad-Flat No-lead (QFN) package in which the internal components are soldered directly to a copper lead frame, as shown in Figure 1. Wire bonds and a moulding compound form the other elements of the encapsulated package.
This structure allows the heat generated by the internal components to be dissipated directly by the copper in the lead frame, which has a thermal conductivity of 385W/mK. It is around 1,000 times more thermally conductive than a PCB, which has a typical thermal conductivity of 0.343W/mK.
As a result, the copper lead frame can help the heat to dissipate much more efficiently than a PCB-based module. Furthermore, a thicker copper lead frame may spread the heat over a larger area, thus accelerating the transfer of heat to the system’s board. It is also important to note that the moulding material can have a similar heat-spreading effect, increasing the area in which heat is transferred from the internal components, and providing an additional path to the lead frame.
The effect of this internal structure on thermal performance is shown in Figure 2, which features Intersil’s ISL8203M power module.
Mounted on a standard four-layer evaluation board with 2oz/1oz copper layers, the module is stepping down a 5V input to a 3.3V input to a 6A load. There is no cooling air flow, and the ambient temperature is 25°C. Even in these difficult conditions, the module’s maximum temperature is only 66.8°C.
The structure which Intersil has adopted in this family of power modules enables it to address the most extreme needs for power density with devices in ultra-compact packages, as shown in Table 1. Offering a complete DC-DC power supply, each module has been optimised for use in low-power, low-voltage applications while meeting demanding requirements for electrical and thermal performance. These complete power modules provide designers with a ready-made solution for when power density, time-to-market, reliability and the availability of design capabilities all affect the OEM’s choice of component.
|ISL8202M||ISL8203M (Single)||ISL8205M||ISL8203M (Dual)|
|Maximum Output Current per Channel||3A||3A||5A||6A|
|Input-voltage Range||2.6V to 5.5V||2.85V to 6.0V||2.6V to 5.5V||2.85V to 6.0V|
|Output Voltage Range||0.6V to 5.2V||0.8V to 5.0V||0.6V to 5.2V||0.8V to 5.0V|
|Topology||Current-mode PWM/PFM||Current-mode PWM||Current-mode PWM/PFM||Current-mode PWM|
|Light-load Efficiency Mode||Yes - PFM||No||Yes - PFM||No|
|Package (mm x mm x mm)||QFN 4.5 x 7.5 x 1.85||QFN 6.5 x 9 x 1.83||QFN 4.5 x 7.5 x 1.85||QFN 6.5 x 9 x 1.83|
Table 1: Intersil’s family of modules for low-voltage, low-power applications